Fluid evolution in CM carbonaceous chondrites tracked through the oxygen isotopic compositions of carbonates

Lindgren, P., Lee, M.R. , Starkey, N.A. and Franchi, I.A. (2017) Fluid evolution in CM carbonaceous chondrites tracked through the oxygen isotopic compositions of carbonates. Geochimica et Cosmochimica Acta, 204, pp. 240-251. (doi: 10.1016/j.gca.2017.01.048)

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The oxygen isotopic compositions of calcite grains in four CM carbonaceous chondrites have been determined by NanoSIMS, and results reveal that aqueous solutions evolved in a similar manner between parent body regions with different intensities of aqueous alteration. Two types of calcite were identified in Murchison, Mighei, Cold Bokkeveld and LaPaz Icefield 031166 by differences in their petrographic properties and oxygen isotope values. Type 1 calcite occurs as small equant grains that formed by filling of pore spaces in meteorite matrices during the earliest stages of alteration. On average, the type 1 grains have a δ18O of ∼32–36‰ (VSMOW), and Δ17O of between ∼2‰ and −1‰. Most grains of type 2 calcite precipitated after type 1. They contain micropores and inclusions, and have replaced ferromagnesian silicate minerals. Type 2 calcite has an average δ18O of ∼21–24‰ (VSMOW) and a Δ17O of between ∼−1‰ and −3‰. Such consistent isotopic differences between the two calcite types show that they formed in discrete episodes and from solutions whose δ18O and δ17O values had changed by reaction with parent body silicates, as predicted by the closed-system model for aqueous alteration. Temperatures are likely to have increased over the timespan of calcite precipitation, possibly owing to exothermic serpentinisation. The most highly altered CM chondrites commonly contain dolomite in addition to calcite. Dolomite grains in two previously studied CM chondrites have a narrow range in δ18O (∼25–29‰ VSMOW), with Δ17O ∼−1‰ to −3‰. These grains are likely to have precipitated between types 1 and 2 calcite, and in response to a transient heating event and/or a brief increase in fluid magnesium/calcium ratios. In spite of this evidence for localised excursions in temperature and/or solution chemistry, the carbonate oxygen isotope record shows that fluid evolution was comparable between many parent body regions. The CM carbonaceous chondrites studied here therefore sample either several parent bodies with a very similar initial composition and evolution or, more probably, a single C-type asteroid.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Lee, Professor Martin and Lindgren, Dr Paula
Authors: Lindgren, P., Lee, M.R., Starkey, N.A., and Franchi, I.A.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):1872-9533
Published Online:09 February 2017
Copyright Holders:Copyright © 2017 The Authors
First Published:First published in Geochimica et Cosmochimica Acta 204:240-251
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
491881Spatial and temporal scales of aqueous alteration in icy planetesimalsMartin LeeScience & Technologies Facilities Council (STFC)ST/G001693/1SCHOOL OF GEOGRAPHICAL & EARTH SCIENCES
602461Reconstructing thermal and fluid alteration histories of planetary materials.Martin LeeScience & Technologies Facilities Council (STFC)ST/K000942/1SCHOOL OF GEOGRAPHICAL & EARTH SCIENCES
697411A journey from the solar nebula to planetary bodies: cycling of heat, water and organicsMartin LeeScience & Technologies Facilities Council (STFC)ST/N000846/1SCHOOL OF GEOGRAPHICAL & EARTH SCIENCES