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Volatile Fractionation in the Early Solar System and Chondrule Matrix Complementarity

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Volatile Fractionation in the Early Solar System and Chondrule Matrix Complementarity Volatile fractionation in the early solar system and chondrule͞matrix complementarity Philip A. Bland*†‡, Olivier Alard§¶, Gretchen K. Benedix*ʈ, Anton T. Kearsley†, Olwyn N. Menzies*, Lauren E. Watt*, and Nick W. Rogers§ *Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom; †Impacts and Astromaterials Research Centre, Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom; §Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; ¶Laboratoire de Tectonophysique, Centre National de la Recherche Scientifique–Institut des Sciences de la Terre, de l’Environnment, et de l’Espace de Montpellier, Universite´de Montpellier II, 34095 Montpellier, France; and ʈDepartment of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 Edited by Robert N. Clayton, University of Chicago, Chicago, IL, and approved July 15, 2005 (received for review March 8, 2005) Bulk chondritic meteorites and terrestrial planets show a mono- using a two-component model related to chondrule formation, tonic depletion in moderately volatile and volatile elements rela- where a volatile-rich component (matrix, of CI composition) tive to the Sun’s photosphere and CI carbonaceous chondrites. accreted with volatile-depleted chondrules (the assumption be- Although volatile depletion was the most fundamental chemical ing that volatility-dependent evaporative loss occurred during process affecting the inner solar nebula, debate continues as to its chondrule formation). Wasson and Chou (2), observing a mono- cause. Carbonaceous chondrites are the most primitive rocks avail- tonic decrease in volatile abundance with decreasing condensa- able to us, and fine-grained, volatile-rich matrix is the most tion temperature, proposed the incomplete condensation model, primitive component in these rocks. Several volatile depletion in which a gas of solar composition dissipated during conden- models posit a pristine matrix, with uniform CI-like chemistry sation. In this scenario, volatile depletion occurred before chon- across the different chondrite groups. To understand the nature of drule formation and is generally related to cooling of a hot inner volatile fractionation, we studied minor and trace element abun- disk. Attempts were also made to explain volatile depletion by dances in fine-grained matrices of a variety of carbonaceous evaporation during parent body heating (3). Subsequent exper- CHEMISTRY chondrites. We find that matrix trace element abundances are imental studies showed that volatile fractionation patterns can- characteristic for a given chondrite group; they are depleted not be reproduced by heating chondritic materials (4), while relative to CI chondrites, but are enriched relative to bulk compo- numerical modeling of nebula conditions further supported sitions of their parent meteorites, particularly in volatile sidero- incomplete condensation (5). The incomplete condensation phile and chalcophile elements. This enrichment produces a highly model is now broadly favored (6). The two-component model nonmonotonic trace element pattern that requires a complemen- remains alive, however, with the recent suggestion that the tary depletion in chondrule compositions to achieve a monotonic bulk. We infer that carbonaceous chondrite matrices are not primitive chondrites contain fine-grained materials (accretion- pristine: they formed from a material reservoir that was already ary rims around chondrules and matrix) of approximately uni- depleted in volatile and moderately volatile elements. Additional form CI-like composition, which escaped high-temperature pro- thermal processing occurred during chondrule formation, with cessing (7). In addition, the X-wind model (8) suggests that CAIs Ϸ exchange of volatile siderophile and chalcophile elements be- and chondrules formed close to the proto-Sun ( 0.06 a.u.), tween chondrules and matrix. This chemical complementarity before being carried out to fall onto a ‘‘cold’’ accretion disk shows that these chondritic components formed in the same composed of thermally unprocessed fine-grained nebula mate- nebula region. rial. This is a variant of the two-component model: in this scenario, nonfragmental matrix (matrix without chondrule frag- carbonaceous chondrite ͉ chondrule formation ͉ volatile depletion ments) should be of similar CI-like composition in different unequilibrated meteorites; matrix should not be compositionally related to chondrules in a given meteorite; and bulk volatile arbonaceous chondrites were first recognized as primitive ͞ Csamples of the early solar nebula in the 1960s, when it became depletion should be a function of the chondrule matrix ratio. apparent that a group of these meteorites contained moderately Several other models (9, 10) also advocate chondrule formation volatile elements (condensing between Ϸ1,350 K and 650 K) and in the inner solar nebula and later mixing with matrix further out. volatile elements (condensing at Ͻ650 K) in similar abundance to Huss et al. (11) consider that the survival of presolar grains in the solar photosphere (1). These elements are severely depleted in carbonaceous chondrites, and the correlations between presolar the terrestrial planets and in most other meteorite groups. Chon- grain abundance patterns and volatile-element depletions in drites are constructed from chondrules, the igneous products of bulk meteorites, precludes incomplete condensation from a solar transient heating events, and Ca–Al-rich refractory inclusions gas as the mechanism for bulk volatile depletion. Instead, they (CAIs). These high-temperature components are embedded in a propose that the basic chemical characteristics of the different volatile-rich, fine-grained, mineralogically complex matrix, which is chondrite groups were produced by variable heating and partial a host for presolar grains. In addition to the different compositional evaporation of presolar dust before chondrule formation. Vol- groups (e.g., CV, CO, CR, CM, and CI), carbonaceous chondrites atile loss during chondrule formation and possibly local con- are subdivided from types 1 to 6 based on the degree of secondary densation were superimposed on this primary evaporative de- aqueous and thermal processing that they have experienced, type 3 pletion. Finally, Yin proposed that the signature of volatile being the most primitive, with aqueous alteration increasing to type 1, and types 3 to 6 showing increasing thermal alteration. CI1 chondrites, aqueously altered, and composed almost entirely of This paper was submitted directly (Track II) to the PNAS office. matrix, have compositions indistinguishable from solar. Other Freely available online through the PNAS open access option. carbonaceous meteorites, containing less matrix (and more chon- Abbreviations: CAI, Ca–Al-rich refractory inclusion; LA, laser ablation; ICP, inductively drules), are depleted in volatiles to varying degrees. coupled plasma. The varying volatile contents in chondritic meteorites ‡To whom correspondence should be addressed. E-mail: [email protected]. prompted Anders (1) to explain volatile element depletion by © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0501885102 PNAS ͉ September 27, 2005 ͉ vol. 102 ͉ no. 39 ͉ 13755–13760 Downloaded by guest on October 1, 2021 element depletion in chondritic meteorites predates any nebula tions. Finally, we observe a good agreement between solution and process and is instead inherited from the interstellar medium.** LA analysis, further evidence that our LA-ICP-MS data do not Trace elements have widely varying chemical affinities and suffer from an analytical artifact. We also find good agreement with condensation temperatures, so their abundance places primary earlier instrumental neutron activation analysis data for Allende, constraints on models describing the formation and evolution of Ornans, and Renazzo matrix (13, 15, 17). chondritic materials. Yet, while primitive meteorites are com- posed of materials that have experienced a huge range of thermal Results environments, models of volatile depletion are based on bulk Elemental compositions of matrices in the CV3, CO3, CR2, CM2, trace element data, where all components are analyzed together and ungrouped carbonaceous chondrites Acfer 094, Tagish Lake, in a bulk. Matrix is the most primitive component in carbona- and NWA1152 are plotted in Fig. 1, with abundance vs. conden- ceous chondrites, but trace element data are available only for sation temperature. Literature data for bulks and relevant chon- four meteorites, with a limited element list: Allende (13, 14), drule, matrix, and rim data (13, 15, 17, 18) are also shown (our Ornans (15), Mokoia (16), and Renazzo (17). Finally, the extent ICP-MS data, ratioed to CI1 chondrite and Yb, are provided in to which chondrules and matrix are genetically related is the Tables 2–5, which are published as supporting information on the subject of continuing debate. Some evidence of chemical PNAS web site). Fig. 2 summarizes volatile data from selected complementarity has been produced for the CR2 chondrites meteorites (with elements in order of volatility) and shows average (17), but for other meteorite groups the relationship between matrix compositions for the different groups and LA-ICP-MS data chondrules and matrix is less clear. for the CI1 chondrites Orgueil and Alais.
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