Lee, M.

Smith, C.L.

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 Smith, C.L. (2010) Mg-Phyllosilicate Pseudomorphs after Calcite in the Pollen (CM2) Carbonaceous Chondrite: New Insights Into Aqueous Alteration. In: 73rd Annual Meteoritical Society Meeting, July 26-30, 2010, New York City, NY.       0B0Bhttp://eprints.gla.ac.uk/59743/     Deposited on: 6 February 2012   MG-PHYLLOSILICATE PSEUDOMORPHS AFTER CALCITE IN THE POLLEN (CM2) CARBONACEOUS CHONDRITE: NEW INSIGHTS INTO AQUEOUS ALTERATION M.R. Sofe1, M.R. Lee1 and C.L. Smith2. 1Department of Geo-graphical and Earth Sciences, University of Glasgow, G12 8QQ, U.K. E-mail: Mahmood.Sofe@ges.gla.ac.uk. 2Department of Mineralogy, Natural History Museum, London SW7 5BD, U.K.  Introduction: CM2 carbonaceous chondrites contain carbon-ates as rare and small crystals. These are mainly calcite, but in-clude aragonite and dolomite. Carbonate minerals were formed during low temperature (~20-70°C) aqueous alteration in the par-ent body or bodies of CM chondrites (e.g. [1-4]). Some aspects of carbonates such as their formational conditions, timing of pre-cipitation relative to each other (were the carbonates precipitated all at once, or were there several generations?), and relative to alteration of the matrix remain poorly understood. This informa-tion can provide new insights into the mechanisms of carbonate growth and alteration process that took place within CM parent body(ies). Here we describe evidence for multiple generations of carbonate crystallization in Pollen (CM2 chondrite) and show that one generation has been pseudomorphed by phyllosilicates. Methods: Carbonates within fine grained matrix were located in a polished thin section of Pollen (CM2) using calcium X-ray maps combined with backscattered electron (BSE) images. These grains were studied by SEM electron backscatter diffraction (EBSD), for accurate determination of crystallographic orienta-tions, SEM cathodoluminescence (CL) imaging and spectroscopy to identify zoning, and laser Raman spectroscopy for mineral identification. Electron probe microanalysis (EPMA) was also used for quantitative chemical analysis of Ca-carbonates, Mg-phyllosilicate pseudomorphs and the matrix.  Results and discussion: Pollen contains 2.02 volume % ara-gonite and calcite, mainly as isolated single grains, but in rare cases also as aggregates. We can identify one generation of ara-gonite and three generations of calcite from their petrographic relationships with each other and with matrix phyllosilicates. The first calcite generation forms zoned crystals that are rimmed by serpentine-tochilinite and contain iron sulphide inclusions. This generation was then partially replaced by Mg-rich phyllosilicates to leave serpentine-tochilinite rimmed pseudomorphs, which comprise 3.15 vol% of the meteorite. A later generation of calcite then replaced chondrule and matrix olivines/pyroxenes and these grains are free of serpentine-tochilinite rims. Aragonite is in-ferred to mark the end of carbonate crystallization and possibly formed synchronously with the third calcite generation. Ca-carbonates in Pollen record a complex history of crystalli-zation. The formation of Mg-phyllosilicate pseudomorphs by re-placement of rimmed calcite grains shows that carbonate crystal-lization was discontinuous, but synchronous or overlapping with matrix phyllosilicate formation. Our observations of other CM2 carbonaceous chondrites suggest that complex histories of car-bonate and phyllosilicate crystallization, dissolution and re-placement are the norm and require further detailed study. References: [1] Clayton R. N. and Mayeda T. K. 1984. Earth and Planetary Science Letters 67: 151 161. [2] Brearley A. J. et al 2001. Abstract #1458. 32nd Lunar & Planetary Science Confer-ence. [3] Lee M. R. and Ellen R. 2008. Meteoritics & Planetary Science 43: 1219-1231. [4] Guo W. and Eiler J. M. 2007. Geo-chimica et Cosmochimica Acta 71: 5565-5575. 73rd Annual Meteoritical Society Meeting (2010) 5112.pdf

Mg-Phyllosilicate Pseudomorphs after Calcite in the Pollen (CM2) Carbonaceous Chondrite: New Insights Into Aqueous

Alteration

http://eprints.gla.ac.uk/59743/1/id59743sofe-1.pdf

Mg-Phyllosilicate Pseudomorphs after Calcite in the Pollen (CM2) Carbonaceous Chondrite: New Insights Into Aqueous Alteration

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