Lee, M.

Lindgren, P.

Sofe, M.

English

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Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk       Sofe, M., Lee, M., and Lindgren, P. (2011) Aragonite in the CM Carbonaceous Chondrites: A Proxy for the Magnitude of Aqueous Alteration. In: 74th Annual Meteoritical Society Meeting, August 8–12, 2011, London, UK.       0B0Bhttp://eprints.gla.ac.uk/59750/     Deposited on: 6 February 2012   ARAGONITE IN THE CM CARBONACEOUS CHONDRITES: A PROXY FOR THE MAGNITUDE OF AQUEOUS ALTERATION  M. R. Sofe, M. R. Lee and P. Lindgren. School of Geographical and Earth Sciences, University of Glasgow, Glasgow G128QQ, UK. E-mail: msofe@glasgow.ac.uk.  Introduction: The degree of aqueous alteration of CM car-bonaceous chondrites can be tracked by changes in properties including in the volume of fine-grained matrix, the volume and chemical composition of its constituent phyllosilicates, and the extent of alteration of chondrules [e.g. 1-3]. The carbonate min-erals aragonite, calcite and dolomite were also formed during al-teration and whereas calcite is seen in all CMs, dolomite has been recorded only from the more highly altered meteorites [e.g. 4] and little is known about the occurrence of aragonite. Here we have sought to understand better the significance of the corre-spondence between carbonate mineralogy and degree of altera-tion by undertaking a systematic study of inter-meteorite differ-ences in the abundance and properties of aragonite. Methods: Six CM2s were studied, namely Pollen, Murray, Mighei, LON 94101, Nogoya and QUE 93005 (listed in the ap-proximate order of increasing degree of alteration), together with the CM1 SCO 06043. Dolomite was identified by SEM imaging and X-ray microanalysis, and aragonite was distinguished from calcite using electron backscatter diffraction and Raman spec-troscopy.  Results: Aragonite occurs in the matrices of five of the me-teorites studied and is absent from the most highly altered sam-ples (i.e. QUE 93005 and SCO 06043). Only Nogoya contains both aragonite and dolomite. The approximate abundance of ara-gonite, expressed as grains/mm2, is: Pollen (0.20), Murray (0.30), Mighei (0.23), LON 94101 (0.10), Nogoya (0.17). The mean sizes of these aragonite grains are also similar: Pollen (21 μm), Murray (22 µm), Mighei (33 µm), LON 94101 (25 µm), Nogoya (14 µm). The largest grain (101 μm) was found in Mighei. Unlike calcite, the aragonite grains usually lack serpentine-tochilinite rims, although many of them have been partially replaced by ma-trix phyllosilicates. Most of the crystals occur in loose clusters and their c axes have a common alignment.  Discussion: As aragonite crystals are embayed by phyllosili-cates they formed prior to the end of aqueous activity, and their common alignment suggests crystallization under uniaxial lithostatic pressure at depth within the parent body. Given that aragonite occurs only in the less altered meteorites and dolomite in the more highly processed CMs, with the crossover at Nogoya (classified as CM2.2 by [2]), these minerals track the degree of aqueous alteration by their presence or absence. We suggest that crystallization of aragonite rather than calcite is favoured in solu-tions with Mg/Ca >~1, and significantly higher Mg/Ca ratios will yield dolomite. If aragonite and dolomite crystallized after cal-cite, then the Mg/Ca of bicarbonate rich solutions must have in-creased with time, and reached significantly greater values in those meteorites that were more highly altered overall. Acknowledgements: We thank the Natural History Museum (London) and NASA-JSC for loan of the samples studied. References: [1] McSween, H.Y. 1979. Geochimica et Cos-mochimica Acta 43:1761–1770. [2] Rubin A.E. et al. 2007. Geo-chimica et Cosmochimica Acta 71:2361–2382. [3] Howard K.T. et al. 2009. Geochimica et Cosmochimica Acta 73:4576–4589. [4] Johnson, C.A. and Prinz, M. 1993. Geochimica et Cosmo-chimica Acta 57:2843–2852. 74th Annual Meteoritical Society Meeting (2011) 5250.pdf

Aragonite in the CM Carbonaceous Chondrites: A Proxy for the Magnitude of Aqueous Alteration

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Aragonite in the CM Carbonaceous Chondrites: A Proxy for the Magnitude of Aqueous Alteration

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