WORKSHOP ON PARENT-BODY AND NEBULAR MODIFICATION OF CHONDRITIC MATERIALS
LPI Technical Report Number 97-02, Part 1 Lunar and Planetary Institute 3600 Bay Area Boulevard Houston TX 77058-1113 LPIITR--97 -02, Pan 1
WORKSHOP ON
PARENT-BODY AND NEBULAR MODIFICATION
OF CHONDRITIC MATERIALS
Edited by
M. E. Zolensky, A. N. Krot, and E. R. D. Scott
Held at Maui, Hawai'i
July 17-19,1997
Hosted by Hawai'i Institute of Geophysics & Planetology University of Hawai'i
Sponsored by Lunar and Planetary Institute University ofHawai'i National Aeronautics and Space Administration NASA Integrated Systems Network
Lunar and Planetary Institute 3600 Bay Area Boulevard Houston TX 77058-1113
LPI Technical Report Number 97-02, Part 1 LPIITR--97-02, Part 1 Compiled in 1997 by LUNAR AND PLANETARY INSTITUTE
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Zolensky M. E., Krot A. N., and Scott E. R. D., eds. (1997) Workshop on Parent-Body and Nebular Modification of Chondritic Materials. LPI Tech. Rpt. 97-02, Part I, Lunar and Planetary Institute, Houston. 71 pp.
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Cover: Olivine with amorphous olivine rims in a primitive (?) clast in the Ningqiang chondrite. View measures I Ilm across. LP] Technical Report 97-02. Part] 111
Preface
This volume contains abstracts that have been accepted for presentation at the Workshop on Parent-Body and Nebular Modification ofChondri tic Materials, July 17-19, 1997, in Maui, Hawai'i. The workshop was organized by co-conveners M. E. Zolensky (NASA Johnson Space Center), A. N. Krot (University ofHawai'i), and E. R. D. Scott (University ofHawai'i). Members ofthe Scientific Organizing Committee were A. Bischoff (Institut for Plane to logie, Munster), P. Cassen (NASA Ames Research Center), B. Fegley Jr. (Washington University), K. Keil (University ofHawai'i), J. Kerridge (University ofCalifornia-San Diego), H. Nagahara (University ofTokyo), H. Palme (Universitiit zu Koln), S. Russell (Smithsonian Institution), and K. Tomeoka (Kobe Univers,ity). Logistics and administrative and publications support for the workshop were provided by the staff ofthe Lunar and Planetary Institute.
LPI Technical Report 97-02, Part I v
Contents
Thermally Metamorphosed Antarctic CM and CI Carbonaceous Chondrites in Japanese Collections, and Transformation Processes ofPhyllosilicates J. Akai and S. Tari ...... 1
Aqueous Alteration of Carbonaceous Chondrites: Evidence for Preaccretionary Alteration A. Bischoff...... 2
Magnetite in Vigarano: An Iron-57 M6ssbauer Spectroscopy Study P. A. Bland, M. A. Sephton, A. W. R. Bevan, F. J. Berry, J. M. Cadogan, and C. T Pi/linger ...... '" ... 3
Metamorphic Transformations of Opaque Minerals in Chondrites M Bourot-Denise, B. Zanda, and R .. Hewins ...... 5
Unraveling Nebular and Parent-Body Effects in Chondrite Matrixes: Mineralogical and Compositional Constraints A. J. Brearley ...... 7
Chlorine-bearing Melt Inclusions in Olivine from Unequilibrated Ordinary Chondrites J. C. Bridges ...... 8
A Search for Accretionary Textures in CM Chondrites L. Browning ...... 9
Use of Oxygen Isotopes to Constrain the Nebular and Asteroidal Modification of Chondritic Materials R. N. Clayton ...... lO
Turbulent Size Selection and Concentration of Chondrule-sized Objects: Reynolds Number Invariance and Implications J. N. Cuzzi, R. Hogan, A. Dobrovolskis, and J. Paque ...... 11
Effect of Revised Nebular Water Distribution on Enstatite Chondrite Formation K. E. Cyr, M L. Hutson, and J. 1. Lunine ...... 12
Interstellar Hydroxyls in Meteoritic Chondrules: Implications for the Origin of Water in the Inner Solar System E. Deloule, J.-c. Doukhan, and F. Robert ...... 13
Theoretical Models and Experimental Studies ofGas-Grain Chemistry in the Solar Nebula B. Fegley Jr...... 14 vi Workshop on Modification of Chondritic Materials
The Chondrite-Achondrite Transition: Decoupling of Oxygen Isotope and Geochemical Changes C. A. Goodrich ...... 15
Progressive Alteration ofCM2 Chondrite Matrixes: Determining Relative Phyllosilicate Contents by X-Ray Diffraction M M Grady, M. Batchelder, G. Cressey, and M J Genge ...... 17
Primitive Matrix Components of the Unique Carbonaceous Chondrite Acfer 094: Clues to Their Origin A. Greshake ...... 18
Chemical Alteration of Chondrules on Parent Bodies J N. Grossman, C. M. 0 'D. Alexander, and J. Wang ...... 19
Thermal Quenching of Silicate Grains in Proto stellar Sources S. L. Hallenbeck, J A. Nuth, and F. J. M Rietmeijer ...... 20
Iron-rich Aureoles as Recorders ofIn Situ Aqueous Alteration in the CM Carbonaceous Chondrites Murray, Murchison, and Allan Hills 81002 N. P. Hanowski and A. J Brearley ...... 21
Transportation of Gaseous Elements and Their Isotopes in a Thermally Evolving Chondritic Planetesimal K. Hashizume and N. Sugiura ...... 22
An Experimental Study of Magnetite Formation in the Solar Nebula Y Hong and B. Fegley Jr...... 23
Lightning and Shock Heating as Candidate Processes for Chondrule Formation M Horanyi ...... 24
Fayalitic Halos Within Forsterites from Carbonaceous Chondrites X Hua and P. R. Buseck ...... 25
Presolar Grains as Tracers of Nebular and Parent-Body Processing of Chondritic Material G. R. Huss ...... 26
Chronologic Constraints on Secondary Alteration Processes l. D. Hutcheon ...... 27
Elemental Redistribution by Aqueous Fluids in Un equilibrated Ordinary Chondrites: Tieschitz and Semarkona Compared R. Hutchison, C. M. 0 'D. Alexander, and J. C. Bridges ...... 27 LPI Technical Report 97·02;Part 1 vii
Anhydrous Alteration of Allende Chondrules in the Solar Nebula Y. Ikeda and M. Kimura ...... 29
The Kaidun Meteorite: Evidence for Pre- and Postaccretionary Aqueous Alteration A. V. Ivanov, G. Kurat, F. Brandstatter, L. F. Migdisova, and N. N. Kononkova ...... ~ ...... 29
Alteration of Plagioclase-rich Chondrules in C03 Chondrites: Evidence for Late-stage Sodium and Iron Metasomatism in a Nebular Environment R. H. Jones ...... 30
A Transmission Electron Microscope Study ofthe Matrix Mineralogy of the Leoville CV3 (Reduced-Group) Carbonaceous Chondrite: Nebular and Parent-Body Features L. P. Keller ...... 31
Relationship Between Anhydrous and Aqueous Alterations in CV3 Chondrites M Kimura and Y. Ikeda ...... 33
Mineralogical and Chemical Modification ofCV3 Chondrites During Fluid-assisted Metamorphism in the CV3 Asteroid A. N. Krot, E. R. D. Scott, and M. E. Zolensky ...... 34
The Meteorite Parent-Body Alteration Model and the Incompatible Reality G. Kurat ...... 36
The Alteration ofNickel-bearing Sulfides During Thermal Metamorphism on Ordinary Chondrite Parent Bodies D. S. Lauretta, K. Lodders, and B. Fegley Jr...... 36
Nebular and Parent-Body Processes in Chondrites: Labile Trace Elements as Indicators and Thermometers M E. Lipschutz ...... 38
What Do Enstatite Meteorites Tell Us About the Solar Nebula? K. Lodders and B. Fegley Jr...... 39
Fragmental Aggregation in the Nebula: A Basic Nebular Process G. E. Lofgren ...... 40
Parent-Body Metamorphism ofCV3 Chondrites: Counterarguments Based on Accretionary Rims ~d Calcium-Aluminum-rich Inclusions G. J. MacPherson and A. M. Davis ...... 42 Vlll· Workshop on Modification of Chondritic Materials
Evaporation Behavior ofMinerals and Silicate Melt in Vacuum and in Hydrogen Gas H. Nagahara ...... , ...... 43
Rubidium-Strontium Isotopic Systematics ofChondrules from the Antarctic CV Chondrites Yamato 86751 and Yamato 86009: Additional Evidence for Late Parent-Body Modification N Nakamura, T. Kani, and G. Kondorosi ...... 44
Experimental Study on Formation of Secondary Minerals in Calcium-Aluminum-rich Inclusions in Carbonaceous Chondrites K. Nomura and M Miyamoto ...... ;...... 44
Impact Melting, Metal-Silicate Fractionation, and Volatile-Element Mobility on the L-Chondrite Parent Body M. D. Norman ...... 45
Formation of Single-Domain Iron Particles: Implications for the Nebula J. A. Nuth and P. A. Withey ...... '" ...... 47
Oxygen-Fugacity Indicators in Carbonaceous Chondrites: Parent-Body Alteration or High-Temperature Nebular Oxidation? H. Palme ...... 48
Thermodynamic Modeling ofAqueous Alteration in CV Chondrites M 1. Petaev and M . V. Mironenko ...... 49
Antarctic LL Chondrites A. M Reid ...... 50
Alteration ofPre solar Dust Based on Transmission Electron Microscope/Analytical Electron Microscope Studies of Chondritic Interplanetary Dust Particles and Nonequilibrium Simulation Experiments . F. J M. Rietmeijer, F. Guofei, and J. M. Karner ...... 51
Correlation ofWater of Hydration with Diameter in Tholen E-class and M -class Asteroids A. S. Rivkin, D. T. Britt, L. A. Lebofsky, E. S. Howell, and B. E. Clark ...... 53
Alteration of Calcium-Aluminum-rich Inclusions: Times and Places S. S. Russell and G. J MacPherson ...... 54
Evidence from the Bovedy (L3) Chondrite for Impact-generated Chondrules 1. S. Sanders ...... 55 LPI Technical Report 97-02, Part I ix
Asteroidal Modification of C and 0 Chondrites: Myths and Models E. R. D. Scott, A. N. Krot, and L. B. Browning ...... 56
Nebular or Parent-Body Alteration ofChondri tic Material: Neither or Both? D. W G. Sears and G. Akridge ...... , ...... 57
Evolved Gas Analysis: A Technique to Study Cosmic Alteration of Chondrites? Th . Stelzner and K. Heide ...... 58
Volatile-Element Enrichments in Interplanetary Dust Due to Nebular Processes? T. Stephan, D. Rost, and E. K. Jessberger...... 59
Do Iodine-Xenon Ages Tell Us Anything About the Site of Secondary Alteration? T. D. Swindle ...... '" ...... 60
Aqueous Alteration and Dehydration Processes in the Carbonaceous Chondrites K. Tomeoka ...... 61
Manganese-Chromium Systematics in Sulfides of Un equilibrated Enstatite Chondrites: Parent-Body vs. Nebular Processing and Implications for Accretion Times M. Wadhwa, E. K. Zinner, and G. Crozaz ...... 62
Conditions for Forming Calcium-Aluminum-Rich-Inclusion Rim Layers: Preliminary Experiments D. A. Wark '" ...... 63
Oxygen Fugacity in the Solar Nebula J. T. Wasson ...... 64
Refractory Forsterite in Carbonaceous Chondrites: An Unaltered Condensate from the Solar Nebula S. Weinbruch, H. Palme, B. Spettel, and 1. M. Steele ...... 65
Fayalitic Olivine in CV3 Chondrite Matrix and Dark Inclusions: A Nebular Origin M K. Weisberg and M Prinz ...... 66
Volatile Trace-Element Composition and Shock in Equilibrated H Chondrites S. F. Wolfand M E. Lipschutz ...... , ...... 67
A Unique Chondrule Consisting ofForsterite and Clear Glass Groundmass with Compositional Zoning in Ordinary Chondrites K. Yanai ...... 68 x Workshop on Modification of Chondritic Materials
The History of Metal and Sulfides in Chondrites B. Zanda, Y Yu, M. Bourot-Denise, and R. Hewins ...... 68
Aqueous Alteration of Carbonaceous Chondrites: Evidence for Asteroidal Alteration M E. Zolensky ...... 70 LPI Technical Report 97-02, Part I
Abstracts
THERMALLY METAMORPHOSED ANTARCTIC CM AND CI CARBONACEOUS CHONDRITES IN JAPANESE COL LECTIONS, AND TRANSFORMATION PROCESSES OF PHYLLOSILICATES. 1. Akai and S. Tari, Department of Ge ology, Faculty of Science, Niigata University, Ikarashi 2-nocho, 8050, Niigata 950-21, Japan.
Since the first discovery ofthermal metamorphism ofCM and CI chondrites [1,2] a lot ofinterest has been focused on these materials and some data have been accumulated [1-10]. Thermal metamor phism is very characteristic of CM and CI chondrites, in contrast to non-Antarctic carbonaceous chondrites. In the metamorphism, phyllosilicates change to olivine, through some intermediate struc tures [4,5,7-9]. The cause of this metamorphism has also been variously estimated [11-14]. Some shock effect experiments have been carried out to ascertain this possibility [15]. However, the possibility of heating by shock events was considered less likely. Hiroi et al. [13,14] suggested possible thermal metamorphism in many of the Co, G-, B-, and F-type asteroids. They interpreted it as due to thermal metamorphism after extensive aqueous alteration in Fig. 1. (a) TEM lattice image of patchy structure in serpentine minerals the parent body.The different degrees ofthermal metamorphism may found in Y 793595. (b) Corresponding ED pattern suggesting 14-A diffrac depend partly on the different water content and depth. More data are tion maxima. necessary to ascertain the validity of these scenarios or suggest others. The ratio of thermally metamorphosed carbonaceous chon drites/unheated carbonaceous chondrites may become the funda mental datum. by the author are also included): ClI : Y 82162; C12 : Y 86720, The objectives ofthis study are (I) Using as many Antarctic CM B 7904; CM2: Y 74662, Y791198, Y 793321, Y 793595, Y 82042, and CI carbonaceous chondrite specimens as possible, we want to Y 82054, Y 82098, Y 86695, A 881334, A 881458, and obtain the ratio in Antarctic meteorites. In this study we searched for A 881955. the mineralogical evidence of metamorphism in the constituent These specimens can be grouped as follows, based on the degree minerals. (2) We further describe constituent minerals by TEM. ofthermal metamorphism, although some have not been fully exam (3) Summarizing previous data also, the authors will describe the ined yet. Estimated temperatures are also shown for some specimens. thermal transformation process of phyllosilicates. Intensely metamorphosedspecimens includeY 86720 (700°-850°C), The following 14 Antarctic carbonaceous chondrite specimens Y 82162 (600°-800°C), B 7904 (750°-900°C), Y 82054, Y 82098, were examined (carbonaceous chondrites that have been examined Y 86695, and A 881334. Weakly or very weakly metamorphosed
. :::t:""'{' ... ~...... 6~0'~<~ : .7b.o~;~
- .....;. ~:: -
. EO .P'I'u.m . ha;.l :o. p~:t::t:e~::: d'l; sapp&.arance. e£ l'Ial:o T....~ 'o.itf-. .C:!$~ .p. pe:a ::.··.: a~ff. and",?~=<:e of. A "PE'e..t s }Str;~a...... 1th a 9-1)1 AdiZ'zus< c. . . s'h~!:" olivi ne s!X)ts di.f~i:;a,c -tion lllJ/.ntaxb... diff:.!Sl! ell'.'ine ~v.ots
Fig. 2. Schematic transfonnation process of serpentine during thermal metamorphism. 2 Workshop on Modification of Chondritic Materials
specimens include Y 793321 (300°-500°C), Y 793595 «250°C?), ing nebular and parent-body processes. The study ofminerals altered A 881458 «250°C?), and Y 82042 (slightly to unmetamorphosed?). and formed by aqueous processes before accretion or within the Unheated specimens include Y 74662, Y 791198, and A 881955. meteorite parent body(ies) is offundamental importance considering These data indicate that thermally metamorphosed Antarctic CM various aspects in the evolution of solid matter in the early solar and CI carbonaceous chondrites dominate compared to non-Antarc system. Below, evidence for preaccretionary aqueous alteration of tic chondrites. These characteristics correspond to the past flux of carbonaceous chondrites will be discussed in detail. meteorite falls to the Earth. In this study, very weak metamorphism The following remarks have to be made in advance: was suggested in several specimens (Y 793595, A 881458, etc.). I. Concerning the topic of this contribution two very important More detailed examinations may be necessary in some cases to detect questions have to be considered: When do nebular processes end very weak metamorphism on other specimens that have been previ and planetary (parent-body) processes start? Should we use the term ously reported as unheated. Thus, degrees of metamorphism from "nebular" or"preaccretionary" to characterize processes that happend very weak to intense are expected, assuming inner heating processes before accretion of the final (CI, CM, CR, etc.) parent body? Here, in the parent body. the term "preaccretionary" will be used, because this term would Recent investigation of Y 793595 indicated that it contained allow processes that occurred in small precursor planetesimals (see polygonal serpentine, which is rarely found in terrestrial environ below; [1,2]). ments. The other characteristics ofphyllosilicates and related layer 2. This abstract will summarize evidence for preaccretionary lattice minerals are similar to those in non-Antarctic carbonaceous aqueous alteration. Describing and discussing preaccretionary fea chondrites: tochilinite, and regularly mixed layer minerals of tures resulting from aqueous alteration does not mean that the author tochilinite and serpentine structures, etc., were found for these un rules out any kind of aqueous alteration on the meteorite parent heated or very weakly metamorphosed CM2 specimens. bodies as an important process in the evolution ofseveral groups of In Y 793595 and A 881458, which may have experienced very carbonaceous chondrites. Many aqueous alteration features in CM weak metamorphism, characteristic streaks with 14-A diffraction and (perhaps all) in CI chondrites certainly result from parent-body maxima from patchy structures were associated with main 7-A spots processes [e.g., 3-8]. (Fig. I). This may correspond to an initial stage ofdecomposition of Observations in C Chondrites: In CI chondrites no indica 7 -A serpentine structure. Summarizing these data, the decompositional tions for preaccretionary aqueous alteration are found. Any pre sequence (stages) of serpentine minerals during thermal metamor accretionary signature has been erased by (I) severe aqueous alter phism were estimated: Ordered 7-A serpentine --7 patchy textures ation processes certainly due to the presence of liquid water (e.g., with broad 14-A diffraction spots --7 decomposed intermediate tran early formation of carbonate veins [7,9]) and (2) impact brecciation sitional structure with halo diffraction --7 domain formation ofoliv (individual clasts usually
bodies for the evolution of early solar system materials has been M. K. et a\. (1995) Proc. NIPR Symp. Antarct. Meteorites, 8, II. summarized by Kerridge et al. [25]. These authors state that evidence [21] Ichikawa O. and Ikeda Y. (1995) Proc. NIPR Symp. Antarct. either for nebular or planetary aqueous alteration processes would Meteorites, 8, 63. [22] Brearley A. J. (1993) GCA, 57, 1521. have fundamental implications for ideas concerning nebular dynam [23] Keller L. P. and Buseck P. R. (1990) LPS XXI, 619. ics, gas-solid interactions in the nebula, and accretionary processes. [24] Hashimoto A. and Grossman L. (\987) GCA, 51, 1685. Considering various phases and paragenesis in carbonaceous [25] Kerridge J. F. et al. (\994) Meteoritics, 29, 481. chondrites affected by aqueous alteration, it is in many cases difficult to distinguish between nebular and parent-body effects. However, based on mineralogical evidence [I] and Tomeoka et al. [2] provide MAGNETITE IN VIGARANO: AN IRON-57 MOSSBAUER clear evidence that phyllosilicates in rims formed prior to the forma SPECTROSCOPY STUDY. P. A. Blandl , M. A. Sephton2, tion of the parent body. Due to the paragenesis in the rims and sizes A. W. R. Bevanl , F. J. Berry3, J. M. Cadogan4, and C. T. PillingerS, of the rims, a parent-body origin can be ruled out (compare, e.g., lWestem Australian Museum, Francis Street, Perth, Western Aus Figs. 10e and II of[l] and discussions of[I,2]). Also, based on tralia 6000, 2Department of Geochemistry, Faculty of Earth Sci chemical studies, evidence has been found that certain phyllosili ence, University of Utrecht, P.O. Box 80021, 3508 TA, Utrecht, cates probably formed in the nebula [15]. Schirm eyer et a\. [15] The Netherlands, 3Department ofChemistry, The Open University, found that phyllosilicates within several Ca,AI-rich inclusions of Milton Keynes MK7 6AA, UK, 4S chool ofPhysics, The University CM chondrites contain Li, whereas mineralogically very similar of New South Wales, Sydney, NSW 2052, Australia, SPlanetary phyllosilicates in the accretionary rims ofthe CAl (I 0 ~m apart from Sciences Research Institute, The Open University, Milton Keynes the Li-bearing grains) do not. A nebular origin for certain phyllo MK.7 6AA, UK. silicates in CAls is supported by the finding of hydrous phases within CAls of CV chondrites [23,24]. On the other hand, some Introduction: CV chondrites are subdivided into two main observations such as the formation of calcite [25] are difficult to subgroups based on their apparent state ofoxidation. The distinction explain by nebular gas-solid interactions. between"oxidized"and"reduced" meteorites is defined petrographi Certainly, parent-body aqueous alteration of CM and CI chon cally in the relative proportions of metal vs. magnetite (determined drites exists. The degree of parent-body alteration varies among the by point counting) and in the Ni content of sulfides [I]. While this chondrites. It is also certain that some aqueous alteration took place approach has been of use in interpreting aspects of the evolution of before accretion ofthe final CM parent body(ies). It may, however, the CV chondrites, point counting has a serious limitation as a be difficult to explain all observations by gas-solid reactions in the quantitative tool for deriving modal analyses, particularly in meteor nebula (e.g., calcite formation). Therefore, an alternative model ites with a large volume of matrix: the resolving power of the should certainly be considered. If aqueous alteration occurred in microscope. Although Mossbauer spectroscopy only detects Fe relatively small and uncompacted precursor planetesimals that were containing phases, it has the significant advantage of detecting any subsequently destroyed and dispersed by collisions prior to the Fe nuclei bound in a lattice. The intensity ofan individual absorption accretion of the dust mantles and the meteorite parent body(ies), in a Mossbauer spectra is directly related to the proportion ofthe total most, ifnot all, ofthe observations discussed above can convincingly Fe at that site in a lattice (and often the proportion of the total Fe in be explained (compare [I D. an individual phase). Thus, given an accurate determination ofweight In summary, preaccretionary aqueous alteration is a fundamental percent Fe [e.g., 2] in the sample, a Mossbauer analysis may be process in the evolution ofprimitive chondri tic materials. The forma recalculated to give weight percent of Fe-containing phases. Given tional history ofCM (and probably other C) chondrites is certainly the density of the meteorite, and relevant mineral phases, we can more complex than thought a decade ago. Figures 20 and 21 of [I] convert weight percent values to an approximate volume percent. provide clear evidence for preaccretionary aqueous alteration. Ev In this study we compare Mossbauer spectra of Vigarano, re eryone who opposes the model ofpreaccretionary aqueous alteration corded before and after a hydrous heating experiment, to constrain should first convincingly explain the features within these figures; the abundance ofFe-containing phases in the sample and to observe this will be very difficult. how phases respond to (artificial) "aqueous alteration." References: [I] Metzler K. et al. (\992) GCA, 56, 2873. Methodology: Hydrous heating ofVigarano was accomplished [2] Tomeoka K. et al. (1991) 16th Symp. Antarct. Meteorites, 37. by loading 1.8 gofthe powdered meteorite into a stainless steel insert [3] Kerridge J. F. and Bunch T. E. (1979) in Asteroids (T. Gehrels, (T316, I-ml i.v.) with 0.4 ml high-purity degassed water (water/rock ed.), p. 745, Univ. ofArizona, Tucson. [4] Tomeoka K. and Buseck ratio = 0.22). The insert was purged with N2 gas and placed in a high P. R. (1988) GCA, 52, 1627. [5] Grimm R. E. and McSween H. Y. pressure reactor (4740, Parr Inst. Co; 71-ml i.v.) with 20 ml water to Jr. (1989) Icarus, 82, 244. [6] Lee M. R. (1993) Meteoritics, 28, 53. minimize pressure differentials across the insert wall. This arrange [7] Endress M. and BischoffA. (1996) GCA, 60,489. [8] Hanowski ment was then heated to 320°C for 72 hr. Before and after heating the N. P. and Brearley A. 1. (1997) LPS XXVIII, 503. [9] Endress M. et sample was analyzed by Mossbauer spectroscopy. Spectra were al. (1996) Nature, 379, 70 I. [10] Bunch T. E. and Chang S. (\ 980) recorded at 298 K with a microprocessor-controlled Mossbauer GCA,44, 1543. [II] Brearley A. J. and Geiger T. (1993) Meteor spectrometer using a S7Co/Rh source. Drive velocity was calibrated itics, 28, 328. [12] Browning L. B. et al. (1995) LPS XXVI, 182. with the same source and a metallic Fe foil. Spectra were fitted with [13] Browning L. B. and Keil K. (1997) LPS XXVIII, 163. a constrained nonlinear least-squares fitting program of Lorentzian [14] Nakamura T. et al. (1991) 16th Symp. Antarct. Meteorites, 40. functions. [15] Schirmeyer S. et al. (\997) LPS XXVIII, 1253. [16] Ikeda Y. While it's unlikely that these heating conditions reproduce aque and Prinz M. (1993) GCA, 57, 439. [17] Bischoff A. et al. (\993) ous alteration on the parent body, the mineral transformations pro GCA, 57, 1587. [18] Bischoff A. et al. (\993) GCA, 57, 2631. duced may bear some relation to those produced by natural aqueous [19] Endress M. et al. (1994) Meteoritics, 29, 26. [20] Weisberg events. 4 Workshop on Modification ofChondritic Materials
Results and Discussion: M6ssbauer spectrum recorded before precise tool for mineral identification, no other common oxide or hydrous heating ofVigarano is shown in Fig. I. The percentage area oxyhydroxide shows a sextet absorption at 45-46T at 298 K. The of the spectra [before (A) and after heating (B)] assigned to olivine, association of a smaller sextet at 49T allows us to be reasonably pyroxene, troili te, magnetite (fi tted to two sextets), and paramagnetic confident in our assignment. Thus, in our 298 K spectra ofVigarano, Fe3+ is shown in the table. Also shown are the data from both spectra we assign the two broad sextets (45.8T and 49.IT; ratio 2.1:1) to recalculated for weight percent based on a total Fe content for the magnetite. bulk meteorite of24.71 wt% [2 J. Volume percent is calculated based What is apparent from our analysis is that we find substantially on specific gravity of minerals (see Table I), and a density for more magnetite in Vigarano than has previously been reported Vigarano of 3.0. Doublets associated with olivine and pyroxene are (2.5 vol% compared to 0.3 vol% in the earlier study of McSween treated as a single absorption for the purposes ofthis paper, as peaks rI)). One possible explanation for this disc~pancy is that much ofthe are not well separated. The Mossbauer parameters of the paramag magnetite in this meteorite exists as a fine-grained matrix phase. In netic Fe3+ absorption (isomer shift 0.47, quadrupole splitting 0.66) a detailed study ofthe Vigarano matrix, Lee et al. (3] found intersti are typical ofanumberofoxides and oxyhydroxides( e.g., ferrihydrite, tial to matrix silicates, spinels, Fe-sulfides, and Fe-Ni alloys, areas small-partic1e goethite, lepidocrocite). Low-temperature work would $2 11m2 in size filled by intergrowths of a finely crystalline fibrous be required to unambiguously resolve this phase. material. These authors used selected-area electron diffraction to Iron-57 M6ssbauer spectra ofmagnetite typically show two sex identify this material as ferrihydrite; however, as noted by (3] and [4], tets (six-line absorptions) at 298 K: one (at approximately 49T) d spacings for ferrihydrite and magnetite are very similar. We pro arising from Fe3+ in tetrahedral "A" sites in the spinel structure, the pose that much ofthis interstitial matrix phase is actually magnetite. other (at 45-46T) from mixed Fe2+lFe3+ in octahedral "B" sites. Ifthe paramagnetic Fe3+phase that we identify in our first M6ssbauer Because ofelectron hopping, the latter Fe2+1Fe3+ gives an averaged spectrum is ferrihydrite, this would indicate a similar amount of subspectrum. "B" sites contain equal numbers ofFe2+ and Fe3+ ions, ferrihydrite to magnetite in the matrix. so ideally we expect a 2: I ratio in the spectral area of the 45T sextet In our second spectrum, recorded after hydrous heating of to the 49T sextet. Although M6ssbauer spectroscopy is not always a Vigarano, we observe an increase in the relative proportion of mag-
TABLE I.
D Spec. (A) wt% (A) vol% (A) Spec. (B) wt'Y'o (B) vol% (B)
Olivine (Fa) 4.39 34.6 8.5 5.8 30.9 7.6 5.2 Pyroxene (Fs) 3.96 22.5 5.6 4.2 25.2 6.2 4.7 Troilite 4.65 12 .1 3.0 1.9 8.7 2.1 1.4 Magnetite ("A" site) 5.18 5.6 4.3 2.5 9.6 6.4 3.7 Magnetite ("B" site) • 11.7 • • 16.3 • • Para. F e3+ (ferrihydrite) -3.3 13.6 3.4 3.1 9.3 2.3 2.1
• Spectral areas for both magnetite sites are given. To derive weight percent and volume percent we sum these values.
<: o -:: 2.0 ~ ~ 3.0
4.0
~.O I ~ ! --'---'-----.-~. ·8 C Velo::ity (_lsI
Fig. 1. LPI Technical Report 97-02, Part I S
netite, predominantly at the expense oftroilite and the paramagnetic [S] Kullerud G. and Yoder H. S. Jr. (1963) Carnegie Inst. Wash . Fe3+ phase (the combined spectral area of Fe-Mg silicates remains Yearb.,2IS-218. essentially unchanged). If ferrihydrite is being transformed to mag netite, it may indicate that magnetite in Vigarano is the product ofa relatively-high-temperature aqueous event (our experiment was at METAMORPHIC TRANSFORMATIONS OF OPAQUE 320°C, compared to estimates of <150°C for an aqueous alteration MINERALS IN CHONDRITES. M . Bourot-Denisel, B . event that produced ferrihydrite and smectite in the matrix ofVigarano Zandal,2, and R. Hewins2, lMus{mm national d'Histoire natureIle, [3]). 61 rue Buffon, 75005 Paris and lAS, Orsay, France, 2Departrnent of Conclusions: Our work indicates that Vigarano contains Geological Sciences, Rutgers University, Piscataway NJ 088SS 2.5 vol% magnetite, possibly as a fine-grained matrix phase. This 1179, USA. finding raises questions as to the genetic significance ofa distinction between "oxidized" and "reduced" ev chondrites, as the overall Introduction: Transformations in silicate phases define the abundance of magnetite we observe in Vigarano is similar to that major petrographic divisions ofchondrites [1,2]. The corresponding quoted for "oxidized" evs. Ifindeed magnetite exists in two distinct transformations of opaque phases throughout the metamorphic se morphologies in these meteorites (large grain size in oxidized evs, quence have been comparatively little studied [e.g., 3] and Perron et small grain size in reduced), it may be this morphological difference aI. [4] showed that some textural changes took place from type 3 to that is significant, rather than abundance, i.e., different processes type 6. We here attempt to systematically describe the changes have formed magnetite in oxidized and reduced CVs. A magnetite undergone by opaque minerals, and establish a metamorphic scale producing reaction (in addition to simple aqueous alteration) is parallel to that based on silicates. We hope to derive (I) a better "sulfurization." At 800°C olivine reacts with S to produce troilite, understanding of the state ofopaque minerals at peak temperatures magnetite, and enstatite [S]. We note that in oxidized-group meteor in order to better constrain cooling rate estimations and (2) an ites magnetite, and pentlandite are finely intergrown [\]. Perhaps this adequate way ofcomparing experimental metamorphic analogs with textural relationship is evidence ofa similar, though lower-tempera metamorphosed chondrites (since measured/estimated temperatures ture, "sulfurization" reaction. are insufficient due to the timescale differences involved). We have References: [I] McSween H. Y. Jr. (1977) GCA , 41, 1777 chosen to limit ourselves here to the main textural transformations, 1790. [2] Mason B. (\963) Space Sci. Rev., 1, 621-646. [3] Lee though behavior ofNi, Co, and trace elements [S] is also relevant. M. R. et aI. (1996) Meteoritics & Planet. Sci., 31, 477-483. Method: We have studied the textural appearance and relation [4] Barber D. J. and Hutchison R. (1991) Meteoritics, 26, 83-9S. ship of metal and sulfides in a series of sections ofordinary chon-
Fig. 1. BSE images of "metallic chondrules" in (a) Semarkona (LL3.0), (b) Bishunpur (LL3.1), (c) St. Mary's County (LL3.3), and (d) Chain pur (L13.4). 6 Workshop on Modification ofChondri tic Materials
Fig. 2. Reflected light views of (a) Dhajala (H3.8), (b) Forest-Vale (H4), (c) Richardton (H5), and (d) Estacado (H6). Field of view is 1.05 x 0.69 mm.
drites ofvarious petrographic types. Because of the lack ofH chon and adjacent metal grains between chondrules gradually coalesce. drites ofthe lowest petrographic types and ofthe superimposition of All the silicate material intially enclosed by the coalescing grains brecciation effects in most LLs ofthe highest ones, we have chosen eventually gets expelled and the grains acquire a uniform clean, to break our series into two overlapping parts: metamorphic effects clear-cut aspect. No round shape survives and the shape of the in type 3 chondrites have been studied in LILL chondrites of types opaque minerals seems to be governed by the interstices they fill . ranging from 3.0 to 4, whereas the effects from 3.4 to 6 have been between the silicates. With the disappearance ofchondrules by type studied in H chondrites. 6, the grains become more evenly dispersed and smaller than the Results: Our results for the two metamorphic series are dis large metal grains of types 4 and 5. played in Figs. I and 2 respectively. The detailed study of opaque Interpretation: The changes from 3.0 to 3.5 seem to be mostly mineral distribution in 3.0 Semarkona indicates that, except for a few driven by reduction of grain boundary area as well as temperature selected locations, metal and sulfide phases are constantly and inti induced breakdown of sulfide followed by S2 vapor migration as mately associated, a likely result ofnebular corrosion effects ofH2S described in [8]. This process becomes very important by 3.7. The gas on kamacite partly before but mostly after chondrule formation apparent coalescence ofmetal grains from3.8 to 5 seems to be driven [6]. Apart from the fme-grained matrix and a few tetrataenite grains by grain boundary diffusion ofFe and Ni (as indicated by this process also found in the matrix, metal grains with no associated sulfides are starting simultaneously with the appearance ofzoned taenites), and only found inside type I (FeO-poor) chondrules. On the other hand., the gradual expulsion of the enclosed silicates is related to the sulfide grains with no metal are restricted to the interior of type II reduction of surface free energy. From 5 to 6, recrystallization be (FeO-rich) chondrules and a few isolated sulfide grains in the matrix. comes the controlling process. The size and distribution of the Figure la shows that the metal/sulfide contact is very highly con opaque minerals seem to be governed by the rate ofnucleation rather torted in such a material. This spatial relationship ofmetal and sulfide than their initial grain size. They are still interstitial to the silicate is gradually changed by metamorphism. We identify three main crystals, but they look more compact because the silicates have steps: (1) Up to 3.5 (Figs. Ib-d), metal and sulfides stay closely become more polygonal. associated but their contact progressively changes from contorted to Conclusions: Opaque phases in chondrites undergo changes straight. Metal grains have a round shape both inside and outside with metamorphism that can be followed step by step, allowing the chondrules. Sulfide appears in increasing quantities associated with optical determination of a petrographic type matching that deduced the metal beads inside chondrules. (2) From 3.5 to 4 (Figs.2a-b) from the silicate phases with a reasonable precision. In the first metal and sulfide start separating from one another and sulfide grains approximation, there is no observable difference between the opaque tend to connect together (this effect becomes more obvious by 3.7). mineral textures observed in H and LL chondrites, which indicates Opaque grains inside chondrules lose their round shape to become that our method could potentially also be applied to metal-bearing more angular. The first zoned taenite grains appear and metal grains carbonaceous chondrites. start coalescing [4,7] by 3.8 (Fig. 2a). (3) From 4 to 5 (Figs. 2c-d), References: [1]VanSchrnusandWood(1967)GCA, 31,747. the separation ofmetal and sulfide progresses almost to completion [2) Sears et al. Nature, 287, 791. [3) McSween et al. (1978) Proc. LPI Technical Report 97-02, Part 1 7
LPse 9th, p. 1437. [4] Perron et al. (1989) LPSxx. 838. [5] Zanda parison with carbonaceous chondrite matrix material [2,5, 10]. There et al. (1994) Science, 265, 1846. [6] Zanda et aI., this volume. is no consensus over the origin of this amorphous material, but [7] Spry (1 969)Metamorphic Textures, Pergamon, 156. [8] Lauretta derivation from chondrule mesostasis [3,4] or formation by disequi et al. (1997) EPSL, submitted. librium condensation processes [5,6] have both been suggested. Most importantly, there does not appear to be a reasonable mecha nism by which this type ofmaterial could have formed within a parent UNRAVELING NEBULAR AND PARENT-BODY EFFECTS body. Amorphous materials represent a highly disordered, meta IN CHONDRITE MATRIXES: MINERALOGICAL AND stable state and will invariably recrystallize during mild annealing or COMPOSITIONAL CONSTRAINTS. A. J. Brearley, Insti alter rapidly in the presence of hydrous fluids. Thus the survival of tute of Meteoritics, Department of Earth and Planetary Sciences, amorphous material indicates that parent-body processing has been University ofNew Mexico, Albuquerque NM 87131 , USA (brearley@ minimal. unm.edu). Unequilibrated Mineral Compositions: Analytical electron microscope studies of the matrixes of these same chondrites show Chondri tic meteorites have experienced extremely complex evo that the mineral phases present, dominantly olivine, are highly lutionary histories, involving formation of their different compo unequilibrated. Olivines with very disparate compositions can coex nents in the nebula and subsequent processing prior to and post ist within micrometers ofone another [5]. In the UOCs, fine-grained accretion into parent bodies. Each of these processes has imprinted matrix olivines range in composition Fao_91 ' This is also true of its own mineralogical or chemical record on the different compo matrix olivine in ALH 77307 (CO), which has a compositional range nents of chondritic meteorites, such as chondrules, CAIs, and fine FlIo-7o, In contrast, this is not the case for Acfer 094, which contains grained matrix materials. Differentiating between the effects ofnebular only very Fo-rich olivine. However, this meteorite is relatively highly and parent-body processes on matrix materials has presented one of weathered, so it is possible that FeO-rich olivines have been terres the more challenging problems in chondrite petrology, in part be trially altered. Altematively, it may be even more primitive than ALH cause of the difficulties of fully characterizing such fine-grained 77307. An additional and compositionally very distinct type offine materials. An additional complexity is that matrixes are likely to grained olivine has also been recognized in several UOCs and car respond in a highly sensitive fashion to parent-body processes. In bonaceous chondrites. These so-called LIME olivines [11] are essence, matrixes can be regarded as the "canary in the coal mine" of forsteritic in composition, but have very elevated MnO contents (up chondri tic meteorites, changing rapidly during thermal metamor to 1.4 wt%). Low-Ca pyroxene compositions in matrixes are some phism or aqueous alteration. This sensitivity can be attributed to a what unequilibrated, but in general are restricted to relatively Mg
combination ofseveral different chemical and physical properties of rich compositions (UOCs, Fs O_2S [4,9]; CO, [6] this component is rich in Si02, MgO, and FeO with minor on a very small group of un equilibrated chondrites. amounts ofNi and S. These differences in composition are consistent The effects ofparent-body processes on chondrite matrixes have with the much higher bulk A120 3, K20, CaO, and N~O contents of been studied to various degrees in the different chondrite groups. unequilibrated ordinary chondrite (UOC) matrixes [4,7-9], in com Aqueous alteration is relatively well understood [e.g., 13] and has 8 Workshop on Modification of Chondritic Materials typically obliterated or obscured evidence ofnebular processes. The CHLORINE-BEARING MELT INCLUSIONS IN OLIVINE general effects of metamorphism are also reasonably clear, but the FROM UNEQUILIBRATED ORDINARY CHONDRITES. details are still somewhat poorly understood. Huss et al. and Mc J. C. Bridges, Department ofMineralogy, Natural History Museum, Sween [7,14] noted that the matrixes ofthe UOCs and CO chondrites Cromwell Road, London SW7 5BD, UK [email protected]). become progressively more translucent with increasing petrologic type, presumably due to grain growth during metamorphism, and Introduction: Melt inclusions are a common but little studied mineral compositions become more homogeneous. In the matrixes of component ofchondrules within UOCs. They are most visible within UOCs the. effects ofparent-body metamorphism are certainly appar olivine phenocrysts. These inclusions are composed ofsilicate glasses ent by petrologic type 3.4 [4]. For example, although Bishunpur with variable compositions. In thin section they characteristically (type 3.1) matrix is dominated by amorphous material, in Sharps and have elliptical outlines and a mauve color. They formed as a result of Chainpur (both 3.4), the matrixes consist largely ofdensely packed, melt being trapped during crystallization ofthe surrounding phase fine-grained, equilibrated olivine (':"Faso) with minor amorphous olivine in the case of chondrites-and therefore can potentially material [4]. These observations are consistent with the behavior of provide information about chondrule melt compositions. matrixes in CO chondrites, except that extensive crystallization of Preliminary results of analyzing the composition of melt inclu olivine has occurred by petrologic type 3.1 rather than 3.4 [15,16). sions and the effects of artificial heating and cooling are reported This difference may be because the more olivine normative matrix in here. Some melt inclusions were found to contain Cl. The presence CO chondrites recrystallizes more readily than the feldspathic amor of halogen-bearing and alkali-rich phases in chondrites has often phous material in UOCs. These observations show that the textural been attributed to alteration processes -nebular or planetary (1,2). and compositional effects of metamorphism manifest themselves In contrast it has been suggested elsewhere [3,4] that some CI rapidly on matrix materials. In the CO chondrites, clear composi bearing phases within UOCs crystalJized from chondrule melts and tional trends are observed in the matrix as a function of petrologic are not the result of alteration. The inclusions studied here provide type. For example, the bulk matrix and olivine compositions become further information about the distinction between primary CI-bear increasingly Mgrich, consistent with equilibration between Mg-rich ing phases and those that formed through alteration. chondrulesand Fe-rich matrix [14,16]. Techniques: Thin sections of Bishunpur (LL3.1), Chainpur CV chondrites, such as Allende, have often been considered to (LL3.4), Tieschitz (H3.6), and Pamallee (LL3.6) were searched for record a relatively pristine record of nebular processes. However, recent observations suggest that a reappraisal of these meteorites is in order. Ithas been argued that the fayalitic olivines in CV chondrite matrixes are nebular condensates based on their distinct, tabular morphologies and high minor-element contents (17). However, the presence of inclusions ofpentiandite, chromite, and poorly graphi tized C [18] in matrix olivines suggests that formation by dehydra tion of hydrous phases such as serpentine [19] is more likely. Conclusions: Although the matrixes ofchondrites are extreme ly sensitive to the effects of parent-body processes such as aqueous alteration and metamorphism, a very small group ofthe most highly unequilibrated chondrites do appear to have retained a relatively well-preserved record of nebular processes. Acknowledgments: Funded by NASA grant NAGW-3347 to 1. 1. Papike, PI. References: (1] Scott E. R. D. et al. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), pp. 718-745, Univ. of Arizona, Tucson. [2] Brearley A. J. (1996) in Chondrules and the Protoplanetary Disk (R. H. Hewins et aI., eds.), p. 137, Cambridge Univ. [3] Ashworth J. R. (1977) EPSL, 35, 25. [4] Alexander C. M. O'D. et al. (\ 989) EPSL, 95, 187. [5] Brearley A. J. (1993) GCA, 57, 2291. [6] Greshake A. (\ 997) GCA, 61, 437. [7] Huss G. R. et al. (1981) GCA, 45, 33. [8] Matsu nami S. (1984) Proc. NIPR, 35,126. [9] Nagahara H. (1994) GCA, 48, 2581-2595. [\0] McSween H. Y. Jr. and Richardson S. M. (1977) GCA, 4, 1145. [II] Klock W. et al. (1989) Nature, 339, 126. [12] Brearley A. J. (1989) GCA, 53, 2395. [13] Zolensky M. E. and McSween H. Y. (\998) in Meteorites and the Early Solar System (1. F. Kerridge and M. S. Matthews, eds.), p. 114. [14] McSween H. Y . (1977) GCA, 44, 477. [15] Keller L. P. and Buseck P. R. (1991) GCA, 65, 1155. [16] BrearleyA. J. (1994) LPS xxv. 165. [\7] MacPherson G. et al. (\985) GCA, 44, 2267. [18] Brearley Fig. 1. . Melt inclusion (25-~m diameter) enclosed within an olivine grain. A.1. (1996)Meteoritics & Planet. Sci., 31, A21. [19] Kojima T. and The bubble at the right hand side is a void space formed during shrinkage Tomeoka K. (1993) Meteoritics, 28, 649. of the glass. Sample is a porphyritic chondrule from Tieschitz. LPI Technical Report 97-02, Part I 9 TABLE 1. Composition of melt inclusions within oliving (wt%). trapping during chondrule formation. This may be due to the pres ence of a relatively high pressure of volatile-rich gases at the same Pamallee Pamallee Chainpur Range time as chondrules were forming. It has been suggested elsewhere SiO, 62.2 60.7 66.8 45.3-76.0 that such a process may have operated becauseCI-, alkali-rich phases AlP3 12.5 10 .3 13.1 6.9-12.5 within chondrules, and bulk enrichments ofalkali elements in some TiO, 0.5 0.3 <0.5 chondrules, are not always easily explicable by alteration [3 ,6). The Crp3 0.4 7.5 <4.6 hydrous alteration and associated corrosion sometimes seen in UOCs, FeO 2.8 4.2 2.1 0.8-7.6 which is responsible for some remobilization of alkali elements and MgO 6.2 1.2 1.5 0.3-12.4 halogens, occurred at a late parent-body stage [5]. Determining the MnO 0.2 <0.2 setting for the initial, primary concentration ofvolatile phases within CaO 7.9 0.3 0.2-11.0 some chondrules will depend on which model concerning the site of Nap 3.6 2.1 4.2 2.4-4.8 Kp 2.7 4.4 3.9 1.\-4.4 chondrule formation-nebular or planetary-is favored. The latter CI 0.1 0.1 <0.1 setting is preferred here because localized compositional heteroge Total 99.1 91.1 91.6 neities, as shown by the presence of some volatile-rich pha.ses and other chemically differentiated components in UOCs [7], are more Range of compositions calculated from 29 melt inclusions within sections easy to explain through processes ofplanetary differentiation, rather of Bisbunpur, Chainpur, Pamallee, and Tieschitz. than within the nebula. Conclusions: Volatile elements, notably CI, are present in small but significant amounts in melt inclusions trapped within olivine glassy melt inclusions within the olivine of chondrules. Chemical from UOC chondrules. These inclusions are unaffected by alteration compositions were determined with a Hitachi 2500 SEMIEDS sys and so suggest that someCI-bearing, volatile-rich gases were trapped temat 15kVand2nA. Long counting times were used (I 50 s) in order during chondrule formation. Melt inclusions provide evidence that to achieve relatively low detection limits (-0.07 wt% for CI). Only not all halogen-bearing (and alkali-rich) phases within chondrules those melt inclusions that were not on cracks were used, in an effort can be attributed to alteration. The initial trapping of volatile-rich to minimize the risk of contamination. gases is believed to have occurred in a planetary/parent-body setting A demountable thin section of Tieschitz was prepared and, fol as is therefore, by this model, all subsequent remobilization ofthese lowing optical identification of melt inclusions, was studied using a elements seen in some UOCs. heating/cooling stage. The effects of heating and cooling the melt Some ofthe melt inclusions will be analyzed by an irradiation and inclusions were monitored with an optical microscope. laser fusion technique. This will provide further information about Results: Petrography. Melt inclusions were found in all the their halogen and volatile-element contents. sections studied. They were identified by their mauve color and Acknowledgments: J. Wilkinson ofthe Royal School ofMines, elliptical outlines, and range in size up to 80 Ilm maximum diameter Imperial College, is thanked for providing the heating/cooling stage (Fig. I). Most ofthe melt inclusions also contained "bubbles" at the data. margins. These occupy <30 vol% of the inclusions. Although the References: [I] Kimura M. and Ikeda Y. (1995) Proc. NIPR inclusions are predominately glass, some appeared to have under Symp. Antarct. Meteorites, 8, 123-138. [2] Krot A. N. et al. (1995) gone limited devitrification as they contained a few Ca-pyroxene Meteoritics, 30,748-775. [3] Bridges J. C. et al. (1997) Meteoritics dendrites and Cr spinels. No signs of corrosion or alteration were & Planet. Sci., submitted. [4] Bridges J. C. et al. (I 995) Proc. NIPR present. Symp. Antarct. Meteorites, 8, 195-203. [5] Hutchison R. et al. The melt inclusions have a variable range of compositions. One (1994) Meteoritics, 29,476-477. [6] Kurat G. et al. (1984) EPSL, common characteristic, however, is the relatively high K20 contents 68, 43-56. [7] Bridges J. C. and Hutchison R. (1997) Meteoritics & for chondri tic material (up to 4.4 wt%). This is, for instance, higher Planet. Sci., 32, in press. than the typical K20 contents of chondrule mesostases. Chlorine was detected in some melt inclusions (0.1 wt%). The composition of 3 individual melt inclusions from Tieschitz and Parnallee are given in Table I, together with a range of compositions from 29 melt A SEARCH FOR ACCRETIONARY TEXTURES IN CM inclusions located in Bishunpur, Chainpur, Tieschitz, and Parnallee. CHONDRITES. L. Browning, Hawai'i Institute of Geophysics Heating stage. Melt inclusions within olivine from Tieschitz and Planetology/School ofOcean and Earth Science and Technology, were cooled to -196°C and heated to 600°C. Between these tem University ofHawai'i at Manoa, Honolulu HI 96822-2219, USA. peratures the "bubbles" at the margins of the inclusions did not undergo any changes in volume or leakage of fluids . This is consis Introduction: It has been reported that examples of pristine tent with their being void space, created during contraction of the asteroidal materials can be observed in most CM chondrites [1). melt pockets as they cooled during chondrule formation. These "primary accretionary rocks," ac.cording to [I], experienced Discussion: The glassy, unaltered nature ofthe melt inclusions no aqueous alteration or brecciation on the CM asteroidal parent shows that remobilization of alkali elements seen in some compo body and can be identified by their texture-a densely packed nents (e.g., matrix) ofTieschitz and other UOCs [5] has not affected agglomeration of rim/core components with no interstitial matrix them. Instead the K and CI contents of the inclusions were deter materials. These authors claim that the meteorite Y 791198 is mined when the chondrules were still molten. Therefore, the concen made up exclusively of "primary accretionary rock" (or "primary tration and sometimes remobilization oflabile elements within chon rocks") while fragments ofsimilarly pristine material are present as drules of UOCs took place in several stages. An early stage was lithic clasts in most other CM samples. 10 Workshop on Modification oj Chondritic Materials Metzler et al. [I] propose the following multistage scenario to that CM materials were aqueously altered in some preaccretionary explain the origin of primary accretionary rocks: Both altered and environment, as well as the CM parent body. unaltered CM phases were present in the solar nebula as isolated The current study indicates that agglomeritic textures are con grains; coarse-grained CM components accreted a mantle (or "ac fined to discrete portions ofY 791198 and do not define the general cretionary rim") of fine-grained alteration phases as they passed fabric of this meteorite. Alternate textures include coarse-grained through one or more nebular dust clouds; and finally the CM aster core components without rims and relatively broad regions of fine oidal parent body accreted as an agglomeration of these rim/core grained materials (i.e., "matrix" ) that do not appear as rims. Obser components. Primary rocks are lithified samples ofthese accretion vations indicate that some lithic fragments in other CM chondrites ary agglomerates. possess these textures while others don't. Moreover, regions con There are several interesting implications of this model. As one taining dense ly packed agglomerates ofrimlcore components are not example, it requires that aqueous alteration generally preceded the always contained within these clasts. The nature ofthe agglomerates formation ofall CM rim textures, including chondrule rims, and the themselves also varies widely. For example, some are made of accretion of the CM asteroidal parent body. Because all CM rims discreet rim/core components, but the majority are smaller and ap contain abundant phyllosilicate minerals that dehydrate at tempera pear to be lumps of rim or matrix materials with no obvious core tures on the order of a few hundred degrees Celsius [2], their model component. Browning et al. [6] point out that different types of also implies that relatively cool rim materials accreted onto similarly agglomerates may have formed by different processes and suggest cool core components. This contrasts sharply with the widely held that some of these smaller agglomerates may have formed when hypothesis that rim materials in other types of chondrites were individual grains were imperfectly replaced during in situ serpen sintered onto hot chondrule cores immediately following the chon tinization. In contrast, the densely packed agglomeritic texture of drule-forming event [3]. Although the location and relative timing primary accretionary rocks is interpreted by [I] as evidence for a lack of CM alteration has been discussed on several different grounds of in situ brecciation. Unraveling the origin of the various agglom [1,4,5], the primary rock textures described by Metzler et al. [I] have erate types and the significance of densely packed agglomeritic not been reexamined by different researchers. This work represents textures requires more work. an effort to identify, characterize, and reevaluate the origin of pri To summarize, primary accretionary rock textures are present in mary rock textures in Y 791198 and 10 CM falls. Y 791198 and other CM chondrites, but probably do not represent Analytical Techniques: Two thin sections of the meteorite pristine accretionary materials. Moreover, although the thin sections Y 791198, plus sections from 10 CM falls, were characterized by examined in this study contain many of the petrologic features petrographic and scanning electron microscopy. Compositional data described by [1], a comparison with other CM chondrites suggests are currently being collected using an electron microprobe. that Y 791198 may not be as unusual as previously thought. Results: Several observations argue that Y 791198 experi References: [I] Metzler et al. (1992) GCA, 56, 2873-2897. enced in situ aqueous alteration. For example, multiple phyllosili [2] Johannes (1968) Contrib. Mineral. Petrol., 19, 309-315. cate veinlets «2Ilmthick) extend from the interior ofone chondrule [3] Hewins R. et aI., eds. (1996) Chondrules and the Protoplane and then cut across a major fraction of the surrounding chondrule tary Disk, Cambridge Univ. [4] Browning et al. (l996) GCA, 60, rim. These cross-cutting veinlets indicate that the rim was already 2621-2633. [5] Brearley and Geiger (l991) Meteoritics, 26, 323. emplaced around the chondrule when alteration occurred. Similar [6] Browning et al. (I 997} GCA, submitted. features are observed in the other CM chondrites examined in this study. Although no relationship has been detected between the chem istry or mineralogy of CM rims and various core types [I], these cross-cutting vein lets show that fluid-rock reactions caused the local USE OF OXYGEN ISOTOPES TO CONSTRAIN THE redistribution of dissolved components between at least some CM NEBULAR AND ASTEROIDAL MODIFICAnON OF core and rim materials. Similar occurrences of veinlets are also CHONDRITIC MATERIALS. R. N. Clayton, Enrico Fermi commonly observed around individual olivine fragments in Y 791198 Institute, University of Chicago, Chicago IL 60637, USA (r and other CM chondrites. These phyllosilicate veinlets generally [email protected]). occur as fragile dendritic patterns that are interwoven with the sur rounding fine-grained materials. The dendritic patterns of these On the scale ofindividual chondrules and CAl, 0 isotopic varia veinlets probably reflect the fluid pathways that dissolved compo tions appear to be dominated by exchange and mixing interactions nents took on the CM parent body, while their delicate nature argues between two principal reservoirs: a 160-rich dust reservoir and a against thep lausibility oftheir having survived the type ofmultistage 160-poor gas reservoir [1]. Each of these reservoirs may have had a physical processing described by [I]. In a similar fashion, most somewhat variable isotopic composition as a consequence of their calcite grains in Y 791198 are extensively embayed and possess very interaction with one another. At low temperatures in the nebular delicate spiderIike morphologies that are best reconciled with an cloud, chemical interactions between these reservoirs were probably origin by in situ precipitation/dissolution processes. Analogous cal not significant. However, isotopic exchange of 0 between solid or cite morphologies are present in most other CM chondrites, sug molten silicates with nebular gases (CO and HzO) must have oc gesting that these formational processes are typical ofCM alteration. curred during transient heating events such as chondrule formation. This evidence for in situ aqueous alteration indicates that Laboratory experiments show that this process occurs on a time-scale Y 791198 is not made up exclusively ofprimary accreted materials ofminutes [2]. At some times orplaces in the solarnebula, conditions from the CM parent body, as suggested [1]. However, because the favored condensation of liquid or solid water to form wet planetesi effects of aqueous processing are superimposed upon preexisting mals. This provided an environment for low-temperature chemical materials in complex ways, these results do not negate the possibility and isotopic interactions [3]. LPI Technical Report 97-02, Part I . 11 An important property of stable isotope exchange processes is not obliterate this signature, since dehydration produces very small the strong dependence of isotopic fractionation factors on tempera changes in 0 isotopic composition [14]. The observed isotopic . ture [4]. For 0 isotopes, equilibrium fractionations between the compositions ofthese materials show no evidence for low-tempera phases at the temperatures ofmolten silicates are generally less than ture hydration, but rather follow extrapolations of the variations in 3%0, whereas fractionations between minerals and water are com carbonaceous chondrite chondrules. Thus, the reservoir interactions monly greater than 20%0 at O°C [5). that produced the observed compositions must have involved the On the basis ofmeasurementsofconstituents ofthe carbonaceous nebular gas at temperatures high enough that the mass-dependent chondrites, it has been proposed that the initial dust component in the fractionation should be small. A rough estimate ofthe lower tempera solar nebula was enriched in 160 relative to the larger gaseous reser ture limit is about 300°C. Most importantly, the gaseous reservoir voir by about 70%0 [3). Thus, high-temperature processes, in which must have been the same one with which chondrules and CAl inter mass-dependent fractionation effects are small compared to reser acted, i.e., the solarnebular gas, rather than an isolated parent-body voir differences, lead to mixing lines in the 0 three-isotope diagram. fluid. Thus the 0 isotope data for dark inclusions and CV matrix do These lines for chondrules and CAl typically have slopes close to not themselves suggest a hydration-dehydration origin. If such a I, reflecting the compositions of the two primary reservoirs. scenario is required by other evidence, the 0 data imply that the In the H, L, and LL ordinary chondrites, 0 isotopic composi hydration step took place in the nebula at temperatures above 300°C. tions ofchondrules follow a single, slope 1 mixing trend, indepen References: [I] Clayton R. N. (1993)Annu. Rev. Earth Planet. dent of the Fe group ofthe host meteorite [6]. Thus, ordinary chon Sci., 21, 115-149. [2] Yu Y. et al. (1995) GCA, 59, 2095-2104. drite chondrules appear to be drawn from a single population with [3] Clayton R. N. and Mayeda T. K. (1984) EPSL, 67, 151-161. common chemistry. The distinction among Fe groups ofthe subse [4] UreyH. C. (1947)J Chern. Soc. Lond., 562-581. [5] Onuma N. quently assembled parent bodies is probably the result of sorting et al. (1972) GCA, 36, 169-188. [6] Clayton R. N. et al. (1991)GCA, processes [7]. The 0 isotope evidence implies that chondrules ac 55,2317-2337. [7] Dodd R. T. (1981) Meteorites : A Petrologic quired their isotopic composition in high-temperature events in the Chemical Synthesis, Cambridge. [8] Clayton R. N. and Mayeda T. K. nebula, before parent-body formation. (1985) LPS XVI, 142-143. [9] Weisberg M. K. et al. (1995) Proc. Chondrules in unequilibrated enstatite chondrites also form a NIPR Symp., 8, 11-32. [10] Bridges J. C. et al. (1994) Meteoritics, linear array in the 0 isotope diagram, which does not overlap that of 29,448-449. [11] Choi B.-G. et al. (1997) LPS XXVlIJ. 227-228. ordinary chondrite chondrules (8). The range of variation is small, [12] Muehlenbachs K. and Clayton R. N. (1972) Can. J Earth Sci., and thus the slope of the mixing line is not well constrained. The 9, 172-184. [13] Krot A. N. et al. (1995) Meteoritics, 30, 748-775. separation from the 0 chondrite and C chondrite compositions im [14] Clayton R. N. et al. {I 997) Meteoritics & Planet. Sci., 32, in plies a dust component with a somewhat different isotopic composi press. tion. Some chondritic materials have t.170 exceeding the highest val ues found in chondrules ofthe ordinary chondrites. These samples TURBULENT SIZE SELECTION AND CONCENTRATION provide lower limits on t.170 of the gaseous nebular component. R OF CHONDRULE-SIZED OBJECTS: REYNOLDS NUM chondrites have chondrules with t.170 = 3.0 [9]; silica xenoliths in BER INVARIANCE AND IMPLICATIONS. J. N. CuzzP, some ordinary chondrites also havet.17 = 3 [10], and magnetite with R. Hogan2, A. Dobrovolskis3, and J. Paque4, INASA Ames Research t.17 = 5 has been found in unequilibrated ordinary chondrites [II]. Center, Moffen Field CA 94035, USA ([email protected]. The various carbonaceous chondrite groups exhibit the isotopic gov), 2Symtech Inc., Mountain View CA 94035, USA, 3Board of effects ofboth high- and low-temperature interactions between res Studies in Astronomy, University of California, Santa Cruz CA, ervoirs. In CV and CM chondrites, both CAls and chondrules USA, 4SETI Institute, Mountain View CA, USA. follow a slope I mixing line, indicative ofexchange between nebular gas and solids or liquids at high temperatures [1]. Chondrule rims in It is generally agreed that individual chondrules formed as entities Allende also show this behavior, which implies acquisition of the in a gaseous nebula prior to being accumulated into a meteorite rims in the nebular environment. parent body, where they incur various forms ofmodification. There Low-temperature water-rock interaction (often leading to the are major unanswered questions about the properties of the nebula production of hydrous minerals) is most evident in CI and CM environment in which chondrules formed. This process, by which chondrites, where it produces large heavy-isotope enrichment in 0 the most primitive meteorites are formed overwhelmingly from [3). The same processes are operative (under similar conditions) in chondrules, must then be an aspect of"nebula processing." Textures terrestrial sea-floor "weathering" in which basalts are altered to clay in certain fragments ofprimitive meteorites might be summarized as minerals [12]. The heavy-isotope enrichment is due to the mass being primarily chondrules and clastic, chondrule-sized fragments of dependent isotopic fractionation between clay minerals and water. other minerals. Each ofthese is covered with a rim ofdust containing Small degrees ofheavy-isotope enrichment due to incipient aqueous physical and chemical properties that are independent ofthe compo alteration are also evident in C03 and some UOC [6]. sition and mineralogy of the underlying chondrule. This (unfortu It has been postulated that many occurrences offayalitic olivine nately rather rare) texture was caJled "primary accretionary texture" in chondrules, chondrule rims, dark inclusions, and chondrite matrix by [I] to reflect their belief that it precedes subsequent stages of result from dehydration of phyllosilicate precursors (13). If the fragmentation, comminution, mixing, heating, and other forms of proposed phyllosilicates had been formed in low-temperature alteration occurring on the parent body(ies). The size distribution of parent-body processes such as those that produced CI and CM these chondrules and fragments, and the properties of their dusty phyllosilicates, they should have acquired the heavy-isotope enrich rims, are key clues regarding the primary nebula accretion process. mentcharacteristic ofthese processes. Subsequent dehydration would Key chondrule properties (e.g., mean size, density, and rims) remain 12 Workshop on Modification ofChondritic Materials relatively well defined, even in the more abundant meteorites that previously [2]. We discuss the implications for particle concentra have clearly suffered internal mixing, abrasion, grinding, and miner tion in nebula turbulence and some remaining uncertainties. alogical alteration or replacement (due presumably to the collisional References: [I) Metzler K. et al. (1992) GCA, 56,2873-2897. growth and heating process itself). [2] Cuzzi 1. N. et al. (1996) in Chondrules and the Protoplanetary It has been our goal to infer the key nebula processes indirectly Disk (R. H. Hewins et aI., eds.), Cambridge Univ. [3] Hogan et al. from the properties of the earliest primitive meteorites by using a (1997) in preparation. (4) Hughes D. W. (1978) EPSL, 38, 391-400. theoretical framework in which the nebula possesses a plausible level [5] Paque 1. and Cuzzi 1. N. (1997) LPS XXVlll. [6] Meneveau C. of isotropic turbulence. We have shown that turbulence has the and Sreenivasan K. R. (1991) J. Fluid Mech., 224, 429-484. property of concentrating one particular particle size by orders of magnitude. The preferentially concentrated size depends primarily on the intensity of the turbulent kinetic energy (represented by the Reynolds number of the nebula). Specifically, the preferentially EFFECT OF REVISED NEBULAR WATER DISTRIBU concentrated particle has a stopping time equal to the turnover time TION ON ENSTATITE CHONDRITE FORMATION. K. E. ofthe smallest eddy [2]. The intensity level ofturbulence implied by Cyr, M. L. Hutson, and J. 1. Lunine, Department of Planetary Sci chondrule sizes can be maintained by a small fraction of the energy ences, Lunar and Planetary Laboratory, University ofArizona, Tuc released by the radially evolving disk. (It must be noted that the son AZ 85721, USA ([email protected]). details ofthe transfer ofenergy actually occurring remain obscure.) We have carried our studies of the turbulent concentration pro The unique chemical and mineral composition of enstatite chon cess to a deeper level and have obtained several new and interesting drites has been difficult to explain and accurately reproduce in results. We have established that two critical aspects of the process chondrite formation models. Enstatite chondrites consist of highly can be described in a way that is Reynolds number independent [3]. reduced phase assemblages: the silicates contain almost no FeO In our previous results, we needed to rely on large extrapolations while the Fe metal contains significant amounts of Si. Further, between computational turbulence regimes and nebula turbulence enstatite chondrites also contain a variety of unusual minerals, the regimes (5 orders of magnitude in Reynolds number). majority ofwhich are not found in ordinary or carbonaceous chon First, we ran numerical calculations of particle density fields in drites [I]. It is generally believed that the mineral assemblages seen turbulence for a range of particle sizes with uniform initial spatial in the enstatite chondrites could not have resulted from equilibrium distribution. We then determined the distribution of particle sizes condensation ofa solar composition gas at the low pressures, 10-L within the high-density regions. We have not only shown that the 10-6 atm, thought to have existed in the solar nebula [2]. shape of the particle distribution resident in dense clumps is quite In her Ph.D. dissertation [I], M. L. Hutson constructed a forma similar to that found in chondrites, but it is also Reynolds number tion model for enstatite chondrites based on the assumption that independent (according to our mapping of factor 3-4 in Reynolds condensation occurred in nebular regions ofnonsolar composition. number). Chondrule size distributions provided by our numerical The modal mineralogy and bulk chemistry of the chondrites were models might by expected to persist even at much higher nebula best fit by a model requiring two chemical fractionations from gas of Reynolds numbers. Results will be shown and compared with size solar composition. The first was "refractory element" fractionation distributions from [8] and from our own experiments [5]. In the [3], which removed condensates at a temperature ofaround 1270 K. newer data, chondrules are disaggregated so their size and density The second was the removal of water from 1-5 AU by diffusional can be measured separately. This is critical because in the aerody transport. Loss of water vapor in gas increases the reducing nature namic sorting provided by turbulence, the defining parameter is the and the ClO ratio ofthat gas. Hutson found that the depletion of80 aerodynamic stopping time, which is proportional to the product of 85% of the cosmic water abundance from 1-5 AU alone could the particle radius and density. explain many ofthe chemical and mineralogical signatures ofensta Second, we have developed a new way of describing the particle tite chondrites; however, it was not sufficient, and thus the prior density fields that is also invariant to Reynolds number. The math fractionation process was also posited. ematics offractals has been shown to be particularly appropriate to Hutson based her water-depletion analysis on Stevenson and describe the spatial distribution of certain properties 6fturbulence, Lunine [4]. In their model water vapor is tranported across the specifically the dissipation rate ofturbulent kinetic energy [6]. This condensation front by eddy diffusion and rapidly condenses out as means that the spatial distribution ofthe turbulent energy dissipation ice. The ice is assumed to suffer little effect from gas drag or other rate is scale-independent, and that higher intensities have a different transport processes and so remains in the condensation zone. The spatial structure, or dimensionality, than lower intensities (i.e., are model predicts that the inner 5 AU ofthe nebula becomes severely concentrated in smaller regions). We have shown that that same depleted in water vapor in as little as 105 yr. mathematics applies to the particle density field when the particle However, in recent work [5] we found that gas drag effects in [4] size is close to the preferentially concentrated size (3]. The density had been underestimated and that ice particles could drift back field (or concentration factor) for these particle sizes can then be significant distances inward of 5 AU and sublimate, reinjecting the described by a probability distribution that has only two elements: nebula with water vapor. Ourexpanded water transport model, which a Reynolds-number-independent scaling function (which we deter incorporated both diffusion and drift processes, still predicts an mine in our numerical calculations at three relatively low Reynolds overall depletion in water vapor, but with a zone of local vapor numbers) and the Reynolds number itself. The entire particle density enhancement on the orderof20-100% from 1-2 AU, which gradu probability distribution can be predicted at any Reynolds number ally drops off out to 5 AU. Thus, unlike [4], we find a radical from this universal function. The particle concentrations predicted dependence to the water vapor depletion pattern and thus to the this way are in general accord with simple extrapolations published reducing nature and C/O ratio in the inner nebula. LPI Technica/ Report 97-02. Part I 13 We will reinterpret Hutson's enstatite chondrite formation model of HzO in solar system objects hinges on a correct interpretation of and her conclusions in light of the revised nebula water depletion this isotopic enrichment. model, in particular considering the changes in condensation se Mineralogical Identification of Water in Chondrules: quence ofmaterials and the overall effect ofthe radial dependence of Transmission electron microscopy diffraction patterns ofexcentro water depletion on nebular chemistry. radial pyroxene chondrules (type I) from Bishunpur show that the References: [I] Hutson M. 1. (1996) Ph.D. thesis, Univ. of zone-axis containing the [010]* reciprocal direction show an ex Arizona, Tucson. [2] Wood J. A. and Morfill G. E. (1988) in Mete treme disorder in the sequence of (0 10) planes. The corresponding orites and the Early Solar System (J. F. Kerridge and M. S. Mat high-resolution micrographs reveal a structure constituted by two thews, eds.), p. 329, Univ. of Arizona, Tucson. [3] Larimer J. W. types of narrow bands with approximately equal areas, both types and Wasson J. T. (1988) in Meteorites and the Early Solar System being exactly parallel to the trace of the (010) pyroxene plane. One (J. F. Kerridge and M. S. Matthews, eds.), p. 394, Univ. of Ari type of bands clearly shows lattice fringes typical of the clino zona, Tucson. [4] Stevenson D. J. and Lunine J. I. (1988) Icarns, enstatite structure. The other type of bands is attributed to lamellae 75, 146. [5] Cyr K. E. et al. (1997) learns, submitted. ofamphiboles. This assumption is supported by the electron micro probe analyses: the rules for cation site occupancy in pyroxenes indicate that the ratio of the atomic concentration of octahedral to INTERSTELLAR HYDROXYLS IN METEORITIC CHON tetrahedral cations is comets and chondritic meteorites, show a systematic D enrichment However, hydroxylated pyroxenes did notundergo isotopic reequili relative to the protosolar nebula (see Table I). Deciphering the origin bration with this hydrothermal H20 . Therefore the pyroxenes have TABLE I. The highest DIH ratios measured in LL3 matrix clays (interstellar water) are assumed to represent a minimum for the interstellar value; the lowest DIH value (protosolar water) in LL3 chondrules stands for water that underwent an isotopic exchange with the protosolar H and is calculated from the statistical ' mean ofDIH ratios measured in the range 62-99 x 10-6. DIH (xI0-6) Speciation Location References Galactic 40 ± 10 H Big-Bang (primordial) [I] 23 ± 5 H Quasar 1937-1009 [2] 16 ± I H Local interstellar medium [3] 210 ± 120, 150 Hp Interstellar clouds [4] Protosolar Nebula 36 ± 10, 31.5 ± 4.0 H, Presolar gas 4.5 x 109 ago [5] Planets and Comets 20 ± 8 H, Jupiter [6,7] 15 ± II H, Saturn [8] 149 ± 3 H,o Bulk Earth [9] 320 ± 30 H,o Comet PlHalley [10] 280 ± 40 Hp Comet Hyakutake [I I] LL3 Meteorites Inlerstellar Water 730 ± 120 -OH In clays [12] Proloso/ar Waler 88 ± II -OH In chondrules Present work Carbonaceous Meteorites 140 ± 10 -OH Mean statistical value 130 < DIH < 170 (66%) 14 Workshop on Modification of Chondritic Materials retained their preaccretional interstellar isotopic signature. These al. (1996) Planet. Space. Sci., 44, 1579-1592. [7] Niemann H. B. et data demonstrate that at least two sources of H20 were intimately al. (1996) Science, 272, 846-849. [8] Noll K. S. and Larson P. L. mixed in the solar system. This is attested by the markedly different (I 990) Icarus, 89, 168-189. [9] Cordieretal. (1988). [10] Eberhardt DfH ratios preserved in chondrule pyroxenes and in their associated P. et al. (I 995)Astron. Astrophys., 302, 301-318. [II] GautierD. et mesostasis. al. (1996) in ACM 96, p. 46, Versailles, France, July 8-12, 1996. Interstellar Chondrule Precursors Recorded by Their High [12] Deloule E. and Robert F. (1995) GCA, 59, 4695-4706. DfH Ratios Preserved in Pyroxene: As clearly demonstrated by their high DfH ratios, hydroxylation ofpyroxenes took place before the incorporation of chondrules in the parent body. Two possible THEORETICAL MODELS AND EXPERIMENTALSTUDIES interpretations can be proposed to account for their high H20 con OF GAS-GRAIN CHEMISTRYIN THE SOLARNEBULA. B. centrations: (I) H20-rich solids were present among the chondrule Fegley Jr., Planetary and Chemistry Laboratory, DepartmentofEarth precursors; (2) chondrules formed in a H20-rich environment and and Planetary Sciences, Washington University, One Brookings were subsequently altered after or during their cooling. In both cases, Drive, St. Louis MO 63130-4899, USA ([email protected]). high DIH ratios suggest that this H20 was of interstellar origin. No geochemical test is known that would discriminate unequivo Introduction: . "Here today, gone tomorrow" describes the cally between I and 2. We favor the interpretation according to which ephermeral existence of the solar nebula. The question ofhow long the chondrule melt was water supersaturated, amphibole crystalliz the gaseous nebula existed is fundamental for models of nebular ing under disequilibrium conditions during the rapid cooling of the chemistry because the underlying assumption of all . condensation chondrule. Such a hypothesis has been tested experimentally by calculations [1-3] is that enough time was available for equilibrium flash heating an enstatite-smectite mixture at 1900°C, followed by to be reached between gases, gas and grains, and between different rapid cooling of this synthetic chondrule. If correct, this interpreta grains. However, different chemical reactions proceed at different tion implies that H20 was initially present among the chondrule rates, which generally vary exponentially with temperature. Hence, precursors as phyllosilicates (or ice?) similar to those previously it is unclear that all reactions, especially low-temperature reactions reported in the matrix ofthese meteorites. between gas and grains and between different solid grains could Implications for the Solar System DfH Ratio: Interpretation reach equilibrium before the nebular gas dissipated and chemistry of the DIH ratio in bulk meteorite samples over the last 40 years was ceased. Here I revisit this question and discuss reactions forming Fe confronted by the problem of possible terrestrial contamination of sulfides, magnetite (Fe304), and hydrated silicates. anhydrous minerals. Our results solve this problem and show that Nebular Lifetime: Astronomical observations ofyoung stars, the majority ofH in the anhydrous meteoritic minerals is indigenous isotopic studies of meteorites, and theoretical models of accretion to the minerals. This conclusion is based on a central observation: The disks suggest a nebular lifetime of0.1-1 0 m.y. [4,5]. A widely used distribution of isotopic ratios in chondrules exhibits minimum earlier estimate was 1013 s (- 300,000 yr) [6]. (DIH = 88 ± II x 10-6) and maximum values (DIH = 479 ± 8 x Gas-Grain Reactions: Condensation calculations [1-3] pre 10-6 in pyroxenes), which cannot conceivably be attributed to terres dict that troilite, Fe304, and hydrated silicates form by reaction of trial contamination. Two mechanisms are proposed for these two nebular gas with solid grains at low temperatures. The gas molecules end members: (I) The low DIH ratio (88 ± II x 10-6) probably must collide with solid grains to react with them. Initially, reaction results from an isotopic exchange between protosolar H20 and HD. rates depend on the temperature, gas partial pressure, and grain size For heuristic purposes this H20 is designated as "protosolar water." (surface-to-volume ratio). Higher temperatures, higher pressures, (2) DIH ratios> 200 x 10-6 can be reached only via interstellar and smaller grains increase the reaction rate. "Fluffy" and porous chemistry. Crude calculations show that freaches a value of 15 (i.e., grains react faster because the surface-to-volume ratio is increased. 479 x 10-6/3 I x 10-6) at T = 130 ± 10K in less than a million years, Conversely, lower temperatures, lower pressures, and larger grains via ion-molecule reactions. This type ofinterstellar chemistry likely decrease reaction rates. Reaction rates shift from linear kinetics took place in the molecular cloud that predates the formation of the (chemical control) to parabolic kinetics (diffusion control) and slow protosolar nebula. Such an isotopic distribution provides a tool to down once a sufficiently thick nonporous product layer coats the distinguish between in situ and preaccretional alteration processes. unreacted solid. The critical thickness varies for each reaction and Interstellar and Cometary Water Budgets on Earth: Based depends on the temperature. on these isotopic estimates and assuming that the mixing process SCT Models: I used simple collision theory (SCT) to quantify between the different H20 carriers identified in LL3 chondrites is a these ideas and to model gas-grain kinetics fortroilite, magnetite, and general rule for solar system objects, the cometary (DIH = 300 ± hydrated silicate formation in the solar nebula and in the subnebulae 50 x 10-6) and interstellar contributions (DIH = 730 ± 120 x 10-6) believed to exist around Jupiter and Saturn during their formation [7 to the Earth (DIH = 149± 3 x 10-6) can be calculated: Less than 10% 9]. The models assume fully dense, monodispersed spheres (0.1 )Lm ofH20 on the present-day Earth was inherited from the early comet radius) comparable in size to interstellar dust and fine-grained mete ary bombardment, and thus >90% was outgassed from the primitive orite matrix. The kinetic theory of gases and kinetic data from the mantle. Ofthe resulting 100%, 14 ± 3% is of interstellar origin and literature were used to calculate gas collision rates with grains and the thus 86 ± 3% resulted from the oxidation of the protosolar gas. fraction of collisions that led to chemical reaction. The chemical References: [I] Gloeckler G. and Geiss 1. (I 996) Nature, 381, lifetime (tehorn) for a reaction must be less than or equal to the nebular 210-212. [2] Tytler D. et al. (1996) Nature, 381, 207-209. lifetime (~eb) for the reaction to proceed during the nebu lar lifetime. [3] Linsky J. L. (I 993)Astrophys. J , 402, 694-709. [4] Gensheimer Iron Sulfides: Simple collision theory modeling predicts that P. D. et al. (1996) Astron. Astrophys., 314, 281-294. [5] Gautier D. FeS formation is rapid (-320 yr in the nebula) [7], sowe studied FeS and Morel P. (1997) Astron. Astrophys., in press. [6] Lecluse C. et formation first [10-14]. Our experimental data show that corrosion LPI Technical Report 97-02, Part I 15 ofFe metal and Fe-Ni alloy (Fe meteorite metal) in HtH2S mixtures Icarus, 122, 288. [II] Lauretta D. et al. (1996) Proc. NIPR Symp. is rapid, even at H2S concentrations of 23, 50, and 100 parts per Antarct. Meteorites, 9, 97 and 111. [12] Lauretta D. et al. (\996) million by volume (ppmv), which span the expected value of 33 Meteoritics & Planet. Sci., 31, A78. [13] Lauretta D. et al. (1997) ppmv H2S in the solar nebula. We observe the transition from linear LPS XXVllI. [14] Lauretta D. et aI., this volume. [15] Hong Y. and to parabolic kinetics and find characteristic chemical signatures in Fegley B. Jr., this volume. [16] Zolotov M. Yu. and Fegley B. Jr. sulfides formed from Fe-Ni alloys: (\) bulk FelNi in sulfide and (1997) unpublished results. metal are similar, if not identical, (2) increasing Ni conf:entration from the metal-sulfide to the sulfide-gas interface, (3) pentlandite inclusions in the monosulfide solid solution. These features can be THE CHONDRITE-ACHONDRITE TRANSITION: DE destroyed during thermal metamorphism, and hold the potential for COUPLING OF OXYGEN ISOTOPE AND GEOCHEMICAL distinguishing nebular from parent-body sulfides [11,14]. Detailed CHANGES. C. A. Goodrich, RRI Box 98, Chester VT 05143, microprobe studies ofNi-bearing sulfides in chondrites are needed USA. to see which (if any) Ni-bearing sulfides are nebular remnants and which are parent-body products. Chondrites and Achondrites: Chondrites represent primitive, Magnetite: The SCT models predict that Fe304 formation takes undifferentiated solar system material. In terms of nonvolatile ele -320,000 yr in the nebula, but is more rapid in the higher-pressure ment composition, they closely match the solar photosphere [I]. subnebulae. However, the activation energy (EacJ forwiistite forma Their textures (chondrules) result from nebular processes [2]. And tion had to be used to calculate the tchern for Fe304 because the Eac, although they have been metamorphosed they have not been melted for Fe30 4 formation kinetics in H21H20 mixtures is not known. on their parent bodies [I]. In contrast, the bona fide achondrites Intuitively, we would think that "rusting" is rapid, but reaction of (HEDs, SNCs, and lunar meteorites) resemble terrestrial igneous -1 Ilbar H20 vapor with Fe at I30°C and below is probably slow. rocks in texture, mineralogy, and bulk composition, allowing us to Our initial experiments [15] show that Fep4 forms by reaction of understand them as products of igneous differentiation on their Pe with H20, but also suggest that the linear rate is slower than that parent bodies (4 Vesta, Mars, Moon). They have ages that are for FeS formation. Also, the transition from linear to parabolic younger than those of chondrites [3,4], consistent with this under kinetics and the effect ofNi plausibly decrease the Fe oxidation rate standing. even more. I consider bulk Fe304 formation in the solar nebula un On a 8 170-8180 plot, chondri tic materials plot along lines of likely, although F e304 rims may form on metal. Magnetite formation slope -I and differentiated materials plot along lines of slope -1/2 is kinetically and thermodynamically favorable in the jovian sub (0.52). The slope -I lines are believed to be mixing lines, reflecting nebulae. an inhomogeneous distribution of 160-rich material in the solar Hydrated Silicates: The SCT models predict that hydrated nebula [5]. Slope -1120 isotope lines result from mass fractiona silicate formation (serpentine, talc) takes -4.5 b.y. in the solar tion, which occurs during chemical processes such as igneous differ nebula. Hydrating rock in a near-vacuum at or close to room tem entiation. The SNC and HED groups each plot along their own slope perature is a slow process. The SCT models predict that hydrated -1/2 mass-fractionation lines, distinct from the Earth-Moon line, silicates can form in the subnebulae around Jupiter and Saturn indicating that their parent bodies formed from distinct 0 reservoirs. [8,9]. The higher pressures in the subnebulae lead to faster reaction There is evidence that (with the exception of volatile elements) rates. Also, hydrated silicates become stable at higher temperatures, achondrite parent bodies were similar to chondrites in bulk chemical since condensation temperatures generally increase with increasing composition [6-8]. In particular, Ca/AI (wt) ratios inferred for the pressure. Mica dehydration experiments in my laboratory [16] EPB and Mars (1.09) are identical to those of chondrites [9]. Thus, show that H20 loss is more rapid than breakdown to the thermody there has emerged a general view that the terrestrial planets and namically stable reaction products. Cation diffusion is the slow step asteroids accreted from more or less chondri tic material [10] with for forming the predicted anhydrous assemblage. This implies that heterogeneous 0 isotopes [II], and that they evolved (in proportion cation diffusion is also the rate-limiting step for hydration of to their size) toward isotopic homogenization and progressively anyhydrous minerals. Formation of hydrated silicates in jovian more fractionated rock types as heating occurred. The primitive protoplanetary subnebulae, inside parent bodies, or in temporary achondrites and ureilites, which appear to be transitional between atmospheres (of large planetesimals) that could form by adiabatic chondrites and bona fide achondrites, provide an opportunity to test gas capture from the solar nebula seems more plausible than nebular this view. gas-grain reactions. Primitive Acbondrites and UreiIites: Primitive achondrites Acknowledgments: This work is supported by NASA grant (winonaites-IAB inclusions, brachinites, acapulcoites, lodranites) NAGW-3070. I thank K. Lodders, D. Lauretta, Y. Hong, and have been defined as meteorites that are close to chondri tic in bulk M. Yu. Zolotov for helpful discussions. composition but have igneous textures and appear to have lost an References: [1] Larimer J. (1967) GCA, 31, 1215. [2] Gross (Fe,Ni)S melt and in some cases a low-melting-T silicate melt [12]. man L. (\972) GCA, 36, 597. [3] Lewis J. S. (1972) EPSL, 15, Thus, they are either tacitly [12] or explicitly [10,13] considered to 186. [4] Podosek F. A. and Cassen P. (1994) Meteoritics, 29, 6. be residues ofvery low degrees ofpartial melting. Recently [14] have [5] Cameron A. G. W. (1995) Meteoritics, 30, 133. [6] Cameron informally redefined primitive achondrites as meteorites "derived A. G. W. (1978) Moon and Planets, 18, 5. [7] Fegley B. Jr. (\ 988) from parent bodies that avoided isotopic homogenization and thus inLPI Tech . Rpt. 88-04, 51. [8] Fegley B. Jr. and Prinn R. G. (1989) retained some of their primary chemical and isotopic characteris in The Fonnation and Evolution ofPlanetary Systems, p. 171, Cam tics," and have included ureilites in this class. Depletion in lithophile bridge Univ. [9] Fegley B. Jr. (\ 993) in The Chemistry ofLife 's incompatible and plagiophile elements can be used as a measure Origins, p. 75, Kluwer, Dordrecht. [10] Lauretta D. et al. (1996) of degree of "primitiveness" (Fig. I). Depletion increases in the 16 Workshop on Modification oj Chondritic Materials 10 c ~ ~ ~ 0 0 c 0 0 113 • 113 • 0 I@ 0.1 !lIB •• • • • 0.01 0.01 0.1 10 SmIYbfCI 4170 Fig. 1. Ureilites (black boxes), lodranites (boxes with crosses), winona Fig. 3. See Fig. I for description of symbols. ites-IAB (open boxes), acapulcoites (open triangles), brachinites (open circles), Allende chondrules (black triangles), and chondrule rims (crosses). 100 113113 ~ 95 • 113113 2.5 .-., 90 0 A~ r- ~ • "... e ~ 113 ~ OJ) E 85 •• •• 0 •• • • 80 • .r... •• Y • • -2.5 • 75 • • -5 70 -5 0 5 10 -2.5 -2 -1.5 -I -0.5 0 0180 4170 Fig_ 2_ See Fig. I for description of symbols. Fig_ 4_ See Fig. I for description of symbols. order (brachinites, acapulcoites) ~ winonaites and lAB inclu (winonaites: slope -0.55 r2 = 0.92; brachinites: slope -0.49 r2 = sions ~ lodranites ~ ureilites. 0.94). Of acapuIcoites and lodranites, which plot on a single slope If the view that primitive achondrites and ureilites have been -1/2 (0.47) line and so may be from the same parent body, it is depleted to various (small) degrees with corresponding degrees of lodranites that are more depleted and so should show the least scatter, (incomplete) 0 isotopic homogenization makes sense, then there and yet they show more scatter than acapulcoites (acapuIcoites form should be a simple relation between degree of depletion of a group a line with slope -0.57 and r2 = 0.83, while lodranites scatter about and the degree to which its 0 isotopic compositions deviate from a a poor line of slope -0.27 and r2 = 0.24). These observations are slope -I line and tend toward a slope -112 line on a 1)170-5180 precisely the opposite of what would be expected. diagram. The least-depleted groups should plot on slope-I lines and In addition, if mg-lll70 correlations are of nebular origin, the the most-depleted groups should tend toward slope -1/2 lines. This quality of the correlation should decrease with increasing degree of is not the case (Fig. 2). Ureilites, which are the most depleted, plot depletion ofa meteorite group. Chondritic materials should show the along the slope -I CAl line. Winonaites, brachinites, and acapulco best correlation and ureilites should show the worst. Again, this is ites, which are the least depleted, plot on nearly slope -1/2 lines not the case (Figs. 3,4). The ureilite correlation is ofthe same quality LPI Technical Report 97-02, Part J 17 as that of Allende chondrule rims (r2= 0.76 for ureilites and 0.71 phyllosilicate composition [6], a parameter that can only be obtained for Allende rims), and much better than that ofAllende chondrules after painstaking electron microprobe analysis of many sections. (r2 = 0.29) (Fig. 3). Of acapulcoites and lodranites (Fig. 4), acapul Recent advances in the application of X-ray diffraction tech coites should show a better correlation, but they show no significant niques to the characterization of mixtures ofminerals have resulted correlation (r2 = 0.14) compared to lodranites (r2= 0.38). And the in the development ofa rapid., whole-pattern profile-stripping method lodranite correlation should be better than that ofureilites (Fig. 4), for the quantification of multi phase samples [7]. This method has but it is not (but see [15]). Again, the observations are the opposite been further developed to deconvolute mixtures of clay minerals in of what would be expected. environmentally important deposits [8]. Analyses can be carried out There is not a simple transitional sequence ofcoupled 0 isotopic on either powdered samples or thin sections, with a data acquisition and geochemical changes among the primitive achondrites and time ofonly 5-10 min (although a useable pattern for identification ureilites. This raises some questions about our understanding of the purposes can be obtained in seconds). Given that the main matrix chondrite-achondrite transition: (I) Are the incipient melting condi component in CM2 chondrites might well be described as a mixture tions that explain the textures and compositions ofwinonaites and of clay minerals, it was thought that application of the XRD tech acapulcoites sufficient to equilibrate 0 isotopes? If not, (2) Is it nique might be appropriate as a rapid and simple method for tracing possible that winonaites and acapulcoites accreted from material that the mineralogical variations in carbonaceous chondrite matrixes was already 0 isotopically equilibrated but not chemically frac brought about by aqueous alteration. To this end, a suite of CM2 tionated? If so, (3) Why did ureilites not equilibrate 0 isotopes? chondrites will be examined; initial results for Cold Bokkeveld and (4) Is it possible that ureilites accreted from material that was not Murray are reported here. Cold Bokkeveld is the most altered CM2 chondri tic in bulk composition, in particular with superchondritic chondrite in the alteration sequence proposed by Browning et aI., CalAI? (5) Are mg-l1170 correlations really nebular? while Murray occurs midway through the sequence [6]. Acknowledgments: This work was supported by my mother. Method: A preliminary optical examination ofthin sections of References: (I] McSween H. Y. (1987) Meteorites and Their Cold Bokkeveld and Murray identified areas approximately 5 x Parent Planets, Cambridge Univ. [2] Grossman 1. N. (1988) in 5 rnm2 relatively free from large olivine grains, clasts, etc. X-ray Meteorites and the Early Solar System (J. F. Kerridge and M. S. diffraction data were acquired with an Enraf-Nonius diffractometer Matthews, eds.), p. 680, Univ. of Arizona, Tucson. [3] Jacobsen system utilizing a curved position-sensitive detector (PSD) with S. B. and Wasserburg G. J. (1980) EPSL, 50, 139. [4] McSween simultaneous detection of diffracted X-rays over 1200 of arc; tube H. Y. (1994) Meteoritics, 29, 757. [5] Clayton R. N. et al. (1973) operating conditions were 45 kV and 45 mAoThe monochromated Science, 181, 485. [6] Jones J. H. (1984) GCA, 48, 641. [7] Dreibus X-ray beam (CU-KCL1radiation) was delimited by a combination of G. and Wanke H. (1985) Meteoritics, 20, 367. [8] Treiman A. H. horizontal and vertical slits, resulting in beam dimensions -5 rom x et al. (1986) GCA, 50, 1071. [9] Ahrens L. H. (1970) EPSL, 10, 1. 0.25 rnm; a stationary bearn-sample-detector geometry was em [10] Taylor G. J. et al. (1993) Meteoritics, 28, 34. [II] Clayton ployed. Each section was aligned using a collimated laser to position R. N. et al. (1991) GCA, 55, 2317. [12] Prinz M. et al. (1980) LPS the selected area under the glancing-angle incident X-ray beam. XI, 902; (1983) LPSXZV; 616. [13] Benedix G. K. et al. (1995)LPS Two analyses were made of a polished thin section surface of XXVI, 99. [14] Clayton R. N. and Mayeda T. K. (1996) GCA, 60, Cold Bokkeveld, firstly for an acquisition time of 10 min with the 1999. [15] McCoy T. J. (1997) GCA, in press. section stationary, the second with the section rotating in its own plane for an acquisition period of I hr. Although a clearer pattern, with less noise and better count statistics resulted from the experi PROGRESSIVE ALTERATION OF CM2 CHONDRITE ment oflonger duration and larger area irradiated, no advantage was MATRIXES: DETERMINING RELATIVE PHYLLOSILI gained, thus subsequent analysis of the section of Murray was CATE CONTENTS BY X-RAY DIFFRACTION. M. M. carried out for an acquisition time of I 0 min with a stationary sam Grady, M. Batchelder, G. Cressey, and M. J. Genge, Department of ple. Fluorescence from Fe is induced by the CU-KCL1 radiation; Mineralogy, The Natural History Museum, Cromwell Road, London correction for this effect gives a total Fe content for the specimen. SW7 5BD, UK. Sequential pattern matching between sample and patterns from single phases, followed by profile stripping allows identification of Introduction: Fine-grained matrix is the most volumetrically each phase present, and their relative abundances [7,8]. abundant component in CM2 chondrites (1,2]. It is a complex mix Results: Preliminary results from Cold Bokkeveld and Murray ture ofphyllosilicate minerals, sulfides (including tochilinite), car indicate that in both meteorites the two majorphyllosilicates present bonate, and high-temperature mineral fragments [3]. CM2 carbon were, as anticipated, a serpentine-group mineral (28 - 12°) and aceous chondrites have suffered hydrothermal alteration to various smectite (28 - 6°). Comparison between the two chondrites showed levels, a process that might have occurred either on the parent body that although both meteorites contained similar quantities ofsmectite, [1,4] or in a nebular setting [5], but the conditions (P, T, eH, pH) Cold Bokkeveld has approximately twice as much serpentine as under which the alteration took place are still subject to debate [2]. Murray, as befits its more altered nature [6]. A complete sequence of The extent to which matrix alteration has occurred is an important CM2 chondrites will be measured. Matching the meteorite profiles parameter that must be determined before secondary alteration ef with patterns from single mineral standards covering the spectrum of fects can be distinguished from primary parent-body variations. serpentine-group compositions from cronstedtite (Fe-rich) to ser Unfortunately, however, the extremely fine-grained and highly pentine (Mg-rich) phyllosilicates should allow determination ofthe intergrown nature of carbonaceous chondrite matrix has made con cronstedtite-to-serpentine ratio of each meteorite, reflecting the ex structing a reliable index ofalteration a difficult task. The most recent tent to which Fe-bearing phyllosilicates have been converted to Mg and comprehensive index relies on calculation of an average matrix rich end members during alteration. 18 Workshop on Modification of Chondritic Materials References: [I] McSween H. Y. (1979) GCA, 43, 1761-1770. In order to study the fine-grained matrix material «3 ~m), four [2] Zolensky M. E. et al. (1993) GCA, 57, 3123-3148. [3] Zolensky matrix-rich areas of the meteorite were investigated by TEM. The M. E. and McSween H. Y.(1988) in Meteorites and the Early Solar matrix is dominated by an amorphous material that is also present in System (1. F. Kerridge and M. S. Matthews, eds.), pp. 114-143, a sample produced by crushing, excluding an origin as a result ofion Univ. of Arizona, Tucson. [4] Tomeoka K. and Buseck P. R. beam irradiation (Fig. I). The most abundant phases enclosed in the (1985) GCA, 49, 2149-2163. [5] Metzler K. et al. (1992) GCA, amorphous material are tiny rounded olivines (200-300nm), rounded 56,2873-2898. [6] Browning L. B. et al. (\996) GCA, 60, 2621 pyroxenes (300-400 nm), and F e,Ni-sulfides (100-300 nm). Some 2633. [7] Cressey G. and Schofield P. F. (\996) Powder Diffr., 11, 15-80-nm-sized olivines (Fa Annealing ofthe amorphous material. In order to recrystallize Introduction: How well do chondrites preserve primitive Mg-rich olivines from a Fe-rich groundmass, highly reducing condi chondrule compositions? Chondrules in most meteorites have been tions are required, which would simultaneously result in the forma chemically altered by secondary processes after they solidified. tion ofFe-metals. Such are absent in the fine-grained matrix ofAcfer Thermal metamorphism on parent bodies affected most chondrites, 094. However, considering the matrix ferrihydrite as an alteration causing elements to migrate across chondrule borders, e.g., olivine product of former Fe-metals, recrystallization of the amorphous and pyroxene equilibrate as Mg and Fe diffuse across grain and material cannot be completely ruled out. chondrule boundaries at metamorphic temperatures. Secondary min Condensation in the solar nebula. The condensation ofolivines erals such as plagioclase and phosphates form metamorphically, containing more than 10-2 Fa mol% requires strongly oxidizing accompanied by a similar migration of alkalis, Ca, and rare earth conditions. Such conditions may have been present locally in the elements. Shock metamorphism on the parent body also may cause solar nebula, possibly due to vaporization of volatile components elements to move into or out ofchondrules as phases such as metal [10]. On balance, the fact that mixing ofa disequilibrium condensa and sulfide become mobilized. A third parent-body process, aqueous tion product (amorphous material) and an equilibrium condensate alteration, severely affected chondrules in CM and CR chondrites, (olivines) so intimately would be quite difficult, a formation by again almost certainly resulting in the transport ofelements between partial annealing is favored. chondrules and matrix. Depending on the extent of thermal and Matrix Pyroxenes: Two different pyroxenes are present: shock metamorphism and aqueous alteration, the compositions that enstatites (Fs0-3) and Ca-rich pyroxenes (W020-45)' The few Ca-rich chondrules had just after accretion may be partially or completely pyroxenes are fragmental grains ofprobably clastic origin. Enstatitic erased, and will certainly be obscured. pyroxenes are mostly abundant and typically occur as 300-400-nm Thus, researchers use the unmetamorphosed, unshocked, least rounded individual grains. Nearly all pyroxenes studied are altered, petrographic type 3 meteorites such as the unequilibrated intergrowths of orthoenstatite and clinoenstatite. The odd and even ordinary chondrites (UOCs) and CV3, C03, EH3, and EL3 chon number ofclinoenstatite (100) repeats and the presence ofdifferent drites to measure the compositions that chondrules had at the time of orientated twins proves that they formed by rapid cooling (1000°- accretion, and thereby to learn about their formation and nebular 5000°CIhr) from high temperatures [II]. Excluding fragmentation history. But, do even these chondrites preserve chondrule composi and recrystallization, the small Mg-rich pyroxenes (Fso_3) with tions? rounded morphologies are thought to have condensed in the solar Metamorphism: The UOCs themselves encompass a wide range nebula at highly oxidizing conditions. However, high cooling rates of metamorphic grades. Petrologic subtypes 3.3 to 3.9 have a nar are contrary to an assumed slowly cooling nebula. Therefore, the rower range ofFelMg ratios in their olivine than do lower subtypes, formation of the rounded pyroxenes remains uncertain. and experienced mild thermal metamorphism on the parent body Ferrihydrite and Phyllosilicates: Ferrihydrite, chlorite, and resulting in movement of cations across chondrule borders [I]. A serpentine were identified as the only hydrous phases in Acfer 094. similar range ofmetamorphic effects is present in the type 3 carbon Whereas the formation ofchlorite and serpentine found in the amor aceous and enstatite chondrites [2,3], leaving only a few good can phous material is attributed to a first recrystallization, ferrihydrite didates for preserving primitive chondrule compositions. and serpentine occurring in cracks and pore spaces have formed by Metasomatism: Bleaching. Among the most primitive UOCs hydrous alteration. Based on mineralogical criteria it is impossible to (including Semarkona, Bishunpur, Krymka, Sharps, and Tieschitz), distinguish between parent-body and terrestrial alteration. the effects oflight aqueous alteration have been known for a number Although some similarities exist, comparing the fine-grained ofyears, but most ofthe affected phases are in the extremely fine matrix of Acfer 094 with the matrixes of the wiequilibrated C03 grained matrix or as rare, altered patches in chondrules [4-6]. How chondriteALHA 77307, and the unusual type 3 chondriteKakangari, ever, we recently rediscovered and documented [7J the observation Acfer 094 remains unique. The presence of an amorphous phase, that the finest-grained chondrules, either cryptocrystalline (C) or most probably representing an unprocessed pristine condensate, radial pyroxene (RP) in texture, in most UOCs and type 4 chondrites supports the idea that such material ofvariable chemical composition have thick "bleached" zones at their surfaces and along cracks [8-9]. could be the precursor of other matrix components [12]. The unaffected areas of these chondrules commonly have norma References: [I] Brearley A. (1989) GCA, 53, 2395-2411. tive compositions consisting of -90% low-Ca pyroxene plus -10% [2J Brearley A. (1993) GCA, 57, 1521-1550. [3] Bischoff A. and highly albitic plagioclase(glass). The bleached zones have much less Geiger T. (1994) LPS xxv, 115-116. [4] Newton 1. et al. (1995) normative feldspar and are porous. The ion probe shows that in Meteoritics, 30, 47-56. [5J Greshake A. (1997) GCA, 61, 437 Semarkona these zones are also rich in H20 and halogens. The albitic 452. [6] Gao X. et al. (1996) Meteoritics & Planet. Sci., 31, A48. glass has most likely been dissolved and transported out of the [7] Alexander C. et al. (1989) EPSL, 95, 187-207. [8J Banfield 1. et chondrules by circulating aqueous fluids, leaving holes and small al. (1991) GCA, 55,2781-2793. [9J Rietrneijer F. (1996)LPSXXVII, amounts ofalteration products behind. Thus, the bulk compositions 1069-1070. [10] Palme H. and Fegley B. (1990) EPSL, 101, 180 ofsuch chondrules are not primary, and have been lowered in alkalis, 195. [IIJ Brearley A. and Jones R. (1988) Eos Trans. AGU, 69, AI, and Si; only analyses ofthe cores ofthese chondrules give data 1506. [12] Scott E. et al. (1996) LPS XXVII, 1161-1162. relevant to chondrule formational conditions. We note that if this type of process affected coarser-grained chondrules, etching away just the outer few micrometers of large areas of glass, it might be CHEMICALALTERAnONOFCHONDRULESONPARENT extremely difficult to identify. BODIES. J.N.Grossmanl,C.M. O'D.Alexander2,andJ. Wang2, Where did this aqueous alteration occur? In Semarkona, there is IU .S. Geological Survey, 954 National Center, Reston VA 20192, convincing evidence that this process happened on the parent body USA ([email protected]), 2Camegie Institution ofWashington, after compaction: Some chondrules have only partially bleached Washington DC 20015, USA ([email protected]). surfaces, and are altered where matrix contacts the chondrule, but not 20 Workshop on Modification of Chondritic Materials where one chondrule touches another; the porous matrix acted as a altered glass is found in the pyroxene-rich outer zones of these conduit for fluid flow. In Bishunpur, which is more crushed and (commonly) igneously zoned chondrules, making it tricky to recon brecciated than Semarkona, there exist chondrule fragments that are struct, and impossible to measure directly the preaccretionary bulk bleached along the original surface, but not along the broken surface, compositions of the whole chondrules. indicating impact processing after alteration. In the higher-petro The glass in a few 10w-FeO chondrules appears to be completely logic-type ordinary chondrites, bleached chondrules are found in the altered. These chondrules do not show zoning of volatiles in their absence of hydrated minerals, implying that metamorphism (and glass, but are instead uniformly high in volatiles throughout. In the dehydration) postdated an episode of aqueous alteration, so it is one example we studied, the glass composition lies along the vola possible that virtually all ordinary chondrites experienced some tile-rich ends of the alteration trends shown by zoned chondrules. In aqueous alteration during their history. Semarkona, the group A5 chondrules, characterized by highly albitic Metasomatism: Alteration of glass. Another effect of sec mesostasis [13], may also be thoroughly metasomatized objects. In ondary processes acting on chondrules in primitive UOCs (thUS far one large example, glass inclusions trapped in the olivine have very studied in Semarkona and Bishunpur) is the compositional zoning of low NatAl ratios compared to the mesostasis surrounding the olivine. mesostases in 10w-FeO chondrules [10,11]. Glass near the outer The mesostasis is also high in H20 and other volatiles in ratios similar surfaces of these chondrules is commonly much richer in volatile to that found in the altered parts oflow-FeO chondrules. Some other lithophile elements than glass deep inside the chondrules: alkalis high-FeO chondrules (type II or group B) contain similar glass (Na, K, Rb, and Cs), H (probably as H20), halogens (CI and F), and inclusions in their olivine, hinting at metasomatic effects in these as Mn all show this effect, with enrichment factors of 10 or more well. observed in some cases. The DIH ratio is also much higher in volatile We believe that the presence of volatile-rich, hydrated glass in rich glass zones. The glass of the same chondrules shows weaker chondrules is generally diagnostic of parent-body aqueous alter zoning ofmajor elements, apparently caused by the fractional crys ation. It is important to note that this does not rule out the possibility tallization ofclinopyroxene near the chondrule surface during solidi that nebular alteration also affected these or other chondrules. fication; cooling was too rapid for melt near the surface to equilibrate Conclusions: There are abundant effects ofaqueous alteration with melt within the chondrule. This primary igneous process gave on the parent body in chondrules from UOCs, making it more diffi rise to bright yellow cathodoluminescence in the outer zones ofthese cult than previously thOUght to read the compositional record left chondrules and also must have resulted in a slight enrichment ofthe from the time of chondrule formation. Much of the data in the mostly incompatible volatiles (assuming that they were not all lost at literature may need to be reinterpreted in light of these results. this stage; see below). In any case, the observed magnitude ofvola Indeed, there may not be any ordinary chondrites with chondrules tile-element zoning is far too great to be caused by this primary completely free from secondary parent-body processing. igneous effect (and for elements like Na, diffusion in silicate melts is References: [I] McCoy et al. (1991) GCA, 55, 601-619. probably too fast to preserve such effects); the DIH fractionation also [2] Scott and Jones (1990) GCA, 54, 2485-2505. [3] Zhang et al. could not be igneous in origin. We must appeal to secondary pro (1995) JGR, 100, 9417-9438. [4] Hutchison et al. (1987) GCA, cesses to explain these. 51, 1875-1882. [5] Alexander et al. (1989) GCA, 53, 3045-3057. Volatiles must have entered the chondrules in a subsolidus alter [6] Alexander et aI. (1986) Meteoritics, 21, 328. [7] Grossman ation process. Simple diffusional migration ofthe elements into the (1996) LPS XXVII, 467-468. [8] Kurat (1969) in Meteorite Re chondrules, such as might happen during nebular condensation or search, pp. 185-190. [9] Christophe Michel-Levy (1976)EPSL, 30, recondensation, or during thermal processing on the parent body, is 143-150. [10] Grossman (1996) Meteoritics & Planet. Sci., 31, unlikely because zoning profiles are similar for elements with very A55-A56. [11] Grossman et al. (1997) LPS XXVIII, 481-482. different diffusion coefficients and condensation temperatures. The [12] Deloule and Robert (1995) GCA, 59, 4695-4706. [13] DeHart presence ofup to 1% H20 in the volatile-enriched zones is evidence etal. (1992) GC4, 56, 3791-3807. that the alteration process occurred in the presence of an aqueous fluid phase that reacted with the glass and transported the volatile elements. Clearly, the glass has not completely reacted away, as it is still isotropic (albeit browner in color) and cathodolwninescent. THERMAL QUENCillNG OF SILICATE GRAINS IN PRO We conclude that the parent body is the most likely setting for TOSTELLAR SOURCES. S. L. Hallenbeck,l 1. A. Nuth1, and this alteration process as well. The matrix material surrounding F. J. M. Rietrneijer2, IMail Code 691, NASA Goddard Space Flight chondrules would be an excellent source of volatile elements that Center, Greenbelt MD 20771, USA, 2Departrnent of Geology, Uni could be transported into chondrules by the fluid. Indeed, comple versity of New Mexico, Albuquerque NM 87131, USA. mentary fractionations are observed between chondrules and matrix for a nwnberofvolatile elementratios in Semarkona. The matrix also Turbulent convection ofhot vapor and dust into cooler regions of has a high DIH ratio [11,12], and would be a good source ofD for the protostellar disks can result in abrupt thermal quenching of freshly metasomatic fluid. It seems likely that this fluid is the same one that condensed grains. Laboratory experiments on initially amorphous altered the C and RP chondrules: the anorthitic glass in 10w-FeO dust analogs provide a unique opportunity to study the thermal chondrules probably responded quite differently to the fluid than the processing of silicate grains in protostellar sources. Infrared spec albitic glass in most C and RP chondrules. troscopy and analytical electron microscopy of these laboratory Glass in the cores of many 10w-FeO chondrules appears to be samples will aid in the identification ofprimitive materials preserved unaffected by aqueous alteration: zoning profiles are commonly U in chondritic meteorites and comets, and may be a useful way of shaped across the chondrules, with flat, unzoned central regions relating infrared observations ofcometary dust to the chemical com (which we note are not completely volatile free). But the most- position and mineralogical content of such grains. LPI Technical Report 97-02, Part I 21 6,------~ No olivine (Mg2Si04) or pyroxene (MgSi03) compounds were present at this stage in the annealing process. 5 In the primitive solar nebula the conditions necessary to produce quenched silicates may only have existed in relatively low-density _ 4 environments such as in bipolar outflows above the interface be ... tween the accreting Sun and the nebular accretion disk. In denser .c regions one would expect that the cooling rate would be too low to -:::, 3 preserve such highly amorphous materials due to the high nebular .E opacity. However, in late stages ofplanetary accretion, following the 2 loss of much ofthe insulating gas and dust of the nebular accretion phase one might find conditions suitable for the preservation ofsuch signatures. In particular, immediately following the impact of two planetary-scale bodies, such as has been hypothesized to have formed 04-~--~~--r_~~--~_r the Earth-Moon system, one expects the formation of a massive, 9.60 9.70 9.80 9.90 10.00 silica-rich atmosphere, the outermost portions of which should be rapidly quenched by radiative cooling to space. Dust lost from such 10001T (11K) an environment could be accreted by cometessimals prior to their ejection into the Oort Cloud or Kuiper Belt. Such grains could also Fig. 1. Arrhenius plot of the annealing time required for the 10-llm lR be accreted as asteroidal regolith on growing planetissimals and feature of the amorphous silicate to shift from 9.3 to 9.7 Ilm over the could be preserved as meteorite matrix. Finally, it may be possible to temperarure range 1000-1038 K. indirectly gather evidence for planetary accretion by observing the infrared spectra ofoptically thin dust in T Tauri stars and searching for the signature of quenched dust produced by planetary-scale Magnesium silicate smoke was prepared in a condensation flow impacts. apparatus [I] from Mg metal and a mixture ofSiH4 diluted in He and References: [I] Nelson R. J. et al. (1989) Proc. LPSe 19th, O2 at -770 K at a total pressure of 80 torr. The freshly condensed pp. 559-563. smoke was deposited on a collection sheet at -300 K and removed from the vacuum chamber after the furnace had cooled. The IR spectrum ofthe initial Mg silicate condensate, a chaotic mixture of IRON-RICH AUREOLES AS RECORDERS OF IN SITU Mg metal and silicon oxides with a Mg:Si atomic ratio of 1:3, AQUEOUS ALTERATION IN THE CM CARBONACEOUS displayed two broad bands of 9.3 and 21.3 Ilm, attributed to Si-O CHONDRITES MURRAY, MURCHISON, AND ALLAN stretching and O-Si-O bending vibrations respectively. HILLS 81002. N. P. Hanowski and A. J. Brearley, Institute of Samples ofthe Mg silicate were annealed in vacuum and moni Meteoritics, Department of Earth and Planetary Sciences, Univer tored by IR spectroscopy as a function ofannealing time, focusing on sity of New Mexico, Albuquerque NM 87131, USA. the evolution of the 10-J.lm feature. As annealing proceeded, the maximum shifted to longer wavelengths and was eventually ob Introduction: Iron-rich aureoles in aqueously altered CM car served at 9.7 /lm, the value typically reported for silicates in the bonaceous chondrites represent compositionally distinct areas of interstellar medium and in circumstellaroutflows ofO-rich stars. The fine-grained secondary material (e.g., matrix) surrounding altered rate ofspectral evolution was measured as a function oftemperature metal inclusions. These inclusions have diameters up to > I 00 J.lm and from 1000 to 1038 K. At 1000K, II days ofannealing were required occur in matrix, chondrules, and chondrule fragments. The aureoles for the maximum to shift from 9.3 to 9.7 J.lm. At 1027 K, the maxi show characteristic elemental enrichments and depletions due to mum was observed at 9.7 J.lm after 6 hr, while only 2 hr ofannealing element ~obilization into and out ofaltered metal inclusions. Sev was needed at 1038 K in order to produce the same spectral changes. eral Fe-rich aureoles (>500 /lm in diameter) have been identified An Anhenius plot of the annealing time required before the maxi previously as the result ofin situ aqueous alteration ofmetal inclu mum in the IR spectra ofthe amorphous silicate had reached 9.7 Ilm sions in the CM chondrite [IJ. Here we present additional occur over the temperature range 1000-1038 K is presented in Fig. I. rences ofsuch aureoles in the CM chondrite Murchison (with a low Given the very rapid rate at which the 9.3-J.lm maximum shifts to alteration degree similar to Murray) and the highly altered CM 9.7 Ilm, observations ofdust with maxima in the range between 9.3 chondrite, ALH 81002. An understanding of the mechanisms in and 9.7 J.lm in either optically thin protostellar sources or in comet volved in the formation ofFe-rich aureoles has considerable poten ary dust would imply the existence of very high thermal gradients tial for further elucidating the conditions and duration of the alter combined with rapid mass transport necessary to quench the anneal ation process. ing ofthis early state. Observations: Backscatteredelectron (BSE) images from three Transmission electron microscope analysis of the Mg silicate thin sections of Murchison show between two and six well-devel samples revealed zoned entities 2-3 Ilm in diameter that contained oped, Fe-rich aureoles several hundred micrometers in diameter in cores of pure silica that were either polycrystalline tridyrnite or each section. Some of these aureoles can be directly related to dense, fine-grained amorphous silica smoke. These silica .cores may altering metal inclusions in the matrix or chondrules. The majority, be experimental artifacts of the vapor condensation process. The however, is apparently imaged in a plane that does not include the cores were mantled with mixed Mg silicates with a bulk composition altered metal inclusion (Fig. la). Quantitative X-ray maps usually of dehyroxylated serpentine (Mg3Si207) or smectite (M&,Sig0 22). show that the fine-grained material inside the aureoles has an Fe 22 Workshop on Modification of Chondritic Materials are the most characteristic feature ofFe-rich aureoles in ALH 8 J002 and the detection ofaureoles in BSE images ofthis meteorite would be extremely difficult without them. The altered metal inclusion associated with one aureole in ALH 81 002 is compositionally similar to inclusions in Murchison and Murray (but with a higher S content of -20 wt%) and affects the altered mesostasis of the chondrule in . which the inclusion is located. Altered mesostasis in proximity to this in-clusion «150 )lm from the inclusion) is slightly enriched in Fe (-5 wt%) and S (-2 wt%) compared to more distant altered meso stasis. Discussion: The presence ofFe-rich aureoles in Murchison and ALH 81002 is consistent with evidence from Murray (I] that their aqueous alteration occurred (probably exclusively) in a parent-body environment. The undisturbed appearance of the aureoles and the fact that they enclose chondrules and chondrule fragments indicate early in situ alteration of the metal inclusions after brecciation ceased to playa significant role. The compositional behavior of the altered metal inclusions in Murchison and ALH 81002 (depletion in Fe and enrichment in S, Mg, and Si) appears to complement the behavior oftheir surrounding aureoles (enrichment in Fe and deple tion in S, Mg, and Si). This is consistent with earlier mass balance calculations on inclusions and aureoles in Murray [I] indicating a coupled exchange between them. Although some aureoles, espe cially in Murchison, bear a strong resemblance to Fe-rich chondrule rims, they are clearly the result of a chemical exchange process. Chondrule rims in CM chondrites generally have a slightly higher S content than surrounding matrix (3], which is the reverse ofwhat is observed in Fe-rich aureoles. With analogy to terrestrial alteration processes, the radial mor phologies and limited (submillimeter) range of elemental exchange Fig. 1. (a) Backscattered electron image of an Fe-rich aureole in the eM indicated by most Fe-rich aureoles suggest a diffusion process in carbonaceous chondrite Murchison. (b) Quantitative X-ray map of the Fe which elements are probably exchanged in solution in an H20-rich concentrations in the aureole shown in (a) (maximum intensity corre fluid without advective transport [4]. The relatively large aureoles in sponds to -35 wt% Fe). (c) Quantitative X-ray map of the S concentra ALH 81002 indicate that their spatial extent may be related to the tions in the same aureole (maximum intensity corresponds to -3 wt% S). higher degree of alteration in this meteorite, reSUlting from more extensive interaction with aqueous fluid The Fe enrichment is clearly coupled with S depletion (and minor Mg and Si depletion) in all the aureoles found in Murray, Murchison, content 2-10 wt% higher than equivalent material outside the aure and ALH 81002. This process must involve the preferential break oles. The Fe enrichment is often highest in the outer margins of the down of S-bearing phases in the fine-grained matrix. One possible aureoles. Areas of Fe enrichment (Fig. Ib) also show a depletion in candidate for the source of the S is tochilinite, which is believed to S from-3 wt% (a typical S content ofmatri x and rim material in CM be an important matrix phase in CM chondrites [5]. However, the chondrites [2,3]) to 1500 )lm) than any aureole present in Murray or Murchison. The aureoles in ALH 81002, as well as those in Murchi son and Murray, enclose adjacent fragments, fine-grained material, TRANSPORTATION OF GASEOUS ELEMENTS AND and often other chondrules. The Fe enrichment in aureoles in ALH THEIR ISOTOPES IN A THERMALLY EVOLVING CHON 81002 is usually <5 wt% and the S depletion <2 wt%. They show, DRITIC PLANETESIMAL. K. Hashizume1 and N. Sugiura2, however, a distinct Fe enrichment (often> 15 wt%) along the outer 1DepartmentofEarth and Space Science, Osaka University, Toyonaka, edges ofthe aureoles, forming a reaction front. These reaction fronts Osaka 560, Japan ([email protected]), 2Department of LPI Technical Report 97-02, Part 1 23 Earth and Planetary Physics, University ofTokyo, Bunkyo, Tokyo these initial conditions, we simulated two-dimensional gas flows in 113, Japan ([email protected]). radial (r) and longitudinal (9) directions. During the thermal meta morphic event, we ca!culated chemical reactions between gas species Ordinary chondrites are considered to have experienced thermal and their dissolution into metal phases. We assumed that the 0 metamorphism in small bodies. We are interested in the behavior of pressure is buffered by redox reaction between fayalite and metallic volatile elements in such a thermally evolving planetesimal. Volatile Fe. elements generally have high vapor pressures at high temperature. In In order to simulate the gas transportation, we formulated the porous bodies, with a high gas permeability, volatile elements are transportation equations, modifying the equations described in [2,4] transported efficiently over a long range. The behavior of volatile to better simulate the gas transportation of minor species and iso elements transported by penneable gas flow can be handled by an topes. Two equations are formulated depending on the two flow equation whose form is similar to that of the equation of thermal modes, viscous flow (I «L) and molecular flow (I» L), whereL is the diffusion. We can follow the transportation of heat and volatile effective diameter ofthe flow path, which is comparable to the pore elements in planetesimals, when the parameters in these equations, size or the crack opening size, and I is the mean free path ofthe gas initial conditions, and the chemical behavior ofvolatile elements are molecules. The flow equation in the viscous case is described as given. Numeric simulations forC transportation ina thermallyevolv ing chondritic planetesimal have been previously studied, conclud Vi(vis) = -[kJUo xdPo/dy + D x d(lnX)/dy] ing that the efficiency of C transportation from the interior of the planetesimal to its surface largely depends on the gas permeability where v is gas flow velocity, P is gas pressure, X is gas fraction (=P/ near the surface [I]. In this study, we carried out numeric simulations Po), k is gas permeability, u is viscosity, and D is the gas diffusion for transportation of C, N, and H in a planetesimal using a similar coefficient. The suffixes 0 and i are the total fraction and respective scheme. Using the simulation, we intend to explain several new gas species (therefore Po = l:Pi, for example). The first term in the constraints recently observed among equilibrated ordinary chon equation is called Darcy's law. The gas permeability k of ordinary drites [2]. chondrites is estimated by [5]. In the viscous case, the flow speed We discovered that N in equilibrated H chondrites is mainly determined by Darcy's law is proportional to the total pressure trapped in taenite (f.c.c. y-Fe,Ni), probably dissolved in the intersti gradient. Since we assume isochemical starting materials, there is no tial sites of the metals. The N concentrations in taenite were around total pressure gradient in the edirection. Therefore, the homogeni 10 ppm, and the isotopic compositions ranged-40 < olSN < +120%0 zation ofN isotopes within the 9 direction can be driven only by gas among the chondrites [2]. From the diffusion coefficient ofN in y diffusion, the second term ofthe equation. Comparing kofUo x P oand Fe,Ni, we estimated the closure temperature of N in taenite to be D, both parameters with a dimension ofm2/s, D is smaller than the approximately 500°C. We concluded that N presently trapped in first parameter at a high-pressure region such as Po = 107 (Pa). taenite was in equilibrium with the N components surrounding the Therefore, we expect inefficient isotopic homogenization effects metal grains, possibly the gas phase (N2) existing among pores during the viscous flow mode. The isotope homogenization occurs during the thermal metamorphic event, and were trapped in there much more efficiently in the molecular flow mode, whose flow when the parent body cooled down to 500°C. We noted several equation is described as unresolved problems in [2], which we focus on in this study. The problems are as follows: (I) Can we explain the N concentrations in Vi(mol) = -[(I - LlI) k;/ui x dP/dy + Lli x vi(vis)] taenite by dissolution equilibrium between the N2 gas and the metal phase? At 500°C, the N gas pressure is required to be around I bar However, the total mass transported during the molecular mode is to be equilibrated with 10 ppm of N dissolved in taenite, if the less than the amounts transported during the viscous mode because dominant N gas species were N2. If the .gas transportation in the H the molecular mode is applied onlywhen the total pressure is low, for chondrite parent body occurred very efficiently, the gaseous ele example, below 105 (Pa). ments may be totally lost from the interior ofthe parent body. (2) Can References: [I) Sugiura N. et al. (1986) EPSL, 78, 148-156. we maintain N isotopic heterogeneity within a single thermally [2] Hashizume K. and Sugiura N. (1997) GCA, 61, 859-872. evolving parent body? Ifthe gas transportation (isotope homogeni [3] Sugiura N. and Hashizume K. (1992) EPSL, Ill, 441-454. zation) occurred very effectively at the interior, we may not expect [4] Sugiura N. et al. (1987) Mem. NIPR Spec. Issue, 46, 216-225. any isotopic heterogeneity remaining among the equilibrated ordi [5] Matsui T. et al. (1986) Meteoritics, 21. 109-116. nary chondrites, even if there existed a large spatial-scale isotopic heterogeneity among the accreted materials [2,3]. We carried out numeric simulations solving the transportation AN EXPERIMENTAL STUDY OF MAGNETITE FORMA equations described in the next paragraph, combined with calcula TION IN THE SOLAR NEBULA. Y. Hong and B. Fegley Jr., tions for heat transportation and chemical reactions. Many of the Planetary Chemistry Laboratory, Department ofEarth and Planetary basic parameters are the same as in [I], and the thermal model is Sciences, Washington University, St. Louis MO 63130-4899, USA entirely the same as in [I]. We assumed an isochemical starting ([email protected]; [email protected]). material, containing volatiles (H, C, N, and 0) homogeneously. We also assumed a homogenous heat source in the parent body, in this Introduction: Chemical equilibrium calculations predict mag case 26AI, whose amount is enough to make the maximum tempera netite formation at low temperatures [1,2) in the solar nebula via the ture at the center of the parent body 850°C. In addition, we added net thermochemical reaction: 3Fe (alloy) + 4H20 (gas) = Fe304 slightly different amounts of ISN so that the N isotopic composition (magnetite + 4H2 (gas). Using a simple collision theory (SCT), in the starting material differs from the planetesimal. Starting from Fegley concluded that Fep4 formation under nebular conditions 24 Workshop on Modification of Chondritic Materials Temperature (C) peared, the sample returned to its original appearance, and XRD 2 showed only Felines. These observations show that magnetite formed 5 by the oxidation of Fe by H20, and that magnetite can be reduced back to Fe when H 0 is not present. Further kinetic experiments are j4 Solar (H.lH20) ratio - (670 - 1300) 2 t underway. ~3 Summary: Thermodynamic calculations show a magnetite sta t bility field at ~ 100°C for the solar H21H 20 ratio range. Our experi t2 mental data show magnetite fonnation from Fe oxidized in H21H 0 :. 2 • mixtures. The reaction 3Fe (alloy) + 4H20 (gas) = Fe304 (magnet 11 F~O. (magnetite) ite) + 4H2 (gas) can be reversed when the H21H20 ratio is reduced, proving Fe oxidation by water vapor. 0 10 20 30 Acknowledgments: This work is supported by NASA grant 1000O/T (I<) NAGW-3070. We thank K. Lodders and D. Lauretta for helpful discussions, and D. Kremser and R. Poli for technique assistance. Fig. 1. Equilibrium between Fe (metal) and Fe304 (magnetite). The References: [I] Larimer (I 967)GCA, 31, 1215-1238. [2] Gross shaded area is the solar H:zIH20 ratio range. Magnetite is stable at $IOO°C under nebular conditions. The experimental points are shown by dots. man (1972) GCA, 36, 597-619. [3] Fegley (1988) inLPI Tech. Rpt. 88-04, 51-60. [4] Fegley and Prinn (1989) in The Formation and Evolution of Planetary Systems, pp. 171-211, Cambridge Univ. [5] Lauretta et al. (1996) Icarus, 122, 288-305. [6] Robie and Hemingway (1995) USGS Bull. 2131. [7] Grevesse and Noels from O.l-Ilm-radius Fe grains required-1013 s and thus mayor may (1993) in Origins and Evolution ofthe Elements, pp. 14-25, Cam not be kinetically inhibited over the solar nebular lifetime of_10 13 s bridge Univ. [8] Kubaschewski and Hopkins (1953) Oxidation of [3,4]. However, no work on Fe30 4 formation kinetics at low tempera Metals and Alloys, Butterworth, London. tures in HiH20 gases is available to test the SCT model predictions. Our goal is to study the kinetics and mechanism ofFe oxidation at low temperatures in gas compositions relevant to the solar nebula to LIGHTNING AND SHOCK HEATING AS CANDIDATE evaluate the possibility of magnetite formation in the solar nebula. PROCESSES FOR CHONDRULE FORMATION. M. Experimental Methods: Magnetite formation was studied by Honinyi, Laboratory for Atmospheric and Space Physics, Uni heating high-purity Fe foil (99.998%) in HiH20 gas mixtures at versity of Colorado, Boulder CO 80309-0392, USA. different temperatures for known periods of time. This approach is analogous to that in our experimental studies ofFe-sulfide formation We report on our continued theoretical and experimental inves [5]. Thin Fe foil (-0.5 cm x 0.5 cm x 0.025 cm) was placed next to tigations on dusty gas and plasma dynamics that lead to short aPt-PI Rh thennocouple in the isothermal zone ofthe furnace, with duration momentum and energy exchange heating events. We have 40 cm3 min- I (STP) HiH20 flow passing by the Fe foil. The H20 extended our previous test particle simulations to describe aerody vapor content was controlled by the temperature of an H20 satura namic shocks and lightning discharges. We are now considering the tor and the flow rate was regulated by electronic mass flow control self-consistent coupling (mass, momentum, energy, and electrostatic lers. Gas chromatography and water uptake by Mg(CI04)2 both gave charge) between the gas/plasma and the evaporating/condensing H20 concentrations in agreement with the expected value at the chondrule precursor grains. We estimate heating/cooling rates, size saturator temperatures. The reaction products were analyzed by X distributions, and the depth of the molten-solid interface for the ray diffraction (XRD) and the reaction extent was monitored by the surviving grains as a function ofthe initial parameters and grain size weight gain of the reacted sample. distributions. Thermodynamic Calculations: The equilibrium between Fe In collaboration with S. Robertson (University ofColorado) and metal and magnetite was calculated using updated thennodynamic B. Walch (University of Northern Colorado) we have modified an data and elemental abundances [6,7]. The results are shown in existing experimental plasma physics facility to produce conditions Fig. I. Iron is stable above the equilibrium line, while magnetite is that imitate lightning discharges in the early solar system. We have stable below the equilibrium line, and both phases coexist on the line. developed an experimental setup where small precursor grains can be Within the solar HiH20 ratio range, magnetite is stable at ~IOO°C. exposed to electrostatic discharges. The first two sets ofexperiments (The distribution ofC between CO and CH4affects the solar HiH20 (-I 0 samples) showed little or no heating. We are now working on ratio.) several improvements: (I) fabricating smaller precursors, (2) better Results: The black dots in Fig. I show our experimental con electrostatic isolation, and (3) better placement of the samples. We ditions. X-ray diffraction shows that Fe30 4is formed on our samples. rely on our collaborators to analyze the produced artificial chon No wiistite or hematite were detected in these runs. Magnetite forma drules. R. Hewins (Rutgers University) provided precursor grain tion is reversible and depends on the HiH20 ratio. Pure Fe foil material and examined the returned samples. heated at 205°C for 7 days in a gas mixture with an H21H 20 ratio Independently of the details of their generation, lightning dis -41 gained 60 Ilg cm-2and was partially converted to Fe304' X-ray charges produce an ionized plasma column. The initial plasma pa diffraction showed only Fe and Fe3041ines. The color change on the rameters (density, electron and ion temperature, and the size of the surface of the reacted sample indicates the thickness of the Fe30 4 discharge channel) fully determine the subsequent evolution. In our layer [8]. The reacted sample returned to its original weight after model calculations we quantitatively describe the history of the heating at 205°C for 4 days in purified H2. The oxide color disap discharge channel simultaneously with the thermodynamics of the LPI Technical Report 97-02. Part I 25 embedded dust grains. The expected plasma conditions of a dis relative to kamacite indicates Fe depletion. We propose that the halos charge in the early solar system cannot be reproduced in our labora formed by the oxidation of the Fe-Ni metal in the inclusions and tory experiments. We adapt the theoretical models to the laboratory subsequent F e2+ diffusion into adjoining forsterite. plasma conditions to verify the calculations. Using the measured Petrography and Mineral Chemistry: The fay ali tic halos occur historyofthe plasma parameters ofour experimental device as model either near the surfaces of chondrules or along fractures. All halos inputs we calculate energy deposition rates and predict critical grain within a given host grain have the same shapes in polished surfaces, sizes that could produce fully or partially melted samples. but they can differ from one grain to another. Some halos appear Lightning discharges are likely to occur in high-dust-density round, while others are elliptical. By observation of halos in many regions ofthe solar nebula where dust grains have a wide particle size olivine grains, we conclude that their three-dimensional shapes are distribution. One expects that a larger population ofparticles, smaller ellipsoidal. Using optical microscopy, we determined that the long than the characteristic chondrule size, fully vaporize and produce axes ofthe elliptical halos are roughly parallel to the c axis ofthe host plasma and neutral gas, mass loading the original plasma. Also, the olivine. The largest halos are 20 /lm long and 6 ~m wide. Round bigger particles can get entrained and drain the momentum of the inclusions of metal and sulfide occur in the centers of most halos. expansion. In collaboration with A. Korosmezey (KFK!, Budapest, Some inclusions are elongated; multiple adjacent halos look like Hungary) we now self-consistently model the expansion of a dusty several ellipsoids superimposed on one another. plasma discharge. The energy deposition, heating, and mass loss The Fe contents are highest next to the metal and sulfide inclu from the grains is coupled with continuity equations for the mass, sions and decrease gradually with distance from the inclusions. The momentwn, and energy of the expanding plasma. most Fe-rich parts ofthe halos have compositions of-Faso, and those The requirement of low electrical conductivity points toward of the host forsterite are -Fal.o, The rate of compositional change regions in the disk with high dust densities where the grains become along specific directions within a given halo depends on their crys the major charge carriers. In these regions, consecutive shocks or tallographic orientations; the gradients are flatter parallel to crystal density waves could cumulatively build up large electrostatic fields. lographic c than parallel to other directions. The plots of composi In a shocked high-gas-temperature region the ionization fraction tional profiles can be well approximated by the error function. In the increases significantly and dust particles can swiftly collect the free case ofthe multiple fayalitic halos around the elongate inclusions, the electrons and ions via collisions except for the charges residing on the F a contents di ffer from one analyzed spot to another, even though all dust grains, since grain-grain collisions are likely to be rare. As the the spots are at roughly similar distances from the inclusions. shock passes or dissipates, the gas cools and recombines. Different Inclusions in the centers ofthe fayalitic halos are polymineralic, sizes andlor compositions ofthe grains can result in different charg probably mixture ofNi-rich metal, Ni-rich sulfides, and minor apa ing histories. For example, due to the higher secondary orphotoelec tite (whitlockite or other meteoritic phosphates), and silicates. tron production, small particles (micrometer-sized) are more likely Discussion: We propose that oxidation and diffusion were im to collect positive charges and bigger grains (millimeter-sized) are portant factors in halo formation. The standard Gibbs energies (in more likely to end up negatively charged. Naturally, at any point joules per mole) were calculated for the formation ofFeO and NiO charge neutrality is maintained as a function of temperature between 300 and 2000 K. The results show that Fe is more readily oxidized than Ni over this entire temperature range. The residual metal after the oxidation will be Ni e(n, - n;) + Q(a)n(a)da = 0 J:= rich, which is consistent with the composition ofthe metal (Ni up to -50 wt%) in the inclusions. Oxidation of the metal inclusions was where e is the charge ofan electron, ne and ni are the electron and ion accompanied by the production ofNi-rich sulfides. densities respectively, Q(a) is the average charge (positive or nega Both the relative dimensions and the shapes ofthe compositional tive) ofthe particles with radius a, and n(a) is the grain size distribu profiles indicate that the fayalitic halos formed via diffusion. We tion with the amin the smallest and a max the largest grain considered. assume that the forsterite initially contained small metal inclusions. Drag sorting separates the oppositely charged small and large par An increase in f02 resulted in the development ofsome F e2+. Based ticles, perhaps leading to the buildup of large-scale electric fields. on experiments [1] and calculations [2], the diffusion of divalent cations in olivine is highly anisotropic and is fastest parallel to the c crystallographic direction. For Fe2+, diffusion parallel to c is 4x FAYALITIC HALOS WITHIN FORSTERITES FROM CAR faster than along a and 5x faster than along b [I]. Using the re BONACEOUS CHONDRITES. X. Hual and P. R. Buseck2, lationship, x2 =Dt, we calculated that xe:xb:x. =20:9: 10. The most IDepartment of Geology and Chemistry, Arizona State University, elongated halos have dimensions roughly proportional to the calcu TempeAZ 85287-1404, USA,2DepartrnentofBiochemistry,Arizona lated ratios, although some halos have larger ratios. State University, Tempe AZ 85287-1404, USA. We generated time-temperature curves by assuming that the maximum diffusion distances are 5, 10, 15, and 20 ~m respectively. Introduction: Spindle-shaped fayalitic halos occur in forsterites The plot shows that (I) it would take> I 0 10 yr to form even the of the Allende, Kaba, Mokoia, Vigarano, and Ningqiang carbon shortest halo (5 /lm) by diffusion at -560 K; this lifetime is longer aceous chontrites. The halos (1) are ellipsoidal or round in shape, than the age of the solar system (4.6 x 109 yr); (2) if we take the (2) have round, bleblike inclusions ofNi-rich metal and sulfide in nebula lifetime as the upper limit for the diffusion, the lowest tem their centers, and(3)havetheir long axes parallel to each other within perature required to form a 5-/lm halo is -650 K, and -700 K is given host crystals. The Fe and Mg compositional profiles are well needed for a 20-/lm halo; and (3) the halos could have formed with approximated by the error function, which is consistent with a diffu in a matter of a few years or tens of years at temperatures above sional origin. The relatively high Ni contents ofthe metal inclusions 1000K. 26 Workshop on Modification of Chondritic Materials The meteorites that contain the fayalitic halos are all CCs and morphism may be provided by the P3 noble-gas carrier in presolar contain low-temperature minerals such as layer silicates and organic diamonds, which appears to release its gases over a limited tempera species. The upper temperature limit for such materials depends on ture range, independent of the nature of the surroundings [3] . the specific species, but it seems highly unlikely that the host mete Tracers of Nebular Processing: In the least metamorphosed orites could have been heated sufficiently to permit the halos to have meteorites ofeach class, matrix-normalized abundances of pre solar formed in situ. The temperatures inferred as feasible from the above diamonds vary by only a factor of-2.2 [1,2]. There is also increas alternatives suggest a parent-body origin is unlikely because all ingly compelling evidence that each chondrite class acquired the organic components would have been destroyed and phyllosilicates same mixtures ofdiamonds [3-5] and SiC [6-8]. These observations would have been dehydrated at the temperatures required to produce show that a singlewidespreadreservoirofpresolar grains was sampled the halos. by all chondrite classes. This reservoir is most plausibly the dust from Weare thus left with the possibilities that the halos formed in the the solar system's parent molecular cloud. nebula prior to incorporation on their respective parent body(ies), or The CI chondrite, Orgueil, and the matrixes of CM2 chondrites the forsterites have been recycled from an earlier parent body that was have the highest matrix-normalized abundances of silicon carbide, heated sufficiently to allow the halos to form. Such a recycled origin and graphite, have among the highest diamond abundances, and have would help explain the observed nonequilibriurn coexistence of diamonds with high N contents and the highest contents ofP3 noble forsterite and fayalite [3], but it makes it difficult to understand the gases [1-4]. These characteristics indicate that CI chondrites and the primitive bulk compositions of these meteorites. matrixes of CM2 chondrites contain the least-processed sample of Another unresolved problem is what happened to the Mg that was the presolar grains' reservoir from the Sun's parent molecular cloud. presumably replaced by the Fe in the halos? It is possible that Si02 The CI chondrites and the CM2 matrixes also have the bulk chemical was introduced [4] or that Mg left via a gas phase through minor compositions that are most like the composition of the Sun [e.g., fractures. This hypothesis is in agreement with the unpublished 9,10]. As the bulk compositions of the meteorite classes become experimental results of Pal me et a\. (personal communication). It is more fractionated relative to the CI composition, the abundances and clear that important questions remain regarding these intriguing characteristics ofpre solar grains also become increasingly different features. from those in CI chondrites. For example, EH chondrites are much References: [I] Buening D. K. and Buseck P. R. (I 973) JGR, enriched in total Fe and are much more reduced than CI chondrites. 78,6852. [2] LasagaA. C. (1980) Am. Mineral., 65,1237. [3] Hua The EH chondrites also acquired higher matrix-normalized abun X. and Buseck P. R. (1995) GCA, 59, 563. [4] Palme H. and Fegley dances of diamond and SiC than CI chondrites and no detectable B. Jr. (1990) EPSL, 101, 180. graphite [1,2]. The CV3 chondrites are enriched in refractory ele ments such as Ca and AI and in 160 relative to CI chondrites. They also have higher diamond abundances and much lower SiC and PRESOLAR GRAINS AS TRACERS OF NEBULAR AND graphite abundances than CI chondrites. In both ofthese classes, the PARENT-BODY PROCESSING OF CHONDRITIC MA most resistant presolar grains (diamonds and, under reducing condi TERIAL. G. R. Huss, Lunatic Asylum of the Charles Arms tions, SiC) have higher abundances in the more chemically fraction Laboratory, Division of Geological and Planetary Sciences, Mail ated material, while the more reactive graphite and the carrier ofP3 Code 170-25, California Institute of Technology, Pasadena CA noble gases are strongly depleted. These correlations suggest that the 91125, USA. same processes were responsible for both the abundance differences and the differences in bulk composition and provide insight into Presolar grains such as diamond, SiC, graphite, and AI20 3 are those processes. present in the least-altered members of all chondrite classes [1,2]. A nebular model in which the bulk compositions of chondrite These grains reside in the fme-grained "matrix" and did not experi classes reflect different degrees ofnebular condensation [e.g., 9,11] ence the chondrule-forming process. Because most types ofpre solar is inconsistent with presolar-grain abundance data. Pre solar grains grains are chemically unstable in putative nebular conditions and in cannot survive in a solar-composition gas hot enough to evaporate chondritic meteorites, they serve as sensitive monitors ofnebular and most nebular dust [e.g., 12]. If presolar grains represent a minor parent-body conditions. component added to the nebula after condensation, then very effi Tracers ofMetamorphism: Isotopic and trace-element char cient mixing in the meteorite-formation region would have been acteristics ofsome presolar material and the abundances of presolar required to produce the near-constant diamond abundances in mete grains in "matrix" are functions of the metamorphic history of the orite matrixes. This mixing would have to have taken place without host meteorite [1-4]. Currently recognized presolar grains show a destroying the chemical fractionations produced by condensation. range of resistance to metamorphism. If all meteorite classes ac Not only is such a mixing scenario unlikely, this model does not quired similar initial mixtures of presolar grains, then the relative predict or explain the observed correlations between bulk composi abundances of the different types of presolar grains can be used as tions of chondrites and abundances and characteristics of presolar probes of metamorphism in the host meteorite. In most meteorite grains. Efficient mixing apparently occurred prior to nebular chemi classes, graphite and the diamond fraction that carries P3 noble gases cal fractionations and processing of the presolar-grains complex. If are least resistant; SiC is moderately resistant; and diamond and so, then chondrite groups could not have resulted from partial con AI20 3 are most resistant to parent-body metamorphism. However, in densation ofhot nebular gas followed by isolation ofthe condensates the highly reduced EH chondrites, SiC and diamond are both more from the nebula [e.g., 9,11]. stable, and SiC becomes more resistant to metamorphism than dia The currently known presolar grains are only the most recogniz mond [1,2]. These relationships provide a relative scale ofme tam or able partofa much larger complex ofpre solar materia\. This presolar phic intensity. An absolute temperature scale for low-grade meta dust, processed in various ways in the solar system, but not evapo LPl Technical Report 97-02, Part J 27 rated and recondensed, may have been the principal raw material for phates in the IIIAB irons, presumably by (secondary) oxidation ofP meteorites and planets. Bulk presolar matter would have had CI originally contained in metal [6]. Evidence of radiogenic 26Mg in (= solar) bulk composition, with the elements partitioned between plagioclase from Ste. Marguerite [7] provides additional evidence different kinds of solids ranging from stellar condensates like SiC for the rapid growth of chondrite parent bodies with cooling below and graphite tointerstellar amorphous material and dirty ices. Similar the AI-Mg closure temperature within -6 m.y. of CAl formation. abundances of refractory presolar grains would be expected in all Data from the l291_129Xe and U-Pb systems are complementary, classes because chondrites would be modified samples of a single supporting the early onset scenario while also indicating that meta initial mixture. Nebular evaporative processing acting on the initial morphic activity on some chondrite parent bodies continued over a presolar mixture could have produced the chemical fractionations protracted interval ofup to -60 m.y. [8,9]. Unfortunately, while the that define the meteorite classes while at the same time producing the range of metamorphic ages from both the I-Xe and U-Pb systems is variations in abundances and properties of preso\ar grains [1 ,2]. If very similar, most attempts to correlate I-Xe and Pb-Pb ages of so, then the characteristics and relative abundances ofdifferent kinds individual meteorites have proved unsuccessful. An important ex of presolar materials in unmetamorphosed chondrites are potential ception is recent work demonstrating good agreement for phosphate probes of nebular processing. mineral separates [10]. Most ofthe chronological data relating to the Acknowledgments: Supported by NASA grants NAGW 3040 times of formation of secondary minerals in chondrites, excluding and 3296 to G. J. Wasserburg. Caltech Division Contrib. #5821 CAls, indicate relatively long times and appear to "date" parent-body (965). processes, since timescales appear incommensurate with anticipated References: [I] Huss G. R. and Lewis R. S. (1995) GCA, 59, nebular lifetimes. 115-160. [2] Huss G. R. (1997) inAstrophysicalImplications ofthe Acknowledgments: Work performed under the auspices of Laboratory Study ofPresolar Grains (T. Bematowicz, ed.), AlP. the DOE by LLNL under Contract W-7405-Eng-48. [3] Huss G . R. and Lewis R. S. (1994) Meteoritics, 28, 811-829. References: [I] Richardson S. M . (1978) Meteoritics, 13, [4] Russell S. S. etaL(1996) Meteoritics & Planet. Sci., 31,343 141-159. [2] Van Schmus W. R. and Wood J. A. (1967) GCA, 355. [5] Huss G. R. and Lewis R. S. (1994) Meteoritics, 28, 791 31, 747-765. [3] Perron C. et al. (1988) Meteoritics, 27, 275. 810. [6] Huss G. R. et al. (1997)GCA, 61 , submitted. [7] Huss G. R. [4] Endress M. et al. (1996) Nature, 379, 701-703. [5] Hutcheon et a!. (1994) LPS xxv. 585-586. [8] Huss G. R. et al. (1995) LPS I. D. and Phinney D. L. (1996) LPS XXVll, 577-578. [6] Hutcheon XXVI, 645-646. [9] Larimer J. W. and Anders E. (1967) GCA, 31, I. D. et a!. (1992) LPS XXIll, 565-566. [7] Zinner E. K. and Gopel 1239-1270. [10] Anders E. and Grevesse N. (1989) GCA, 53, 197 C. (1992)Meteoritics, 27, 311-312. [8] Swindle T. D. and Podosek 214. [II] Grossman L. and Larimer J. W. (1974) Rev. Geophys. F. A. (1988) in Meteorites and the Early Solar System (J. F. Ker Space Phys., 12,71-101. [12] Mendybaev R. A. et al. (1997) LPS ridge and M. S. Matthews, eds.), pp. 1127-1145, Univ. of Arizona, XXVII, 937-938. Tucson. [9] Gopel C. eta!' (1994) EPSL, 121, 153-171. [10] Braz zle P. R. and Hohenburg C. M. (1996) Meteoritics & Planet. Sci., 31, A20-A21. CHRONOLOGIC CONSTRAINTS ON SECONDARY AL TERATION PROCESSES. I. D. Hutcheon, Isotope Sciences Division, Lawrence Livermore National Laboratory, Livermore CA ELEMENTAL REDISTRIBUTION BY AQUEOUS FLUIDS 94551, USA (hutcheon I @Un1.gov). IN UN EQUILIBRATED ORDINARY CHONDRITES: TlESCHITZ AND SEMARKONA COMPARED. R. Hutchi A central goal ofmeteoritics is understanding the chronology of sonl , C. M. O'D. Alexander2, and 1. C. Bridgesl, IDepartment of events leading to the transformation (a.k.a. modification) of inter Mineralogy, Natural History Museum, Cromwell Road, London stellar precursor material inherited by the nascent solar nebula to SW7 5BD, UK, 2Department of Terrestrial Magnetism, Carnegie form first meteorites and then small planetary bodies by processes of Institution of Washington, 5241 Broad Branch Road NW, Wash evaporation and condensation, melting and recrystallization, accre ington DC 20015, USA. tion, brecciation, and metamorphism. There is a particular need to distinguish, to the extent possible, chronological differences between Introduction: A number of UOCs show signs of hydrous nebular and parent-body processes related to specific features in alteration [1,2]. Kurat [1] suggested that in Tieschitz, feldspathic or chondri tic meteorites. The most direct and readily interpreted infor nepheline-rich mesostases of some pyroxene-rich chondrules had mation on the timescale of chondrite metamorphism comes from been selectively removed by leaching to produce voids. He found no radiometric age dating of "secondary" minerals, i.e., phases pro evidence for redeposition ofthe leached material elsewhere, but this duced as a result of metasomatic events in the solar nebula or on was disputed [3]. The products of aqueous alteration were discov parent bodies. Carbonates in CI and CM chondrites and most Ca ered in Semarkona (and Bishunpur) by Hutchison et al. [2]. Chon phosphates'and many feldspars in equilibrated ordinary chondrites drule mesostases, anhydrous silicates, and sulfide have been partially are secondary minerals that have been used for this application [1 altered to smectite [4] with little production of voids. Tieschitz 3]. Two short-lived radionuclide systems have provided strong evi matrix contains a second generation of Fe-rich olivine, whereas in dence that metamorphic processes began less than 10 m.y. after the matrix ofSemarkona Fe-rich olivine is absent [4,5]. We com formation of the first solar system solids. Excesses ofs3 Cr, from the pare the mineralogical and chemical characteristics of the matrixes decay ofS3Mn ('t1l2 = 3.7 Ma) in carbonates from Orgueil, indicate of the two meteorites to try to identify the processes that acted on an early onset of aqueous activity on the CI planetesimal, only the respective parent bodies. -15 m.y. after the formation ofCAIs [4,5]. This timescale is also Tieschitz: Tieschitz has a chemical composition between H commensurate with that inferred for the formation of Fe-Mn-phos and L groups, is petrographic subtype 3.6 [6], and is texturally 28 Workshop on Modification oj Chontirilic Materials unique. The presence of fine-grained opaque rims-dark matrix between the two types ofmesostasis without compositional change, [3]-around chondrules and other objects is not uncommon in which indicates that a single chondrule liquid was involved. They UOCs, but additionally, in Tieschitz, white matrix fills channels make straight, sharp contacts with bright, smooth mesostasis, but between the chondrules. These channels are a few tens ofmicrome their contacts against dark mesostasis are irregular and marked by ters wi de and their filling generally abuts the dark rims ofthe bound voids. ing chondrules. In polished thin section, white matrix is transparent Dark, blocky mesostasis probably formed in situ by the aqueous (white), with low birefringence and low reflectivity. In the SEM it alteration of primary bright mesostasis. White matrix may have appears dark and blocky on a 5-20-mm scale [3]. Grains of albite formed by the alteration of mesostases expelled from deforming and nepheline may be present [7] and the norm has 50% albite and chondrules [see 3, Fig. 2] with no dendritic augite. Dendrites would 25% nepheline [8]. Rare ferromagnesian mineral fragments occur have inhibited the expUlsion of mesostasis from chondrules. and unidentified, Na-, AI-rich phases may be abundant [5]. Christophe Semarkona: Semarkona belongs to petrographic subtype 3.0 M icbel-Levy [3] agreed wi th [ 1] that some chondru Ie meso stases had [6]. As in Tieschitz, aqueous alteration affected chondrules and been removed, but argued that the material had been redeposited to matrix together. Semarkona, however, contains few optically ob form white matrix. Hutchison et al. [7] disagreed and suggested that servable voids, but porosity is revealed in the TEM (Fig. 3 in [4]). white matrix formed by the crystallization of mesostases expelled Textural relationships indicate that smectite and other products di from hot, deforming chondrules. New observations, however, led rectly replaced mesostasis, olivine, pyroxene, and sulfide, without Hutchison et al. [9,10] to accept that leaching and elemental large-scale elemental redistribution. Because ofthe volume increase redeposition had also occurred. entailed in converting anhydrous precursors to smectite, some el Reinvestigation ofTieschitz [9, I 0] was prompted by the acciden ementalloss or redistribution is required. For example, during hy tal discovery of two types of meso stasis coexisting within five por drous alteration, Ca was not taken up by smectite but was fixed in phyritic olivine (PO) chondrules. The proportion ofchondrules with calcite [4]. voids was determined from a photomosaic ofback scattered electron Discussion: The question arises, could Tieschitz have under images representing a 35-mm2 area: 30 of 101 chondrules contain gone aqueous alteration to a similar degree to that experienced by discernible voids. As observed by [3], some voids have acicular Semarkona, followed by dehydration and mild thermal metamor crystals-parts of augite dendrites-extending inward from their phism? Alexander et al. [5] noted that Semarkona matrix is signifi walls, so the voids are natural. Thus, primary voids or voids due to cantly enriched in Al (x 1.6-2.0), Na (x2.4-2.9), and K (x6.8-7.0), leaching are an important feature ofthe meteorite. In the SEM, some and depleted in Ca (x0.4-0.5) relative to the bulk meteorite. In mesostasis appears to be dark and blocky, like white matrix, whereas contrast, Tieschitz dark matrix is only slightly enriched in Al (x 1.3) most is bright and smooth. Augite dendrites are present throughout and Na (xl.4-1.8), highly enriched in K (x6.3-6.5), and undepleted both types of mesostasis but are absent in white matrix. In a por in Ca relative to the bulk meteorite. Is it possible, then, that Semarkona . phyritic olivine-pyroxene (POP) chondrule the two types are pre matrix could have been chemically fractionated by dehydration and sent without augite dendrites. Dark, blocky meso stasis proved to recrystallization to yield the mineral assemblages found in Tieschitz be chemically similar to white matrix, being rich in A120 3, Na20, white and dark matrix? K20, Ba, Cl, and F and poor in Ca [10] relative to bright, smooth Simple chemical calculations argue against this. The silicate mesostasis. component ofSemarkona matrix (Table I,no. I) could yield 66 wt% The five PO chondrules are typical of Tieschitz. Their longest olivine, F047, within the range of Tieschitz matrix olivines [5], plus dimensions range from 0.6 to 1.0 mm, they are largely surrounded by 34 wt% ofa felsic component. The felsic component is then recalcu opaque, fine-grained rims and they are somewhat misshapen. Depar lated to 100 wt% for comparison with white matrix (Table I, nos. 4 ture from a circular outline is the result of abrasion or of mutual and 5). White matrix is significantly poorer in Si02and K20 than the indentation by neighboring objects. Chondrule outlines are flat felsic component ofSemarkona matrix. Loss ofthese oxides from the tened and the dark rims attenuated or absent at their contacts. In all felsic component could indeed produce a composition similar to five PO chondrules the phenocrysts are zoned, Ca-poor pyroxene is white matrix. Tieschitz may therefore have suffered a net loss, albeit absent, and, as in most Tieschitz chondrules [3], augite dendrites are present throughout the mesostases. In one chondrule the olivines have cores ofFo90 with F075 rims, zoned to F057 at the chondrule TABLE I. margin. In this chondrule the augite dendrites have a uniform com position close to W03sEn43Fs19' with 1-2 wt% Cr20 3, whether set in ,- 2· 3- 4· 5· bright and smooth or dark and blocky mesostasis. Si02 46.7 22.8 23.9 70.5 58.7 As already noted, except for the presence of augite dendrites, Alp) 5.2 5.2 15.3 19.1 blocky mesostasis in the five PO chondrules resembles white matrix. FeO 28.9 28.9 5.0 Kurat [1] and Christophe Michel-Levy [3] observed that the meso MgO 14.4 14.4 3.5 stases of pyroxene-rich chondrules are most suceptible to leaching; CaO 1.26 1.26 3.7 4.0 such chondrules are more likely to contain voids, especially near Nap 2.77 2.77 8.2 9.3 their margins. One porphyritic olivine-pyroxene (POP) chondrule Kp 0.76 0.76 2.2 0.4 Sum was found with its mesostasis largely intact. This chondrule has no 99.9 66.1 33.9 99.9 100.0 augite dendrites but has both types of meso stasis. Olivine pheno - I = Semarkona matrix silicate (Table 2, no. 8, in [2]) recalculated to crysts (F069_52 ) are overgrown by platy, Ca-poor pyroxenes (W03 100 wt%; 2,3 = analysis I apportioned into olivine, Fo." and felsic com ponents; 4 = the felsic component, 3, recalculated to 100 wt%; 5 = white En67) with thin, irregular overgrowths ofCa-rich pyroxene (WO<17 matrix [5, Table 3, no. I]. En53Fs>30)' The pyroxene phenocrysts extend across the contact LPI Technical Report 97-02. Part 1 29 small, ofSi02 and K20. The question is, when? Ifmatrix is largely due to replacement ofthe plagioclase component ofcryptocrystalline the product ofcomminution ofchondrules [5], Tieschitz chondrules, groundmasses by nepheline and sodalite. In addition, groundmass on average, may have been less siliceous and less potassic than those plagioclase in Allende chondrules sometimes suffers replacement by in Semarkona. Certainly, many Tieschitz chondrules have nepheline nepheline or sodalite. This replacement was caused by substitution normative mesostases [7] whereas many Semarkona chondrules have of Ca of plagioclase component by 2Na that were introduced from quartz-nonnative mesostases [5]. Alternatively, leaching by aqueous outside chondrules: fluid may have removed some Si02 from Tieschitz. In any case we are left with the conclusion that the two chondrites differ in primary CaSi2AI 20 g + 2Na -7 NazSi2AI20 g + Ca silicate chemistry or in postaccretional history. An component Gas Nepheline Ca phases References: [I] Kurat G. (1969) in Meteorite Research (P. M. Millman, ed.), pp. 185-190, Reidel, Dordrecht. [2] Hutchison R. et where the Ca atoms in the right hand of the reaction equation were al. (1987) GCA, 51, 1875. [3] Christophe Michel-Levy M. (1976) partly expelled outside the chondrules and partly used to produce Ca EPSL, 30, 143. [4] Alexander C. M. O'D. et at. (1989) GCA, 53, rich phases in the same chondrules. The Ca-rich phases are 3045. [5] Alexander C. M. O'D. et at. (1989) EPSL, 95, 187. hedenbergite, andradite, grossular, kirschsteinite, and wollastonite, [6] Sears D. W. G. et at. (1980) Nature, 287, 791. [7] Hutchison R. and commonly occur in close association with nepheline and sodalite et al. (1979) Nature, 280, 116. [8] Wlotzka F. (1983) in Chondrules [2]. The secondary altered groundmasses including nepheline and and their Origin (E. A. King, ed.), pp. 296-318, LPI, Houston. sodalite often contain fine-grained ferroan olivine grains with [9] Hutchison R. (1992) Meteoritics. 27, 236. [10] Hutchison R. et FoSO _70, suggesting that the replacement took place at the same time al. (1994) Meteoritics, 29, 476. as the replacement ofphenocrystic enstatite by ferroan olivine. Heat ing experiments to produce nepheline from plagioclase crystals and feldspathic glass beads were performed to estimate the fonnation ANHYDROUS ALTERATION OFALLENDE CHONDRULES conditions, and the results suggest that the formation ofnepheline in IN THE SOLAR NEBULA. Y. Ikeda and M. J We found abundant evidence of aqueous alteration in an EH References: [I] Ivanov A. V. et aL (1989) Geochem. IntI., 26, chondrite fragment #02.04 from the Kaidun heterogeneous breccia 84-91. [2] Ivanov A. V. et aL (\993) Geochem. IntI., 30, 11-19. [I]. The fragment consists ofenstatite, albite, silica, graphite, kamacite, [3] Kurat G. et a1. (1997) Meteoritics & Planet. Sci., submitted. perry-ite, schreibersite, troilite, niningerite, and "hydrodaubreelite," and contains three types of Fe-rich oxide phases, which apparently formed from preexisting metal and schreibersite. Analyses obtained ALTERATION OF PLAGIOCLASE-RICH CHONDRULES by EMP are all characterized by low totals, which indicates the IN C03 CHONDRITES: EVIDENCE FOR LATE-STAGE presence of a light element, very likely H. The phases are distin SODIUM AND IRON METASOMATISM IN A NEBULAR guished by both their chemical composition and their location within ENVIRONMENT. R. H. Jones, Institute ofMeteoritics, Univer the fragment. Phases I and II are characterized by a large variation of sity ofNew Mexico, Albuquerque NM 87131, USA (rjones@unm. both major (Si02 2.6-41.6 wt% and Fe 23.1-69.5 wt% respec edu). tively) and all minor element contents and usually have a high con tent ofCI «2.4 wt%) and a N~01K20 ratio >1. Phase I has high Introduction: A suite of plagioclase-bearing chondrules in Ni «24.6 wt%) and S «16.5 wt%) contents and low Alp) «1.5 C03 chondrites shows evidence for extensive Na and Fe metasoma Wt%) and MgO « 1.4 wt%) contents. In contrast, phase II has low tism. An important question to address is whether this metasomatism Ni « I wt%) and S contents « 1.7 wt%) and high Alp) «21 wt%) took place in the nebula, as a result of interactions with a low and MgO contents «16.5 wt%). temperature gas, or whether ittook place on the CO parent body after Phase III is characterized by a rather homogeneous chemical accretion. The CO parent body is known to have undergone an composition and low contents of minor elements. It is rich in Si02 episode of thermal metamorphism, so it is possible that some or all (41.3 wt%) and FeO (43.3 wt%), poor in all other elements, has a ofthe observed alteration was associated with this episode. However, Na20IKP ratio near I, and is free is of CL this style ofmetasomatism is also commonly associated with CAIs, Phases I and II occur in the peripheral zones ofNi,Fe-metal grains in which it is understood to have happened in the nebula. Petro located near the surface of the fragment. Sometimes a complete graphic observations suggest that the metasomatism did indeed oc replacement ofthe metal grain is observed, but the primary "metallic" cur in the nebula, and that parent-body metamorphism left a second morphology of the grain is preserved. No regularity in a real distri ary overprint on the metasomatized chondrules after accretion. bution ofphase III was found. It is present mainly as irregular grains, Observations: We have studied nine chondrules from C03 in which small enstatite grains are occasionally included. This situ chondrites that have a "basaltic" texture, with primary pyroxene and ation is rather typical for Ni,Fe-metal grains ofthe unaltered sample. plagioclase phenocrysts. These chondrules were first described by The chemical composition, morphology, and location of phases [I] in Lance. A similar object from Omans was described by [2]. We I and II permit us to identify them as the product of aqueous altera have searched 19 thin sections of a total of 13 different CO chon tion ofNi,Fe metal and schreibersite, which very likely took place drites, and the nine chondrules we found are in four different late and at low temperatures. This low-temperature alteration could chondrites: Kainsaz, subtype 3.1 [3,4], Lance, subtype 3.4 [3], and have taken place in the Kaidun parent body [2]. Another possible ALHA 77003, and its paired chondrite, ALH 83108, subtype 3.5. place could have been some porous zones within the rock that These chondrules have complex primary mineralogies and crys permitted fluids to percolate. tallization histories. They are reduced and contain Fe,Ni metal and The chemical homogeneity of phase III appears to indicate that FeS, and their silicate mineralogy consists of plagioclase (AngO_90) formation took place under equilibrium conditions, a situation that is and pyroxene phenocrysts that usually consist of orthopyroxene different from phases I and II. The difference in the formation cores with augite overgrowths. Many ofthe chondrules also contain conditions is reflected in a different chemical composition. This discrete units of clinoenstatite with olivine enclosed in a poikilitic equilibrium alteration must be due to higher temperatures and/or texture. Primary compositions of olivine and pyroxene are inferred longer duration as compared to those that formed in phases I and II. to have high Mg/(Mg+ Fe) ratios (Fa and Fs <2), consistent with the Considering the widespread survival of highly reactive and re presence ofmetaL The primary groundmass contains silica and glass. duced phases in the Kaidun breccia [1,3], it is hard to conceive that One chondrule contains an inclusion of plagioclase and nepheline the alterations described here all took place in the Kaidun rock. At with numerous spinel inclusions. A similar inclusion was observed least, the high-temperature alteration must be of pre-Kaidun age, in one ofthe chondrules described by [I]. Russell et aL [5] describe whatever the origin ofthat complex chondri tic breccia might be. The a CAl in Isna with a similar texture, which is a potential precursor for type of alteration found in this fragment is typical ofaqueous alter the spinel-rich inclusions in the plagioclase-bearing chondrules. ations in many other meteorites: (I) Alteration did not uniformly The chondrules appear to have undergone extensive secondary affect the meteorite, or even the fragment in question. It therefore Na and Fe metasomatism. Introduction ofNa into the chondrules must have occurred before the final assemblage. (2) Alteration did resulted in partial nephelinitization ofplagioclase. Nepheline occurs not convert metal completely into oxides, reflecting the very limited as an intergrowth texture within the plagioclase crystals (Fig. I). extent ofthis aqueous alteration in meteorites. No evidence for large Introduction ofFe resulted in Fe-Mg exchange in olivine and spinel amounts ofwater penetrating the meteorite exists, in spite ofthe fact to form zoned grains, and reactions with silica in the groundmass to that it contains abundant CUCM-type chondrite fragments. The kind form ferrosalite (Fe and Ca-rich pyroxene). of alteration seen here and in other meteorites is, therefore, much Location of Metasomatism: It is important to understand more easily rationalized in terms of reaction of solids with vapor whether Fe and Na were introduced contemporaneously. Iron-mag before accretion of the meteorite. nesium exchange in olivine and spinel also occurs during parent Acknowledgments: This work was supported by RFBR grant body metamorphism, as a result ofequilibration ofFeO-poor miner 97-05-64378 in Russia and by FWF in Austria. als in chondrules and FeO-rich matrix. It is necessary to disentangle LPI Technical Report 97-02, ParI 1 31 same chondrite. For the two chondrules in Kainsaz, low degrees of nephelinitization and preservation ofsignificant amounts ofsilica in themesostasis indicate asimilar, low degree ofmetasomatism. Quali tatively, the chondrules in Lance show similar degrees ofalteration to each other, including almost complete replacement of silica and intermediate degrees ofnephelinitization. However, one chondrule . has significantly lower Fa in olivine than the others (Fig. 2). In ALHA 77003 and ALH 83 \08, three chondrules show distinctly different degrees of alteration, from almost no nepheline in one chondrule to very high proportions ofnepheline in the other two. For the chondrule with the low proportion of nepheline, olivine compo sitions (Fa3?) are similar to compositions in a heavily metasomatized chondrule a few millimeters away in the same thin section (Fa36). This implies that the degree of Fe metasomatism is considerably higher than the degree ofNa metasomatism, and that the two pro cesses are decoupled. The degree ofalteration ofsilica in the ground mass ofsuch a chondrule would help to clarify ifthis were the case, but unfortunately no silica or its reaction product has been observed Fig. 1. Metasomatic textures in a Kainsaz chondrule. in this chondrule. In alI the other chondrules studied, the degrees of Fe and Na metasomatism appear to be quite well correlated. These observations suggest that the metasomatism did not take place on the parent body of the CO chondrites. It appears to have 50 taken place in the nebula, before accretion. This is consistent with the Olivine compositions style of alteration observed in CAls in the CO chondrites [5}. 40 Timing of Metasomatism: If metasomatism took place in the .,,-,.~".chondru!cs. ~ nebula this has further implications for the nebular environment. o Host chondritc. type IA chondRlles. r Obviously, metasomatism must have occurred after formation ofthe 30 "'-'" plagioclase-bearing chondrules. A study of Mg isotopes in the two ' mally affected by thermal metamorphism and aqueous alteration and trix. Much ofthe metal in Leoville matrix shows an unusual morphol so appear to be mineralogically more primitive than the more altered ogy, occurring as highly anhedral grains (nearly skeletal) with nu oxidized subgroup, and their mineralogy and petrography may pro merous embayments and reentrant features. Most are single crystals, vide key data on nebular and parent-body processes that were oper although some of the larger metal grains are polycrystalline. ating in the early solar system. While a number ofdetailed studies on Sulfides (mostly troilite along with minor low-Ni pyrrhotite) are an oxidized subgroup ofCV3 chondrites have been reported, little is uncommon in Leoville matrix, and tend to be coarse-grained relative known about the reduced members of the CV group. In this report, to the other matrix phases. the results ofa transmission electron microscope (TEM) study ofthe Leoville matrix lacks the Fe-rich and Ni-rich phases such as matrix mineralogy of the Leoville CV3 (reduced-group) carbon hedenbergite, fayalitic olivine (F a> 60), magnetite, pentlandite, and aceous chondrite are described. high-Ni taenite (awaruite) that are common in the oxidized CV Methods: Regions ofinterest were extracted from petrographic chondrites. In addition, Leoville has been strongly deformed, result thin sections and prepared for TEM analysis by ion milling. The ion ing in chondrule flattening, compaction ofmatrix, and the deforma milled specimens were analyzed using a JEOL 20 I 0 TEM equipped tion of individual mineral grains [e.g., 2]. with a NORAN thin-window energy-dispersive X-ray (EDX) spec Discussion: The TEM results on Leoville matrix show that it is trometer. Fourier transform infrared (FTIR) transmission spectra mineralogically very primitive, consistent with thermoluminescence were collected using a Perkin-Elmer infrared microscope equipped data, which indicate a petrologic type 3.0 [3]. Although Leoville dis with a thermal emission source. Spectra were obtained from 30-J.lm2 plays a shock overprint, the deformation was not accompanied by regions of matrix in the ion-milled sections over the wavelength significant thermal metamorphism. The lack ofthermal effects com range 2.5-15 J.lm. bined with the absence of aqueous alteration products suggests that Results: Leoville matrix is dominated by fine-grained (typi many of the phases in Leoville matrix should retain chemical and cally I were observed). Low-Ca pyx is very Mg rich (>E~6) and also suggested by its etched appearance. Chemical etching on the contains trace Cr and Al in addition to the minor Ca and Fe. The low parent body is unlikely because of the paucity ofaqueous alteration Ca pyroxene occurs as isolated single crystals and in clusters of products in matrix. Futhermore, the PGC follows the irregular out crystals. In both occurrences, the pyx is intimately associated with line of the metal grains, which indicates that the metal had the fine-grained metal and fayalitic olivine. Sparse grains ofhigh-Ca pyx morphology prior to being rimmed byPGC. The metal morphologies (augites) are intergrown with the fine-grained olivine in matrix and show some similarities to grains in lOPs that are believed to have range in composition from Di4oHd6o to DigOHd2o. been physically etched by ionizing radiation in a nebular or presolar Matrix is essentially devoid of feldspar/feldspathoids, and no environment [6]. phyllosilicates have been observed to date, although minor rust Previous work has shown that Leoville is more strongly shocked staining occurs throughout the section and is dominated by a poorly than other CV chondrites [2], and while matrix olivines have been crystalline ferrihydrite-like phase. The only commonly encountered highly strained due to the deformation, there is no extensive forma AI-rich phase in Leoville matrix is hercynitic spinel. The hercynites tion of the characteristic (100) defects that are common in olivine tend to be small, anhedral, and Cr-bearing. The hercynites are Fe rich from the matrixes ofoxidized CVs. Because lattice offsets are asso with Mg/Mg + Fe (at.) clustering about 0.32. ciated with the olivine (100) defects (e.g., in Bali and Grosnaja), it Metal is the dominant opaque phase in Leoville matrix and was proposed that the defects resulted from deformation on the CV appears to be uniformly distributed in matrix. Grain sizes are vari parent body [e.g., 7]. However, these defects are uncommon in the able, but most ofthe metal is submicrometer in size and consists of highly deformed olivines in Leoville matrix, suggesting that these kamacite (-5 atom% Ni) and taenite (with 40-55 atom% Ni). Elec defects in the oxidized CVs are somehow related to the aqueous tron diffraction confirms the presence ofbcc and fcc metal and the alteration processes that bave affected these meteorites or were lack ofordered variants (e.g., tetrataenite) and FeNi carbides. Poorly exposed to a different strain rate environment. graphitized carbon (pGC) occurs as discontinuous rims up to 20 nm Conclusions: TEM studies of the Leoville CV3 chondrite thick surrounding many of the submicrometer taenite grains in ma show that matrix contains a number of likely nebular products that LPI Technical Report 97-02, Part I 33 were accreted on the Leoville parent body, including enstatite-metal toward groundmasses, despite the fact that phyllosilicate or plagio aggregates and metal-PGC grains. A parent-body shock event is clase occur in the groundmasses. The ferroan rims are usually 10 superimposed on the matrix minerals and has resulted in consider 20 ~m wide, consistent with those in Allende olivines. Some olivines able strain. There are several differences in the matrix mineralogy of (F099- 96) in fragmental chondrules and isolated minerals in both CV s Leoville (reduced group) relative to the oxidized members ofthe CV directly contact ferroan matrixes without zonation. chondrites. The atomic Mg/Mg + Fe ratios of pyroxenes are 0.97-\.00 in Acknowledgments: The Leoville samples used in the study Kaba and Mokoia chondrules. Thus, the ferroan olivine rims were were kindly provided by E. K. Gibson. This work was supported not equilibrated with coexisting pyroxenes in Mokoia. The olivine by NASA Contract NASW-5046 and by MVA, Inc. zonation was secondarily formed under subsolidus conditi.ons like References: [I] McSween H. Y. (1977). [2] Nakamura T. et al. that in Allende chondrules [I]. Low-Ca pyroxenes in Mokoia are (1992)EPSL, 11 4, 159. [3] Guimon R. K. et al. (1995) Meteoritics, abundantly replaced by olivines (FoS3_77), similar to those in Allende 30, 704. [4) Bradley 1. P. et al. (1984). [5] Brearley A. J. (1990) [I], whereas pyroxenes in Kaba do not show such texture. GCA, 54, 831. [6] Bradley J. P. eta\. (I 996)LPSXXVlI, 149. [7] Kel Primary groundmass phases are devitrified glass and plagioclase ler L. P. et al. (1994) GCA, 58, 5589. (Anso _9s) in Kaba and Mokoia. They typically occur in the central parts of chondrules. All chondrules in Kaba do not contain feld spathoid. On the other hand, nepheline and sodalite are encountered RELATIONSHIP BETWEEN ANHYDROUS AND AQUEOUS in some Mokoia chondrules. They replaced primary groundmass ALTERATIONS IN CV3 CHONDRITES. M. Kimura and Y. phases, like those in Allende. Two Mokoia chondrules contain Ikeda, Institute of Astrophysics and Planetary Science, Faculty of hedenbergite (Eno_2FS49-50W049-so) and wollastonite among pheno Science, Ibaraki University, Mito 310, Japan ([email protected]. crysts. Such Ca-rich phases are byproducts ofthealkali-Ca exchange ibaraki.ac.jp). reaction [2]. Therefore, chondrules in Mokoia show typical features of anhy Introduction: Almost all chondrules in Allende CV3 chon drous alteration such as secondary olivine zonation, replacement of drite were secondarily subjected to anhydrous alteration: (I) alkali enstatite by ferroan olivine, alkali-Ca exchange reaction to form Ca exchange reaction to form nepheline, soda lite, and Ca silicates feldspathoids and Ca silicates. The former two reactions are noticed such as hedenbergite and wollastonite, (2) secondary zonation of in all chondrules, and the alkali-Ca reaction features are noticed in oli vine, and (3) replacement ofenstatite by ferroan olivine [1 ,2]. The more than 30% of chondrules. other ox idized CV3 chondrites, such as Axtell, Y 86751 , Ningqiang, Stage of Alteration Reactions: In Mokoia chondrules, phyl and ALH 81258, also experienced such reactions ([3,4] and unpub losilicates formed around zoned olivine, low-Ca pyroxene replaced lished data). The anhydrous alteration weakly took place in Efremovka, by ferroan olivine and feldspathoids. Mokoia chondrules experi Leoville, and Vigarano of the reduced subgroup [5]. enced both anhydrous and aqueous alterations. On the other hand, On the other hand, aqueous alteration to produce phyllosilicate chondrules in Kaba do not show any features of anhydrous altera took place in some oxidized CV3 chondrites such as Kaba and tion. Phyllosilicate directly replaced primary low-Ca pyroxene and Mokoia [6,7). Thus, various alteration reactions have been noticed groundmass. for CV chondrites. In order to explore the relationships between the Aqueous alteration has been considered to take place in CV chon anhydrous and aqueous alterations of chondrules, we carried out drite parent body at low temperatures, below 100°C [6] . Such low mineralogical and petrological study of chondrules in Kaba and temperatures are consistent with no zoning of olivine in Kaba, be Mokoia, in comparison with those in the other CV chondrites. cause diffusion of Fe and Mg hardly occurs below about 600°C in Aqueous Alteration in Kaba and Mokoia: Our observations the nebular lifetime [I]. On the other hand, olivines in Mokoia also show that chondrules in Kaba and Mokoia were extensively chondrules show distinct zonation. We suggest that the aqueous subjected to aqueous alteration. Phyllosilicates occur commonly in alteration, possibly in the parent body, may have followed anhydrous the peripheral parts of chondrules. Saponite is identified in Kaba alteration at about 600°-800°C [2], possibly in the nebula [8]. chondrules, consistent with previous works [e.g., 6]. On the other Now we distinguish several kinds ofalteration reactions for CV hand, Na phlogopite as well as saponite commonly occur in chon chondrites: anhydrous alteration, aqueous alteration, and dehydra drules, as already noted [7]. Margarite rarely occurs in chondrules in tion for some dark inclusions [9]. CV chondrites of the reduced Mokoia. subgroup were hardly to weakly subjected to these reactions [5]. Phyllosilicates typically replaced glass and plagioclase in ground Chondrites of the oxidized subgroup show these alterations in vari mass. Phyllosilicates often replaced pyroxenes in both CVs. How ous degrees. The anhydrous alteration took place extensively in ever, low-Ca pyroxenes distinctly show replacement texture in com some CVs such as Allende and others. Kaba was only subjected to parison with high-Ca pyroxenes. In all chondrules of Kaba and the aqueous alteration, whereas Mokoia experienced the anhydrous Mokoia, olivines do not show any replacement texture by phyllo reactions followed by the aqueous alterations. silicates. Resistance to the alteration increases in the order glass and Acknowledgments: We thank R. Hutchison for loaning thin plagioclase < low-Ca pyroxene < high-Ca pyroxene < olivine. sections of Kaba and Mokoia. Anhydrous Alteration in Mokoia: Although both Kaba and References: [I] Ikeda and Kimura (1995) Proc. NIPR Symp. Mokoia experienced aqueous alteration, their mineralogy of anhy Antarct. Meteorites, 8, 97-122. [2] Kimura and Ikeda (1995) Proc. drous phases is different. Phenocrystic olivines in Kaba chondrules NlPR Symp. Antarct. Meteorites, 8, 123-138. [3] Murakami and are F095_IOO without chemical zoning toward groundmasses. On the Ikeda (1994) Meteoritics, 29, 397-409. [4] Krot et al. (1995) Mete other hand, olivine phenocrysts in Mokoia chondrules are FOSI _99 in oritics, 30,748-775. [5] Kimura and Ikeda (1 996)Proc. N1PR Symp. the cores, but F062_9S in the rims. They usually show normal zoning Antarct. Meteorites, 10, in press. [6] Keller and Buseck (1990)GCA, 34 Workshop on Modification of Chondritic Materials 54.2113-2120. [7] Tomeoka and Buseck (1990) GCA. 54. 1745 dized CV3s, support the origin ofNi- and Co-rich metal by prefer 1754. [8] Ikeda and Kimura, this volume. [9] Kojima and Tomeoka ential oxidation of Fe in asteroidal environment. (1996) GCA. 60. 2651-2666. Formation of Pure Fayalite: Hua and Buseck [19] described pure fayalite in chondrules, CAIs, and matrixes in Kaba and Mokoia and suggested a multistage nebular process for its origin; it includes MINERALOGICAL AND CHEMICAL MODIFICATION OF oxidation ofmetal to magnetite at low temperature «400 K) by the CV3 CHONDRITES DURING FLUID-ASSISTED META reaction MORPHISM IN THE CV3 ASTEROID. A. N. Krotl, E. R. D. Scotti, and M. E. Zolensky2, lHawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai 'i, Honolulu HI 96822, USA, 2Earth Science decomposition of enstatite during a high-temperature reheating and Solar System Exploration Division, NASA Johnson Space event under reducing conditions to form SiO gas Center, Houston TX 77058, USA. Calcium-aluminum-rich inclusions (CAIs), chondrules, matrixes, and dark inclusions (DIs) in CV3 chondrites experienced late-stage and formation of fayalite under oxidizing conditions alteration processes resulting in many alteration features previously ascribed to the nebula [1,2]. These features include (I) oxidation and sulfidation ofNi-and Co-poor metal and troilite to magnetite, awaruite, Ni-rich taenite, Co-rich kamacite, wairauite, pyrrhotite, and pentlan No physical model explaining these highly variable redox condi dite; (2) replacement of magnetite by fayalite, salite-hedenbergitess tions in the late-stage solar nebula was proposed. pyroxenes, and andradite; (3) fayalitic olivine (Fa30-60) rims around Our mineralogical study shows that fayalite replaces magnetite forsterite grains; (4) fayalitic olivine (Fa30_ 60) veins in forsterite and sulfide nodules in chondrules, fine-grained rims, and matrixes in low-Ca pyroxene; (5) tabular fayalitic olivine (Fa30_60) in matrix; (6) Kaba and Mokoia. The fine-grained rims are commonly crosscut by salite-hedenbergitess-andradite inclusions in matrix; (7) Fe-alkali fayalite-magnetite-sulfide veins that may extend into the matrix halogen metasomatic alteration ofchondrules and CAIs resulting in where they connect with fayalite-sulfide-magnetite inclusions. Based formation of grossular, anorthite, nepheline, sodalite, wollastonite, on these observations, we conclude that fayalite-bearing assem salite-hedenbergitess, andradite and kirsch-steinite; and (8) various blages postdate chondrule formation, accretion offine-grained rims phyllosilicates (margarite, clintonite, Na-phlogopite, saponite, chlo and matrixes in Kaba and Mokoia, and hence, formed in an asteroid. rite, serpentine, montmorillonite, talc, bio-pyriboles). Inspired by Salite-hedenbergitess and andradite replace magnetite nodules and the aqueous alteration-dehydration model suggested by Kojima and fayalite in chondrules, CAIs, and matrixes in Kaba and Mokoia. We Tomeoka [3] for the Allende DI AII-AF, we suggested that all infer that formation ofthe magnetite, fayalite, salite-hedenbergitess, oxidized CV3s [4] experienced variable degrees of fluid-assisted and andradite resulted from a fluid-rock interaction by reactions metamorphism resulting in the alteration features listed above [I]. Although the following mineralogical, petrographic, and chemical 2FePis) + 3Si0 (aq) + 2H2(g) = 2 (4) studies ofCV3s and their DIs strongly support this suggestion [5 3Fe2Si04(s) + 2HP(g) 14], the time, duration and physical-chemical conditions of alter ation, including p, T, and fluid composition (PH, f , concentra F~Si04(s) + 3Si0 (aq) + 2Ca2+(aq) + 2H20(aq) = 02 2 (5) tions of dissolved species), are still poorly constrained. 2CaFeSi20 6(s) + 2H2(g) . Formation ofMagnetite, Nickel- and Cobalt-rich Metal, and Nickel-rich Sulfides: There are two major textural types ofmag and netite in CV3s: (I) magnetite nodules associating with Ni- and Co rich metal and Ni-rich sulfides in chondrules and matrix, and 2Fe 0 (s) + 9Si0 (aq) + 9Ca2+(aq) + lOH 0(aq) = 3 4 2 2 (6) (2) framboidal magnetite grains in matrixes in Mokoia and Bali 3Ca3F~(Si04h(s) + lOH2(g) [15,16]. The framboidal magnetite associates with phyllosilicates and is commonly attributed to aqueous alteration [15]; the origin of Silicon, Ca, Mg, and some Fe may have been released from primary magnetite-metal-sulfide nodules remains controversial [I]. The ob minerals in matrix, chondrules, and CAls replaced by phyllosilicates served differences in °isotopic compositions ofthe magnetite nod [15,20] and transported in the fluid, not in the nebular gas. ules and olivine phenocrysts in Allende chondrules [13] exclude the Formation of Fayalitic Olivine (Fa30_60): There are several formation of the magnetite nodules by crystallization from metal textural types of fayalitic olivine in CV3s: (1) tabular-to-equant sulfide-oxide droplets [17] and support their origin by oxidation of grains in matrix [21-23], (2) rims around forsterite grains [21-23], metal after chondrule formation [18]. Although late-stage oxidation (3) veins in forsterite [21-23], (4) olivine replacing low-Capyroxene of metal in the solar nebula cannot be excluded (experiments on [24], (5) rims around metal nodules in forsterite [22], and (6) veins kinetics of magnetite formation by oxidation of Fe-Ni alloys may crosscutting chondrules and fine-grained rims [6]. It seems likely help to resolve this issue), the presence of oxidized and reduced that all these textural types are genetically related [6]. The most opaque assemblages mixed in several brecciated CV3s [1,19] favors widely favored nebular scenario for the origin of fayalitic olivine heterogeneous oxidation in an asteroid. The observed correlation of involves high-temperature formation during condensation and/or the degree of aqueous alteration of the Efremovka DIs [14] and reaction with an oxidizing nebular gas; the latter was suggested to be composition of metal grains, which are similar to those in the oxi produced by evaporation ofdusty nebular regions [2,21-23,25]. The LPI Technical Report 97-02, Part I 35 asteroidal models invoke various mechanisms: (I) solid-state oxida (2) The salite-hedenbergitess ± andradite ± wollastonite ± kirsch tion [24] by the reaction steinite assemblages in Allende chondrules and CAIs formed during Fe-alkali-halogen metasomatism possibly by fluids released during Fe(s) + MgSi03(s) + H20(aq) = FeMgSiOis) + H2(g) (7) dehydration of phyllosilicates by the reactions (2) progressive aqueous alteration ofmatrix and chondrule silicates, CaAI2Si 20 S(s) + Na20(g) = 2NaAISi04(s) + CaO(g) (10) followed by metamorphic dehydration ofthe product phyllosilicates [6], and (3) metasomatic Fe-Mg exchange reactions, coupled with 3CaAI Si 0 g(s) + 3Na 0(g) + 2NaCI(g) = 2 2 2 (II) vapor growth, in the poorly consolidated portion ofan early accreted 3CaO(g) + 6NaAISi04 x 2NaCI(s) parent body [26]. The mineralogical observations strongly support ing an asteroidal setting for the origin of fayaIitic olivine include and (I) microstructures ofmatrix olivine similar to those observed in de hydrated phyllosilicates in CM chondrites [6,8,9,11,27], including 3NaAISiPS(s) + NaCI(g) = (12) voids, inclusions of pentlandite, and poorly graphitized C (PGC) 3NaAISi04 x NaCI(s) + 6SiOz and M . S. Matthews, eds.), p. 746, Univ. of Arizona, Tucson. the nebula in an immaculate state and subsequently become exposed [30] Ikeda Y. and Kimura M. (1995) Proc. NIPR Symp. Antarct. to the bad and destructive world of parent bodies. Meteorites, 8, 97. [3 J] Ikeda Y. and Kimura M. (1996) Proc. NIPR Naturally, observations can be interpreted in an alternative way Symp. Antarct. Meteorites, 9, 51. [32] Kimura M. and Ikeda Y. and., actually, we are forced to interpret most of them in a different (1996) Meteoritics & Planet. Sci., 31, A 70. [33] Tomeoka K. and way when we consider all observations. For example, most CCs Buseck P. R. (1982)Nature, 299, 326. [34] Tomeoka K. and Buseck consist of constituents (chondrules, aggregates, etc.) set into a fine P.R. (1982) Nature, 299, 327. [35] Tomeoka K. and Buseck P. R. grained carbonaceous matrix. These constituents usually bear the (1982) GCA, 54, 1745. [36] Hashimoto A. and Grossman 1. (1987) scars ofincomplete equilibrium with some environment that, appar GCA, 51,1685'. ently, was quite different from that in which they originally formed. If this new environment was the one that was created locally in the parent body then one can expect that constituents ofsimilar miner alogical and textural type should react in the new environment in a THE METEORITE PARENT-BODY ALTERATION MODEL similar way, with the result that they become "altered" to similar AND THE INCOMPATIBLE REALITY. G. Kurat, Naturhis degrees. In nature this is clearly not the case. The rule is rather that torisches Museum, Postfach 417, A-I 014 Wien, Austria. constituents of a given CC show all sorts of different "alterations" and rarely the same intensity. Almost totally hydrated objects are Meteorites and, in particular, chondrites frequently contain ob located right next to almost unchanged ones, .highly sulfidized ones jects that have signs of beginning, advanced, or almost complete are next to unchanged, metal-rich ones, highly oxidized objects changes in their mineralogy or mineral chemistry, a metamorphosis bearing magnetite are approaching or even touching non oxidized that is commonly referred to as "alteration." Such changes are to be ones, etc. What is even more impressive is the widespread survival expected to be omnipresent in meteorites because these rocks are the of highly reactive phases, which were formed in strongly reducing products of a large variety of processes that were operating in the environments, in the highly oxidizing environment ofCCs (Iow-Ni solar nebula. Adaption ofmineral assemblages and mineral chemis metal, phosphides, sulfides of lithophile elements, etc.). Further tries to changing physicochemical conditions is a natural process more, hydrous objects (like anhydrous ones) commonly display that, ofcourse, must have been working also in the solar nebula with delicate growth and aggregation features and a very large composi its widely varying temperature and pressure regimes. Until the [mal tional variety on a micrometer scale. Moreover, because, e.g., product was formed, the constituting matter had to evolve from serpentinization ofolivine or pyroxene should produce at least two ' temperatures above the boiling point of most phases or compounds products, the serpentine and either a Mg mineral (Mg hydroxide or to temperatures below the freezing point of volatile constituents, carbonate in the case ofolivine) or free silica (or a Si-rich mineral in such as H20, CO, CH4, CO2, etc. Equilibration of solid phases with the case ofpyroxene) we can expect to find them at the place of the a gaseous environment depend on the grain size of the solids, crime, but they are never where they should be. Actually, the phases diffusivities of the species considered, and the changes in tempera that theoretically should accompany the hydrosilicates are missing in ture. Thus, equilibrium is rarely achieved in meteorites, as amply almost all cases. Why? shown by the nature ofun equilibrated chondritic meteorites. Incom plete response to varying environmental conditions is often inter preted as low-temperature alteration within the parent body. THE ALTERATION OF NICKEL-BEARING SULFIDES The attempts to stay in equilibrium with the environment cannot DURING THERMAL METAMORPIDSM ON ORDINARY be successful for all phases under all circumstances. More likely, we CHONDRITE PARENT BODIES. D. S. Lauretta, K. Lodders, can expect unsuccessful attempts and, actually, we can observe many and B. Fegley Jr., Planetary Chemistry Laboratory, Department of such examples in almost all meteorites. This normal sequence of Earth and Planetary Sciences, Washington University, St. Louis MO events, as documented by unsuccessful equilibration, has, however, 63130-4899, USA. commonly been interpreted as unlucky " secondary alteration." The place where such bad things can happen best is widely believed to be Introduction: Sulfurization ofFeNi alloys under solar nebula the meteorite's parent body, where it was exposed to conditions it conditions produces Ni-bearing sulfides [1,2]. These sulfides have was not designed for. several distinctive characteristics including pentlandite [(Fe,Ni)9Ss] All meteorites show signs of unsuccessful attempts to reach inclusions within monosulfide solid solution [mss, (Fe,Ni) l_xS] crys equilibrium either with their peers, or with the environment, or both. tals, increasing Ni content with distance from the remnant metal, Even the best "equilibrated" OCs, for example, have a number of multilayer sturctures, and oriented sulfide crystals. The extent to phases that are out ofequilibrium with their colleagues. This fact is which some (or all) ofthese features are altered by thermal metamor usually repressed bymeteoriticists, perhaps because it simplydoesn't phism is unknown, but important to quantify so we can distinguish fit the model. Carbonaceous chondrites had less luck and that's why nebular from parent-body processes in the meteorite record. We they are rich in examples ofunsuccessful attempts ofconstituents to performed experimental simulations of dry thermal metamorphism reach equilibrium with each other and a progressing, changing world. ofan ordinary chondrite parent body to determine the effect of this That world is commonly believed to be a planetary or planetesimal process on sulfide chemistry and morphology. world for reasons that remain in the dark. This way we arrived at a Ex'perimental Method: The starting material of the thermal pretty curious situation: the solar nebula, the parent of all meteor metamorphism experiments was composed ofsilicates with the nor ites, which had to change its conditions drastically during its evolu mative mineralogy of LL chondrite silicates [3], filings from the tion, is not eligible as the CUlprit in ruining our meteoritic constitu Canyon Diablo (CD) Fe meteorite, and Ni-bearing sulfides produced ents. According to widespread belief, they rather must emerge from by gas-solid reaction between CD metal and H2-H2S gas [I]. These LPI Technical Report 97-02, Part J 37 100 x Ni Local equilibration. The experiments begin with metal and sul Fe+Ni+Co fide separated by a silicate matrix. After two weeks at 500°C, we observed a very limited extent ofreaction. However, the Ni concen (a) Initial sulfide COO'4lQSition 1 mss + pent tration gradient is gone and the mss and pentlandite are locally mss • "1 ° ° 30 • .,0 1 ° ° equilibrated. The compositional variation across a mss-pentlandite 1 "" .... 1 assemblage is given in Fig. I b, and the bulk compositions are plotted • :0 ,0 in Fig. 2b. 1 1 Sulfur mobilization. After three months reaction at 500°C there O~~~~--~--~--~--~--~I---L--~~O o 20 40 60 80 '00 was noticeable S mobilization. The system moves from local equilib rium toward global equilibrium by creating metal-sulfide assem 'ii (b) 50.0 ~o ~c 2 W8!~ 1 pent' 1 1 1 47.5 1 1 1 1 45. 1 1 1 1 1 1 1 1 1 (a) Inijial Composition Nl·50S 1 mss 1 • • • ::t~_'~.LI~,~mss~'·~I~~~~i~'~I~'~~'~I~~~~~~~~,I, , I, I, ..... ,0 20 30 40 50 r • • o "'PoroIInciIoFe·33.63 Ni- 33.03 (c) ::[".'.!'~"C'd~~' "I' "I 'l,'~,l s·33.33 eo.J.metal : 1 1 ~ • I 1 mss ~ : ...... : (b) 50 Fe - 50 "r------5OO--:•.".C-2-w-ee-ks----""""~ Ni - 50 S 1 20 40 60 80 '00 ,20 ------~~. (d) 1 3 months 1 9OO"C 1 1 . • Fe - 33.12 •m81al• I1 1 metal NI- 33.01 1 1 S- 33." 1 1 1 1 1 mss 1 1 • 1 . • 1 1 • • :j I I ! I It , , , 1 , Ni-50S 0.0 1 1 1 1 J (c) 5OO·C 3 months 0 ,0 20 30 40 50 Distance ()un) Fig. 1. Traverse across sulfide grains at different stages of reaction. Composition is given as atomic percent Ni (cations basis). (a) The initial suI fide composition. The concentration gradients are characteristic of sulfides fonned under nebula conditions. (b) Heating for short times removes the concentration gradients leaving behind an equilibrated mss (d) 50 Fe - 50 9OO·C 3 months pentlandite assemblage. (c) Longer duration heating produces FeNi2 and Ni-poor troilite. (d) Taenite (-12% Ni) exsolves at higher temperatures. components were mixed together in proportions similar to those in LL chondrites. The components were pressed into a pellet and heated in an evacuated, sealed silica tube at 500°C or 900°C for two weeks and three months. weight percent Results: The Ni content of the starting sulfides is shown in Fig. la and the composition is plotted on a Fe-Ni-S diagram (wt%) Fig. 2. The composition of experimental sulfides plo\led on Fe-Ni-S in Fig. 2a. The sulfide is divided into two compositional regions. The ternary diagrams (wt%). Dashed lines connect equilibrium phases. The bulk region originally adjacent to metal is composed solely ofmss (solid composition of each assemblage is plotted as an open circle. The lever rule circles). The Ni content ofthis region varies from 0.2 to 3 atom% Ni. determines the relative abundance of each phase. (a) Composition of sulfide produced by sulfurization ofan FeNi alloy. The sulfide is composed The outer region is composed of mss grains with pentlandite inclu of both mss and pentlandite. (b) The sulfide composition after heating for sions and is Ni-rich (10-30 atom% Ni). We compare these compo two weeks at 500°C. The compositional variation is gone, and an mss sitions to the heated assemblages. The extent ofalteration varies with pentlandite equilibrium assemblage has fonned. (c) A sulfide grain that was the temperature and duration of the experiment. These experiments heated for three months at 500°C. FeNi2 has exsolved from the sulfide produced three distinct results: (I) local equilibration ofthe sulfide leaving Ni-free sulfide. (d) A sulfide grain that was heated for three months grains, (2) limited S mobilization, and (3) extensive cation diffusion at 900°C. The equilibrium metal composition has a much lower Ni content and S vapor transport. These are discussed in detail below. than at 500°C. 38 Workshop on Modification of Chondritic Materials TABLE I. Partition coefficients. least some of their properties must record this primary process. Oxygen isotopic compositional data indicate that the various chon Phases Temperature (0C) Ni Co dritic groups accreted from nebular material that varied isotopically, mss-u 500 0.020 0.239 hence chemically, since it should have been easier for the nebula to mss-y' 500 0.007 0.068 be homogenized isotopically than chemically. Mineralogic and pet mss-y 900 0.027 0.130 rographic characteristics of chondrites indicate that many experi enced extended, postaccretionary, heat-associated genetic episodes (e.g., thermal metamorphism, shock heating). Contents ofthermally labile trace elements in them should reflect such secondary episodes blages from originally distal metal and sulfide grains. In order to provided that the heating events occurred under partly or totally open establish this global equilibrium, the Ni-bearing sulfides release S2 conditions. Here we review the evidence provided by thermally vapor resulting in metal precipitation at the sulfide edges. The Ni labile elements as to specific genetic processes important in the content of this metal is determined by thermodynamic equilibrium formation of the major chondrite groups. between metal and sulfide. At 500°C FeNi2 ('Y') exsolved from the From circumstantial evidence and experimental results obtained sulfide leaving behind troilite containing <0.5 atom% Ni. The com from extended heating of low-petrographic-grade chondrites under positional variation across such an assemblage is given in Fig.lc, and simulated parent-body conditions, certain trace elements are consid the metal and sulfide compositions are plotted on an Fe-Ni-S ternary ered as moderately to highly labile [I]. These elements-which phase diagram in Fig. 2c. The dashed line in Fig. 2c indicates the tie include Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, TI, and In (ordered by line between these two phases. A similar tie-line was suggested by putative volatility during nebular condensation)-are mainly quan [4]. The S released from the evaporating sulfides condenses as tified by RNAA. These elements are volatilized from primitive chon sulfide rims on isolated metal grains. Thus, these sulfides form by a drites heated over a wide temperature span, beginning at 400°C (for gas-solid reaction between FeNi metal and S-bearing gas. This is the most labile) to 2: 1000°C for the least labile elements [I]. To the similar to sulfide formation in the solar nebula. However, as the extent that these experiments approximate open-system parent-body sulfide layers grow around the metal grains, they incorporate nearby processes, their results should enable us to identify materials that silicates, a morphology unique to sulfides formed on a parent body. experienced such processing. In the absence of evidence of such Extensive transfer ofmaterial. Evidence of extensive S mobili secondary processing, composi tional trends for such elements can be zation is observed after three months reaction at 900°C. Since FeNi2 taken to be of nebular origin. is not stable at 900°C, taenite containing -12 atom% Ni exsolved Carbonaceous chondrites provide the clearest samples to test from the sulfide (Figs. Id,2d). The growth ofsulfide layers on metal whether parent-body processes altered compositions established by grains is widespread. Thin sulfide trails grew throughout the sample nebular processes. This is only possible because C I-normalized connecting initially separate metal-sulfide assemblages. Since Fe levels of labile trace elements are constant in most carbonaceous and Ni diffusion through sulfides is rapid, the network of sulfide chondrites [2] and consistent with a two-component model similar trails allows for extensive cation movement. As a result, metal grains to, but somewhat different from, that ofAnders and co-workers [29]. are nearly equilibrated throughout these sample. Noble gas, petrographic, and Mossbauer data for three such chon Conclusions: Thermal metamorphism can alter sulfides that drites support the conclusion, based upon data for mobile trace formed under solarnebula conditions. The extent ofalteration varies elements, that these were thermally metamorphosed in their parent with temperature and time. Initially, individual sulfide grains equili bodies [4,5]. Spectral reflectance data for these three and artificially brate, erasing Ni concentration gradients established during sulfide heated Murchison samples indicate that the surfaces of many C, G, formation in the nebula. Longer heating times result in S loss and B, and F asteroids contain a substantial complement of thermally metal ex solution from the sulfides. F eNi2 forms at 500°C and taenite metamorphosed materials excavated from their interiors [30,31]. with -12% Ni forms at 900°C. The equilibrium relationships ob Parent material(s) for L4-6 chondrites apparently condensed served in the samples allow us to the determine partition coefficients and accreted at temperatures lower than those that produced for Ni and Co between mss and metal (Table I). H4-6 chondrites. Primary contents ofmobile trace elements in most Acknowledgments: This work was supported by NASA grant L4-6 chondrites were altered by shock (Table I). An accompanying NAGW-3070. report discusses the situation in H chondrites [32]. References: [I] Lauretta et al. (1997) LPS XXVII. 783. The situation for E3-6 chondrites is less clear. Since EH and EL [2] Lauretta et al. (1996) Proc. NIPR Symp. Antarct. Meteorites, 9, chondrites differ in contents ofmajor, refractory elements, it may be 97. [3] McSween and Bennett (J99 I)lcant.s, 90,107. [4] Blumetal. that contents of labile trace elements also reflect nebular processes (1989) GCA, 53, 543. [33]. However, contents ofmobile trace elements could also reflect open-system, thermal metamorphism and differentiation in the par ent bodylbodies (Table I). NEBULAR AND PARENT-BODY PROCESSES IN CHON References: [I] Lipschutz M. E. and Woolum D. S. (1988) in DRITES: LABILE TRACE ELEMENTS AS INDICATORS Meteorites and the Early Solar System (J. F. Kerridge and M. S. AND THERMOMETERS. M. E. Lipschutz, Department of Matthews, eds.), pp. 462-487, Univ. of Arizona, Tucson. [2] Xiao Chemistry, Purdue University, West Lafayette IN 47907-1393, X. and Lipschutz M. E. (1992) JGR, 97, 10197-10211. [3] Takeda USA. H. et al. (1984) EPSL, 71,329-339. [4] Paul R. L. and Lipschutz M. E. (1989) Z. NaturJ, 44a, 979-987. [5] Paul R. L. and Lipschutz Obviously, all planetary samples (including chondrites) derive M. E. (1990) Proc. NIPR Symp. Antarct. Meteorites, 3, 80-95. ultimately from material that condensed from the nebula so that at [6] Ikramuddin M. and Lipschutz M. E. (1975) GCA, 39, 363-375. LPI Technical Report 97-02, Part 1 39 TABLE I. Agents for establishing mobile trace-element contents in chondrites and relevant heating studies. Chondrite Type Heating Studies Nebular Process( es) Parent-Body Process(es) C [6-10] 61 CI-C6 [2] Mobilization by thermal metamorphism in I CI, 2 C2 [4,5] H [II] H4-6 [12-14]· L [ 15] unshocked L4-6 [?] Mobilization by shock in most or all (?) L4-6 [16,17]· E [18,19] E3 (?) Mobilization and lor fractionation ofFeS-F~ eutectic by thermal processes in E4-E6 [19,20] Unequilibrated ordinary chondrites [11 ,15] H3, L3, LL3 [21]· H regolith breccias [II] Implantation of mobile trace elements in regolith [22-24]· Rose City Fractionation of metal (and sulfide?) [13]· Y 74160 (LL7) [ 15] Mobilization (?) and fractionation ofFeS-Fe eutectic by thermal process(es) [3] Chondrules [25] H inclusion in Barwell (L5-6) [25] Fractionation of FeS-Fe eutectic by shock (?) [26] Igneous inclusions in ordinary chondrites Fractionation of metal (and sulfide?) in inclusions within equilibrated H and L chondrites [27 ,28] • Additional references cited in these papers. [7] Matza S. D. and Lipschutz M. E. (1977) Proc. LSC 8th, differentiated achondrites. EHs may provide more information about pp. 161-176. [8] Matza S. D. and Lipschutz M. E. (1978) GCA, the nebula, ELs may record metamorphic parent-body processes, and 42, 1655-1667. [9] Herzog G. F. et al. (1979) GCA, 43, 395-404. aubrites give insights into metal-sulfide fractionation on a very [10] BartG. et al. (1980) GCA, 44,719-730. [IIJlkramuddin M. et reduced parent body. Here we focus on conditions required for ED al. (1977) GCA, 41, 1247-1256. [12] Lingner D. W. et al. (1987) formation in the solar nebula. GCA, 51, 727-739. [13] Wolf S. F. and Lipschutz M. E. (1995) Larimer [1-3] showed that more-reducing conditions than in a JGR, 100, 3335-3349. [14] Wolf S. F. et al. (1997) JGR, in press. solar gas are required to form Fe-free silicates, osbornite (TiN), Si [15] Ikramuddin M. et al. (1977) GCA, 41,393-401. [16] Walsh bearing Fe-Ni metal, and oldhamite (CaS). The redox state is given T. M. and Lipschutz M. E. (1982) GCA, 46, 2491-2500. [17] Hus by the CIO ratio, which is 0.48 for a solar gas [4]. In asolar gas all ton T. 1. and Lipschutz M. E. (1984) GCA , 48, 1319-1329. C is in CO while °is about evenly divided into CO and H20 and [18] Ikramuddin M. et al. (1976) GCA, 40, 133-142. [19] Biswas oxide condensation reactions such as 2 Al (g) + 3 H20 = AI20 3(s) + S. et a1. (1980) GCA, 44, 2097-2110. [20] Binz C. M. et al. (1974) 3 H2 occur. Figure I shows major-element condensation at PlOt = GCA, 38, 1579-1606. [21] Binz C. M. et al. (1976) GCA, 40, 10-3bar as a function ofCIO ratio, which was varied by decreasing 59-71. [22] Bart G. and Lipschutz M. E. (1979) GCA, 43, 1499 the °abundance from solar. Initial condensates from a solar gas are 1504. [23] Xiao X. and Lipschutz M. E. (1991) GCA, 55, 3407 corundum, hibonite, grossite, perovskite, melilite (shown by 3415. [24] Lipschutz M. E. et a1. (1993) Meteoritics, 28, 528 gehlenite), spinel, forsterite, and enstatite. Iron metal condensation 537. [25] Sears D. W. G. et a1. (1995) Meteoritics, 30, 169-181. is independentofC/O. Increasing C/O decreases H20 (g), and oxides [26] Hutchison R. et al. (1988) EPSL, 90, 105-118. [27] Sack R. 0 . only form at lower T when the reaction CO + 3 H2 = Hp supplies et al. (\ 994) JGR, 99, 26029--26044. [28] Yolcubal I. et a!. (1996) H20. JGR. submitted. [29] Takahashi H. et al. (\978) GCA, 42, 97-\06. At CIO -0.91 to -0.95 osbomite forms instead of perovskite. [30] Hiroi T. et al. (1993) Science, 261, 1016-1018. [31] Hiroi T. Oxides of AI, Ca, and Si remain initial condensates because enough et al. (1996) Meteoritics & Planet. Sci., 31, 321-327. [32] Wolf H20 is still available. Interestingly, the relatively reduced CH chon S. F. and LipschutzM. E., this volume. [33] Keil K. (\989) Meteor drites contain osbomite and the oxides (e.g., grossite [5 ,6]) predicted itics, 24, 195-208. for this intermediate C/O range. At CIO > 0.95 TiC replaces TiN as an initial condensate; AIN and CaS replace Ca-AI oxides, and SiC replaces gehlenite. Graphite and cohenite (F e3C) condensation requires CIO > I. If C/O is increased WHAT DO ENSTATITE METEORITES TELL US ABOUT by increasing the C abundance TiN appears at CIO -0.95 and TiC, THE SOLAR NEBULA? K. Lodders and B. Fegley Jr., Plane AIN, and CaS form at ClO -0.98. tary and Chemistry Laboratory, Department of Earth and Plane The CIO range where ECs may have formed is constrained by tary Sciences, Washington University, Campus Box 1169, One their mineralogy. Graphite and Fe)C are not very abundant indicating Brookings Drive, SI. Louis MO 63130-4899, USA. that CIO was ~l because graphite and F~C require C/O > 1. The occurrence of CaS puts the lower bound at CIO -0.95. The highly reduced enstatite chondrites (EC) and achondrites Larimer [2] noted that condensation predicts minerals not ob (aubrites) are exotic, but important, meteorites. Enstatite chondrites served in ECs. These are TiC, AIN, and, to some extent, SiC. These comprise about 1-2% ofall chondrites (carbonaceous chondrites are initial condensates may be absent (or less abundant) because they -2-5%). The enstatite meteorites form three groups: (I) EH chon continued to react with the gas atlower T. This idea is consistent with drites, typically metamorphic type 3 and 4; (2) EL chondrites where the high-volatile-element abundances in ECs [7]. At lower tempera metamorphic types 5 and 6 are prominent; (3) aubrites, which are . tures, TiC converts to TiN and AIN changes to albitic plagioclase. 40 Workshop on Modification of Chondritic Materials 0.75 0.80 0.85 0.90 0.95 1.00 1.05 abundance of 0 (as HP) is required in Neptune to explain the observed CO concentration in its atmosphere [13]. Cold-trapping 1800 A H20 from the inner solar system would also explain how the reduced CaTi0 1600 3 component postulated in the two-component accretion models for the terrestrial planets formed [14]. 1400 Fe-melal More modeling is needed to decide whether the reduced condi tions were created by carbonaceous dust enrichment orH20 removal, 1200 g or by a combination of both. Acknowledgments: ~ This work is supported by NASA grant :J NAGW-452J. 1600 eCD . a. References: [I] Larimer J. W. (1968) GCA, 32, 965. [2] Lari E CD l- mer J. W. (1975) GCA, 39, 389. [3] Larimer J. W. and Bartholomay e .2 M. (1979) GCA, 43,1455. [4] Grevesse N. and Noels A. (1993) in ~ 1400 e Origin and Evolution of the Elements (N. Pranzos et a!., eds.), CD B '0e pp. 15-25, Cambridge Univ. [5] Scott E. R. D. (1988) EPSL, 91, I. 0 C,.) [6] Bischoff A. et a!. (1993) GCA, 57, 2631. [7] Larimer J. W. and Ganapathy R. (1987) EPSL, 84, 123. [8] Baedecker P. A. and 1600 Wasson J. T. (1975) GCA, 39, 735. [9] Huss G. R. and Lewis R. S. (1995) GCA, 59,115. [10] Cameron A. G. W. (1995) Meteoritics, 1400 30, 133. [II] Lissauer J. J. (1987) Icarus, 69, 249. [12] Stevenson D. J. and Lunin J. I. (1988) Icarus, 75, 146. [13] Lodders K. and C 1200 Fegley B. (1994) Icarus, 112,368. [14] Wiinke H. (1981) Philos. Trans. R. Soc. London, A303, 287. 0.75 0.80 0.85 0.90 0.95 1.00 1.05 c/O Ratio Fig. 1. Condensation temperatures for initial condensates as a function of FRAGMENTAL AGGREGATION IN THE NEBULA: A 3 C/O ratio at PIOI = 10- bar. BASIC NEBULAR PROCESS. G. E. Lofgren, Mail Code SN4, NASA Johnson Space Center, Houston TX 77058, USA (gary.e. [email protected]). The Ti-bearing troilite in enstatite meteorites may result from reac Summary: Fragmental and clastic aggregates have been iden tions like TiC + 2 H2S = TiS2(in FeS) + CH4 or TiN + 2 H2S = TiS2 tified in highly unequilibrated ordinary chondrites [1,2]. These ag (in FeS)+ 0.5 N2 + 2H2 at 10werT. SiC forms at high T, but at C/O '= gregates are composed of pulverized chondrule debris including 0.98, only -20% Si is in SiC before all Si is consumed by forsterite individual olivine and pyroxene crystal fragments, chondrule frag and enstatite. ments, and, rarely, whole chondrules together with varying amounts How were reducing conditions obtained in the solar nebula? of fine-grained nebular matrix material, troilite, and metallic Fe-Ni. Removal of 0 by oxide condensation increases C/O in the gas to These aggregates not only attest to extensive recycling as part of -0.57, but this is too low to produce reduced condensates, and the chondrule formation, but also suggest a process that affects chon major elements are then already condensed. Injection of carbon drule compositional variations. The chondrule compositions are a aceous dust or removal of H20 more plausibly increased the C/O function ofrandom mixtures ofthese components in a manner similar ratio [1-3,8]. Isotopic variations in chondrites indicate that the solar to that described by Alexander [3]. Rims that are considered of nebula was not ~omogeneous. Higher abundances ofpresolar SiC in nebular origin [4] enclose many of the aggregates, suggesting that Qingzhen (EH3) than in Orgueil (CI) [9] may indicate that larger they formed in the nebula. These aggregate particles may be unmelted amounts ofcarbonaceous dust (which could raise the C/O ratio) were or may show varying degrees ofmelting from incipient to extensive. present in the EC-forming region. Drawbacks of dust addition are Small degrees of melting produce coherent aggregates with clastic that (I) the C/O range where EC minerals form is very narrow to granular textures. Larger degrees of melting produce igneous (-0.98 to -1), and (2) increasing C/O from solar to -I requires textures. Total or near total melting would produce melt droplet large amounts of carbonaceous dust. This dust probably contains chondrules. The rims show different melting histories from the main SiC in ddition to other C-bearing phases. Thus, the Si abundance aggregates suggesting multiple heating episodes. Thus these aggre also increases and could account for the higher SilMg weight ratio gates most likely are chondrule precursors. They form by processes in EHs (-1.5) than that in CI chondrites (-1.1). However, the SilFe similar to agglomeratic [5] and fine-grained type I Semarkona chon ratio in EH and CI chondrites is very similar, which argues against drules [6]. These findings support the existence ofa solarnebula with Si enhancement but points to Mg fractionation. a long and complicated history with many heating events and suffi Ifcarbonaceous dust did not cause the reducing conditions, about cient movement and density ofparticles to allow numerous collisions halfofthe 0 had to be lost from the EC-forming region, possibly by and comminution of chondrules and the subsequent aggregation of H20 ice condensation. Solar nebula models place H20 ice conden the generated debris followed by reheating and formation of new sation at 5.2 AU and beyond [10]. Early accretion of ice and rock chondrules in repetitive cycles as suggested by [5,7]. As these aggre may have caused a runaway accretion of Jupiter so that the jovian gates approach larger dimensions, they are the beginning ofnebular planets served as cold traps for H20 ice [11,12]. A 440x solar accretion and asteroid formation. LPI Technical Report 97-02, ParI I 41 Fig. 1. Fragmental aggregate composed ofBP chondrule fragments with Fig. 3. Clastic aggregate composed of angular to subrounded grains of attached pyroxene grains enclosed in a nebular rim. olivine and pyroxene enclosed in a nebular rim. Fig. 2. Fragmental aggregate composed of clastic aggregates and BO and Fig. 4. Clastic aggregate composed of grains of olivine and minor BP chondrule fragments. pyroxene enclosed in a nebular rim. Abundant metal segregated toward outer rim of particle. Particle Descriptions: The fragmental aggregate is the prod large fragments are sharp and arcuate. There is no unequivocal uct of an important process in the nebula. It formed by accidental nebular rim, but a partial rim is present. collisions ofthe six major components ofnebular debris listed above Clastic aggregates are a subgroup of the fragmental aggregate. and mayor may not be enclosed in a clastic, metal/sulfide, or nebular They are composed of crystal fragments, and chondrule fragments rim. All fragments in the aggregate did not necessarily come together are rare. A coarse-grained aggregate, shown in Fig. 3, contains Mg at the same time, consequently some of the fragments or groups of rich olivine and Fe-rich pyroxene with a large grain size variation. fragments may have acquired rim material before final aggregation. Crystals are subangular to subrounded. The pyroxenes are finer A fragmental aggregate composed of four chondrule fragments is grained than olivine. Iron-rich grains tend to be in the outer part of show in Fig. I. The largest fragment (f-4) is coarsely barred pyroxene the particle. There is rare sulfide in the particle and a sulfide-rich rim and the other fragments are finer BP of similar composition. The with some F emetal and nebular matrix material. A much more metal boundaries between the fragments are sharp and free ofdebris. There rich, clastic aggregate is shown in Fig. 4. It consists ofa largely metal are clastic pyroxenes of similar composition attached sporadicalJy free core ofcoarse, rounded olivines, F03_11 , surrounded by an outer around the perimeter and the entire fragment is enclosed in a nebular zone of finer, more-angular olivines, richly interspersed with con rim of nebular matrix [4] mixed with sulfide. A more diverse frag centrations ofmetal and sulfide. The circumference ofthe particle is mental aggregate is show in Fig. 2. It is composed of a clastic marked by an uneven distribution of finer pyroxene grains and a aggregate with a large olivine crystal (left), a BO fragment (center), nebular rim encloses the whole particle. There is a group of fine and smaller olivine crystals and BP fragments (right). The clastic grained clastic aggregates that appear dark in plane light mostly aggregate consists of anhedral to subhedral olivine and pyroxene because they are so fine grained. They usually have an irregular grains with some angular faces and appears to have a clastic texture. outline and may be metal rich or have a metal-rich rim. Some contain The large olivine is distinctly more Mg rich. The BO fragment shows abundant rounded metal grains, some have none. Where partial signs of partial melting prior to aggregation. The boundaries ofthe melting occurs, subsequent overgrowths that develop upon cooling 42 Workshop on Modification of Chondri tic Materials give some ofthe crystals euhedral outlines. Ifthere is partial melting, and the Protoplanetary Disk (R. H. Hewins et a\., eds.), pp. 173 then the metal is usually spherical. With increased partial melting, the 180, Cambridge Univ. [8] Krot A. and Wasson 1. (1995) GCA, 59, more rounded the particle the more the metal migrates outward. The 4951-4966. particles described by [5,6] are included in this category, though some ofthe aggregate olivine particles of[5] may be coarser grained. Rims or partial rims are composed primarily of clastic olivine and PARENT-BODY METAMORPHISM OF CV3 CHON pyroxene (angular crystals 5-20 !lm), but also including Fe metal and DRITES: COUNTERARGUMENTS BASED ON ACCRE sulfides-mostly troilite and other fine-grained rim or matrix-type TIONARY RIMS AND CALCIUM-ALUMINUM-RICH IN debris. These rims may be partially melted and have slightly rounded CLUSIONS. G.1. MacPherson' and A. M. Davis2, 'Department to subrounded crystals orthe crystals could have euhedral overgrowths ofMineral Sciences, U.S. Museum ofNatural History, Smithsonian and a glass or fine-grained mesostasis. With more extensive partial Institution, Washington DC 20560, USA ([email protected]), melting they would become the igneous rims described by [8]. 2Enrico Fermi Institute, University of Chicago, Chicago IL 60637, Discussion: The processes documented here support existing USA ([email protected]). models developed by [5-7] for aggregation ofparticles and multiple cycles ofheating in the nebula. Further work is needed to determine Krot and coworkers [1] proposed that many features of CV3 the effects ofthese heating events on fine-grained, unmelted materi chondrites are the result ofextensive metamorphism and metasoma als in order to distinguish them from parent-body heating associated tism on the CV parent body, including the alkali-rich secondary with shock and other kinds of metamorphic heating. The processes mineralization in calcium-aluminum-rich inclusions (CAIs). Also, that form these aggregates also suggest that the nebula must have a the distinctive platy olivine matrix in Allende and its kin was pro density of particles what would allow sufficient collisions to frag posed to result from dehydration offormer phyllosilicates. We argue ment chondrules to grain sizes of a few micrometers to tens of that the model of [I] is too general in seeking to explain diverse micrometers and the subsequent aggregation. features that in fact formed by a variety of processes in different References: [I] Lofgren G. E. (1996) Meteoritics & Planet. locations, some of them nebular. Sci., 31, A8\. [2] Lofgren G. E. (1997) LPS XXVIII; Russell P. Prominent in Allende (and Vigarano and other CV3s) are dark and Lofgren G. E. (1997) LPS XXVIII. [3] Alexander C. M. O'D. accretionary rims around CAIs. The following features, summarized (1994) GCA, 58, 3451-3467. [4] Metzler K. et a\. (1992) GCA, 56, from the detailed study of [2], are salient: (I) the rims are layered, 2873-2897. [5] Weisberg M. K. and Prinz M. (1996) in Chon with the layers differing not only in mineral proportions and grain drules and the Protoplanetary Disk (R. H. Hewins et a\., eds.), size but also in mineral compositions; (2) both within individual pp. 119-127, Cambridge Univ. [6] Hewins R. H. et a\. (1996)LPS layers and from layer to layer, extreme disequilibrium mineral assem XXVII. 537-538. [7] Rubin A. and Krot A. (1996) in Chondrules blages exist that could not have survived extensive high-temperature Fig. 1. (3) Backscattered electron image and (b) Na X-ray map of a portion of an Allende fluffy type A inclusion, showing the distribution of nepheline and soda lite in the accretionary rim of the inclusion. The dark object filling the left of the images is the CAl; the accretionary rim is the unifonnly fine-grained gray region just outside the CAl. Note that the inner part of the accretionary rim, nearest the CAl, does not contain Na-rich phases, only the outer portion. LPl Technical Report 97-02, Pari 1 43 parent-body metamorphism of the type proposed by [I], such as References: [1] Krot A. N. et al. (1995)Meteoritics, 30, 748 extensive compositiQnal zoning (F0 56_94) within olivine crystals that 775. [2] MacPherson G. J. et al. (1985) GCA, 49, 2267-2279. are <10-20 mm in size, and direct contact of diopside and heden [3] Davis A. M. et al. (1994) LPSxxv, 315-316. [4] MacPherson bergite crystals (the former surrounds the latter; [2, Fig. 9]) with no G. J. and Davis A. M. (1993) GCA, 57, 231-243. [5] Hutcheon I. D. discernible compositional gradients at the point of contact; (3) the and Newton R. C. (1981) LPS XII, 491-493. [6] Allen J. M. et al. layers are highly porous, as are the meteorite matrixes; (4) olivine (1978) Proc. LPSC 9th, 1209-1233. crystals in the accretionary rim layers have the same distinctive barrel- orlens like shapes as olivine crystals in the meteorite matrixes; and (5) crystals in the rim layers and matrixes do not show growth EVAPORATION BEHAVIOR OF MINERALS AND SILI interference textures. Many properties ofthe accretionary rim layers CATE MELT IN VACUUM AND IN HYDROGEN GAS. H. and the matrix in Allende are so similar that the two cannot have Nagahara, Geological Institute, University ofTokyo, Hongo, Tokyo unrelated origins, and the properties of both are not compatible with 113, Japan. either high-temperature metamorphism of originally anhydrous as semblages or with common formation from dehydration of Evaporation is one of the major phase transformations at low phyllosilicates. Extensive high-temperature thermal metamorphism pressure of the solar nebula and stellar environments. Recent pro cannot explain the preservation offine-scale mineral chemical zon gress of well-controlled evaporation experiments and a theoretical ing or the extreme disequilibrium assemblages within the rims; the approach have enabled us to recognize the evaporation processes. In dehydration model cannot explain the existence of fine-scale grain the present study, kinetics ofevaporation obtained from experiments size and mineral chemical layering, the crystal chemical zoning or the are summarized with the link of theoretical consideration. disequilibrium mineral assemblages, the absence of crystalline Experimental Work: In the kinetic theory of gas molecules, intergrowths expected in an assemblage that purportedly grew in evaporation rate of a substance is related to its equilibrium vapor place, or the characteristic lensoid shapes of the olivines. pressure, weight of the gas molecule, and temperature. Kinetics of The term "alteration" in CV3 CAIs encompasses a wide range of the surface reaction, however, lowers the evaporation rate below that mineral assemblages and petrographic settings that do not all neces predicted from the equilibrium vapor pressure, which is shown by the sarily have the same origin. The first stage of alteration in Allende evaporation coefficient. What should be studied in experiments are CAIs was the replacement of Ak-rich melilite and anorthite with (1) the mode of evaporation, (2) evaporation rate, and (3) isotopic grossularmonticellite and wollastonite. A study ofsuch altered areas fractionation factor. These experimental results enable us to estimate indicates that they inherit theirrefractory trace-element patterns from the temperature and pressure history of the object, and particularly melilite, not fassaite and anorthite, and that no exchange ofrefractory composition of the ambient gas. elements with an external reservoir occurred during alteration [3]. Silicates. Mode of evaporation (congruent), evaporation rate in Sodium and halogen infiltration was linked in a second alteration vacuum and H gas including temperature dependence, and isotopic step, and led to the formation ofnepheline and sodalite. This assem fractionation factor have been studied for forsterite [1-3]. The mode blage is common in the oxidized subgroup CV3 but sparse in the of evaporation (incongruent) of enstatite is being studied [4], but reduced subgroup. Melilite in type B CAIs commonly contains small little is know about kinetic behavior, particularly in the presence of but consistent amounts of Na in solid solution, which correlates H gas. The mode ofevaporation, evaporation rate in vacuum and in positively with the akermanite content of the melilite. This Na, H gas, and isotopic fractionation factor for Si02 in vacuum were unexpected in pristine high-temperature nebular condensates or their studied [1,5]. Hibonite has been intensively studied in the relation melted derivatives, apparently is the result ofremelting ofthe inclu ship between CAls [6,7]. Very limited work has been done for other sions following one or more periods of the alkali-rich secondary silicates. mineralization [4]. Nepheline and sodalite also occur in the accre Sulfide. The mode ofevaporation (incongruent) and evaporation tionary rim structures described above. The dispersed grains are not rate oftroilite in vacuum and H gas have been studied. Change ofthe uniformly distributed within the accretionary rims or in a general dominant gas species is suggested for different H pressure condi halo ofNa-rich phases outside the CAIs but, rather, are confined to tions [8). specific rim layers (Figs. la,b) where they do not appear to be re Silicate melt. Although many experiments have been carried out placing any minerals in situ. This stratigraphically controlled distri [9-12], the role ofkinetics has not been fully evaluated. Dependence bution suggests the Na-rich phases accreted as such and are not the of a on melt composition and temperature has not been studied products of parent-body metasomatism. Measurements of AI-Mg systematically. Melt of the Si02-MgO system has been fairly well isotope compositions suggest that (I) formation ofgrossular in type studied, and dependence of the evaporation coefficient ofMg/Si on B CAIs occurred >0.5 to >2.4 m.y. after initial CAl formation [3,5]; the bulk composition is suggested [13]. The evaporation of trace and (2) the CAl remelting episodes occurred as much as 2-3 m.y. elements from silicate melt is quite different from that of major after initial CAl formation [4] . The melting was almost certainly a elements, which have a smaller kinetic barrier [14). nebular event, meaning that the alkali metasomatism must also have Application: Most minerals and silicate melt evaporate incon been nebular unless some mechanism is found for recycling indi gruently to cause chemical and isotopic fractionation. Two other vidual CAls from a parent body to the nebula and back again, kinetic processes play important roles when the chemical evolution possibly multiple times. A different "alteration" assemblage occurs ofan object is considered. One is material transfer (diffusion) in the in cavities within CAls and includes whiskers of wollastonite [6]. condensed phase [15,16]. Theoretical application oftbe possibility The extreme length/width ratios of these crystals suggests growth of isotopic fractionation was made [16]. The other is crystallization from a vapor at low degrees ofsupersaturation, difficult to reconcile in silicate melt. Except for minerals that evaporate congruently, most with parent-body metamorphism. minerals and silicate melt change the bulk composition due to partial 44 Workshop on Modification of Chondritic Materials evaporation. It causes crystallization isothermally or even during heating [17]. Kinetics on the surface significantly control the evapo 87Sr/86sr 4.5Ga ration rate and texture of the objects, which may be critical for the ~-~~~.-': • <..nondrulc • estimation of heating conditions of chondrules and CArs [18]. i Y-86751 • 0.718 "...... References: [I] Hashimoto A. (I 990) Nature, 347, 53. [2] Na e Wholflllo.:t. . 0 O>onUruIe • • gahara H. and Ozawa K. (1996) GCA , 60, 1445. [3] Davis A. et al. • (1990) Nature, 347, 655. [4] Tachibana S. et al. (1997) Proc: NIPR • Symp. Antarct. Meteorites, 21, 164. [5] Nagahara et al. (1993) 0.708 Meteoritics, 28, 406. [6] Hashimoto A. (1991) LPS XXII, 529. • • [7] Floss C. et al. (1997) LPS XXVIII. [8] Tachibana S. and Tsuchi • yama A. (1996) Proc. NIPR Symp. Antarct. Meteorites, 21, 164. (a) [9] Hashimoto A. et al. (1983) Geochem. J., 17, III. [II] Wang J. 0.698 '----~_-'-~~~_'_~...... _~~...J et al. (1994) LPS XXV; 1457 and 1459. [12] Floss C. et al. (1996) 87Srt86Srr------"-7--~ GCA,60, 1975. [13] Nagahara H. and Ozawa K. (1996)Proc. NIPR 0.718 o Symp. Antarct. Meteorites, 21, 125. [14] Hashimoto A. (1988)LPS XIX, 459. [15] Wang J. et al. (I 993) LPSXXIV, 1479. [16] Nagahara 4.SGa H. and Ozawa K. (1997) LPS XXVIII. [17] Nagahara H. and Ozawa c K. (I 996) LPSXXVII. [18] Ozawa K. and Nagahara H. (1997) LPS 0.7~ XXVIII. 0- 0 0 ."." (b) RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMATICS OF ,,.. 0.698 CHONDRULES FROM THE ANTARCTIC CV CHON 0.0 0.1 0.2 0.3 0.4 DRITES YAMATO 86751 AND YAMAT086009: ADDI 87RbI'I6sr TIONAL EVIDENCE FOR LATE PARENT-BODY MODI l l FICATION. N. Nakamura ,2, T. Kanil, and G. Kondorosi , Fig. 1. Rubidium·strontium evolution diagram for the CV chondrites. IGraduate School of Science and Technology, Faculty of Science, Kobe University, Nada, Kobe 657, Japan, 2Departrnent ofEarth and Planetary Sciences, Faculty of Science, Kobe University, Nada, ucts?). So it is possible that the Rb-Sr system of the Y 86009 Kobe 657, Japan. chondrules were affected by terrestrial weathering. However, the Antarctic weathering tends to leach out Rb and so to lower the 87Rbl Chondrules and CAIs, as well as other components of the CV3 86Sr ratio [6], which is not the case for the Y 86009 chonrules (see chondrites, were modified by different degrees of late-stage alter Fig. I b). We therefore suggest that the chondrules were subjected to ation processes [I]. Most Allende (CV) chondrules, as well as CAIs, severe late-stage Rb-Sr isotopic perturbation on the parent body. also indicate Rb-Sr isotopic perturbation [2,3] caused by late open References: [I] Krot A. N. et al. (1995) Meteoritics. 30. 748 system modification on the parent body at a significantly later time 775. [2] Gray C. M. et al. (I 973)Icarus, 20. 213-239. [3] Tatsumoto than 4.5 Ga. In order to clarify the time and nature of the event, we M. et al. (1976) GCA, 40. 617-634. [4] Nakamura N. et al. (1993) carried out combined Rb-Sr isotopic analyses and petrographic Meteoritics, 28. 407-408. [5] Murakami T. and Ikeda Y. (1994) examination of the Allende chondrules [4]. Extending our Rb-Sr Meteoritics, 29. 397-409. [6] Nishikawa Y. et al. (1990) Mass isotopic study ofcarbonaceous chondrites, we have carried out Rb Spectroscopy. 38. 115-123. Sr analyses for the bulk samples and II chondrules from the two Antarctic CV chondrites Y 86751 (oxidized subgroup) [5] and Y 86009 (unclassified subgroup), and report the preliminary results EXPERIMENTAL STUDY ON FORMATION OF SECON compared with those of Allende. DARY MINERALS IN CALCIUM-ALUMINUM-RICH IN Two BO chondrules with less Na-rich phases and one PO chon CLUSIONS IN CARBONACEOUS CHONDRITES. K. drule with more Na-rich phases were analyzed. As shown in Fig. la, Nomura and M. Miyamoto, Mineralogical Institute, Graduate all the Y 86751 data, including one bulk meteorite, plot closely on the School ofScience, University ofTokyo, Hongo, Tokyo 113, Japan. 4.5-Ga reference line, indicating that the bulk meteorite and the chondrules have been in a closed system since their formation at Introduction: Carbonaceous chondrites contain Ca-AI-rich 4.5 Ga. Note that a few Allende chondrules and the bulk meteorite inclusions (CAIs), which mainly consist ofrefractory minerals such show a similar isotopic feature, but the majority ofchondrules show as gehlenite, spinel, perovskite, and hibonite, which are predicted on deviation from the 4.5-Ga line to a higher Rb/Sr ratio (Fig. la). the basis of thermodynamic calculations [I] to be the first phases Because data for Y 86751 are still limited, additional analyses may formed from the early solar nebula. However, many CAIs contain be required to confirm the observed feature for Y 86751 . secondary minerals such as nepheline, calcite, and phyllosilicates. Contrary to Y 86751, the Rb-Sr systems for Y 86009 chondrules, The formation process ofsecondary minerals in CAIs is still contro as well as the bulk meteorite, are extensively perturbed. Briefpetro versial, whether they were produced by the reaction with the solar graphic observations ofcbondrules suggest that Y 86009 chondrules nebula [e.g., 2] or by aqueous alteration in parent bodies [e.g., 3]. carry more or less Na-rich alteration phases but their abundances We performed hydrothermal experiments on gehlenite, spinel, appear to be much less than the Allende chondrules. In addition, we and diopside, which are common in CAIs in carbonaceous chon noted uncharacterized phases in a few chondrules (weathering prod drites, to study aqueous alteration in parent bodies. Because the LPl Technical Report 97-02. Part J 45 discovery of thennally metamorphosed carbonaceous chondrites thermal experiments. One explanation for these anhydrous minerals suggests that thennal metamorphism after aqueous alteration in is that secondary minerals in CAIs were produced by aqueous alter parent bodies affects the mineral assemblage of CAIs, we also per ation and the subsequent mild thermal metamorphism in parent fonned heating experiments on the products obtained by hydrother bodies. mal experiments to study subsequent thermal metamorphism after In heating experiments, H20 molecules in hydrogrossular de aqueous alteration. crease with increasing heating temperatures. The differential thermal Experimental: Hydrothermal experiments were performed for curve for hydrogrossulargave one endothermic peak at about 500°C. gehlenite, diopside, and spinel. The mixtures of gehlenite powder A weight loss between 400° and 600°C in the thermogravimetric and AI 20 3and/or Si02reagents were also prepared to study the effect curve suggests that the endothermic peak in the differential thermal of AI20 3 and/or Si02 on the aqueous alteration of gehlenite. Solu curve is due to dehydration of hydrogrossular. tions with NaOH, NazC03, and HCI were used. The experiments On the basis of our experiments, the alteration processes are were performed at 200°C for 7 days. summarized as follows: Both Na- and Si-rich fluid was supplied by Heating experiments on hydrogrossular obtained by the hydro decomposition of matrix glassy materials. This fluid dissolved thermal experiments as starting materials were performed with an gehlenite to become rich in AI enough to crystallize nepheline hy electric furnace at 400°, 500°, 600°, and 700°C for 24 hr. Thermal drate and/or analcime. Under the oxidizing condition, organic C in analyses including DTA (differential thermal analysis) and TG parent bodies are expected to have reacted with 02in fluid to generate (thermogravimetric) were also performed to study the thermal meta C032-, which reacted with Ca released from gehlenite to crystallize morphism ofhydrogrossular obtained by the hydrothermal experi calcite. Under the reducing condition in parent bodies, gehlenite was ments. dissolved by fluid to release Ca, AI, and Si from the surface into fluid. The mineral assemblage ofrun products was determined by using These ions crystallized hydrogrossular, and excess Ca and Si crystal the X-ray powder diffraction (XRD) method. lized Ca-Si hydrate such as tobermorite. All hydrous minerals pro Results and Discussion: Gehlenite was decomposed to hydro duced by aqueous alteration were dehydrated during the subsequent grossular and acicular Ca-Si material (probably Ca-Si-hydrate) in a thermal metamorphism in parent bodies to be converted into anhy INNaOH solution. In a IN Na2C03 solution, gehlenite was decom drous minerals. posed to calcite and hydrosodalite. Calcite was euhedral and sur References: [I] Grossman L. (1972) GCA , 36, 597-619. rounded by the hydrosodalite matrix. This result is consistent with [2] Grossman L. and Steele I. M. (1976) GCA , 40, 149-155. the texture ofthe CAIs often seen in CM chondrites [4] and can be [3] Greenwood R. C. etal. (1994) GCA, 58, 1913-1935. [4] Mac interpreted as follows: Organic C in the matrix reacted with 02 in Pherson G. J. et al. (1983) GCA, 47, 823-839. [5) Tomeoka K. et fluid to generate C032- under the oxidizing condition. Gehlenite in al. (1992) Meteoritics, 27, 136-143. CAIs was attacked by the COl--rich fluid to generate Ca2+-rich fluid enough to crystallize calcite. Gehlenite was decomposed to amorphous AI-Si by leaching ofCa in a IN HCI solution. This may be related to the formation ofphyllo IMPACT MELTING, METAL-SILICATE FRACTIONA silicates in CAIs. Sodium-poor fluid may have produced phyllo TION, AND VOLATILE-ELEMENT MOBILITY ON THE L silicates with calcite whereas Na-rich fluid in parent bodies may have CHONDRITE PARENT BODY. M. D. Nonnan, GEMOC, generated hydrosodalite or other Na-AI-Si hydrate with calcite. School ofEarth Sciences, Macquarie University, North Ryde NSW For the gehlenite-Al20 3mixtures, the product was hydrogrossular. 2109, Australia ([email protected]). For the gehlenite-Si02 mixtures, the products were hydrogrossular and II-A tobennorite [Cas(Si60,sH2) 4H20] for the 10: I and 10:2 "I think we'd all like to know what's in that rock." compositions. F or the 10:4 composition, nepheline hydrate (NaA1Si04 -Agent Mulder I12H20) was obtained in addition to tobermorite. For the 10:8 com position, analcime (NaAISi20 6 H20) and tobermorite were obtained. The L-chondrite parent body suffered a massive, possibly cata The presence of tobermorite from this system suggests that Ca in strophic, impact event about 500 m.y. ago [1-3). We are investigat tobermorite may be supplied by gehlenite because no CaO was added ing the petrologic and geochemical consequences ofthis event through to the starting mixtures. As hydration proceeded, Ca, AI, and Si studies of impact melt and host chondrite material preserved in dissolved in the solution through decomposition of gehlenite to Chico, a I 05-g L chondrite. Chico is composed of~60% impact melt crystallize tobennorite. Analcime or nepheline hydrate may crystal that intrudes the host chondrite as a dike, and may represent a lize by the reaction ofAl (dissolved from gehlenite), Si (supplied by complex of dikes injected into the wall or floor of the crater during starting material), and Na (supplied by solution) because AI is not the 500-m.y. event [2]. added in the starting material of this system. The host chondrite is shocked to stage S6, and contains pockets Neither diopside nor spinel were affected in all solutions with IN ofsilicate melt and veins of shock-mobilized metal. Chondrules are HCI, NaOH, and NazC03' This result shows that both diopside and highly deformed, and are fractured and terminated in sharp contact spinel are stronger than gehlenite against alteration under the hydro with the dike. The melt rock is clast poor, and consists primarily of thermal conditions and is consistent with observation ofCAIs [5]. fine-grained, randomly oriented, euhedra1 olivines and pyroxenes in The minerals closely related to secondary minerals observed in a matrix ofinterstitial glass. Metallic globules up to 2 cm in diameter CAIs were obtained through hydrothermal experiments on gehlenite. are concentrated along the center ofthe dike. Their orientation is sug This result suggests that several secondary minerals in CAIs were gestive of flow differentiation and implies an early stage of metal produced from gehlenite by aqueous alteration in parent bodies. silicate immiscibility. Melt textures are present within these glob However, anhydrous minerals such as wollastonite, nepheline, and ules, which contain Fe-Ni metal, troilite, and schreibersite [2]. Very grossular, which are found in CAIs, were not obtained by the hydro small « 1-51lm diameter) pinpoints ofmetal commonly adhere to the 46 Workshop on Modification of Chondritic Materials surfaces of olivine grains, demonstrating imperfect segregation of . metal from silicate during the melting. 700 0 Variable K/Ca and depletions of Cs, Cd, Bi, Tl, and In in the 600 . Chico impact melt relative to the host chondrite have led to sugges 500 o~ tions that volatile elements may also have been mobilized during the Ir ppb 0 impact event [2,4]. 400 chondrite Chico thus provides a natural laboratory in which to study ele :nJ • ment partitioning and fractionation caused by metal-silicate segrega 200 tion and volatile-element mobility during a large impact event on a • • 100 dike chondri tic parent body. Depletion of volatile elements and fraction • • ... I ation ofmetal from silicate are characteristic features ofthe terrestrial U planets and presumably many asteroids (e.g., the eucrite parent 3.0 body), so, conceivably, similar processes on larger scales may have influenced the differentiation of planetesimals during the early his 2.5 As tory of the solar system. 2.0 r ppm 0 0 To investigate the consequences of this impact event on the L oCb chondrite parent body, we have conducted an INAA study of Chico 1.5 0 using samples from the same cores that were analyzed by [2]. 1.0 • Rare-earth-element patterns ofthe impact melt and the host chon • •• drite are all consistent with an undifferentiated parent body and no 0.5 .,. large-scale crystal-liquid fractionation during the impact melting 0.0 . 15000 event (Fig. I). Absolute concentrations of the REE in splits of the 0 5000 10000 melt rock vary by about 30%, and are negatively correlated with Nippm siderophile elements such as Fe, Ni, and Ir (Fig. 2). Rare-earth element contents of the host chondrite splits also range to relatively Fig. 3. high values, and do not vary with the amount of metal. 120 100 y = 13.05x + 6.63 3.0 Metal Fe/Ni =6.2 80 a: 2.0 Z 8J N Q) iii ~ § 40 o c o 1.0 20 0.8 melt. chondrite host 0 0 0.6 ":-''--'-.&...... -L;:::-'-::!-..L...:;:~'--J...... --''~~...... I 0 3 4 5 6 7 8 La 8m Eu Tb Vb Lu 10000/Ni Fig. 1. Fig. 4. 0.28 24 0 00 026 22 0 ,ike 0 • 0 E 024 0 20 • 0 0 ~ • ~ • • host E 0.22 Z 16 '. (J) • • CO chondrite r • , • 0 0 0.20 16 r 0.16 • 0 5 10 15 20 1000 1 Fig. 2. Fig. 5. LPI Technical Report 97-02, Part 1 47 12 range to both higher and lower values (Fig. 7). Melting or crystalli zation of the melt should not fractionate these elements (as long as 0 • 10 plagioclase is not involved), so the variations in alkali element 00 • 0 content and SmfNa in the dike may be indicating redistribution of 0 8 00 alkalies. Sodium contents lower than those of the host chondrite l!:'" 0 almost certainly reflect volatile loss as this is difficult to achieve os 6 • '"z either by melting or by crystallization. • • ~ Within the dike there is a general correspondence between the 4 •• degree ofsiderophile element depletion, REE enrichment, and alkali • element enrichment, suggesting that the processes of metal extrac 2 tion, melt migration, and volatile-element mobility were physically 6 8 10 1~ ' 14 16 18 20 22 CaO/K2O linked during this impact event in the L-chondrite parent body. References: [I] Haack et a1. (1996) Icarus, 119, 182-19\. Fig. 6. [2] Bogard etal. (1995) GCA, 59,1383-1399. [3] Nakamuraetal. (1990) Nature, 345, 51-53. [4] Yolcubal et al. (1997)JGR Planets, submitted. 0.30 0 028 • FORMATION OF SINGLE-DOMAIN IRON PARTICLES: IMPLICATIONS FOR THE NEBULA. J. A. Nuthl and P. A. 0 N 026 Withey2, I Astrochemistry Branch, Mail Code 691, NASA Goddard to Z • Space Flight Center, Greenbelt MD 20771, USA, 2Department of O. • E 024 0 .0 Physics and Engineering, West Virginia Wesleyan College, Buck en 0 • • hannon WV 2620 I, USA. 0.22 0 , 0 • There is ample evidence in the meteorite record documenting 0.20 significant episodes ofvaporization and recondensationofrefractory 0.7 0.8 0.9 1.0 1.1 12 material. Under the H-rich conditions prevalent in the primitive solar Na20% nebula, one would expect some fraction of the Fe in such vapors to Fig. 7. condense as metal. Even in the absence ofH, Fe-metal particles can be formed, as has been shown to occur when chondritic starting materials are heated rapidly in vacuum in a solar furnace [1]. In situ oxidation-reduction couples can be formed in hot chondritic materi Siderophile elements are strongly affected by the metal-silicate als that could be responsible for the production ofmicroscopic metal fractionation within the dike. Iron, Ni, Co, Ir, Au, As, and Se are particles found in the dusty olivine grains occurring in some chon progressively depleted in the melt relative to the host chondrite, and drules [2-4] as well as for the metal blobs found at the surfaces of trend toward the composition ofametal fragment(M) separated from some chondrules [5). Given the meteoritic and experimental evi the dike (Figs. 2-4). Metal fractionation also drives the melt toward dence suggesting the widespread formation ofFe-metal particles in low Ni/Co and IrlAu ratios (Fig. 5). Although Se is depleted in the the solar nebula we began a series ofexperiments to examine the fate melt, it was not detected in the metal globule and may have been lost of such particles. due to weathering of sulfide. Initial experiments [6] were carried out in the presence ofa high The anticorrelation ofREE and siderophile elements must reflect magnetic field and we reported the rapid formation of enormous, variable proportions ofmetal and silicate melt, but the melt arrays are weblike structures resulting from the magnetically induced coagula too steep to be produced simply by removing metal from the chon tion ofFe dipoles. The structure of the individual strands was quite drite host (Fig. 2). The dike appears to be overly enriched in incom similar to the structure of electrostatically coagulated insulating patible lithophile elements, and the degree ofthis enrichment corre particles studied on USML I and 2 by Marshall and Freund [7] and lates with the degree of siderophile depletion. This may reflect consisted ofhead-to-tail aggregates of thousands of individual par migration of residual silicate melt within the dike, or addition of a ticles linked into very long strings that became cross-linked through metal-poor silicate melt component from the host chondrite. LREE coagulation while suspended in the ambient gas. Subsequent work variations in the host chondrite may also reflect variable amounts of demonstrated that the individual coagulation cross section for mag a melt component. netic dipoles should be several orders ofmagnitude greater than the Potassium and other volatile elements are depleted in the central geometric cross section of the grains [8] and argued that a small portion of the dike [2,4], resulting in high CalK in some samples percentage of magnetic Fe dipoles formed together with insulating (Fig. 6). Most ofthe melt, however, appears to be enriched in K and silicate grains could serve as a catalyst for the rapid accretion ofall have CalK lower than the host chondrite, implying possible volatile the particulates present in the region. Additional experiments carried enrichment, although K may also have been added by the melt out to examine the conditions necessary to form magnetic dipoles component responsible for enrichment of the REE. revealed onevery intriguing fact: Magnetic particles could beforrned Several samples ofthe dike have Na contents and Sm/Na ratios in the absenceofa strong magnetic field [9). In fact, it may bepossible that cluster around the host chondrite values, while other samples to form one class of magnetic particles in the complete absence of 48 Workshop on Modification of Chondritic Materials magnetic fields since single-domain grains are, by definition, more I. The extent ofthealteration is different for different meteorites, thermodynamically stable when the Fe atoms are magnetically aligned but even individual meteorites were nonuniformly affected. The than when they are randomly oriented [10]. boundary between the Allende AF inclusion, considered to be almost Our previous studies indicated that the particles produced in our completely altered by [3], and normal Allende is very sharp [3,4). In experiments are magnetically hard (the degree and direction ofmag addition, although fayalitic rims around forsteritic olivine are ubiq netization is highly stable). Unfortunately these grains were pro uitous in Allende, occasionally olivine grains with discontinuous duced in relatively strong magnetic fields and were most likely rims are found, which excludes alteration in the present environment multidomain particles. Single-magnetic-domain Fe grains only occur (5). In Y 86751 (CV3) there are chondrules with fayalitic rims and within a very narrow size range; smaller grains are super paramag chondrules without rims [6], implying that alteration must have netic and respond rapidly to changing magnetic fields whereas larger occurred before the final assemblage ofbrecciated fragments or the grains contain multiple domains and require a strong magnetic field aggregation of components of nebular origin. to align them into a single dipole. By controlling the concentration 2. The relationship between the various processes collectively of Fe vapor available for nucleation in our system we can exercise termed alteration is unclear. Rim formation alone may require more some degree ofcontrol over the resultant grain size distribution and than one process [7). have observed that the conditions likely to produce the smallest and 3. The steep compositional gradient between forsteritic and largest particles do not show evidence for magnetically induced fayalitic olivine provides a constraint to the last alteration process in coagulation, whereas at intermediate particle size such coagulation time-temperature space. This process occurred under conditions that occurs. We will present magnetic hysteresis loops for various grain would not allow significant volume diffusion [8,5). size distributions in order to prove the presence ofsuperparamagnetic, 4. Type 2 and 3 CCs have locally retained accretionary structures single-domain and multidomain grains in our various samples and to in the form ofrims around chondrules, minerals, and lithic fragments delineate the experimentally determined size range for each magnetic [9,10]. Accretionary rims are still present in AF [4]. phase. We will discuss the implications ofthese results for observa .5. The bulk chemical changes produced by alteration processes tions ofmodem protostellar disks and will discuss possible morpho are minor or even absent. There is no evidence in type 2 and 3 CCs logical evidence in meteorite matrix that could be used to test for the for large-scale redistribution of elements by fluid phases on a centi formation and presence of magnetically coagulated grains in the meter scale, except for unusually high abundances ofNa, Cl, Br, Au, primitive solar nebula. As, and Sb in Allende AF [II). References: [I] King E. (1983) in Chondrules and Their Ori It thus can be concluded that the last alteration event in CV gins (E. A. King, ed.), pp. 180-187, LPI, Houston. [2] Fredrikson K. chondrites that produced the fayalitic rims with extremely steep et al. (1969) in Meteorite Research (P. M. Millman, ed.), pp. 155 F eO-gradients either occurred at low temperatures with variable and 165, Reidel, Dordrecht. [3] Nagahara H. (1981) Nature, 292, 135 limited H20-rock interaction in local compartments of the parent 136. [4] Rambaldi E. (1981) Nature, 293, 558-561. [5] Grossman 1. body and subsequent short heating or at high temperatures by fast (1988) in Meteorites and the Early Solar System (1. F. Kerridge and reaction with an oxidizing gas phase and rapid cooling. Both pro M. S. Mathews, eds.), pp. 680-696, Univ. of Arizona, Tucson. cesses are characterized by low l60 (12). However, there is no simple [6] Nuth J. A. etal. (1994)lcarus, 107, 155-163. [7] Marshall 1. R. relationship between 8170 and the extent ofalteration. Type 3 mete and Freund F. (1996)LPS XXVII, 811 . [8] Nuth 1. A. and Wilkinson orites of the reduced subgroup have higher 8 170 but show less G. M . (I 996)lcarus, 117, 431-434. [9] Withey P. A. and Nuth1. A. alteration [13]. It appears, in summary, that even if some of the (1996) LPS XXVII, 1449-1450. [10] Butler R. F. and BanneIjee mineralogical changes in type 3 CCs are the result of the hydration! S. K. (1975) JGR, 80, 252-259. dehydration mechanism, the extent is limited, Many ofthe mineral ogical and mineral chemical phenomena in these meteorites require oxidizing conditions at high temperatures. In the following, argu OXYGEN-FUGACITY INDICATORS IN CARBONACEOUS ments for gas-solid reactions at high temperatures and oxidizing CHONDRITES: PARENT-BODY ALTERATIONORIDGH conditions is given. Such conditions are not predicted for the canoni TEMPERATURE NEBULAR OXIDATION? H. Palme, Uni cal solar nebula, they would require addition of 0 from vaporized versitiit zu K61n, Institut flir Mineralogie und Geochemie, Ziilpicher material. strasse 49b, 50674 K61n, Germany. There is evidence for oxidizing conditions in Ca,Al-rich inclu sions at high temperatures as recorded by trace elements such as W, Type 3 carbonaceous chondrites (CC) are considered as the best Mo, and Ce (see summary (14)). Refractory metal alloys in CAls are candidates for the identification ofprimitive components formed by oxidized and sulfurized. This requires extensive elemental redistri gas-solid reaction in a nebular environment, primarily because ofthe bution, i.e., solid-state diffusion during or after oxidation [15]. rarity ofH20-bearing phases and the lack ofthermal metamorphism. Formation of fayalitic halos (-50 !-lm) in forsteritic olivines re That the lack of hydrous phases is indicative of the presence of quires solid-state diffusion and thus high temperatures. This event primary nebular mineral assemblages has been recently challenged must have preceded formation of fayalitic rims with sharp transi [1-3] . It has been proposed that Allende and other type 3 CCs have tions. Less steep FeO gradients in olivine, often observed, also experienced extensive hydrous alteration but that later dehydration involve significant volume diffusion and must have formed earlier has removed volatile constituents and produced fayalitic olivine with and at high temperatures [5] . inclusions ofpentlandite and Cr-spinel and other secondary minerals Dohmen et al. [16] have experimentally demonstrated that rim in CAls and chondrules (sodalite, nepheline, andradite, etc.). formation with the release ofgaseous Mg occurs readily on forsterite Characteristic features of the alteration, whether of nebular or grains given high temperatures and appropriate 0 fugacities. The parent-body origin, are described below. presence ofmetallic Fe is sufficient to trigger this reaction; physical LPI Technical Report 97-02, Part J 49 contact with forsterite is not required. The ease with which this an aqueous solution that has altered the rock to a certain degree reaction occurs is surprising and suggests that formation of fayalitic depending upon the water/rock ratio and the temperature of the olivine at high temperatures is a common process. This applies alteration process. Subsequently, the aqueous solution was lost, and equally to the replacement of enstatite by olivine through reaction heating ofthe altered rock partially or completely dehydrated it. Our withFeO. objective was to monitor changes in the mineralogy ofrocks and the Fine-grained spinel-rich inclusion of oxidized CV meteorites chemistry of aqueous solutions (fluids) in these processes. show simultaneous enrichment in FeO, CoO, G~03' and ZnO [17]. Since the composition of the aqueous solution is unknown, we In the highly altered Allende AF these elements have normal bulk have based our estimate on a reasonable assumption that the solution Allende abundances. Their incorporation into spinel is readily ex was in equilibrium with the bulk rock or, at least, with the fine plained by gas-solid reaction, as has been experimentally demon grained matrix. We have chosen the bulk composition ofthe Allende strated for FeO, CoO, and NiO [15]. CV3 chondrite [4] and the matrix composition of Kaba [5]. Calcu The conclusion is that low-temperature alteration, if present in lations were done at 99°C and 150°C and W/R ratios ranging from type 3 CCs, is oflimited extent and that there is abundant evidence 0.0001 to 0.5. We found that at W/R < 0.2, practically all Hp is for high-temperature oxidation of primary mineral assemblages. A consumed by the reactions with the anhydrous rock. At W/R = 0.2, chaotic solar nebula, variable in f02 but uniform in chemical compo a completely altered rock contains-I-3 wt% aqueous solution with sition, is required. This is achieved by the addition of various frac high concentrations ofCI-, Ca2+, and Na+ ions. At higherW/R ratios tions of volatilized chondritic material. the aqueous solution becomes progressively diluted, with only minor References: [I] Krot A. N. (1995) Meteoritics, 30, 748. or no changes in the mineralogy of the altered rock. At W/R < 0.2, [2] Krot A. N. (1997) Meteoritics & Planet. Sci., 32,31. [3] Kojima Hp reacts with the rock to form H2 and Hp-rich fluid, with the T. and Tomeoka K. (1996) GCA, 60, 2657. [4] KuratG. eta!. (1989) HpIH 2 ratio and minor contents ofH2S, CO2, and CO increasing at Z. Naturforsch. , 44a, 988. [5] Weinbruch S. et al. (1990) Meteor higher WIR ratios. The reaction ofH20 with both the metal and C itics, 25, 115. [6] Murakami T. and Ikeda Y. (I994) Meteoritics, 29, results in very high H2 and CH4 fugacities so the system must be open 397. [7] Brearley A. J. (1996) Meteoritics & Planet. Sci., 3 I , A21. (i.e., lose H and methane) to allow H20-rock reactions to proceed. [8] Hua X. et a!. (1988) GCA, 52, 1389. [9] MacPherson et al. Mineralogical changes during reactions between the H20 and (I985) GCA, 49, 2267. [10] MetzlerK. et al. (1992) GCA, 56, 2873. rock strongly depend upon the W/R ratio and the amounts of metal [11] Palme H . et a!. (1989) Z. Naturforsch., 44a, 1005. [12] Wein and C in the rock. Ifcomplete equilibrium is maintained, an addition bruch S. etal. (I 993)GCA, 57, 2649. [13] Clayton R. N.and Mayeda ofsmall amounts ofH20 at 100°C to the rock with the bulk compo T. K. (\ 996) Meteoritics & Planet. Sci. , 31, A30. [14] Rubin A. E. sition of Allende results in a mineral assemblage of wollastonite, (1988) in Meteorites and the Early Solar System (J. F. Kerridge grossular, andradite, nepheline, melilite, Ca-olivine, sodalite, apa and M. S. Matthews, eds.), p. 488, Univ. of Arizona, Tucson. tite, whitlockite, metal, troilite, and C. The first OH-bearing silicate, [15] Palme H. et al. GCA, 58, 495. [16] Dohmen R., this volume. vesuvianite, appears in minor amounts at WIR as low as 0.001. [17] Palme H. and Wark D. (1988) LPS XIX 897. Grossular disappears at WIR =0.1 . At W/R =0.1, metal completely oxidizes to magnetite, and monticelite appears at the expense ofCa olivine, wollastonite, and melil ite. At W/R > 0.2, the mineral assem THERMODYNAMIC MODELING OF AQUEOUS ALTER blage consisting oftroilite, magnetite, andradite, and phyllosilicates ATION IN CV CHONDRITES. M. I. Petaev1 and M. V. coexists with aqueous solution. Mironenk02, IHarvard-Smithsonian Center for Astrophysics, Cam To model H20-rock interaction on a local scale, we selected bridge MA, USA, 2Vemadsky Institute, Moscow, Russia. compositions ofCAls [6], chondrules [7], matrix [4], and metal and recalculated them on a FeO-deficient (mg# = 95) and Na, CI, C, P, The CV carbonaceous chondrites are traditionally considered as Na-free basis. Then we modeled reactions ofthese rocks and the bulk rocks consisting of condensates formed in the nebula of the solar Allende with the aqueous solution (equilibrated with the Kabamatrix composition. The mineralogy and/or chemistry of some textural at 150°C and W/R = 0.2) at W/R ratios of 0.1 , I, and 10. At W/R = components of the CV chondrites, such as CAIs, chondrules, and 0.1 the matrix would consist of metal, troilite, diopside, and olivine grains ofmagnesian silicates, are similar to those predicted by equi (Fa -50) with minor amounts of spinel and pargacite. Chondrules librium condensation models [e.g., 1,2]; thus, these components are would be altered to the assemblage of olivine (Fa -50), diopside, thought to be primive nebular condensates that survived later stages serpentine, talc, clinochlore, troilite, and halite. The alteration of ofalteration in an oxidizing environment. The alteration has resulted CAls results in the appearance of Ca,Mg,AI-chlorites with minor in drastic increases in the FelMg ratio in fine-grained matrix olivine amounts of hematite, calcite, and bassanite; no Na minerals are and the Ni/Fe ratio in metal, and the formation ofsecondary minerals present. The reaction between metal grains and the aqueous solution including garnets, clinopyroxenes, feldspathoids, phyllosilicates, results in partial replacement ofmetal by magnetite with trace amounts and others. Petrographic observations indicate that the alteration has of magnesian olivine and halite; the latter crystallizes from the taken place on a local scale, i.e., CAIs, chondrules, metal grains, and highly saline solution formed due to consumption ofH20 in reaction matrix arereplaced by different minerals. There is general agreement with the metal. At higher W/R values olivine and metal react with among meteoriticists that the secondary alteration was caused by H20 to produce serpentine and magnetite respectively. metasomatism, which might have taken place either in the nebular or Finally, we modeled the dehydration of altered mineral assem an asteroidal environment (see [3] for review). In this study we are blages at various temperatures up to 600°C. At 300°C some amounts testing an asteroidal hypothesis [3]. of Ca,Mg-amphibole and Na-phlogopite still remain in the bulk We assume that the primary, unaltered rock consisting of FeO Allende along with forsterite (Fa -50), diopside, troilite nepheline, free silicates, metal, troilite, and accessory minerals has reacted with spinel, and halite. At higher temperature the bulk mineralogy in 50 Workshop on Modification of Chondritic Materials cludes olivine (Fa -40), diopside, troilite, anorthite, and spinel. No Na- and Cl-bearing minerals are present. Chondrules would consist of0 li vine (F a-20), diopside, and ferroan spine 1. CAIs would consist of melilite, spinel grossular, andradite, and traces of anhydrite. An orthite and nepheline may be also present. Magnetite formed from metal during aqueous alteration should survive dehydration. Our preliminary results suggest that neither aqueous solution nor fluid would supply CI and Na to form nepheline and sodalite from volatile-free compositions. More calculations are in progress. Refereoces: [I] Grossman L. (1972) GCA, 36, 579. [2] Wood 1. A. and Hashimoto A. (1992) GCA, 57, 2377. [3] Krot A. N. et al. (1995) Meteoritics, 30, 748. [4] Jarosewich E. (1990) Meteoritics, 25, 323. [5] Scott E. R. D. et al. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), p. 718, Univ. of Arizona, Tucson. [6] McPherson G. 1. et al. (1988) in 0. 68 0.685 0.68 o.f195 0.7 0.105 0.7' 0.1150 c.n 0. 7'25 0:73 Meteorites and the Early Solar System (J. F. Kerridge and M. S. Fo in olivine Matthews, eds.), p. 746, Univ. of Arizona, Tucson. [7] McSween H. Y. Jr. et al. ( 1982) in Chondrules and their Origins (E. A. King, Fig. 1. MglMg + Fe olivine vs. MglMg + Fe orthopyroxene for Antarc ed.), p. 195, LPI, Houston. tic LL chondrites. ANTARCTIC LL CHONDRITES. A. M. Reid, Department of Table 2 shows the average compositions determined for olivine Geosciences, University of Houston, Houston TX 77204-5503, and orthopyroxene in EET 92013,4 for runs on six separate days. USA. Results: Table 3 shows the average mineral compositions for the 28 Antarctic LL chondrites analyzed to date. Within each indi Iotroductioo: Among the ordinary chondrites the LL-group vidual chondrite there is a remarkable homogeneity in both olivine minerals have the compositional range that allows the best opportu and pyroxene composition. The total range in averaged olivine and nity to both look at the intragroup variability in mineral composi orthopyroxene compositions is shown in Fig. 1, which can be inter tions and also to determine the variation in mineral composition with preted as indicative ofa progressive change in mineral compositions petrographic type. Several excellent studies ofthe LL group [e.g., 1 within the LL suite either over a range ofbulk compositions, or over 5] havebeen restricted only by the relatively small number ofsamples a range ofredoxltemperature conditions for essentially constant bulk available for study. The Antarctic collection contains a significant meteorite compositions. number ofLL chondrites, though the majority are LL6s and there are Using the Ca content of the orthopyroxene as a measure of very few LL4s. Silicate compositions have been determined for 28 metamorphic temperatures achieved, the more Fe-rich silicates record LL chondrites from the Antarctic collection at NASA Johnson Space slightly higher temperatures. The mineral data fall approximately Center (JSC). into two groups with minerals in LL4,5 chondrites more Mg-rich Aoalytical Techniques: Major silicate and oxide phases were than the LL6,7s. Thus the more metamorphosed LL chondrites do analyzed by electron microprobe, using the Cameca SXIOO at JSC. tend to have moreFeinthe silicates and more Ca in the orthopyroxene, For each set ofanalyses the operating conditions were identical and consistent with the suggestion that the sequence LL4 to LL7 is one the same standards were used. In addition, one ofthe most homoge of progressive metamorphic grade and oxidation state [5]. In detail, neous LL chondrites was analyzed on separate occasions in order to however, the data are not wholly consistent with this model: While monitor variation between runs. Table I shows the average compo the orthopyroxene has lower MglFe than the coexisting olivine, the sition and standard deviation for analysesofolivine, orthopyroxene, MglFe partitioning varies among the meteorites and does not corre clinopyroxene, and spinel in the LL7 chondrite EET 92013. For late with the Fe content in the silicates. Using the preliminary petro elements present in amounts greater than 5 wt% the standard devia graphic classification of the Antarctic LL chondrites, there is not a tion on repeat analyses is less than 1.5% of the amount present. consistent correlation between petrologic type and mineral compo- TABLE I. Average mineral compositions in LL7 chondrite EET 92013,4. SiOl TiOl AlP3 Cr20 3 FeO MnO MgO CaO Nap Total Olivine 36.94 0.01 0.0\ 0.02 27.50 0.47 34.59 0.02 0 99.61 n= 208 0.37 0.01 0.05 0.10 0.37 0.03 0.43 0.01 0.01 Opx 54.40 0.19 0.17 0.12 16.38 0.47 26.92 0.88 0.01 99 .56 n = 50 0.28 0.02 0.02 0.05 0.23 0.03 0.25 0.12 0.01 Cpx 53.33 0.43 0.50 0.81 6.31 0.20 15 .96 21.71 0.51 99.79 n = 17 0.32 0.02 0.02 0.05 0.25 0.02 0.16 0.29 0.03 Spinel 0.05 3.86 5.82 54.31 32.49 0.05 1.73 0.03 0.01 98.37 n=8 0.03 0.09 0.08 0.31 0.24 0.13 0.19 0.03 0.01 Numbers in alternate rows are 10. LPI Technical Report 97-02, Part J 51 TABLE 2. Mineral compositions for LL chondrite EET 92013,4. n Si02 Ti02 AI,O) Cr2O) FeO MnO MgO CaO Nap Total Olivine 56 36.90 0.01 0.01 0.04 27.44 0.50 34.26 0.02 0 99 .21 36 36.73 0.01 0.01 0.01 27.67 0.45 34.59 0.02 0 99.56 22 37.08 0.01 0.04 0.03 27.50 0.47 34.43 0.03 0 99.62 37 36.60 0.01 0.01 0.01 27 .75 0.45 34 .85 0.01 0 99.75 31 37.34 0.01 0.01 0.02 27.19 0.45 34.58 0.02 0 99.64 26 37. 19 0.01 0.01 0.01 27.46 0.49 35.10 0.01 0 100.3 Orthopyroxene 17 54.27 0.19 0.16 0.12 16.41 0.49 26.74 0.89 0.01 99.31 2 54.37 0.22 0.18 0.09 16.54 0.44 26.85 0.90 0.02 99.62 9 54.39 0.19 0.18 0.10 16.38 0.47 26.89 0.88 0.01 99.50 8 54.34 0.18 0.18 0.10 16.56 0.45 27.03 0.84 0.01 99.69 5 54.51 0.19 0.17 0.21 16.21 0.44 26.85 0.91 0.01 99.51 9 54.63 0.19 0.17 0.10 16.23 0.48 27.26 0.88 0.02 99.98 Each row is an average of analyses on a separate day. TABLE 3. Average mineral compositions for Antarctic LL chondrites. Si02 TiO, AI,O) Crp) FeO MnO MgO CaO Nap Total Olivine 37.22 26.36 0.44 35 .54 0.04 99.72 Opx 54.50 0.18 0.21 0.16 15 .89 0.44 27.19 0.96 0.02 99.57 Cpx 53 .15 0.41 0.67 0.84 6.28 0.20 16.40 21.01 0.53 99.52 Spinel 0.10 2.98 6.10 55.38 31.75 0.11 1.89 0.03 98.35 sitions (e.g., see Fig. I). In order to provide a complete test of the sulfates to indicate incipient aqueous alteration. Two important ob model of progressively increasing grade and redox state within the servations on the nature ofCP and CF IDPs are (I) their morpholo Antarctic LL chondrite suite, and perhaps resolve some ofthe appar gies, textures, and ultrafine-grained minerals and mineral assem ent inconsistencies, the preliminary classification as to metamorphic blages are unique and not found in any conventional meteorite [I] type will be reassessed, and the modal amounts ofsilicate, metal, and and (2) their infrared spectra closely resemble those of the P and D troilite will be determined as a check on the compositional range outer-belt asteroids and comet nuclei [2]. Thus, they are associated within the group. with protoplanets containing the least-altered solar nebula dusts that References: [I] Fredriksson K. eta!. (1968) in Origin andDis are not sampled by conventional meteorites. Chondritic lOPs from tribution ofthe Elements (L. H. Ahrens, ed.), pp. 457-466, Perga short-period comets may include vestiges ofpresolar and interstellar mon. [2] Heyse J. V. (1978) EPSL, 40, 365-38\. [3] McCoy T. J. dusts among their constituents. Anhydrous IDPs are classified as et at. (1991) GCA, 55,601-619. [4] McSween H. Y. and Patchen olivine- and pyroxene-rich CP, whereas CF IDPs and hydrated IDPs A. D. (1989) Meteoritics, 24, 219-226. [5] McSween H. Y. and are classified as serpentine- and smectite-rich IDPs. These layer Labotka T. C. (1993) GCA, 57, 1105-1114. silicates are secondary phases due to postaccretion aqueous alter ation, and at least part of this IDP classification scheme relies on secondary mineral properties. ALTERATION OF PRESOLAR DUST BASED ON TRANS The grain sizes of Mg,Fe olivines and Mg,Fe pyroxenes in the MISSION ELECTRON MICROSCOPE/ANALYTICAL smallest recognizable textural units in CP and CF IDPs range from ELECTRON MICROSCOPE STUDIES OF CHONDRITIC 2 nm to -50 nm in diameter, occasionally up to -500 nm. All IDPs INTERPLANETARY DUST PARTICLES AND NONEQUI enter the Earth's atmosphere at kilometer-per-second velocities and LIBRIUM SIMULATION EXPERIMENTS. F. J. M. Riet experience a brief (5-1 5 s) period of flash heating (up to-IOOO°C). meijer, F. Guofei, and J. M. Karner, Department of Earth and Plane Based on nucleation and growth theory alone, a typical thermal tary Sciences, University ofNew Mexico, Albuquerque NM 87131, profile ofthis event will be conducive to form these Mg,Fe silicates USA. in an amorphous precursor. There is no proof that these ultrafine olivines and pyroxenes, or a fraction ofthem, formed in this manner. Chondritic interplanetary dust particles (IDPs) collected in the Their formation does not require a sustained thermal regime. It is lower stratosphere include porous (CP) to less porous chondritic possible that they are also secondary minerals. filled (CF) aggregates and compact hydrated chondritic IDPs. Ag Observations: CP and CF IDPs are typically made up of sub gregate lOPs are typically dominated by anhydrous minerals, but circular units between -100 nm and -500 nm in diameter. The first they may contain small amounts of layer silicates, carbonates, and type of unit, "granular units (GUs)," consists of ultrafine-grained 52 Workshop on Modification of Chondritic Materials Mg,Fe olivines, Mg,Fe pyroxenes, and Fe,Ni sulfides in an amor annealing of Mg,Fe,SiO and MgSiO smokes and the hydration of phous C and hydrocarbon matrix [3). To these were added C-free MgSiO smokes. "polyphase units (PUs)" of amorphous to holocrystalline, Ca,AI The condensation study showed that the compositions of the bearing ferro-magnesiosilica materials. They include (I) coarse individual smoke grains coincide with metastable eutectic points in grained PUs that consist of Mg,Fe olivine and Mg,Fe pyroxene the binary equilibrium phase diagrams. The metastable eutectic points (En,Fo = 0.75-1.0), Ca-bearing aluminosilica materials, and an in the MgO-Si02 system match serpentine and smectite dehy Fe,Ni sulfide [4) and (2) ultrafine-grained PUs consisting ofMg,Fe droxylates. Eutectic points in the FeO-Fe20rSi02 system at Fe/ olivines and Mg,Fe pyroxenes, plus Fe,Ni sulfides, Fe,Ni metals, or (Fe + Si) (el. wt%) = 0.4 and 0.15 do not match Fe silicates. During Fe oxides in an amorphous ferro-magnesiosilica matrix. The bulk thermal annealing in vacuo fayalite and ferrosilite (-20 nm in diam compositions range from Mg/(Mg+ Fe)(el. wt%)= 0.3 toO.75 [4,5). eter) formed after 8 hr at 725°C in amorphous ferromagnesiosilica They were mislabeled "tar balls" [6), but are now known as GEMS material. After 167 hr these grains were 100-200 nm. After 167 hr [7) . They occur as S-bearing and S-free units. Sulfur may be lost Mg vapor reacted with Fe oxide to magnesioferrite (MgFe204) with during atmospheric entry heating [5). Concentric circular 1.2-nm local periclase condensation [II). Forsterite + tridymite had reacted smectite lattice fringes may occur in ultrafine-grained PUs in addi to thermodynamically stable enstatite after 30 hr annealing. In the tion to anhydrous minerals [5). A third unit consists oflow-atomic initial stages ofMgSiO smoke hydration, discrete amorphous silica number elements only. Although sheets of carbonaceous materials globules formed in the smokes. Only regions that evolved stoichio occur in CP and CF IDPs [8), there is still little evidence for discrete metric Mg/Si ratios showed kerolite and/or saponite protophyllo carbonaceous units that may fuse more readily than other units, silicates and associated Mg(OHh [12]. including GEMS. Discussion: The bulk compositions of ultrafine-grained units The chondritic IDPs are mixtures ofthese units plus micrometer plot along the serpentine dehydroxylate line. Its termination on the sized silicates and Fe,Ni sulfides [10). None ofthe units is chondritic Mg-Si join of the ternary diagram is constrained by vapor phase for all major elements. In a diagram Mg-Fe-Si (el. wt%) C-free units · condensation. Since Fe during FeSiO condensation was oxidized, define two trends: (I) ultrafine-grained units along the Mg,Fe ser we found no metastable eutectic points matching the Fe serpentine pentine dehydroxylate, (Mg,F ehSiz07, line, and (2) coarse-grained dehydroxylate composition (assuming it is allowed in the equilib units along the Mg-rich part of a smectite dehydroxylate line, rium phase diagram). Fayalite and ferrosilite and intermediate phases (Mg,Fe)6Sig022' Two models to explain the petrological properties readily formed during thermal annealing in vacuo. This result sup ofC-free units are (I) preaccretion irradiation and amorphization of ports that as long as the Fe oxidation remains in the FeO stability silicates and sulfides [7) and (2) closed-system crystallization and field, Mg3Si20 7condensate may evolve to its Fe end member within phase separation within isolated amorphous precursors [4,5). The the limits ofthe equilibrium phase diagram, i.e., Mg/(Mg+ Fe) ratio presence oftypically nonchondritic amorphous materials as precur <0.85, which matches the highest ratios for pyroxenes and olivines sors to layer silicates, (rare) feldspars, and plagioclase that do not in GEMS. The 0 fugacities ofthe environment wherein they evolved occur as discrete subspherical units in chondritic IDPs was already were characterized by wustite. Formation ofeither olivine or pyrox recognized [cf. 3]. These amorphous materials could be fragments of ene will be a function of heating rate [13]. Once started, their forma coarse-grained PUs. tion is linked to local supersaturation and formation of an Fe,Ni The analytical and transmission electron microscope (AEMlTEM) phase. data revealed two critical properties of the chondritic IDPs: (I) the The bulk compositions ofcoarse-grained PUs plot on lines con common presence of amorphous materials with discrete Mg-Fe-Si necting the metastable eutectic points on the Fe-Si join and the ratios and Ca-bearing aluminosilica materials and (2) the ultrafine metastable smectite dehydoxylate eutectic on the Mg-Si join. This size oftheir constituent minerals. Both properties cause a high level relationship supports that they condensed at more oxidizing condi offree energy that will be available during alteration. Our simulation tions than GEMS. The subsequent crystallization and phase separa studies constrain these properties assuming a critical role of non tion into Fe,Mg silicates and amorphous Ca-bearing aluminosilica equilibrium vapor phase condensation in the formation of amor materials proceeded in the FeO stability field. phous materials. We note that amorphous silica and tridymite were The mineralogy of coarse-grained units is presented by two ge prominent phases in the early stages ofthermal annealing and hydra neric reactions tion and that micrometer-sized silica grains occur in at least one CP IDP [3]. Mg3Fe3Sig0 2i +AI,Ca)amorph. = Experiments: We analyzed ultrathin sections of analog sam 2 (MgI,sFel.SSi207)amorph. + 4 Si02(+AI,Ca)amorph. ples in the same manner as the IDPs. These analogs included vapor phase condensates (smokes) in the system MgO-Fe20rSi02 [10]. and Binary smoke samples were produced in the Flow Condensation Apparatus at 500°C in an H2 atmosphere at a pressure of -80 torr. Silicon and 0 are introduced as SiH4 and O2, The starting materials for Fe silicates are liquid Fe(CO)s entrained in H gas fed through the When the original composition was more Fe rich, the sulfides asso liquid before mixing with SiH4 and excess H2 in a mixing bulb. This ciated with these units formed by sulfidation according to gas then flowed to an alumina furnace tube. Magnesium vapor is produced by heating solid Mg in a graphite crucible in the furnace 1.5 Mg2Fe4Sig022(+AI,Ca)amorph. + 3 H2S = tube. The mixed gases flow from the furnace (where some conden Mg3Fe3Sig022(+AI,Ca)amorph. + sation occurs) to the cooler collection chamber wherein the remain 4 Si02(+AI,Ca)amorph. + 3 FeS + 3 H20 ing vapor condenses. Smokes are scraped from a collector plate located near the gas outlet ofthe appparatus. We also studied thermal followed by either one or both ofthe above reactions. The units show LPl Technical Report 97-02, Part I 53 no evidence (yet) for hydration, but H20 produced in this fashion and upsetthe interpretation of these objects as all being fragments of could be used to form kaolinite, e.g. F e-Ni cores. These observations led Rivkin [3] to split the "wet M" asteroids off to form a new asteroid class, the W class. The compo sition of the members of this class is as yet unknown, but may be partly similar to hydrated salts found in the veins of carbonaceous Similar reactions describe the mineralogical properties in GEMs that chondrites. Similarly, observations of hydrated E-class asteroids at may contain saponite. 3 j.1m by [3] call into question the association between this asteroid Conclusions: The C-free units are amorphous condensates class and the igneous, anhydrous aubrite meteorites [4]. that underwent thermal annealing in a closed system, but its timing Further studies ofM asteroids undertaken in 1995 and 1996 give is not constrained by the observations on IDPs. Sulfide formation a total of26 M-class asteroids observed. These data show a distinct requires an open system involving H2S in the solar nebula and also size dependence with regard to the hydration state ofM-class aster causes incipient aqueous alteration. The data do not invalidate a oids. 16 Psyche, which is anhydrous, is the only M-c1ass asteroid model ofpreaccretion irradiation and amorphization, but this model surveyed that is >200 km. In the 65-200-krn-diameter range, 12 does not account for the origin of preexisting silicates and sulfides. asteroids have been observed: 8 hydrated, 3 anhydrous, and I The amorphous smoke grains that formed directly as solids from uncertain. Of the 13 asteroids surveyed in the 0-65-km-diameter the vapor may be regarded as quenched liquids with metastable range, 2 are hydrated, 10 are anhydrous, and I is uncertain. eutectic points probably at ~5OO°C. Subsequent thermal annealing Although the E asteroids are much less common than M aster and hydration at comparable temperatures promoted phase relations oids, and consequently only six ofthese elusive asteroids have been at metastable extensions of eqUilibrium phase boundaries. Under observed, a similar trend is recognizable. Four of the six E-class highly kinetically controlled conditions alternative metastable be havior is to be expected. Thus, alteration products in IDPs will be chaotic, largely unpredictable assemblages until the free energy has dropped to levels commensurate with their local environments. 2 . 0 1.9 ... 22 Kaillope Acknowledgments: Samples were provided by J. Nuth and 1.B "V 16 Psyche his team at NASA Goddard Space Flight Center. This work was 1.7 1 .6 1.5 supported by NASA Grants NAGW-3626 and NAGW-3646. ~ References: [I] Mackinnon I. D. R. and Rietmeijer F. J. M. 1 . 4 ~ 1 . 3 I ~I lIB (1987) Rev. Geophys., 25, 1527. [2] Bradley 1. P. et al. (1992) ~ 1.2 3£ :I:::£ 1 . 1 Astrophys. J. , 394, 643. [3] Rietmeijer F. 1. M. (1992) Trends ~ ' . 0 ~*=--= 0.9 - Minerai., 1, 23. [4] Rietmeijer F. J. M. (1997) LPS XXVIlI, 1173 . 0.8 [5] Rietmeijer F. J. M. (1996) LPS XXVII, 1073. [6] Bradley J. P. 0.7 (1988) GCA , 52, 889. [7] Bradley J. P. (1994) Science, 265, 925. 0 .6 0.5O .':::2-;:0;":. 5;--:;::"0.'";;8---='~. ,:;---;-,.'":4---=,'"" .7-:-:;::"2'":.0:--:2:". 3-:--::2'":.6:--:2::"'".:::-9-:::3'":.2:--:3:"./;=--' [8] Thomas K. L. et al. (1993) GCA , 5 7, 1551. [9] Rietmeijer 'W'avclcDJrlh' (J.,un) F. J. M. (1997) LPS XXVIlI, 1169. [10] Kamer J. M. (1997) M.Sc. thesis, Univ. of New Mexico. [II] Fu G. and Rietmeijer Fig. 1. 0.3-3.5-llm spectra of Tholen M-class asteroids, 16 Psyche and F. J. M. (1994) LPSXXV, 493. [12] RietmeijerF. J. M. (1996) LPS 21 Lutetia, normalized to 0.55 !lm. Although these are the two largest XXVI/, 1069. [13] Rietmeijer F. J. M. (1996) Meteoritics & Planet. asteroids classified as M by [7] , they clearly have quite different hydration Sci., 31, A114. states. The 0.3-2.5-llm data for these asteroids arc from [8,9], and the 1.25-3.5-llm data for the asteroids a(e from references as per Table I. CORRELA TION OF WATER OF HYDRATION WITH DI AMETER IN THOLEN E-CLASS AND M-CLASS ASTER OIDS. A. S. Rivkin I, D. T. Brittl, L. A. Lebofskyl, E. S. HowelJ2, and B. E. Clark3, IDepartment of Planetary Sciences, University of 'If 'If ~j;!1;t19l ~ i'"'l Arizona, Tucson AZ 85721-0092, USA ([email protected]), 2Department of Geology, University of Puerto Rico, Mayagiiez PR 00681 -5000, USA, 3Astronomy Department, Cornell University, ... JtHtIo }Mol Ithaca NY 14853-6801 , USA. ~f-*-j l<+ .. Annydrou* M-oles~_ ~ «old. Water and the OH radical, both in free and bound forms, have "V Hyck"a1ed M--claao (W I...) A.tef'Oid& o trIA aat.roklo. vnc.rt&ln ydraUon .tat. strong absorption features near 3 j.1m, observed on many asteroids qjl~ since the late 19705 [1 ,2]. A knowledge of which asteroids are hydrated and where hydrated asteroids are located gives important 0 . 0 30.0 60.0 90.0 '20.0 '50.0'80.02'0.0240.0270.0300.0 Diameter . knl insights into the conditions prevailing near the beginning of solar system history. Fig. 2. Comparison of sizes of any hydrous M-class and hydrated M (yV In 1988, Jones et al. discovered two M-class asteroids that had class) asteroids. Vertical lines are drawn at 65 km and 200 km. Note that strong absorption features at 3 j.1m. A systematic survey ofM aster while most of the W-class asteroids are larger than 65 Ian in diameter, oids was started in 1991, with the results through 1993 summarized most M asteroids are smaller. This may be due to collisional properties in [3]. These results confirmed the existence of "wet M" asteroids, or a primordial size distribution. The sizes are derived from IRAS [10). 54 Workshop on Modification of Chondritic Materials TABLE I. Hydration states ofE and M asteroids. Calcium-aluminum-rich inclusions (CAls) commonly contain a distinctive suite of secondary minerals. The chemical and isotopic Asteroid Hydrated? Class Reference compositions ofthese minerals can be used to constrain the site and 16 Psyche No M [3,5,6] timing of the alteration event. The style of alteration in CAls is 21 Lutetia Yes W [6] strongly dependent on the meteorite group in which they are found. 22 Kalliope Yes W [6] CV Meteorites: CAls from the oxidized subgroup (e.g., 55 Pandora No? MW [3 ,5,6] Allende) show extensive signs of secondary alkali and Fe enrich 75 Eurydike No M [3] ment. The fine-grained secondary minerals (typica1\y < I 0-20 mm) 77 Frigga Yes W [6] include nepheline, soda lite, montice1\ite, hedenbergite, andradite, Yes W [3,5] 92 Undina and grossular; these typically embay primary minerals and filJ cross 110 Lydia Yes W [6] cutting veins within the CAIs. Some euhedral whiskers ofwolJasto 125 Liberatrix No M [6] 129 Antigone Yes W [6] nite and nepheline are located within cavities. In addition, multilay 135 Hertha Yes W [6] ered Wark-Loveringrim sequences on CAls clearly postdate the CAl 136 Austria Yes W [6] interior, and in that sense can be considered secondary. Fine-grained 161 Athor No M [6] inclusions are typica1\y more altered than coarser-grained 184 Dejopeja No M [6] ones: alteration in these inclusions consists of feldspathoid layers 20 I Penelope Yes W [3] surrounding primary spinel. The temperature ofmelilite+ anorthite 21 6 K 1eopatra No M [3] breakdown to grossular + montice1\ite in type-B Al\ende CAls has 337 Devosa No M [6] been estimated to be 668°C [I]. Hutcheon and Newton argued that M 359 Georgia No [6] the temperature must have remained around this value for a "pro 369 Aeria Yes? WM [6] longed period" to a1\ow fonnation oflarge grossular grains, but the 497 Iva No M [3] 572 Rebekka No M [3] timing of the high-temperature alteration event was probably 771 Libera Yes W [6] <-100,000 yr, otherwise Mg diffusion would ensure the CAl anor 785 Zwetana No M [3,6] thite no longer retains a 26Mg excess [2]. 796 Sarita No M [3] The location ofthe alkali-Fe alteration has been widely debated. 857 Glassenappia No M [6] Most workers believe the alteration took place in a nebular setting. 44 Nysa Yes E [3,6] The sequence of alteration is compatible with equilibration with a 64 Angelina Yes E [3 ,6] cooling, oxidized solar nebula gas [3]. Wark [4] argued for pre 214 Aschera Yes E [6] accretionary alteration because ofthe presence ofalkali-rich halos in [6] 317 Roxane Yes E the meteorite matrix surrounding some CAls. Sodium mapping of 620 Drakonia No E [3] Allende CAls shows that the Na is enriched in accretionary rims, 2035 Stearns No E [6] suggesting CAls became alkali rich prior to incorporation in the We follow [3] in splitting hydrated M asteroids to the W class. parent body. Veins cross cutting CAls typica1\y do not extend into the meteorite matrix, indicating they did not fonn in situ. Euhedral wollastonite whiskers, nepheline needles, and grossular in CAl cavi asteroids are observed to have a 3-f.lm water-of-hydration feature, ties are indicative of condensation from a vapor, and these grains while the other two are anhydrous. The two anhydrous E asteroids probably fonned in the nebula. An alternative viewpoint, champi (620 Drakonia and 2035 Steams) are the two smallest asteroids in the oned by Krot et aI. , argues that the alteration of CV CAls can be group observed, while the hydrated E asteroids are the largest in the explained by a parent-body process of alteration by alkaline-rich spectral class. fluids followed by dehydration [5]. This process is postulated to We wil\ present spectra of these asteroids and discuss the impli have affected the more oxidized CV meteorites, such as Allende, cations of this size dependence with regard to their possible origin more than the other CVs, a conclusion also reached by some other and composition and how this relates to aqueous alteration and studies [e.g., 6]. thenna] history. CO Meteorites: CAls in C03 chondrites have experienced References: [I] Lebofsky (1978) Mon . Not. R. Astron. Soc., considerable secondary alteration, both before and after accretion 182, 17. [2] Feierberg et al. (1985) Icarus, 63, 183 . [3] Rivkin et al. [7,8]. The presence of altered CAls in unmetamorphosed C03s (\995) Icarus, 117,90. [4] Bell et al. (1989) in Asteroids II. Univ. indicates some events occurred in the nebula: fonnation ofWark of Arizon, Tucson. [5] Jones et al. (\ 990) Icarus, 88, 172. [6] Riv Lovering rims, melilite and anorthite breakdown, and Fe enrichment kin et aI., this volume. [7] Tholen (1989) in Asteroids II, Univ. of spinels in hibonite-rich inclusions. In contrast, correlations be ofArizona, Tucson. [8] Ze1\ner etal. (1985) Icarus, 61, 355. [9] Bell tween petrologic type of the host meteorite with Fe content and et al. (1988) LPS XVIIlI, 57. [10] Tedesco et al. (1989) in Aster melilite breakdown in type A and spinel-pyroxene CAls suggest oids II. Univ. of Arizona, Tucson. some alteration occurred during parent body metamorphism [8]. Hibonite seems to be unaffected by the metamorphism experienced by C03s. ALTERATION OF CALCIUM-ALUMINUM-RICH INCLU CM Meteorites: CAls in CM chondrites have suffered ubiqui SIONS: TIMES AND PLACES. S. S. RusselJ and G. J. Mac tous aqueous alteration. The CAl primary mineralogy has been Pherson, Department of Mineral Sciences, MRC NHB-119, U.S. altered to phy1\osilicates (Fe serpentines and Mg serpentines) and Museum of Natural History, Smithsonian Institution, Washington tochilinite, calcite, and calcium sulphate. Secondary minerals typi DC 20560, USA ([email protected]). ca1\y occur in a layer immediately beneath the rim sequence. Some LPI Technical Report 97-02, Part J 55 CM CAls have also suffered fragmentation and recrystallization. It 1913-1935. [10] MacPherson and Davis (1994) GCA, 58, 5599 is not clear what the phyllosilicate is replacing: anorthite is a possi 5625. [II] Weber and Bischoff (1997) Chern. Erde, 57, 1-24. bility. Greenwood et al. [9] suggest that nebula processes' caused [12] Weisburg etal. (1993) GCA, 57,1567-1586. [13] Kimuraetal. fragmentation ofCAls, whereas aqueous alteration took place over (1993) GCA, 57, 2329-2360. [14] Weber et al. (1995) LPS XXVI, a protracted period of time on the parent body. In contrast, [10] 1475-1476. [15] Russell et al. (1997) LPS XXVllI, 1209-1210. argued that the CAls were not altered in the environment in which [16] Swindle et al. (1998) GCA, 52,2215-2227. [17] Davis et al. they are now found, and many are too fragile to have been moved to (1994) LPS xxv, 315-316. [18] Podosek et al. (1991) GCA, 55, their current location by recycling in the regolith, so they favored 1083-1110. [19] Brigham et al. (1986) LPS XVI/, 85-86. formation of hydrous secondary minerals in a nebula environment. CR Meteorites: CAls in the CR chondrite Acfer 059 show no evidence of alteration [II], whereas inclusions in Renazzo and AI EVIDENCE FROM THE BOVEDY (L3) CHONDRITE FOR Rais contain some secondary calcite [12]. IMPACT-GENERATED CHONDRULES. I. S. Sanders, CH Meteorites: Some CAls in the CH chondrite ALH 85085 Department of Geology, Trinity College, Dublin 2, Ireland. show evidence ofrecrystallization due to reheating [\3]. In contrast, inclusions from PCA 91467 and Acfer 182 appear unaltered [14]. Among the more unusual objects in the Irish meteorite Bovedy UnequilibratedOrdinary Chondrites: CAls in ordinary chon are two chondrules whose features suggest that they are the products drites are rare. Unequilibrated-ordinary-chondrite CAls are often of shock melting of preexisting rock. One of them is a millimeter rimmed, and secondary feld-spathoids are occasionally present. In sized, composite object in which one half has the appearance of a one Semarkona CAl, melilite has been partially replaced by soda lite cryptocrystalline pyroxenitic chondrule and the other halfresembles [15]. a coarse-grained, lithic clast. The "chondrule" part has a distinctive Conclusions: Times. While most CAls are believed to have blue-brown color, with radiating crystallites and a semicircular ex formed at around the same time, their alteration was an ongoing ternal outline. It contains a number of inclusions, up to 50!lm long, process that took place over several million years. Iodine-xenon ofirregularly shaped, birefringent, polycrystalline olivine. The "lithic" dating suggests that the alteration took place up to > I 0 m.y. after part comprises large grains ofmosaicized olivine and low-Ca pyrox initial CAl formation [16]. Aluminum-magnesium studies ofgrossu ene. The two halves are interpreted as forming a single object because lar in CV CAls also indicate formation >2.4 m.y. after CAl produc the boundary between them is intricately interdigitated. Delicate tion [17], and a Al-Mg analysis of a recrystallized CAl from CH subparallel protrusions from the olivine resemble residual fingers of chondrite indicates a heating event >2 m.y. after CAl production partially digested silicate in the blue-brown cryptocrystalline phase. [13]. Chemical exchange between anorthite and melilite in type B The entire object is interpreted as a fragment of impact ejecta in inclusions appears to have occurred >2-3 m.y. after CAl formation which a small volume ofshock-induced melt remained attached to a [18]. Similarly, soda lite in fine-grained CV inclusions is postulated fragment of its precursor rock. to have formed after 26AI decay, i.e., several million years after CAl The second chondrule is also blue-brown in color, pyroxenitic in formation [19]. In contrast, sodalite in a Semarkona (LL3.0) inclu composition and cryptocrystalline. It is a 2-mm-long, arrow-head sion apparently fonned very quickly after CAl formation [15]. shaped object whose scalloped margins testify to substantial squeez Places. Many CAls are pristine, but some underwent several ing in the parent body and indentation by neighboring chondrules. It heating events. The presence of altered plus pristine CAls close contains conspicuous, colorless, weakly birefringent inclusions of together in some meteorites (e.g., CMs) suggests that some alteration pure silica. Some of the inclusions are elongate, about 30 !lm wide, occurred before they reached their current site in the parent body and gently curving, while others are globular with radial protrusions, (although this may reflect postaccretionary brecciation). Many fea resembling the shape ofan amoeba. This chondrule is figured in [I], tures of alteration appear to have occurred in the nebula. Wark where it was incorrectly described as being glassy throughout. The Lovering rims predate accretion into the present parent bodies. Some chondrule is interpreted as a droplet of shock melt derived from a primary minerals exchanged with a nebula gas, and some secondary silica pyroxenite parent rock. The elongate inclusions suggest inher minerals condensed from a vapor. Sodalite in Semarkona [15] prob itance from crystals oftridymite in the precursor rock. The amoeba ably formed in the nebula, since 26AI dating suggests it formed be shaped inclusions are interpreted as evidence of the coexistence of fore the accretion ofthe asteroids. In contrast, the long time span of immiscible silica melt and pyroxenitic melt (consistent with phase alteration suggested by I-Xe dating for Allende CAls has been used chemistry in the silica-enstatite system). As it happens, a potential to argue in favor of alteration in a parent body [16]. In addition to parent rock for the chondrule, an igneous-textured silica pyroxenite, nebula processes, metamorphism in parent bodies tended to equili has been found as a clast elsewhere in Bovedy [2]. This clast has a brate CAls with their host rock. Aqueous processing in some mete fractionated composition suggesting that it crystallized on a plan orites may have affected CAls in parent bodies. The location of the etary body. This indirect evidence for early planetary bodies is clearly event responsible for incorporation ofalkalis into CAls, however, is in keeping with the idea of collision and impact melting. still highly contentious. The recognition ofimpact-melt chondrules in Bovedy is ofinter References: [I] Hutcheon and Newton (1981) LPSXII, 491 est because it augments a growing body of evidence that some 493. [2] LaTourette and Wasserburg (1997) LPS XXVlll. 781 chondrules, at least, are not the products of flash melting of dust 782. [3] Hashimoto and Grossman (1987) GCA, 51, 1685-1704. clumps in the solar nebula. For example, Sears [3] points to the [4] Wark (1981)LPS XlI, 1145-1147. [5] Krot et al. (I 995) Meteor presence ofimpact-produced chondrules on the Earth and the Moon. itics, 30, 748-775. [6] McSween (1977) GCA, 41, 1777-1790. Kitamura and Tsuchiyama [4] show that relict grains in chondrules [7] Greenwood et al. (1992) Meteoritics, 27,229. [8] Russell et al. are far more intensely shocked than their coexisting phenocrysts, and (1997) GCA, submitted. [9] Greenwood et al. (1994) GCA, 58, they propose that the relics are survivors of impact melting. Hutchi 56 Workshop on Modification of Chondritic Materials son [5] has long advocated impact as a major force in chondrule chondrules, matrixes, and CAIs [4,5]. Allende, which is relatively production. Rubin [6], while not specifically addressing chondrule anhydrous, appears to have experienced extensive asteroidal alter production, reports many examples ofimpact melting, some ofwhich ation and dehydration [4,5]. In C03 chondrites, type I and II chon have since been shown to date back to the very early solar system [7]. drules have properties consistent with simple diffusive exchange If chondrules were produced by impact, then why are they so with inatrixes as in type 3.0-3.6 ordinary chondrites [6]. However, abundant in chondrites yet so rare on the Moon? A simple but plagioclase-rich chondrules in Kainsaz and Lance experienced meta unhelpful answer is that most chondrules must have been produced somatic alteration [7]. The abundance of secondary nepheline in other than by impact. A more interesting speculation is that the CAIs in C03s correlates with petrologic subtype, also suggesting efficiency of impact-melt production increases exponentially with that alteration and metamorphism are closely linked [8] . Although temperature, such that a hot target (e.g., an early planetary body CK4-6 chondrites are anhydrous and have chondrules that appear to heated by short-lived radioactivity) would melt much more copi have suffered only simple metamorphic equilibration, we find that ously than a cold one, such as the Moon. Furthermore, if the hot chondrite matrixes have coarse fayalite grains, including some planetary body were internally molten or partially molten at the time Allende-like laths, that appear to reflect alteration that may have ofimpact, the scope for abundant chondrule production would seem caused S depletion. limitless. Type 3 ordinary chondrites contain small amounts ofphyllosilicates References: [1] Hill H. G. M. (1993) Meteoritics, 28, 363. [9], carbides, magnetite [10], and metasomatized chondrules [II], [2] Ruzicka A. et al. (1995) Meteoritics, 30, 57-70. [3] Sears D. et which appear to reflect asteroidal alteration. Matrixes in Tieschitz, al. (\ 995) Meteoritics, 30, 577. [4] Kitamura M. and Tsuchiyama Bishunpur, and Chainpur have "dendritic" Fe-rich olivines [12] with A. (1996) in Chondroles and the Protoplanetary Disk (R. H. Hew textures suggestive offluid-assisted metamorphism. Type3-6 ordi ins etal., eds.), pp. 319-326, Cambridge Univ. [5] Hutchison (1996) nary chondrites have variations in FeO concentrations in olivine and in Chondroles and the Protoplanetary Disk (R. H. Hewins et aI., pyroxene, nonnative olivine/pyroxene ratios, and metal abundances eds.), pp. 311-318, Cambridge Univ. [6] Rubin A. E. (1985) Rev. consistent with fluid-assisted oxidation during metamorphism [13]. Geophys., 23, 277-300. [7] McCoy T. J. et al. (\ 995) GCA, 59, Chronologie constraints indicate that metamorphism and alter 161-175. ation both began less than 20 m.y. afterCAI formation [14]. We infer that all chondri tic asteroids experienced some degree of fluid-as sisted metamorphism that was powered by a short-lived heat source, ASTEROIDAL MODIFICATION OF C AND 0 CHON probably 26AI, possibly electrical induction, but not impact heating DRITES: MYTHS AND MODELS. E. R. D. Scott, A. N. [15]. Fluids were largely but not completely lost by 500°C because Krot, and L. B. Browning, Hawai'i Institute of Geophysics and pressures were much lower than on Earth. Planetology, School of Ocean and Earth Sciences and Technology, Chemical Fractionations Among Chondrites are all Nebular University ofHawai'i, Honolulu HI 96822, USA. in Origin: Since the composition of CI chondrites, which is the most heavily altered group of chondrites, matches that ofthe solar Accurate models for asteroidal metamorphism and alteration are photosphere and each chondrite group is held to be isochemical important for helping to distinguish products ofasteroidal and nebu (except for a few volatiles) it is commonly asserted that chemical lar alteration in chondrites, understanding geological processes on fractionations between chondrites must be nebular or terrestrial in chondritic asteroids, and relating chondrite groups to known aster origin. But there is much mineralogical and chemical evidence to oids. Here we review several ideas about alteration and metamor suggest that fluid-assisted metamorphism did change the local bulk phism that appear to be oversimplified or incorrect, in order to help chemical composition of chondrites. identify mineralogical and chemical features in chondrites that are CI chondrites show concentrations ofAI, Ca, Mg, and Ni that are truly products of solar nebular processes. more heterogeneous than those in, e.g., CO chondrites [16]. Alais haS Aqueous Alteration and Thermal Metamorphism are Sepa systematically lower Au and higher Sb concentrations than Orgueil rate Processes: McSween's necessary modification of the Van [16]. In CM chondrites, Na, K, halogens, and H may have been Schmus-Wood sequence for petrologic types [I] has helped to pro mobilized during alteration [17,18]. In CV3 chondrites, oxidized mote the idea that metamorphism and alteration are entirely separate and reduced subgroups have different Na, Mn, and Fe concentra processes that operated in different asteroids. Metamorphism to tions [4]. meteoriticists is the process whereby parent bodies of type 3-6 Compositions of chondritic components were also modified by chondrites were heated in the range 400o-IOOO°C, homogenizing fluid-assisted metamorphism. Chondrules and CAIs in CV3 chon and coarsening silicates by solid-state diffusion after fluids were drites suffered Fe-alkali-halogen metasomatism [4,5]. In LL3 chon largely removed [2]. Alteration is considered to be the process that drites, Semarkona, fine-grained chondrules were depleted in alkalies operated on carbonaceous asteroids at low temperatures (0°-200°C) by aqueous alteration [II]. under conditions where H20 was stable, producing hydrated miner The apparent absence ofchondrites that are enriched in volatiles als, salts, and certain other phases characteristic of CI and CM above CI levels has been used to argue against volatile loss in chondrites [3]. By this process, type-3-like material was converted asteroids as most volatiles would recondense in the cool outer parts to lower petrologic types [I]. Recent studies suggest that the two of the asteroid [19], but their absence may reflect low inherent processes may be part of the same asteroidal heating process. strength or resistance to impact lithification, as for martian meteor Studies oftype 3 chondrites suggest that they commonly experi ites. Volatiles may also have been lost explosively during metamor enced both metamorphism and alteration in asteroids. In CV3 chon phism [20]. drites, fluid-assisted growth of new phases in asteroids dominates Heterogeneous Alteration Features are Characteristic of over simple diffusive equilibration ofsilicates: magnetite, fayalite, Nebular, not Asteroidal, Processes: Even in relatively homoge Ca-rich pyroxenes, nepheline, sodalite, grossular, etc., fonned in neous materials, fluids escape by being focused into distinct con LPJ Technical Report 97-02, Part I 57 duits. The best evidence for the extreme heterogeneity ofasteroidal alteration occurred after aggregation of the present meteorite mate alteration is provided by the Bali CV3 chondrite. Unaltered regions rials, for example, recrystallization of the metal grains so that they with fresh chondrules and unaltered mesostases are found next to interdigitate silicates. However, in most cases it is unclear where the heavily altered veins in which chondrules are largely replaced by alteration occurred and often the discussion centers on whether the phyllosilicates [21]. The veins follow the foliation indicating aster locale was the "parent body" or the "nebula." If it appears that the oidal alteration [21]. Heterogeneous metamorphic and alteration alteration involved one ofthe reactants being in the gas phase, then fearures may result from impact brecciation after metamorphism. it is usually assumed that the locale was the nebula. We have been Thus chondrules that ha ve been partly metamorphosed or altered and exploring the idea that chondrules and chondrites fonned in the subsequently broken to reveal an unaltered surface are not evidence regoliths of asteroid-sized bodies made dynamic by the passage of for nebular alteration and metamorphism. gases from the interior [I]. Figure I describes our proposal as itmight A better understanding ofasteroidal processing should lead to an apply to LL chondrites, but a similar situation applies to the other improved system of classification that clearly identifies groups of chondrite classes, except perhaps the CI and CM chondrites where chondrites from the same asteroid and the nature and extent oftheir metal is absent and bulk Fe/Si values are solar. The interior is asteroidal modification. The present scheme is fine for types3-6 but assumed to be CI- or CM-like in its high volatile contents (10 type I and 2 mean only lvuna-like and Murchison-like. A one 20 wt% HzO). Radioactive or impact heating causes the release of parameter scheme is clearly inadequate for low-grade asteroidal these volatiles and the larger components are temporarily suspended metamorphism as chondrules and matrix minerals did not equilibrate by the aerodynamic drag of the upward-flowing gases. Smaller with the fluid below 600°C. The primary parameters used for classi particles like metal can be carried through the pore space to the fying chondrites into groups, and chemical and 0 isotopic composi surface, thereby creating a zone of material below the surface (the tions, are not uniform within groups, for reasons that are poorly "LL chondrite region" in Fig. I) in which there is an enrichment of understood. These problems are illustrated by the difficulty in as chondrules and depletion ofmetal. The degree ofseparation ofmetal, . signing a group or type to Acfer094, which contains primitive FeO and whether the metal particles rise to the surface or sink the bottom poor matrix material, and is probably the least altered chondrite yet of the regolith, is determined by the relative force of gravity and identified [22]. aerodynamic drag. The scenario we propose seems to explain the Acknowledgments: This work was partly supported by NASA size sorting ofcomponents, and the Fe/Si values characteristic ofthe grant NAGW-328I to K. Keil. major chondrite classes (qualitatively in all cases, quantitatively in References: [I] McSween H. Y. Jr. (1979) Rev. Geophys. some). It also provides a reasonable environment for the formation Space Phys., 17, 1069-1078. [2] McSween H. Y. Jr. et al. (1988) in of chondrules by impact [2]. Here we point out that a dynamic Meteorites and the Early Solar System (1. F. Kerridgeand M. S. Mat regolith also provides a "planetary" environment for producing a thews, eds.), pp. 102-113, Univ. of Arizona, Tucson. [3] Zolensky great many alterations requiring a gas phase. In this sense it has M. and McSween H. Y. Jr. (1988) in Meteorites and the Early elements of both nebula and planetary properties. Some points for Solar System (J. F. Kerridge and M. S. Matthews, eds.), pp.114 discussion: 143, Univ. ofArizona, Tucson. [4] Krot et al. (1995)Meteoritics, 30, I. Ifchondrule rims ("accretionary rims") were produced by the 748-775. [5] Krot et aI., this volume. [6] Scott E. R. D. and Jones recondensation ofvolatiles lost bychondrules [3,4], then the tempo R. H. (1990) GCA, 54, 2485-2502. [7] Jones R. H. and Hutcheon rary atmosphere would be a reservoir to retain volatiles long enough I. D. (1996) Meteoritics & Planet. Sci., 31, A67-A68. [8] Kojima for recondensation on chondrule surfaces without invoking unusu T. et al. (1995) Proc. NIPR Symp. Antarct. Meteorites, 8, 79-96. ally high gas and dust agglomerations in the nebula. [9] Hutchison R. H. et al. (1987) GCA, 51, 1875-1882. [10] Krot et 2. Formation of chondrules by impact into such a loosely con al. (1997) GCA, 61,219-237. [II] Grossman J. N. et al. (I 997) LPS solidated regolith would readily explain why chondrules are so XXVIII, 481-482. [12] Alexander C. M. Q'D. et al. (1989)EPSL, 95, highly oxidized, why many cooled in an atmosphere rich in Na and 187-207. [13] McSween H. Y. Ir. and Labotka T. C. (1993) GCA, 57, 1105-1114. [14] Endress et al. (1996) Nature, 379, 701-703. [15] Keil K. et al. (1997) Meteoritics & Planet. Sci., in press. [16] Kallemeyn G, W. and Wasson 1. T. (1981) GCA, 45, 1217 1230. [17] Wolfet al. (1980) GCA, 44,711-717. [18] Browning 1. and Bourcier W. (1996) Meteoritics & Planet. Sci., 31, A22-A23. [19] Larimer I. W. and Anders E. (1967) 31, 1239-1270. [20] 1. Wilson, private communication. [21] Keller 1. P. et aI . (1994) GCA, 24, 5589-5598. [22] Greshake A. (1997) GCA, 61,437-452. NEBULAR OR PARENT·BODY ALTERATION OF CHON· DRITIC MATERIAL: NEITHER OR BOTH? D. W. G. Sears and G. Akridge, Cosmochemistry Group, Department ofChemistry and Biochemistry, University ofArkansas, Fayetteville AR 72701 , USA ([email protected]). Most chondrite classes contain indications ofalteration since the fonnation oftheir solid components. In some cases it is clear that the Fig. 1. 58 Workshop on Modification of Chondritic Materials other volatiles, why cooling rates were so slow, and why chondrule The evolved gas analysis curves of Allende (CV3) are compa densities were so high [2]. rable to those of unweathered ordinary chondrites. However, the 3. The gas causing the regolith to be dynamic will be largely H20 multistage release of substantial amounts of CO2 from Allende and (with minor CO2), and thus a convenient O-bearing reservoir for of CO from all carbonaceous chondrites above -700°C have to be isotopic exchange [5]. related to the oxidation of C-containing phases like graphite, car 4. The dynamic regolith creates an environment in which a large bides, and diamonds. numberofliquid and gas phase reactions involving major, minor, and trace minerals occurred [e.g., 6,7]. 5. The time interval between formation offirst planetesimals and Ion current (A) chondrules, which is very long by nebula standards (several million years [9]), is explained as the interval between formation of parent body and time at which impacts have sufficient energy for chondrule formation [8]. Ifaccretion was oflong duration (several million years), then the regolith dust layer would also be an accretionary layer and classical nebular and parent-body processes would be occurring simulta neously. However, whether the dynamic regolith idea has any merit 2xl0" 0/ or not, it does illustrate a need to consider environments other than the simple "nebula" and "parent-body" scenarios commonly dis cussed. References: [I] Huang S. et al. (1996) JGR, JOl, 29373. [2] Sears D. W. G. et al. (1995) LPS XXVI, 1263-1264. [3] Sears 5xl0" D. W. G. et al. (1993) Meteoritics, 28, 669. [4] Hewins R. H. (1991) Proc. NIPR Symp. Antarct. Meteorites, 2, 200. [5] Clayton et al. (1983) in Chondrules and Their Origins (E. A. King, ed.), p. 37, 1x10" LPI, Houston. [7] Hutchison R. et al. (1989)GCA, 51, 1875. [8] Krot A. N. et al. (1997) GCA, 61, 219. [9] Podosek F. A. and Cassen 5xl0" P. (1994) Meteoritics, 29, 6. o 200 1200 1400 EVOLVED GAS ANALYSIS: A TECHNIQUE TO STUDY COSMIC ALTERAnON OF CHONDRITES? Th. Stelzner Fig. 1. Evolved gas analysis curves upon heating the strongly weathered and K. Heide, Institut fUr Geowissenschaften, Friedrich-Schiller H5 chondrite Acfer 212 under vacuum conditions. Universitiit, 07749 Jena, Germany. Evolved gas analysis has been used to compare the terrestrial Ion current (A) alteration of ordinary chondrites with the modification features in 4x10".,------, three carbonaceous chondrites (Orgueil, Murchison, and Allende). The interpretation of the evolved gas analysis curves can be complicated due to the often unknown and complex reaction mecha 2xl0" nisms or due to the lack of suitable reference substances. For ex ample, particle size or crystallinity ofFe-oxyhydroxides were found to be important factors affecting the release temperature ofHP [I]. Furthermore, sample-generated 0 can lead to redox reactions like the co+ oxidation of graphite or carbides (unpublished data). F ails ofordinary chondrites usually release only very small amounts of volatiles, whereas weathering leads to significant mineralogical changes reflected by the evolved gas analysis curves (Fig. I). From the carbonaceous chondrites Orgueil (CI) and Murchison 5x10" (CM) large amounts of volatiles are evolved (Fig. 2), which can be attributed to products probably formed by aqueous alteration on the parent body [e.g., 2,3]. The following features distinguish the evolved gas analysis curves 2><10" ofOrgueil and Murchison from those ofstrongly weathered ordinary chondrites: (I) Water release continues to higher temperatures and is probably related to the dehydroxylation of phyllosilicates present ~~~~~~-~~=-~~~-1~00~O-I~~~O~~14OO~ in the meteorites (e.g., saponite, serpentine); (2) the release of CO2 TemperabJre (Oc) and CO between -100°C and 700°C indicates that much higher amounts of carbonates were formed; (3) and Orgueil liberates rela Fig. 2. Evolved gas analysis curves upon heating Orgueil under vacuum tively high amounts of S02 due to the decomposition of sulfates. conditions. LPi Technical Report 97-02. Part J 59 Conclusions: There are distinct features in the gas release pro Arndt et al. [6,7J studied experimentally the question of when files of the investigated carbonaceous chondrites differing from the contamination occurred by exposing IDP analogs on a capture flag profiles ofweathered ordinary chondrites. Some of the peaks cannot to the stratosphere. Bromine was measured with PIXE before and simply be explained by the decomposition of alteration phases iden after flight. Bromine concentrations before exposure were below tified in these meteorites by means of electron microscopic tech detection limit, while after exposure significant Br signals were niques [i.e., 2,3]. A further investigation of the mineral phases and detected. The stratospheric air column passed by the collector even assemblages responsible for these effects should help to improve the quantitatively accounts for the Br contamination. understanding of the chemical and thermal history of carbonaceous Another source of contamination observed in our TOF-SIMS chondrites. study was surface-correlated Be resulting from SEM EDS analysis References: [I J Stelzner Th. and Heide K. (1996) Meteoritics performed on Be substrates. After such analyses, some particles tend & Planet. Sci., 31, 249-254. [2J Tomeoka K. and Buseck P. R. to stick to the substrate and carry Be after they were removed. (I 988)GCA. 52. 1627-1640. [3] Zolensky M. et al. (1993)GCA. 57. For other elements, no unambiguous evidence for or against con 3123-3148. tamination is available so far. Selection Effects: StratosphericIDPs are subject to selection processes. Thus, the collected particles only roughly represent the near-Earth dust population. The first and maybe most severe selec VOLATILE-ELEMENT ENRICHMENTS IN INTERPLAN tion is caused by the stratosphere itself, more precisely by atmo ETARY DUST DUE TO NEBULAR PROCESSES? T. spheric entry heating. Large and high-velocity particles do not sur Stephan, D. Rost, and E. K. Jessberger, Institut fur Planetologie, vive entry into the atmosphere and thus smaller and slower particles Wilhelm-K1emm-Strasse 10, D-48149 Miinster, Germany (stephan@ are overrepresented [8J. nwz.uni-muenster.de). Human selection also occurs, prevailing in most studies, in that mostly only "chondri tic-looking" (black and fluffy) particles are Introduction: Interplanetary dust particles (IDPs) potentially picked from the collector flags [9] and only particles preclassified allow us to investigate the most pristine solar system bodies, comets from SEM EDS analysis as "cosmic" are further investigated. The and asteroids. Therefore, they should be predestined to provide latter classification is mainly based on major-element composition. information on nebular processes. Many stratospheric IDPs are richer This strong bias might be the main reason for the observation that in volatile elements than CI chondrites (cf. compilation by Arndt et cosmic particles are usually chondritic. al. [I J) while the abundances of major elements usually are chon Analytical Techniqu es and Data Treatment: To measure trace dritic. Attempts to explain the enrichments range from postulating a element concentrations in IDPs very often approaches the limits of new type ofvolatile-rich chondritelike matter [2J to invoking atmos detection ofmost analytical techniques. Therefore, the average con pheric contamination processes [3]. Before far-reaching conclusions centrations of trace elements are prone to be overestimated since on nebular processes can be drawn, stratospheric processes, con element abundances below detection limits are typically not con tamination during capture and handling, and artifacts from various sidered. selection effects or from analytical techniques and even from numeri cal data treatment have to be excluded. Contamination: Afterthe probable importance ofstratospheric contamination processes was emphasized based on plausibility rea soning [3J, the first direct experimental evidence for contamina tion-at least for Br, the element with the highest enrichment, up to 1()3 xCI-were Br-salt nanocrystals attached to IDP W7029E5 [4] and a halogen-rich exterior rim ofIDP L2006G I found with time-of flight secondary ion mass spectrometry (TO F -SIMS) [5]. The distri butions ofsecondary halogen ions emitted from a section ofthis IDP upon sputtering with primary Ga+ ions (Fig. I) reveal an outer ring structure for F, CI, and also Br, though the latter image is disturbed by rather high background. Assuming a spherical particle, 20 ~m in diameter, and a continuous 1.5-~m-thick surface layer, this layer represents ",28 vol% of the IDP. This estimate is an upper limit but illustrates the influence that surface contamination might have on bulk chemistry. To test more directly for surface contamination, we analyzed the original surfaces ofstratospheric particles with TOF-SIMS. A major analytical problem results from residual silicone oil from IDP collec tion and handling that often cannot be completely removed, even not by extensive hexane rinsing. Nevertheless, we unequivocally de Fig. 1. Lateral distribution of negatively charged secondary F. CI, and tected F, CI, and Br on the very surfaces ofsix particles': one Fe,NiS Br ions respectively from IDP L2006G L The three elements are clearly rich IDP (U2071H9), four Al20 3spheres (rocket exhaust), and one correlated Field ofview is 30 x 30 Ilm2; high intensity is shown as black. Ca-rich particle of unknown origin. Extensive sputtering reduced Since the background of Sr is relatively high, we also show a smoothed and finally removed the halogens from the AIP3 surfaces, unam Br image (BrsnJ. In this section the halogens occur in a 1.5-llm-thick rim biguous proof of a contamination surface layer. surrounding the 20-l1m (diameter)-sized particle. 60 Workshop on Modification of Chondri tic Maten'als The calculation of averages is itself also a problem, The general chemically remarkably similar to CI chondrites they most probably question is: What does an average tell us if individual IDPs have represent an adequate sample ofprimitive solar system material and individual and different parent bodies? Certainly, one does this even the only available sample of comets. exercise in order to search for large-scale similarities. But, strictu References: [I] Arndt P. et al. (1996) Meteoritics & Planet. sensu, only the mean composition ofparticles that demonstrably Sci., 31, 817-833. [2] Flynn G. J. and Sutton S. R. (I 992) LPSXJ(JII, by whatever means - are related to each other provides infonnation 373-374. [3] Jessberger E. K. et al. (1992) EPSL, 112, 91-99. on the chemical composition oftheirprecursor. However, no criteria [4] Rietmeijer F. 1. M. (l 993) JGR, 98, E7409-E74 14. [5] Stephan to demonstrate a relationship have so far been developed. T. et al. (1994) LPS xxv, 1341-1342. [6] Arndt P. et al. (1996) Most trace-element abundances are only known as element ratios Meteoritics &Planet. Sci., 31, A8. [7] Arndt P., personal communi relative to a major element, Fe [I]. Low Fe can therefore feign high cation. [8] Love S. G. and Brownlee D. E. (1994) Meteoritics, 29, trace-element contents, and elements anti correlated with Fe are over 69-70. [9] Warren 1. L., personal communication. [10] Jessberger estimated. However, by discussing element groups as done by Arndt E. K, et al. (1988) Nature, 332, 691-695. [II] Stephan T. et al. et al. [I] this effect is minimized. (1994) EPSL, 128, 453-467. [12] Flynn G. J. et al. (1993) LPS Another question is: Which type of average, e.g" geometric or XXIV, 497-498. arithmetic, most reliably reproduces the original parent-body com position? Ifall particles had the same mass and Fe concentration, the arithmetic average would be the best choice. Ifabsolute atom num DO IODINE-XENON AGES TELL US ANYTHING ABOUT bers were available for all individual IDPs and ifall selection effects THE SITE OF SECONDARY ALTERAnON? T. D. Swindle, were neglected, then the summed atom numbers would represent the Lunar and Planetary Laboratory, University ofArizona, Tucson AZ parent-body composition [10]. However, in the doable case, i.e., with 85721-0092, USA. the existing limited dataset, the geometric averages are appropriate [I]. One also has to keep in mind that within individual IDPs the The first extinct radionuclide for which evidence was discovered trace-element distributions are largely variable and reflect chemical in meteorites was 1291, which has a 16-m.y. halflife and decays to inhomogeneity on a micrometer scale [I,ll]. 129Xe. Since the 129U 127I should vary with time because ofdecay, the Discussion: Since trace-element abundances are available for I-Xe system is potentially a sensitive chronometer. "I-Xe ages" (129I/ only 89 stratospheric particles, the reliability ofany conclusion from 1271 ratios) have been detennined in nearly 200 samples from more average compositions has to be closely scrutinized [ I). Ifnot all IDPs than 80 meteorites [I]. Although I-Xe ages were originally envi have the same parent body, averaging can be instrumental in detect sioned as "fonnation ages," the data never quite made sense because ing features common to all IDPs, like contamination, or to define the range in ages was larger than expected in the solar nebula, and groups of IDPs that reflect large-scale chemical differences of the there were few correlations with bulk properties. On the other hand, source regions. ifI-Xe ages are interpreted as secondary ages, not only do the "ages" Chlorine and Br enrichments have already been attributed to make sense, but they can tell us something about the timing and atmospheric contamination processes. Arsenic, Rb, and Zr were location of secondary processing. What they seem to tell us is that detected in only a few IDPs since their limits of detection are close much ofthe processing that the I-Xe ages record occurred on parent to the chondritic values. Thus, the significance of the enrichments bodies. remains questionable [I]. The other elements that are enriched, at There are several reasons to think that I-Xe ages are primarily least in chemically defined subgroups ofthe whole IDP set-P, Cu, dating secondary parent-body events on a parent body: Ga, Ge, Se, and Zn-show distributions over -lor 2 orders of I. The range in apparent ages is nearly 50 m.y. for chondrules magnitude that include the respective chondri tic values. Geometric from the unequilibrated ordinary chondrite Chainpur [2], and at least averages yield enrichment factors of about 2 for these elements that 10 m.y. for various objects from CV3 Allende [1,3,4]. Two dMk appear to be significant, possibly questionable for Ge. inclusions from Efremovka give apparent ages that differ by about Sulfur and Ca, on the other hand, are significantly depleted in 6 ± 2 m.y. (5). These an seem too long to be reflecting nebular almost all IDPs and only a few particles with enrichments have been timescales. found [I]. Sulfur depletion by atmospheric entry heating has been 2. In studies of whole-rock LL chondrites [6] and of individual proposed [12]. In CI chondrites, the best available analog to chon chondrules from Chainpur [2] and Tieschitz [7] (and, to a lesser dritic IDP parent bodies, Ca occurs as carbonate. Therefore, Ca extent, Semarkona [8]), the 1291/1271 at the time of I-Xe closure is depletion may result from two effects: First, during disruption of anticorrelated with the 129Xe/132Xe ratio, analogous to the evolution the parent bodies carbonate-rich grains are released that might be too with time ofSr or Nd isotopes. Not only does this imply that these large to survive atmospheric entry. Second, if individual carbonates objects evolved in a closed system, but we can estimate the I1Xe ratio survive they would probably be discarded as terrestrial contamina ofthat closed system, and it is roughly chondri tic, rather than solar. tion since their composition is far from CI-like. Similar selection 3. In Allende, the bulk of the I resides in sodalite [9], which is effects can produce apparent enrichments as well as depletions for clearly a secondary alteration product. Iodine-xenon studies of ob . other elements. jects from Allende have frequently shown highly variable ages within Conclusions: Since little is known about the actual host phase a single object [e.g., 3], almost always showing earlier apparent ages of most trace elements even in CI chondrites, far reaching conclu at higher extraction temperatures, as would be expected in a system sions on enrichments in IDPs cannot be drawn. Selection effects where a series ofevents (or even a continuous cooling) caused the 1 during break-up ofthe parent body, capture by the Earth and by the Xe system in different phases to be set (or reset) at different times. collector, as well as particle picking in the laboratory, may strongly With rare exceptions, no I-Xe ages in Allende are within 5 m.y. ofthe bias the sample assemblage. Nevertheless, since most IDPs are oldest measured I-Xe ages [1,3,4]. The variability suggests that most, LPI Technical Report 97-02, Part 1 61 or all, of what we are seeing is alteration, and the range in ages cally depleted in the order of increasing volatility [15]. From the suggests that it occurred on a parent body. mineralogy and trace-element chemistry, they are likely to have been The most reasonable alternative to the idea that I-Xe ages are heated to 600°-700°C. Of particular interest is that the metamor dating secondary events is to preswne that I-Xe "ages" are really phosed CM chondrites show 0 isotopic compositions distinct from reflecting inhomogeneity in the 129V127I ratio in the solar nebula. the normal CM chondrites but close to the CI chondrites [16]. Although this is possible, it seems unlikely, both because there is no CV Chondrites: Evidence from Dark Inclusions: Type 3 obvious source for the inhomogeneous I (note that interstellar grains carbonaceous chondrites (CV3 and C03) contain little orno hydrous do not seem to have 1291 anomalies) and because the anticorrelation minerals and have been widely believed to have escaped major of muml and I29Xe/ 132 Xe is difficult to explain in the context of degrees of aqueous alteration. However, there is growing evidence nebular inhomogeneity. that the bulk ofthe CV3 chondrites were involved in various degrees This doesn't necessarily mean that there wasn't any secondary of aqueous alteration [e.g., 17-21], and it has recently been sug alteration in the nebula, but it does strongly suggest that at least some gested that some dark inclusions (DIs) in CV3 chondrites were once alteration, particularly that involving soda lite in Allende, occurred affected by extensive aqueous alteration [22-25]. on parent bodies, rather than in the nebula. The process that intro Dark inclusions are lithic clasts that range widely in texture from duced volatiles into the dark inclusions in Efremovka (which have one end member being composed ofchondrules and CAls embedded much more I and Xe than chondrules or CAls so far analyzed) also in a matrix to the other end member consisting mostly offine grains appears to have been a parent-body process. of homogeneous Fe-rich olivine [26]. Previously they have been References: [I] Podosek F. A. and Swindle T. D. (1988) in proposed to be primary aggregates of condensates from the solar Meteorites and the Early Solar System (J. F. Kerridge and M. S. nebula [e.g., 27]. However, a completely opposing interpretation has Matthews, eds.), pp. 1127-1146, Univ. of Arizona, Tucson. emerged from recent studies of some DIs [22-25]. Many ofthe fine [2] SwindleT. D. eta!. (l991)GCA, 55, 861-880. [3] Swindle T. D. grained type of DIs were found to contain numerous rounded to et a!. (1988) GCA, 52,2215-2227. [4] Nichols R. H. Jr. et al. (1990) oval-shaped inclusions in matrixes. The textures suggest that they are LPSXXl, 879-880. [5] Krot A. et al. (1997) LPSXXVlIl, 769-770. pseudomorphs after chondrules and CAls. Veins filled with fibrous · [6] Bematowicz T. J. et al. (1988) GCA, 52, I I 13-1121. [7] Nichols olivine grains were also found to be abundant in the DIs; some veins R. H. Jr. et al. (1991) LPS XXII, 975-976. [8] Swindle T. D. et al. penetrate several chondrule pseudomorphs, providing strong evi (1991) GCA, 55, 3723-3734. [9] Kirschbaum C. (1988) GCA, 52, dence for aqueous alteration after accretion [26]. Nevertheless, no 679-699. hydrous phase indicative of aqueous alteration was found from the DIs. This apparent paradox could be resolved by invoking that these DIs have undergone thermal dehydration after aqueous alteration AQUEOUS ALTERATION AND DEHYDRATION PROC [22-25]. The DIs indeed contain abundant Fe-rich olivine grains ESSES IN THE CARBONACEOUS CHONDRITES. K. with characteristic fibrous morphology similar to phyllosilicates Tomeoka, Department of Earth and Planetary Science, Faculty of [24,25]. The size distribution of chondrule pseudomorphs and the Science, Kobe University, Nada, Kobe 657, Japan. abundance ofCAl pseudomorphs in some DIs suggest that they may have been formerly lithic clasts ofchondritic material, most likely to CI and CM Chondrites: Aqueous alteration is an extremely be the host CV chondrites [24], although other DIs may have been important process for understanding the formation history ofcarbon derived from other precursors [28]. aceous .chondrites. CI and CM chondrites, in particular, consist Parent Bodies: CI and CM parent bodies are regarded to have largely of hydrous phyllosilicates and show abundant evidence of accreted initially as mixtures ofanhydrous silicates and ice [29]. Ice/ aqueous alteration. Where the alteration occurred has long been the rock ratios in these parent bodies would have been very high as subject ofcontroversy. Many recent authors suggest that it occurred suggested by the 0 isotopic study [30]. In the process ofparent-body in the meteorite parent bodies [e.g.,1-6], although some believe that growth, heating must have occurred, and aqueous alteration began by most of it completed before accretion [e.g., 7]. The most convincing reaction between anhydrous silicates and H20 derived from melting evidence for postaccretional aqueous alteration is probably the pres ice. The presence ofveins ofsulfates, carbonates, and phyllosilicates ence ofveins ofsulfates, carbonates, and phyllosilicates [8-1 0]. The suggests that the aqueous alteration occurred in regoliths, and was textures suggest that these vein-forming minerals were formed dur contemporaneous with the period ofregolith gardening. If this hy ing impact brecciation and leaching events on the regoliths of their pothesis is valid, the heat to melt ice may have been supplied in situ parent bodies. Isotopic studies show that at least.some of this alter by shock impacts on the regoliths, although the heat may also have ation took place very early after the formation of the parent bodies been supplied by the decay ofradionuclides from the internal regions [II]. ofthe parent bodies. Because ofthe low gravity in such small bodies, Recently, a growing number ofCI and CM chondrites mainly the water would have been gradually lost by sublimation and vapor from Antarctica have been found to show evidence that they experi ization. Ifthe heat source continued to operate, the H20 would have enced thermal dehydration after aqueous alteration [e.g., 12-14]. In finally run out, then dehydration and thermal transformation of contrast to the normal CI and CM chondrites, they contain abundant hydrous phases should have begun. fine grains of Fe-rich olivine and Fe-(Ni) sulfides in close associa If the suggestion that the precursors of DIs are the host CV tion with phyllosilicates in the matrixes. The phyllosilicates show chondrites is correct, the following implications would be possible. disordered structures characteristic of intermediate states in transi The CV parent body should also have been once involved in exten tion to olivine [13]. The Fe-rich olivines commonly show fibrous sive aqueous alteration and subsequent thermal metamorphism. As morphology, suggesting that they formed by dehydration of phyl DIs were incorporated as clasts and show abundant evidence of vein losilicates [12,13]. Trace elements in these meteorites are systemati mineralization, aqueous alteration may have occurred in the regolith 62 Workshop on Modification oj Chondri tic Materials like CI and CM parent bodies. However, it may not have been so pervasive as in the CI and CM parent bodies; it probably occurred 3500 3000 locally in the parent body where both H20 and heat were available. 20000 2500 Thus, CV materials located where H20 could not have penetrated remained unaltered. Dark inclusions may represent the materials that 2000 1500 existed in the location where H20 was originally abundant but later ~ 15000 ran out, while most of the existing CV chondrites may have come .. 1000 U 500 from a relatively dry, unmetamorphosed place. Therefore, the hetero N on 10000 geneity in alteration condition may reflect the heterogeneous distri I:: MAC88136 bution of ice and rock at the initial state of the CV parent body. ...,U on Spbalerite • 10 References: [I] McSween (1979) GCA, 43, 1761. [2] Bunch IQ 5000 • Ib and Chang (1980) GCA, 44, 1543. [3] Barber (1981) GCA, 45, 945. • It [4] Tomeoka and Buseck (1985)GCA, 49, 2149. [5] Brearley (1995) ·825 GCA, 59, 2291. [6] Browningetal. (1996)GCA, 60, 2621. [7] Metz 6 IXl0 2><10 6 ler et al. (1992) GCA, 56, 2873. [8] Richardson (1978) Meteor 55Mn /52Cr itics, 13, 141. [9] Tomeoka (1990) Nature, 345, 138. [10] Lee (1993) Meteoritics, 28, 53. [II] Endress et al. (1996) Nature, 379, Fig. 1. Manganese-chromium evolution diagram for isolated sphalerite 701. [12] Tomeoka et al. (1989)Proc. NIPR Symp., 2,36. [13] Akai grains, sphalerite #1 (spots a, b, and c) and #825 in MAC 88136 (EL3). (1990) Proc. NIPR Symp., 3, 55. [14] Ikeda (1992) Proc. NIPR The data are consistent with an inferred s3Mn/ssMn ratio of (7.4 ± 0.3) x Symp., 5, 49. [15] Paul and Lipschutz (1989) Z. Naturforsch., 44a, 10-7• Errors plotted are ±Io. 979. [16] Clayton and Mayeda (1989) LPS xx. 169. [17] Tomeoka and Buseck (1990) GCA, 54, 1745. [18] Keller and Buseck (1990) GCA, 54, 2113. [19] Keller et al. (1994) GCA, 58, 5589. [20] Lee et al. (1996) Meteoritics & Planet. Sci., 31, 477. [21] Brearley in MAC 88136 and Indarch) and are thus most likely produced by in (1997) LPS XXVIII. [22] Kojima et al. (1993) Meteoritics, 28, 649. situ decay oflives3Mn. (3) In the four meteorites with S3Cr excesses, [23] Krot et al. (I 995)Meteoritics, 30, 748. [24] Kojima and Tome there are variations in the inferred s3Mn/ssMn ratios between differ oka (1996) GCA, 60, 2651. [25] Krot et al. (1997) Meteoritics & ent sulfide grains within the same meteorite. Furthermore, within Planet. Sci., 32, 31. [26] Johnson et al. (1990) GCA, 54, 819. some individual grains, data points scatter and do not lie on a single [27] Kurat et al. (1989) Z. Naturforsch. , 44a, 988. [28] Weisberg et isochron. These variations are explained by diffusional redistribu al. (1996) LPS XXVII, 1407. [29] Grimm and McSween (1988) tion of Mn and/or Cr after partial or complete decay of s3Mn. Icarus, 82, 244. [30] Clayton and Mayeda (1984) EPSL, 67, 151. (4) Although no strict chronological significance can be ascribed to the differences in inferred s3MnlssMn ratios in sulfides of the four meteorites that have S3Cr excesses, limited time constraints may be MANGANESE-CHROMIUM SYSTEMATICS IN SULFIDES obtained. Specifically, our data indicate that the s3MnlS5Mn ratio in OF UNEQUILIBRATED ENSTATITE CHONDRITES: sulfides in the EL3 chondrites was (7.4 ±0.3) x 10-7 at the time of PARENT-BODY VS. NEBULAR PROCESSING AND isotopic closure, while the minimum inferred value for sulfides in the IMPLICATIONS FOR ACCRETION TIMES. M. Wadhwa1, Indarch meteorite at the time of isotopic closure was (1.7 ± 0.3) x E. K. Zinner2,3, and G. Crozaz2,4, IDepartmentofGeology , The Field Museum, Roosevelt Road at Lake Shore Drive, Chicago IL 60605, USA, 2McDonnell Center for the Space Sciences, Washington Uni 700 versity, St. Louis MO 63130, USA, 3Department ofPhysics, Wash '3 Mn/" Mn _ 6.3xl 0-6 ington University, St. Louis MO 63130, USA, 4Department ofEarth 600 and Planetary Sciences, Washington University, St. Louis MO 63130, ~ 500 USA. t U.. 400 We recently reported the results of an ion microprobe study of Non I:: 300 Mn-Cr systematics in individual sulfide grains of unequilibrated ...,U enstatite chondri tes (UECs) [I]. In that study, we measured Mn/Cr on 200 ratios and Cr isotopes in sphalerite (ZnS) and alabandite (MnS) IQ INDARCH Spbalerite • II grains of f~ur EL3 chondrites (MAC 88136, MAC 88180, MAC 100 .13 88184, and EET 90299) and sphalerite and niningerite (MgS) grains • 15 offive EH3~4 chondrites (Indarch, Qingzhen, Kota Kota, Y 6900 I, 4 0 IXIO 4 2><10 3X10 4 4X10 4 and Y 74370). The main findings from this investigation are summa 55Mn p2 rizedas follows: (1) Sulfide grains analyzed in four ofthe nine UECs Cr (MAC 88136, MAC 88180, MAC 88184, and Indarch) have clear Fig. 2. Data for sphalerite #13 and #15 in lrldarch (EH4) are consistent S3Cr excesses. (2) These excesses can be extremely large, with with an inferred s3MnlssMn ratio of (1.7 ± 0.3) x 10-6, while those for ~s3Crls2Cr ranging up to -18,400%0, the largest S3Cr excess mea sphalerite # II appear to be disturbed, with five of the six data points sured so far. Additionally, in some grains, these excesses in S3Cr plotting along a line (within 20) corresponding to a s3Mn/ssMn ratio of correlate well with the MnlCrratios (see Figs. I and 2 for sphaierites 6.3 x 10-6• Errors plotted are ±Io. LPI Technical Report 97-02, Part 1 63 10-6• This further implies that the time oflast isotopic equilibration The aim of this work is to discover the temperature and duration of of sulfides in the EH meteorite Indarch preceded that of sulfides in the nebular processes that created these rim layers (of spinel + the EL chondrites by at least -3 m.y. perovskite, melilite, and pyroxene). Although it is clear that diffusive reequilibration has indeed It was established 10 years ago [1-3] that rims formed in two affected the Mn-Cr systematics of the sulfide grains in both EH and stages: (I) flash heating of the outer layer of the CAl, leaving a EL, we did not ascribe a parent-body or nebular environment for this refractory residue that is (2) metamorphosed by diffusion ofMg, Si, subsequent processing. However, if the disturbance in the Mn-Cr and °from the nebula to produce rims. This work will study step (2). systematics can be attributed to a specific environment (nebular or Flash Heating: Flash heating [4] vola til ized much ofthe outer parent-body), it would have important implications for the time of 100-400 flm of each CAl to leave a residue with concentrations of condensation of the sulfides and/or accretion of the EH and EL very refractory trace elements 2-8x higher than in the CAL In order chondrite parent bodies. Therefore, we consider here some plausible to produce refractory enrichments of2-8x, the most volatile one half scenarios for the environments that may have affected the Mn-Cr to seven eighths (i.e., 50-87%) ofthe outer CAl material was vola systematics in sulfides in these EH and EL chondrites, and their tilized. The two most volatile major components (30-42% ofCAls) possible implication for the relative accretion times.of the EH and EL are MgO and Si02• Hence, aJl Mg and Si must have been volatilized. parent bodies. The next most volatile component, CaO, comprises 20-40% of In the case of the Indarch EH4 chondrite, it has been previously CAls. Hence, from one half to most of the Ca must also have been suggested that the sulfides equilibrated at temperatures in excess of removed in the 50-87% volatilized matter. The residues would thus 1000°C, and may even have undergone partial melting [2-4]. This have been predominantly AI20 3 with lesser amounts of CaO and event is generally believed to have occurred "postaccretion" on the Ti02• The residue corresponding to a rim:CAI refractory enrichment EH parent body. It most likely marked the time of last isotopic ofapproximately 2.3x has a composition [5] of77.4% A120 3, 20.0% equilibration for this meteorite. Therefore, it is at this time that the CaO, and 2.6% Ti02. Synthetic material ofthis composition, consist 55 53Mn/ Mn ratio in the sulfides in this meteorite was at least (1.7 ± ing ofCaAI40 7 (grossite) with minor CaTi03(perovskite) was used 0.3) x 10-6. This value may, thus, be regarded as the lower limit on to represent the "rim residue" in the experiments below. the 53Mn/55Mn ratio in Indarch sulfides at the time' of accretion. Metamorphism: Rims on CAls typically contain about 16% Regarding the isotopic closure ofsulfides in the EL chondrites, when MgO and 20% Si02 that diffused into the cooling residue from the the 53Mn/55Mn ratio was (7.4± 0.3)x 10-7, there are three possibili nebular gas. The Mg:Si atom ratio in rims is on average -1.2, close ties, i.e., that it occurred at the time of(1) condensation ofthe sulfides to the 1.1 ratio of nebular gas in the pressure-temperature interval in the nebula, (2) accretion ofthe EL parent body, or (3) "postaccretion" after most Ca and Al had condensed and before major Mg and Si metamorphism at a later (unknown) stage. In either of the first two condensation. Magnesium, Si, and 0 each diffused into the refrac- cases, the accretion time for the EH parent body would have to precede that for the EL parent body by at least -3 m.y. However, in case (3), it is not possible to make any inferences regarding the accretion times of the EH and EL parent bodies. It should be noted that ofthe three possibilities, the first one is most likely since major and minor-element zonation patterns preserved in the sulfides in these EL3 chondrites suggest equilibration during cooling in a nebu lar environment [5,6]. Therefore, it seems most likely that the time of accretion ofthe EH parent body preceded that ofthe EL parent body by at least -3 Ma. Finally, it should be noted that the above comparisons between EH and EL chondrites are only valid if 53Mn was homogeneously distributed in the region where the enstatite chondrites formed. Recent work [7] suggests that this may indeed be the case. References: [I] Wadhwa M. et a!. (1997) Meteoritics & Planet. Sci., 32, 281. [2] Mason B. (1966) GCA, 30, 23. [3] Hohen berg C. M. eta!. (1967)Science, 156,233. [4] BirckJ.-L. and Allegre c. J. (1988) Nature, 331, 579. [5] Lin Y. T. et a!. (1991) LPS XXII, 81 I. [6] EI Goresy A. et a!. (1992) LPS XXIII. 331. [7] Lugmair G. W. and Shukolyukov A. (1997) LPS XXVIII, 851. CONDITIONS FOR FORMING CALCIUM-ALUMINUM RICH·INCLUSION RIM LAYERS: PRELIMINARY EX PERIMENTS. D. A. Wark, Westbourne Grammar School, Say ers Road, Hoppers Crossing 3029, Australia, and School of Earth Sciences, University of Melbourne, Parkville 3051, Australia. Similar "Wark-Lovering" rim layers are present on virtually all types of coarse-grained Ca,AI inclusions (CAls) in CV chondrites. Fig. 1. 64 Workshop on Modification of Chondrilic Materials tory residue, with different chemical potentials at each depth, leading nebulae [I]. About 12 years ago there was an upsurge in the devel to formation of specific mineral layers [6] . opment of such models stimulated by several petrographic and cos Experiments: The aim is to find temperatures and diffusion mochemical observations that seemed to require nebular redox con times that produce rim layers. This preliminary work used a polished ditions orders of magnitude greater than present under canonical MgSi03 plate to supply Mg and Si in approximately the correct conditions. A large fraction of these observations involved late proportions to the polished surface ofthe synthesized "rim residue." formed deposits of oxidized Fe, especially fayalite, and many were So far, the following three diffusion couples have been heated in air, made on the Allende CV chondrite. Two papers were particularly and analyzed by SEM: (I) 5 hr at 1400°C, (2) 18 hr at 1225°C (see influential: (I) Fegley and Palme [2] observed that, in many refrac Fig. I), (3) 4 hr at I 300°C. tory inclusions, Mo and Ware depleted relative to other siderophiles Why can a solid source be substituted for nebular gas? The having similar volatilities, and that Wand Mo are much more volatile assumption is that the governing process in forming rims is the rate under oxidizing conditions whereas other siderophiles are scarcely ofdiffusion and reaction in the refractory residue, and thus the means affected; and (2) Peck and Wood [3] observed that ferrous olivine of supplying Mg and Si is, to first order, not important. The condi rims the common forsteritic olivine in Allende and that the minor tions (time and temperature) for producing CAl rim layers can then element (AI, Cr, Ti) contents are too high to allow exsolution from be found in laboratory diffusion experiments. forsterite. Other studies that developed these themes are Hua et al. Results: Experiment A did not produce rim layers, only a [4], Rubin et al. [5], Palme and Fegley [6], Weinbruch et al. [7], and reaction zone 65 J.Lm wide. Experiment B produced the rimlike layers Matsunami et al. [8]; the latter two showed that MnO and FeO were in Fig. I, with the following similarities to those in CAls: (I) Par well correlated in much of the fayalite, inconsistent with condensa allel layers with thicknesses of 4-7 J.Lm and total width of 45 J.Lm; tion from a canonical nebula but roughly consistent with formation (2) same order of italicized phases: hibonite, spinel, plagioclase, under oxidizing conditions. forsterite, and silica, diopsidic pyroxene. In experiment C, 10% Typical pH2/pH20 ratios in these nebular models are 10, i.e., more MgO was mixed with the enstatite to reduce excessive silica, >IOOx lower than in a canonical nebula. The common approach to resulting in gehlenitic melilite replacing the plagioclase, as in CAl explaining such extreme conditions is to assume that (I) the nebula rims. The chief difference so far from CAl rims is the siliceous was originally canonical; (2) dust settled to the midplane and was forsterite layer below the diopside. It also seems that Ca is too high. evaporated there by an unknown heat source, the °from the oxides New experiments with lower CaO (12%) and a source with 1.1:1 being largely converted to HzO ;and (3) grain (and, in some models, Mg:Si atom ratio are underway. These early results suggest that rim chondrule) condensation occurred before the turbulence led to re formation could have required times ofabout 20 hr at about I 200°C. mixing with the nebular gas outside the midplane. Acknowledgments: Heartfelt thanks are due to I. Grey, P. The pendulum began to swing the other way. Blum et al. [9] Rummel, I. Harrowfield, C. Li, C. McRae, G. Mumme, and C. showed that many CV opaque assemblages are best understood as Davidson ofCSIRO Division of Minerals for their assistance. asteroidal alteration products. Although Palme and Fegley [6] had References: [I] Boynton W. V. and Wark D. A. (1985)Mete reported that chromite condensed earlier than F e-Ni under oxidizing oritics, 20, 613-614. [2] Boynton W. V. and Wark D. A. (1987)LPS conditions and thus the high Cr content offayalitic materials was an XVIII, 117-118. [3] Wark D. A. et al. (I 988) LPSXZX; 1230-1231. indication of such conditions, new calculations by Krot et al. [10] [4] Boynton W. V. (1988) Meteoritics, 23, 254. [5] Hashimoto A. showed that chromite condensation behavior was essentially inde (1983)Geochim.J.,17, 111-145. [6] RuzickaA. and Boynton W. V. pendent ofredox conditions. They also noted that those refractory (1995) Meteoritics, 30, 570. inclusions that formed as a result of heating during infall into the nebula might retain their original redox state, in which case the depletions of Mo and W would not be indicative of nebulawide conditions. Wasson and Krot [11] showed that high-Fa olivine asso OXYGEN FUGACITY IN THE SOLAR NEBULA. J. T. ciated with silica in UOC probably formed by aqueous alteration on Wasson, Institute of Geophysics and Planetary Physics, Univer the parent bodies; FezSi04 formed when Fe diffused from the sur sity ofCalifornia, Los Angeles CA 90095, USA. rounding matrix to the silica. The high Mn contents of these grains are readily understood if Mn condensed from the nebula as tiny, A major early success ofcosmochemical modeling was the rec unstable grains that dissolved during aqueous alteration, the Mn ognition that the constituents ofchondrites are readily understood by diffusing to a suitable growing host such as fayalite. This model can sequential condensation in a cooling gas ofcosmic (=solar) compo" also explain the MnO-FeO correlations observed by Weinbruch et al. sition, tempered by the metastable survival ofsome high-temperature [7] and Matsunami et al. [8]. phases. Ofparticular importance is Fe, which prefers metallic bonds Krot et al. [12] carried out an extensive review ofCV alteration at high nebular temperatures (>I 000 K) and bonds to °and S at low products and showed that they are pervasive, i.e. , in chondrules, «700 K) temperatures. Redox conditions in a canonical nebula (one refractory inclusions, matrix, and metal. Their view was that the with a solar composition) are controlled (buffered) by the abundance alteration, whether involving formation ofphyllosilicates, F e-alkali ofgaseous H2 and H20. The currently accepted canonical pH2/pH20 halogen metasomatism, or oxidation/sulfidation of metal was best is 1500, and is relatively insensitive to nebular temperature. Thus, understood by low-temperature processes in the asteroidal parent both the formation ofFe-Ni and its later corrosion by H20 (and H2S) body, as originally suggested for CV chondrites by Housley and are readily understood in terms ofplausible processes in a canonical Cirlin [13], and widely accepted for CM and CI chondrites since the nebula. work of Bunch and Chang [14] and Kerridge et al. [15]. Already two decades ago a few researchers began to try to model There are two reasons it is easier to oxidize metal or aqueously some features of chondrites by condensation from noncanonical alter other phases in the parent bodies than in the solar nebula. The LPI Technical Report 97-02, Part I 65 pHipH20 ratios are much lower in parent bodies, causing the stabil luminescence. At present, two contrasting formation models are ity fields of alteration phases to extend to higher temperatures, and discussed for these olivine grains: (I) condensation from the solar H20 pressures are much higher, leading to higher reaction rates. At nebula [e.g., 1,2], and (2) crystallization from chondrule melts [e.g., 5 9 a nebular pressure of 10- atm pH20 is only 7 x 10- atm; because 3]. Here, we provide additional evidence for distinguishing between H20 was probably the dominant volatile in asteroids, its pressure the two models. could be as high as the overburden pressure, which reaches I atm at We have studied 10 samples from the Allende meteorite contain a depth of I Ian in a chondritic asteroid having a radius of 50 Ian. ing a major fraction of refractory forsterite. The samples included In summary, the evidence for highly oxidizing conditions in the two isolated olivine grains (lOG), one chondrule, and seven chon solar nebula is weak. Although there is evidence (e.g., Mo deple drule fragments. Bulk samples were analyzed by instrumental neu tions) for the formation of refractory inclusions under oxidizing tron activation analysis (INAA). Polished sections of the samples conditions, those formed during infall heating could have been were studied by scanning electron microscopy (SEM), electron probe internally buffered. Other observations, mostly involving fayalite or microanalysis (EPMA), and secondary ion mass spectrometry (SIMS). magnetite, seem better understood in terms ofaqueous alteration on Preliminary INAA results are found in [4]. the parent bodies. All samples contain approximately chondritic abundances ofthe There is no need to continue to invoke implausible events to flash refractory lithophile elements AI, Ca, Sc, V, and the rare earth evaporate nebular solids in the midplane (and to prevent the resulting elements (REE). Comparison between INAA and SIMS reveals that gases from mixing with the other nebular gas). The more important the REE, except Eu, reside predominantly in olivine. Refractory global question now is whether the H20 in the chondritic parent lithophile elements (RLE) are slightly volatility fractionated. In the bodies condensed as ice or as hydrated silicates. two lOG, which consist almost entirely ofrefractory forsterite, RLE References: [I] Herndon J. M. and Suess H. E. (1977) GCA, contents were found to decrease with increasing volatility, while in 41,233. [2] FegleyB. and Palme H. (I 985) EPSL, 72,311. [3] Peck the chondrule sample a complementary pattern was observed, sug 1. A. and Wood 1. A. (I 987) GCA, 51,1503. [4] Hua X. etal. (1988) gesting an origin by condensation. The abundance ofYb within this GCA,52, 1389. [5] Rubin A. E. et al. (1988) in Meteorites and the sequence reflects oxidizing conditions. This is supported by the Early Solar System (1. F. Kerridge and M. S. Matthews, eds.), p. 488. comparatively high Mo contents (0.3-2.4 ppm). [6] Palme H. and Fegley B. (1990) EPSL, 101, 180. [7] Weinbruch The high abundances ofRLE in olivine would either require very S. et al. (1990) Meteoritics, 25, 115. [8] Matsunami S. et al. (1990) high bulkRLEcontents oran unrealistically high degree offractional Proc. NIPR Symp. Antarct. Meteorites, 3, 147. [9] Blum 1. D. et aI., crystallization to achieve the enrichment. Therefore, refractory GCA, 53, 483. [10] Krot A. N. et al. (1993) EPSL, 119, 569. forsterite occurring in chondrules has to be considered a relict phase [II] Wasson 1. T. and Krot A. N. (1994) EPSL, 122, 403. [12] Krot that survived the chondrule-forming process. A. N. et al. (1995) Meteoritics, 30, 748. [13] Housley R. M. and Refractory forsterite has very low Mn (30-40 ppm) and Ni (1 .2 Cirlin E. H. (1983) in Chondrules and Their Origins (E. A. King, 1.7 ppm) contents. The low Mn concentrations also indicate that ed.), p. 145, LPI, Houston. [14] Bunch T. and Chang S. (1980) these grains did not form by igneous processes from precursors of GCA,44, 1543. [15] Kerridge 1. F. (1979) EPSL, 43, 359. Allende chondrule composition. Experimental olivine/melt partition coefficients (see [5,6] and references therein) would require melts with Mn contents below 100 ppm to produce the observed Mn concentrations in refractory forsterite. Chondrules with such low Mn contents are not observed in Allende or in any other carbonaceous REFRACTORY FORSTERITE IN CARBONACEOUS chondrite [7,8]. CHONDRITES: AN UNALTERED CONDENSATE FROM In summary, an origin ofrefractory forsterite by condensation in THE SOLAR NEBULA. S. Weinbruchl , H. Palme2, B. SpetteP, the solar nebula is supported by the high concentrations ofRLE, the and I. M. Steele4 , IDepartment of Material Science, Technical volatility-dependent fractionation of RLE, and the low concentra University of Darmstadt, Petersenstrasse 23, D-64287 Darmstadt, tions of Mn. In addition, refractory forsterite is enriched in 160 Germany, 2Institut fijr Mineralogie und Geochemie, Universitat compared to chondrules, which also excludes an igneous origin [9]. zu K61n, Ziilpicherstrasse 49b, D-50674 KOln, Germany, 3Max Refractory forsterites, thus, seem to represent unaltered condensates Planck-Institut fijr Chemie, Postfach 3060, D-55020 Mainz, Ger from the solar nebula. The lack ofan Yb anomaly and the high Me many, 4Department ofGeological Sciences, University ofChicago, concentrations suggest oxidizing conditions during condensation of Chicago IL 60637, USA. the forsterite grains analyzed here. This is not in contradiction with the low FeO content. Forsteritic olivine is the first Mg silicate to The origin of chondrite components such as matrix, inclusions, condense, even at comparatively high 0 fugacities [10]. chondrules, or isolated mineral grains is frequently obscured by References: [I] Steele I. M. (1986) GCA , 50, 1379-1395. alteration processes on parent bodies and/or in the solar nebula. In [2] Steele I. M. (1988) in Meteorites and the Early Solar System order to better understand the nature ofthese alteration processes and (1. F. Kerridge and M. S. Matthews, eds.), pp. 808-818, Univ. of to quantify the extent they have influenced chondrites, attempts are Arizona, Tucson. [3] Jones R. H. (1992) GCA, 56, 467-482. . made to identify pristine (unaltered) material. [4] Palme H. et al. (1986) LPS XVII, 640-641. [5] Colson R. O . Iron-poor (FeD <1 wt%) olivine enriched in refractory elements et al. (1988) GCA, 52, 539-553. [6] Snyder D. A. and Carmichael Ca, AI, and Ti (hereafter called refractory forsterite) is frequently I. S. E. (1992) GCA, 56, 303-318. [7] Rubin A. E. and Wasson encountered in carbonaceous and in ordinary chondrites [e.g., 1,2]. 1. T. (1987) GCA, 51, 1923-1937. [8] Palme H. et al. (1992) LPS Refractory forsterite occurs as isolated grains in the matrix and XXIII, 1021-1022. [9] Weinbruch S. et al. (1993) GCA, 57, 2649 within chondrules and is readily identified by its blue cathodo 2661. [10] Palme H. and Fegley B. (1990) EPSL, 101, 180-195. 66 Workshop on Modification of Chondritic Materials FAYALITIC OLIVINE IN CV3 CHONDRITE MATRIX 17]. (4) Fayalitic olivines in CV3 chondrite matrix and Dis show a AND DARK INCLUSIONS: A NEBULAR ORIGIN. M. K. range of compositions (Fig. I), suggesting they were not in contact Weisberg and M. Prinz, Department of Earth and Planetary Sci at temperatures high enough to initiate equilibration. Many fayalitic ences, American Museum ofNatural History, New York NY 10024, olivines are zoned and have forsteritic cores, indicating direct re USA. placement by fayalitic olivine without an intermediate (hydrous phyllosilicate) stage. Additionally, zoning and heterogeneity on such Introduction: Olivines with compositions of Fa>35 (dubbed a fine scale could not survive a metamorphic heating event (>500°C) fayalitic olivine) in the matrix, chondrule rims, and dark inclusions [e.g., 16] great enough to completely dehydrate phyllosilicates and (Dis) have been at the heart ofthe nebular vs. parent-body formation transform them to olivines. (5) Dehydration reactions ofserpentine controversy for components in CV3 and other chondrites. The rich phyllosilicates produce enstatite and/or free silica (in addition to "nebularists" have argued for processes of vaporization, condensa olivine), and neither ofthese phases are present in the matrix or DIs. tion, oxidation, and/or metasomatism in a nebular setting to explain (6) A Ningqiang DI that is compositionally and 0 isotopically similar the unusual petrologic, compositional, and isotopic characteristics of to the Allende DIs has amorphous olivine rims that formed by the fayalitic olivine [1-7], while the "asteroidists" have evoked irradiation damage in the nebula [15], indicating a primitive nebular parent-body Fe-Mg exchange and, more recently, hydration/dehy origin for the fayalitic olivine in this case, and suggesting the same dration processes [8-11]. Here we summarize our petrologic data for fayalitic olivine in other CV3 chondrite Dis. (7) Allende chon and observations on the occurrences of fayalitic olivines in matrix drules contain unaltered primary glassy mesostasis [18]. Chondrule and Dis of CV3 chondrites, and discuss their origin. mesostasis would not survive parent-body hydration without becom Matrix: CV3 matrix is a porous aggregate and fayalitic olivine ing hydrated and would become devitrified at the. metamorphic is the major component, generally making up >80 vol%. It ranges in temperatures necessary to completely dehydrate the matrix and form morphology from platy and lath-shaped to anhedral crystals, gener fayalitic olivines. (8) Oxygen isotopic compositions ofAllende matrix ally <5 ~m in size. Fluffy clusters ofirregular olivine < I ~m in size argue for little or no hydration [19]. are also present in matrix. The ratio ofplaty to irregular crystals varies One petrologic argument against a nebular model is that the widely among CV3 chondrites, with platy and lath-shaped crystals fayalitic olivine contains numerous inclusions ofpentlandite, oxides, being dominant in some (e.g., Allende, Axtell, and Mokoia) and rare and spinels that, in some cases, are rimmed by poorly graphitized C to absent in others (e.g., Vigarano). Despite morphological varia [20]; this presents an obstacle for a condensation model since it tions and compositional ranges, all fayalitic olivines in CV3 chon requires formation ofhydrocarbons, at very low temperatures, prior drites have a roughly similar, near-solar FelMn ratio (Fig. I). Chon to growth ofthe olivine. The chondrule pseudomorphs in some DIs drule olivines have different FelMn ratios, and the coarser-grained could be used to support a parent-body dehydration model [e.g., 9], (up to 100 ~m) near-pure fayalites [12], found in some CV3 chon but a condensation model for the fayalitic olivine in these chondrules drites, have still different FelMn ratios with a negative trend. Fayalitic has also been proposed [6,7] and topotactic growth of fayalitic olivines with similar morphologies (platy and lath-shaped crystals) olivine in the nebula may also be possible. On balance, we favor a and compositions are also present in the matrix ofsome unequi Iibrated nebular model and propose that the fayalitic olivine formed by ordinary chondrites (e.g., Krymka, Cbainpur, and Bishunpur), oc vaporization of chondri tic dust to produce a fayalite-rich vapor, curring as single crystals, in clusters, and as epitaxial overgrowths on followed by recondensation of fayalitic olivine. Vaporization of forsterite substrates [13]. olivine, with the exception of pure endmember compositions, is Dark Inclusions (DIs): Dark inclusions are lithic fragments incongruent and therefore heating of forsteritic olivine to its vapor with complex histories that clearly predate their incorporation into ous produces a fayalite-rich vapor [21,22]. Magnesium-rich pyrox their host chondrites. They are texturally diverse and display a range ene will also evaporate incongruently to form olivine and a Si-rich from chondrule- (and inclusion)-rich varieties to chondrule-free matrixlike material. Fayalitic olivines are the major component and are petrologically similar to that in host CV3 matrix, suggesting a similar origin. In addition, fayalitic olivines in some Dis are topotactic 1.0 r.~;:::::;:;;:::::::-::;~~~~r.,..,....,.":t (structurally controlled) replacements ofpreexisting chondrule oliv Fayalitie I A-:-C"'V3--:B~e"'d-uc-e"'d-' ines, resulting in chondrule pseudomorphs. A remarkable DI in the 0.8 Matrix Olivine I • Leoville Ningqiang CV3-related chondrite is petrologically and 0 isotopi ...... ;,!! o cally similar to Dis in CV3 chondrites, but the fayalitic olivine grains i 0.6 are rimmed by amorphous olivine ofthe same composition [14,15]. 00.4 Discussion and Conclusions: The parent-body heating/dehy c: dration model for the formation ofthe fayalitic olivines has consid ::s • Axtell 0.2 o Ball erable uncertainties: (I) There is no direct evidence linking fayalitic .------,10 Kaba olivines to precursor phyllosilicates. In known dehydrated chon II Krymka LL3.1 0 Mokola drites, such as Belgica 7904 and Y 793321, a phase intermediate 20 30 40 50 60 70 80 between phyllosilicate and olivine occurs [16], but no such phase has FeO (wt.%) been found in Allende matrix or Dis. (2) Dehydration ofphyllosilicates cannot explain the wide range of morphologies ofthe fayalitic oliv Fig. 1. FeO vs. MnO (wt%) for fayalitic olivine in CV3 and Krymlca ines in the matrix and Dis and is inconsistent with the epitaxial LL3.1 chondrite matrix. Matrix oiivines in these chondrites have a similar, growth of platy olivine. (3) In some CV3 chondrites the fayalitic near-solar FeOfMnO ratio. The coarser-grained near-pure fayalites in Bali olivines clearly predate the formation of the hydrous phases [e.g., and Kaba have a different FeOfMnO ratio and a negative trend. LP/ Technical Report 97-02. Part 1 67 gas [23]. This event may have been concurrent with chondrule tical techniques to calculate model-independent significance levels formation and the heating ofsome dust to its vaporous may have been for both univariate and multivariate comparisons [9]. a consequence ofthe same heating mechanisms that were responsible Results of univariate comparisons between unbrecciated and for the melting of nebular dust to form chondrules. brecc iated equil ibrated H chondrites and between mildly and strongly References: [I] Peck and Wood (1987)GCA. 51. 1503-1510. shocked ones indicate that using VTEs, the mildly shocked suite of [2] Hua et al. (1988) GCA. 52. 1389-1408. [3] Weinbruch et al. equilibrated H chondrites is indistinguishable from the strongly (I 990) Meteoritics. 25. 115-125. [4] Weinbruch et al. (1993) GCA. shocked one. Comparisons using Student's t-test reveal that none of 57. 2649-2661. [5] Murakami and Ikeda (1994) Meteoritics. 29. the II VTE contents differ at the <0.05 significance level. Only one 397-409. [6] Kurat et al. (1989) Z. Naturforsch .• 44a. 988-1004. element, Ag, differs at <0.1 0, with mean Ag contents being higher in [7] Palmeetal. (1989)2. Naturforsch.. 44a. 1005-1014. [8] Housley the strongly shocked suite of samples. We attribute this to chance. and Cirlin (1983) inChondrules and Their Origins (E. A. King, ed .), Results ofmultivariate comparisons between the mildly and strongly pp. 145-161, LPI, Houston. [9] Kojima and Tomeoka (1995) GCA. shocked suites based on alii I VTEs using LDA and LR give model 60. 2651-2666. [10] Krot et al. (1995) Meteoritics. 30. 748 dependent significance levels of 0.2466 and 0.0491 respectively. 775. [II] Krot et al. (1997) Meteoritics & Planet. Sci .• 32. 31-49. The value for LR, however, increases to 0.320 when a model [12] Huaand Buseck (I 995) GCA. 59. 563-578. [13] Weisberg et al. independent value is calculated; hence, we find no evidence for a (1997) Meteoritics & Planet. Sci .• 32. in press. [14] Weisberg et al. shock-dependent, VTE compositional difference in H chondrites. (1996) Meteoritics & Planet. Sci.• 31. A150-A151. [15] Zolensky These results strongly contrast with the results for analogous com et al. (1997) LPS XXVIII. 1629-1630. [16] Akai (1988) GCA. parisons in L chondrites. 52. 1593-1599. [17] Keller et al. (1994) GCA. 58. 5589-5598. Comparisons between unbrecciated and brecciated suites indi [18] Ikeda and Kimura (1995) Proc. NIPR Symp. Antarct. Meteor cate that these two suites are also indistinguishable using VTE ites. 8. 97-122. [19] Clayton et al. (1997) LPS XVIII. 239-240. contents. Using Student's t-test, one element ofthe II, Zn, differs at [20] Brearley and Prinz (1996) LPSXXVII. 161-162. [21] Naga the <0.05 significance level and an additional element, Cs, differs at hara and Kushiro (1987) EPSL. 85. 537-547. [22] Nagahara et al. the <0.10 level; means for both are higher in the brecciated suite. (1988) Nature. 331. 516-518. [23] Mysen and Kushiro (1988) Am. Multivariate LDA and LR comparisons between unbrecciated and Mineral.• 73. 1-19. brecciated suites using all II VTEs give model-dependent signifi cance levels of 0.2549 and 0.0772 respectively. The LR value, however, increases to 0.131 when a model-independent value is VOLATILE TRACE-ELEMENT COMPOSITION AND calculated; hence, we find no evidence for a brecciation-dependent, SHOCK IN EQUILIBRATEDH CHONDRITES. S. F. Wolf! compositional difference in H chondrites. and M. E. Lipschutz2, lArgonne National Laboratory, Argonne IL In principle, differences in the VTE contents ofmildly and strongly 60439-4837, USA, 2Departtnent of Chemistry, Purdue University, shocked, and brecciated and unbrecciated equilibrated H chondrites West Lafayette IN 47907-1393, USA. might be obscured by other factors, e.g., a relationship involving VTE content and petrographic type [10]. To minimize this possibil Shock loading and breccia formation can be fundamental pro ity, we repeated these comparisons using only H5 chondrites. Mul cesses in meteorite genesis [I]. Shock can affect many chemical and tivariate LDA and LR comparisons of mildly and strongly shocked physical properties used to decipher meteorite history [2]. For ex suites of H5 chondrites using all II VTEs gave model-dependent ample, volatile-trace-e1ement (VTE) contents of mildly shocked L significance levels of 0.0406 and 0.0022 respectively. However, chondrites are significantly lower than in strongly shocked ones [3]. these increase to 0.455 and 0.103, respectively, when model-inde The assumption that this trend also holds for H chondrites, has pendent comparisons are made for H5 chondrites. Multivariate LDA colored interpretations of the relationships between VTE contents, and LR comparisons of unbrecciated and brecciated H5 chondrite and the thermal history of H chondrites and structure of the H suites using aliI I VTEs give model-dependent significance levels of chondrite parent body. In this study, we explore whether VTE con 0.1990 and 0.0069 respectively. The value for LR, however, in tents of equilibrated H chondrites reflect shock metamorphism and creases to 0.270 when the model-independent RS technique is used. brecciation. Thus, for H5 chondrite suites, neither shock nor brecciation has Volatile-trace-element compositional data exist for 90 H chon apparently been important in establishing VTE contents. To examine drite falls: The complete dataset includes Co, Se, Rb, Cs, Te, Bi, Ag, whether we are using too many elements in our comparisons for the In, TJ, Zn, and Cd (listed in increasing order of mobility) [4-6]. We size ofthe existing database we can reduce the number of elements use shock facies data [7] to classify samples as either mildly shocked seriatim, using mobility, as described elsewhere [5,9]. No matter (facies a-c) or strongly shocked (facies d-f). Our database con how few elements we used, we found no evidence for a shock tains 43 mildly shocked «22 GPa) and 12 strongly shocked (>22 dependent difference in VTE contents. GPa) samples. Classification of samples as brecciated or unbrecci The postaccretionary shock and brecciation histories of parent ated is based on published observations [8]. Our database includes bodies ofequilibrated H and L chondrites obviously differ. In addi 70 unbrecciated and 15 brecciated samples. We use univariate statis tion, parent materials of equilibrated H chondrites seem to have tical techniques (Student's t-test) and the multivariate statistical condensed and accreted at temperatures higher than those existing techniques oflinear discriminant analysis (LDA) and logistic regres when L chondrite parent material formed. All ordinary chondrites sion (LR) to compare VTE contents in these two pairs ofmeteorite were not created equal. suites, thus testing the null hypothesis and establishing that any References: [I] StoffierD eta!' (1991) GCA. 55. 3845-3867. observed compositional differences are the result of shock or brec [2] Stoffler D. et al. (1988) in Meteorites and the Early Solar Sys ciation. We also use bootstrap randomization-simulation (RS) statis tem (J. F. Kerridge and M. S. Matthews, eds.), pp. 165-202, Univ. 68 Workshop on Modification of Chondritic Materials of Arizona, Tucson. [3] Huston T. J. and Lipschutz M. E. (\ 984) unique chondrule, clearly distinguished by its texture, minerals, and GCA , 48, 1319-1329. [4] LingnerD. W. et al. (I987) GCA, 51, 727 mineral composition, lies within these chondrules. 739. [5] Dodd R. T. et al. (1994) JGR Planets, 98, 15105-15118. The unique chondrule is -2 mm in diameter, surrounded by a [6] WolfS. F. and Lipschutz M. E. (1997) JGR Planets, in press. thin rim of pyroxene. Olivine is present as two types: (I)a prism [7] Dodd R. T. and Jarosewich E. (\ 979) ESPL, 44, 335-340. like shape with enriched euhedral in the core and (2) granular to [8] Graham A. L. et al. (\ 985) Catalogue of Meteorites, 460 pp. porphyritic occurrences in the mantle to the rim. Both types of oli [9] Wolf S. F. and Lipschutz M. E. (\ 995) in Advances in Analyti vine have an identical composition of forsterite (F 099.SFa o.s), corre cal Geochemistry, Vol. 2, 241-281. [10] WolfS. F. and Lipschutz sponding to those of an enstatite chondrite. Although the ortho M. E. (1993) Meteoritics, 28, 460-461. pyroxenes found in the rim share an almost uniform composition of enstatite (E~7_wFso.s _ 2 Woo.s_l.s), they show no similarities to those found in the enstatite chondrite. The olivine compositions and the A UNIQUE CHONDRULE CONSISTING OF FORSTERITE pyroxenes are both quite different from those of other chondrules, AND CLEAR GLASS GROUNDMASS WITH COMPOSI and also differ from the matri.x of0 livine and pyroxene grains in the TIONAL ZONING IN ORDINARY CHONDRITES. K. host rock. Yanai, Faculty ofEngineering, Iwate University, 3-4-5, Ueda Mori The glass compositions of the chondrule groundmass are also oka 020, Japan. unique, showing typical compositional zoning. Aluminum, Mg, and Ca are enriched at the core, but Si, Fe, Na, and K are poor. Calcium A unique chondrule is recognized in type 3 ordinary chondrites oxide is over 15% in the core, but decreased to 9% at the mantle rim. (OCs); while type 3 OCs contain many chondrules, one in particular On the other hand, N~O is near 0% in the core, but increased to 7% is conspicuous for its unique texture. This chondrule consists of at the mantle rim. mostly euhedral olivine grains, with some drop-shaped F e-Ni metals In this chondrule, phenocrysts (olivine and orthopyroxene) are in a clear-glass groundrnass, which is rimmed by orthopyroxenes. characterized by their uniform compositions, but those of the glass The glass ground mass consists of a transparent core with a pale groundrnass show remarkable compositional variation (chemical brown colored mantle-rim. The composition of glass shows typical zoning); they are different from those ofthe host rock. Itis reasonable zoning ofSi, AI, Ca, Na, K, Fe, and Mg. Below are some results of to consider the features of this chondrule as one of the unique our petrographical and chemical study of this unique chondrule. characteristics ofchondri tic meteorites. The ordinary chondrite, shown in Fig. I, is unequilibrated, clas sified as belonging to the L group because ofits chemicals, minerals, and mineral compositions. Itconsists ofmany chondrules, chondrule THE HISTORY OF METAL AND SULFIDES IN CHON fragments containing olivine and pyroxenes, Fe-Ni metals, troilite, DRITES. B. Zanda,·2, Y. Yu2, M. Bourot-Denise', and R. Hew and matrixes. The olivine compositions range from Feo.6 to FC:!0.7, ins2, 'Museum d'Histoire Naturelle, 61 rue Buffon, 75005 Paris, and whereas the orthopyroxenes range from FSl.4 to FS24.3 with pigeonite Institut d'Astrophysique Spatiale, Orsay, France, 2Department of En87 .s F~ . 2 WOS.3• Various types of chondrules, e.g., granular, por Geological Sciences, Rutgers University, Piscataway NJ 08855 phyritic, poikilitic, crystalline, and radial pyroxene, are present. The 1179, USA. Introduction: Opaque minerals offer a unique approach to disentangling nebular from asteroidal effects in meteorites. Sulfur will be mobilized in every heating/cooling episode, whether nebular or asteroidal, but, unlike any other volatile element, its resulting distribution is easily documented in the reflected light microscope. Iron-nickel sulfurization/desulfurization happened several times in the history of chondrites and several generations of sulfides and metal can potentially be identified: nebular condensates, chondrule melts, postchondrule formation condensates, parent-body products. First Condensation ofOpaque Minerals: Metal. Grossman and Olsen [I] and Kelly and Larimer [2] described the condensation ofNi and Fe, but this early metal may not have survived subsequent S and 0 condensation. Phosphorus, Cr, and Si in metal in carbona ceous chondrites result from chondrule formation [3], not condensa tion (see below). Remnant condensate metal might be found in fine-grained matrix [4]. Sulfides. Sulfur condensation by reaction ofH2S with an Fe-Ni alloy was studied experimentally by [5-7] who showed that the Ni bearing sulfides pentlandite and monosulfide solide solution (mss) would be produced under nebular conditions and suggested that sulfides in Alais [8] may be nebular condensates [5] . The absence of significant Ni in most sulfides in chondrites can be explained by subsequent chondrule formation and thermal metamorphism (see Fig. 1. below). Apart from Alais, Ni-bearing sulfide also occurs in primitive LPI Technical Report 97-02, Part 1 69 matrix [4] and is widespread in the finest-grained (least melted) associated [18], as in Allende [ 19]. These sulfides match the predic "protoporphyritic" chondrules of Semarkona [9,10]. These chon tions off5-7] fornebular condensates. In addition to mss, several Ni drules contain little or no metal, showing that when chondrule pre bearing sulfides are found: pentlandite in Semarkona, but also cursors were assembled, metal sulfurization was essentially com heazlewooditein Allende. They areassociated withawaruite [19,20]. plete. There are no Ni and S concentration gradients in the sulfides, how Magnetite. The condensation of magnetite is only predicted in ever, which have undergone thermal equilibration throughout the restricted conditions [II]. Though systematically present in the fin rock (see below). est chondrules of Semarkona, it probably results from parent-body Parent-Body Processes: Aqueous alteration. Low-tempera alteration [12]. ture aqueous alteration on carbonaceous chondrite parent bodies was Chondrule Formation: The sequence of events during chon pervasive and complex, particularly when thermal metamorphism drule formation can be reconstructed by looking at a sequence of also occurred [21,22]. This process produced tochilinite, sulfates, porphyritic chondrules with increasing grain sizes, from the finest and ferrihydrite from nebular metal and sulfides [21,22]. However, grained (least melted and closest to their precursors) to the coarsest pyrrhotite may be precipitated as a result of olivine dissolution [23] ones (efficiently melted) [10]. The opaque assemblages and their and tochilinite may break down to give sulfide on heating [22]. textures along this sequence match run products along a temperature Limited aqueous alteration on ordinary chondrites resulted in the gradient in a solar fumace [13]. formation ofcarbide and magnetite in the opaque assemblages [12], Melting andbreakdown ofsulfides. The abundant sulfides in the but the dominant secondary process was dry thermal metamorphism. finest chondrules ofSemarkona (up to 14 wt% S) are interstitial to Thermal metamorphism. I. Thermal equilibration: The Ni con the silicates, as in the coolest region of solar furnace charges [13], in tent ofthe troilite in the most primitive chondrites is below detection which the silicates have experienced very little melting [10]. With and that of the pentlandite is homogeneous all through a given increasing melting, sulfides start breaking down and metal starts meteorite. We attribute this to parent-body equilibration, after [24], appearing, exhibiting typical melt textures with the remaining sul and determine temperatures of230°C for Semarkona and 335°C for fides [13]. Such textures are found in the "intermediate" region ofour Allende [20]. A similar equilibration of the sulfide composition is solar fumace charges and throughout the grain-size sequence in type displayed by the gentler reheating experiments of [25]. II(FeO-rich) chondrules. They are restricted to the "cryptoporphyritic" 2. Sulfur migration: As shown by [25], the Ni-bearing sulfides ones in type Is (FeO-poor), where bulk S contents rapidly decrease eventually decompose into Ni-rich metal (FeNi2) and troilite [25], with increasing grain size [10]. We interpret this as volatile loss, as which produces associations similar to those found in the most documented by systematic experiments [10,13] with sulfide-bearing primitive chondrites. Sulfur mobilization remained very limited in chondrule analogs. these meteorites, but it became more extensive as metamorphism Regeneration ofmetal. Iron reduced from chondrule silicates progressed from 3.1 to 3.5: the previously sulfide-free metal blebs occurs in "dusty" olivine grains [e.g., 14] as experimentally repro of type I chondrules contain increasing quantities of troilite [18]. duced, for example, by [15]. In ordinary chondrites, however, chon This is in agreement with the experiments of [25] that show that drule metal is notthe product ofreduction [16] but mostly ofdesulfu sulfide layers grow on the isolated metal grains ofthe startingassem rization [13]. Metal is rare in type II sulfide-bearing chondrules. Its blages. Natural opaque assemblages also change: By 3.1, pent abundance gradually increases along the type I grain-size sequence landite and awaruite disappear, and (apart from the relict kamacite) as more and more extensive S loss has taken place: Blebs are absent the Ni-bearing phase found in the opaque associations is taenite. A in proto-, appear in crypto-, and become abundant in microporphy similar change is observed in the short high-temperature experiments ritic type I chondrules (but may be partly altered to carbides and of [25]. magnetites [12]). As the silicate grain size further increases, Fe loss 3. Sulfur redistribution: By 3.7, a redistribution of the S be from chondrules is observed. The metal grains become coarser, like comes apparent [18]. The sulfide grains tend to connect, and their the silicates, and are distributed closer to the surface of the chon original distribution becomes a little blurry. In particular, opaque drules. Eventually, no metal is left inside the chondrules, whose encasements around adjoining chondrules tend to merge. This effect surfaces become ornamented with metal grains that occur as a con is observed more clearly in experimental charges in which metal and tinuous layer or only a few massive grains. It is easy to form similar sulfides were initially separated: thin sulfide trails grow throughout metal grains by desulfurization in chondrule analogs. Surface grains the samples and eventually connect separate metal-sulfide assem and crown grains all have a very uniform composition for a given blages [25]. The establishment ofa connected sulfide network through chondrule, and their contents ofCr and Si yield f02 matching those out the parent bodies allows extensive cation movement [25], which based on Fe contents of the chondrule silicates [3]. Metal is thus explains why metal compositions equilibrate from 3.5 to 4 and zoned regenerated by desulfurization during chondrule formation, and its taenites first appear by 3.8 [18]. composition is established by equilibration with the silicate melt [3]. 4. Recrystallization: Between3.8 and 5, metal and sulfide grains Large matrix metal grains (often with fine-grained silicate rims) or tend to separate, and adjacent metal grains coalesce, eliminating "metallic chondrules" [17] were lost by chondrules [3]. silicate inclusions [18]. This reduces surface free energy. Metal Sulfor recondensation. Sulfur lost from chondrules recondenses coarsening is reversed at type 6 by chondrule recrystallization, yield on cooling, and metal crowns ornamenting the chondrule surfaces ing separate interstitial/polygonal metal and sulfide grains evenly and metallic cbondrules are readily available sites. Thus, much metal dispersed throughout the rock [18]. regenerated by chondrule formation will be sulfurized again during Impact Shock: The results of varying degrees of impact on chondrule cooling. Sulfides in opaque encasements around chon opaque minerals in ordinary chondrites are summarized in [26]. drules and in large "opaque chondrules" account for roughly 90% of Conclusions: Chondrites contain two successive generations Semarkona's S, and metal and sulfide are almost always intimately of sulfides (pre- and postchondrule formation) and at least one of 70 Workshop on Modification of Chondri tic Materials metal (fonned in chondrules). New generations of these minerals anything other than an aqueous fluid in an asteroidal environment. fonn on parent bodies. Secondly, we have textural evidence ofthe transport ofmineralizing Acknowledgments: This work was supported by NASA and fluids after asteroidal accretion. Finally, we have pseudomorphs of INSU's PNP. We thank A. Rouanet for his help in solar furnace anhydrous phases after hydrous ones. experiments and D. Lauretta for useful discussions and access to his Mineral Products ofAqueous Alteration: The mineralogical abstract for this workshop. products of aqueous alteration most commonly encountered within References: [I] Grossman and Olsen (1974) GCA, 38, 173. primitive extraterrestrial materials include phyllosilicates, hydrox [2] Kelly and Larimer (1977) GCA, 41, 93. [3] Zanda et al. (1994) ides, tochilinites, sulfates, oxides, and carbonates. Most of these Science, 265, 1846. [4] Greshake (1997) GCA, 6,437. [5] Lauretta alteration products are matrix phases, although alteration of larger et al.( 1997) LPS XXVIII. 783. [6] Lauretta et al. (1996) !carus, 122, components (e.g., chondrules, aggregates, and inclusions) also fig 288. [7] Lauretta et al. (1996) Proc. NIPR Symp. Antarct. Meteor ure prominently. For many of these phases asteroidal origin is pre ites, 9, 97. [8] Kerridge et al. (1979) EPSL, 43, 359. [9] Zanda et al. ferred because the conditions for growth would have been kinetically (\996) LPS XXVII, 1485. [10] Hewins et al. (1997) Proc. NIPR inhibited in the canonical solar nebula. For example, the fC02 nec Symp. Antarct. Meteorites, 10, submitted. [II] Wood and Hashi essary to stabilize carbonates was unlikely to be attained in the solar moto (1993) GCA, 57, 2377. [12] Krot et al. (1997) GCA, 61, 219. nebula [2], as was the fH 20 for phyllosilicates [3], not to mention [13] Zanda et a1. (1997) GCA, submitted. [14] Nagahara (1981) hydrous sulfates. Nature, 292, 135. [IS] Connolly et al. (1994) Nature, 371. 136. Type 3 Chondrites: The least-equilibrated chondrites contain [16] Alexander (1996) in Chondrules and the Protoplanetary Disk limited, though significant, direct evidence of aqueous alteration. (R. H. Hewins et a1., eds.), p. 233, Cambridge Univ. [17] Bevan and Matrix, chondrules, and CAls in the CV3 chondrites Kaba, Bali, Axon (1980) EPSL, 47, 353. [18] Bourot-Denise et aI., this volume. Grosnaja, and Mokoia contain considerable amounts ofphy llosilicates, [19] Haggerty and McMahon (1979) Proc. LPSC 10th, 851. principally smectites and micas, while these phases are also present [20] Zanda et al. (1995) Meteoritics, 30, 605. [21] Zolensky and in trace amounts in Allende and Vigarano [4]. These meteorites also McSween (1988) in Meteorites and the Early Solar System (1. F. contain feldspathoids (nepheline and sodalite) of proposed metaso Kerridge and M. S. Matthews, eds.), p. 114, Univ. of Arizona. matic origin. The alteration phases found in CV3 chondrites is [22] Tomeoka et al. (\ 989) Proc. NIPR Symp. Antarct. Meteorites, grossly similar to that in the unequilibrated ordinary chondrites. 2,55. [23] Browning et al. (1994) Meteoritics, 29, 450. [24] Blum Recent work on C03 chondrites has revealed minor amounts of et al. (1989) GCA, 53, 543. [25] Lauretta et aI., this volume. serpentine associated with ferrihydrite, ofprobable asteroidal origin [26] Bennett and McSween (\996) Meteoritics & Planet. Sci., 31, [5], replacing matrix olivine. However, Brearley used the immature 255. protophyllosilicates found in ALHA 77307 to argue that some alter ation had occurred to C03s in the nebula [6]. The nature and origin of this and other poorly crystallized materials in chondrite matrix AQUEOUS ALTERATION OF CARBONACEOUS CHON deserves much more work. DRITES: EVIDENCE FOR ASTEROIDAL ALTERATION. Type 2 Chondrites: The products of aqueous alteration are M. E. Zolensky, Mail Code SN2, NASA Johnson Space Center, ubiquitously present in the CM2 and CR2 chondrites, in amounts far Houston TX 77058, USA. greater than within the type 3s, and in all components (matrix, phenocrysts, chondrules, CAIs). These minerals include abundant Introduction: The primary reactions involved in chemical al serpentines (with extremely variable compositions and structures), teration are oxidation, hydrolysis, and carbonation. Here we are smectites (rare in the CM2s, abundant in the CR2s), clinochlore, Mg concerned with the action ofthe second process, i.e., aqueous alter Fe sulfates, tochilinite, tochilinite-serpentine intergrowths, and car ation. Aqueous alteration may occur over a very wide range of tem bonates [7-9]. Abundant evidence ofreplacement reactions are evi peratures and pressures, including temperatures below O°C. There dent, including olivine and glass being replaced by serpentine, metal are two primary reasons for studying these reactions. First we wish and sulfides being replaced by tochilinite, and Fe-rich serpentine to better understand the overall history of chondri tic materials. Sec being replaced by Mg-rich serpentine [8-10]. ond, aqueous reactions promote chemical redistribution, changes in Carbonates (calcite and dolomite), and lesser sulfates, intimately petrologic relationships and isotopic fractionation, and can compli intergrown with matrix phyllosilicates and tochilinite have been cate thennoluminescence (TL) patterns [I], rendering uncertain the notably reported in Murchison, Nogoya, Cold Bokkeveld, Nawapali, metamorphic grade if this is detennined by TL. There are several and Cochabamba [11-14]. possible major sources ofH20 available for aqueous alteration reac The occurrence ofaragonite in Cochabamba [12] is particularly tions in the early solar system. Water vapor would have been avail interesting because of its metastability with respect to calcite. able for reactions in the primitive solar nebula and, in the absence of Armstrong et al. [2] report calcite intimately intergrown wi th hiboni te evidence for the direct condensation of hydrous minerals in the in the core ofa large CAl, and argue that its presence is best explained nebula, it is possible that these phases could have fonned via the by aqueous alteration on (in?) a planetary body during which °iso hydration (and reconstructive transformation) of preexisting anhy topic exchange may have occurred. drous silicates. However, this scenario cannot account for all aque The organic compounds found withinCM2s [15,16] are believed . ous and related phases observed in meteorites. to have processed during asteroidal evoution, with the present com In this abstract we describe the evidence for aqueous alteration pounds being produced by aqueous and hydrothermal activity. More observed within carbonaceous chondrites requiring an asteroidal (or work on organics will probably be shortly forthcoming. other parent-body) setting. This evidence is ofseveral types. First we Type 1 Chondrites: The cn and CM I chondrites are com have the presence ofspecific minerals unlikely to have formed from posed almost entirely of secondary phases, with the bulk being LPJ Technical Report 97-02, Part I 71 phyllosilicate consisting ofintergrown serpentine and saponite. Pyr zoning or correlations within matrix and altered chondrules and rhotite is the major accessory phase, and it is present in pseudo aggregates (the bulk compositional trends reported for CMs by hexagonal plates and acicular crustals. The acicular pyrrhotites in McSween [26] for example); and (5) lining fractures and filling Kaidun CM 1 carry thick mantles of phyllosilicate not found any cracks-as calcite does in a Murchison CAl [2]. where else [17]. Pseudomorphs After Secondary Minerals and Textures: We Magnetite is another mineral whose presence in chondrites is a have recently recognized that many chondrites hitherto considered potentially important indicator of the conditions of aqueous alter to have experienced a free and easy Ii fe have actually been thoroughly ation, although this is the subject of considerable debate requiring aqueously altered., but with most evidence erased by subsequent additional work. The morphologies of magnetite in Cis (as well as thermal metamorphism, as discussed in more detail by Krot. In the most C2s and C3s, something not generally appreciated) include CV3 chondrites there are inclusions of platy, defect-ridden, inclu framboids, spherulites, and plaquettes, delicate structures that ex sion-bearing, fayalitic olivine, in obvious pseudomorphic replace perimental work indicates may have crystallized from gels or aque ment of phyllosilicates that had, in tum, replaced primitive chon ous solutions [18]). Many magnetite framboids are arranged into a drules, CAls and matrix [27-29]. There are even pseudomorphs after closest-packing relationship that is difficult to reconcile with a nebu veins crisscrossing large areas. The suggestion has been made that lar origin. In Kaidun magnetite framboids are clearly replacing pyr the host CV3s themselves have been similarly altered. rhotite in situ, clearly requiring an asteroidal origin [17]. References: [I] Sears et al. (\980) Nature, 287, 791-795. The CI I chondrites frequently display crosscutting veins of [2] Armstrong et al. (1982) GCA, 46,575-596. [3] Prinn and Fegley Na-, Ca-, and Mg-sulfates (epsomite, hexahydrite, gypsum, and (1987) Annu. Rev. Earth Planet. Sci., 15, 171-212. [4] Krot et al. blodite) [19-21]. Younger carbonate veins can be observed to cut the (\995) Meteoritics, 30, 748-775. [5] Keller and Buseck (1990) sulfates in some cases. In the CMI chondrites the carbonate veins GCA, 54, 1155-1163. [6] Brearley (1993) GCA, 57, 1521-1550. only are present [22]. [7] Barber (1985) Clay Minerals, 20, 415-454. [8] Tomeoka and Studies of Cis (Orgueil) have revealed the presence of alkanes, Buseck (1985) GCA, 49,2149-2163 . [9] Zolensky et al. (1993) aromatic hydrocarbons, aliphatic carboxylic acids, purines, pyrim GCA, 57, 3123-3148. [10] Browning et al. (1996) GCA, 60, 2621 idines, amino acids [23], and insoluble aliphatic and aromatic/ole 2633. [11] Fuchs et al. (\973) Smithson. Contrib. Earth Sci., 10, finic structures [15], all taken to have formed from aqueous solu 39 pp. [12] Muller et al. (\ 979) TMPM, 26, 293-304. [13] Bunch tions. and Chang (1980) GCA, 44, 543-1577. [14] Barber (1981) GCA, Textural Evidencefor Asteroidal Alteration: Aqueous alter 45, 945-970. [15] Cronin (1987) LPI Report 87, p. 10. [16] Peltzer ation within meteorite parent bodies is required by the common et al. (1984) Adv. Space Res., 4, 69-74. [17] ZolensJcy et al. (1996) occurrence ofalteration minerals with the following special textures: Meteoritics & Planet. Sci., 3l, 484-493. [18] Kerridge and Bunch (I) Mineral grains bridging chondiules, aggregates, and phenocrysts (1979) in Asteroids, pp. 745-764. [19] Bostrom and Fredriksson with matrix-for example tochilinite appearing at the interface be (1966) Smithson. Misc. Coli., 15, 1-39. [20] Richardson (1978) tween fractured chondrule grains and matrix in Mighei [8]; (2) veins Meteoritics, 13, 141-159. [21] Fredriksson and Kerridge (1988) bridging chondrules, aggregates, and phenocrysts with matrix - for Meteoritics, 23, 35-44. [22] Zolensky et al. (1997) GCA, in press. example, serpentine-sulfide veins between chondrules, chondrule [23] Hayatsu and Anders (1981) Topics in Current Chemistry, 99, rims, and matrix in CM2 chondrites [24], carbonate veins in CMI 1-37. [24] Browning and Keil (1997) LPS XXVIII, 163-164. chondrites and the CR2 chondrite Al Rais, and sulfate/carbonate [25] Tomeoka and Buseck (1982) Nature, 299, 326-327. [26] Mc veins in CII chondrites [21]; (3) secondary phases being distributed Sween (1979) GCA, 43, 1761-1770. [27] Akai (1988) GCA,52, throughout most, or all, constituents ofa meteorite, often with near 1593-1599. [28] Kojima et al. (1993) Meteoritics, 28, 649-658. identical composition everywhere (for example framboidal magne [29] Krot et al. (1995) Meteoritics, 30,748-776. tite and "saponite" in Allende) [25]; (4) displaying relict chemical