Carbonate assemblages in Cold Bokkeveld CM chondrite reveal complex parent body evolution

1Stefan Farsang,2Ian A. Franchi,,Xuchao Zhao,3Timothy D. Raub,4Simon A.T. Redfern,2,5Monica M. Grady
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13647]
1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK
2School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
3School of Earth & Environmental Sciences, University of St Andrews, Irvine Building, St Andrews, KY16 9AL UK
4Asian School of the Environment, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798 Singapore
5Department of Earth Sciences, Natural History Museum, Cromwell Rd, London, SW7 5BD UK
Published by arrangement with John Wiley & Sons

The paragenesis of carbonates in the Cold Bokkeveld CM chondrite is determined from a detailed petrographic, chemical, spectroscopic, and isotopic study of nine associations of carbonates (aragonite, calcite, and dolomite) with other secondary minerals that occur within the meteorite. Our study reveals the existence of carbonates displaying petrographic features that are distinct from those of type 1 and type 2 carbonates commonly observed in CM2 meteorites. These include carbonates interstitial to octahedral magnetite crystals, for which a new designation of “type 1c” is suggested. The O isotopic values of dolomite (δ18O ranging from +21.1 to +25.8‰ and Δ17O from −4.9 to −4.0‰) are similar to those measured in dolomites from other CM chondrites. The presence of complex carbonates with a CaCO3 core and Mg‐enriched rim implies several generations of fluids and/or their evolving composition on the CM parent body(ies). Petrographic characteristics indicate at least six stages of potentially overlapping carbonate and phyllosilicate formation events. We show that type 1 and type 2 calcite have distinct Raman spectral characteristics. Type 1 calcite is characterized by very broad peaks, whereas type 2 calcite displays narrow peaks similar to those of typical abiotic terrestrial calcite, suggesting high crystallinity. A carbonate Raman spectrum showing features characteristic of both aragonite and calcite likely documents an aragonite‐calcite phase transition. Raman spectroscopy also reveals the presence of organic matter in the majority of carbonates. This indicates that organic carbon was mobilized by aqueous fluids for extended periods.

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