Lunar and Planetary Science XXXIV (2003) 1843.pdf

‘Normal’ Fe-Mg-Al-Cr Spinel Minerals in Chondritic Stratospheric IDPs. D. J. Joswiak and D. E. Brownlee, Dept. of Astronomy, University of Washington, Seattle, WA 98195

Introduction: Spinel minerals with ‘normal’ structure are terrestrial saponite, Orgueil matrix [1] and Allende+coal common in extraterrestrial materials and can form via a and Fe-rich olivine+coal mixtures [unpublished data], (de- large range of processes including nebular condensation, signed to simulate atmospheric entry conditions in IDPs), crystallization from liquid melts and sub-solidus metamor- are also shown in the figure along with average spinel from phic reactions. Owing to their ability to accept a large Type 2 chondrules in carbonaceous chondrites [2]. The range of cations in their crystal structures these minerals spinel field from Type B CAIs in carbonaceous chondrites can be regarded as useful petrogenetic indicators which is indicated by the elongated oval on the spinel-hercynite largely reflect their parental sources. In carbonaceous solid solution join. chondrites, for instance, multicomponent spinels present in Secondary Spinels: Thirteen spinels studied in nine IDPs Type 2 chondrules crystallized from silicate melts (drop- are believed to have a secondary origin which were likely lets) rich in Fe, Cr, Mg and Al whereas nearly pure formed in-situ during atmospheric entry heating. These are MgAl204 spinels present in Type B CAIs formed in a nebu- shown in the ternary diagram as either open or solid circles lar environment consistent with high temperature conden- without an adjacent letter ‘P’; most plot in the interior of sation of refractory minerals. the ternary diagram and contain Cr contents above 15 wt%. In this study we examine non-magnetite, non- These spinels are primarily mixtures of Mg, Fe, Cr and Al magnesioferrite spinel minerals rich in Fe2+, Mg, Al and oxides, and are believed to be liquidus phases crystallized Cr found in IDPs with the hope of shedding insight on how from localized partial melts within the IDPs as low tem- these minerals may have formed. Generally, it appears that perature phases (probably phyllosilicates) were heated to many were formed in the interiors of IDPs via atmospheric melting during atmospheric entry. Typical sizes of these entry heating (secondary spinels). Some of the spinels, spinels are 50 – 100 nm and most are associated with Fe- however, cannot be explained by this mechanism and are rich olivines (or occasionally pyoxenes) and aluminosili- more consistent with having their origins as high tempera- cate glass and sometimes occur as inclusions in these ture nebular condensates (primary spinels). We use spinel phases. Nearly identical assemblages – Mg-Fe-Al-Cr compositions, textural relationships, mineral associations spinel+Fe-rich olivine+aluminosilicate glass – were ob- and comparison to spinels produced in heating experiments served from pulse heating experiments on terrestrial sapo- and those in other extraterrestrial materials to differentiate nites and Orgueil matrix [1] which were designed to simu- primary from secondary spinel minerals in IDPs. late atmospheric entry heating conditions. Stepped He- Analytical Procedures: Spinel group minerals were stud- release data obtained on many of these IDPs provide peak ied in 15 individual chondritic hydrated and anhydrous heating temperatures up to 960 °C [3]. IDPs. The IDPs were microtomed and placed on standard Most of the secondary spinels are present in anhydrous C-coated TEM grids. Chemical data were obtained from silicate IDPs; three that plot on the Mg chromite-chromite EDX spot analyses on 19 spinel grains using a 120 KV solid solution line are all from a single hydrated IDP. JEOL 1200EX STEM. SAED patterns were collected on From SEM photos, none of the IDPs show any significant a few grains to confirm their spinel structures. ‘Normal’ external melt features. The data suggest that minor spinel endmembers – MgAl204-FeAl204-MgCr204-FeCr204 – amounts of saponite or other low temperature minerals were calculated after Fe3+ determineration by charge bal- present in strongly heated IDPs may produce local melt ance. High magnesioferrite or magnetite spinels were not pockets in the interior of the IDPs during atmospheric entry used in this study. In most spinels, the elements Mn and Ti and then recrystallize as a multicomponent spinel min- were typically low in abundance and were not considered. eral+Fe-rich olivine+Al-rich silicate glass. The spinel Due to their small sizes, spectra obtained from some spinel mineral compositions probably reflect the local melt chem- grains were actually composite spectra from overlaps with istry and therefore significant differences in spinel compo- adjacent grains or host phases, particularly olivine. In a sitions may occur even within a single 10 um IDP. few cases where the overlaps were particularly severe, Primary Spinels: Six spinels studied (from 5 different spectra from the ‘host’ phase was subtracted from the com- IDPs) are candidates for having a primary origin, perhaps posite spectra to give an improved spinel analysis. as nebular condensates. These six are labelled in the ter- The results from the measurement of 19 individual spinel nary diagram with a small upper case ‘P’ and have grain grains are plotted in the Mg-Al-Fe2+ ternary diagram. sizes up to 300 nm. Four were observed in hydrated sili- Solid solution fields for spinel-hercynite and Mg chromite- cate dominant IDPs (2 cronstedtite-rich IDPs and one chromite are shown. In the figure, spinel analyses obtained saponite-rich IDP) and plot close to MgAl204 spinel. Of from hydrated IDPs are shown as solid circles while open these, two fall within the CAI spinel field for carbonaceous circles apply to spinels from anhydrous IDPs. Two circle chondrites; one contains 20 wt% Fe and is similar in com- sizes are shown for both hydrated and ahydrous IDPs; large position to Fe-rich spinel from fine-grained Type 2 inclu- circles indicate high measured Cr while small circles indi- sions in Allende with spinel FeO contents up to 23 wt% cate low Cr contents. A capital letter ‘P’ is shown adjacent [4]. The three spinel-bearing HS IDPs cantain phyllosili- to the six IDP spinels that may have a primary origin (see cate minerals that appear relatively intact (7.1 – 7.6 A below). lattice fringes measured for cronstedtite and 11.3 – 11.6 A Spinel data obtained from pulse heating experiments on lattice fringes measured for saponite) even though meas- Lunar and Planetary Science XXXIV (2003) 1843.pdf

‘Normal’ Fe-Mg-Al-Cr Spinel Minerals in IDPs: D. J. Joswiak and D. E. Brownlee

ured peak atmospheric entry temperatures were relatively with Fe-rich olivine and aluminosilicate glass. Their mul- high. No evidence of Fe-rich olivines, characteristic of ticomponent compositions – MgO+FeO+Al2O3+Cr2O3 – secondary assemblages, was observed. Cr contents were reflect crystallization from localized melts from low tem- low in these spinels (0.9 – 1.9 wt%) consistent with spinels perature minerals heated during atmospheric entry. Most from refractory inclusions in carbonaceous chondrites [4]. of the secondary spinels were found in anhydrous IDPs. Two anhydrous IDPs were found to contain Fe-free spinels; Six primary spinels were observed in 5 IDPs; all were char- one plots near MgAl2O4 (and overlaps a secondary spinel acterized by high MgAl2O4 contents or high MgAl2O4 with from a hydrated silicate in the ternary diagram making it either moderate Fe or Cr. These compositions are similar difficult to see). The other primary spinel plots as a large to spinels in CAI’s from carbonaceous chondrites. Their open circle about 1/3 of the distance between MgAl2O4 and MgAl2O4-rich endmember compositions and lack of asso- MgCr2O4 indicating its Fe-free composition but higher Cr ciated Fe-rich olivines typical of secondary minerals, sug- content (23.6 wt% Cr). It occurs in close contact with an gest that these spinels are good candidates to have their aluminous Ti-bearing diopside (2.6 wt % Al; 0.5 wt % Ti). origins in a solar nebula environment. This spinel is similar in composition to spinel grains sepa- References: [1] Joswiak, D. J. and Brownlee, D. E., LPSC rated from the Murchison meteorite where FeO is predomi- XXIX, 1998. [2] Johnson, C. A. and Prinz, M., GCA, 893- nantly low and Cr203 ranges up to 21.5 wt% [5]. 904, 1991. [3] Nier, A. O. and Schlutter, D. J., Meteoritics Conclusions: Non-magnetite spinel minerals were found 28, 675, 1993. [4] Brearley, A. J. and Jones, R. H., Plane- in 15 hydrated and anhydrous IDPs. Most of the spinels tary Materials, Reviews in Mineralogy, Vol. 36, J.J. Papike are believed to have formed in the interiors of the IDPs ed., 3-114. [5] Kuehner S. M. and Grossman, L., LPSC during atmospheric entry heating. The secondary spinels XVIII, 1987. were often found poikilitically enclosed in or associated

Mg-Al-Fe2+ Spinel Ternary Diagram

Al J HS IDP: Cr>15 wt%

CAI spinels in CC's J HS IDP: Cr<2 wt%

E Anh IDP: Cr>15 wt%

E Anh IDP: Cr<5 wt% P JI MgAl 0 PJE J P E FeAl 0 P E I Pulse Heating Exps. 2 4 J 2 4 P E I EE D Ave. Type2 Chondules E P Atom % I

E E E D E MgCr 0 E I E I FeCr 0 2 4 E 2 4 I

I JJ

2+ Mg Fe Mg chromite - chromiteJ solid soln.