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

Lindgren, Paula LU ; Lee, M. R.; Starkey, N. A. and Franchi, I. A. (2017) In Geochimica et Cosmochimica Acta 204. p.240-251
Abstract

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... (More)

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.

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author
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Aqueous alteration, Calcite, CM carbonaceous chondrites, Oxygen isotopes
in
Geochimica et Cosmochimica Acta
volume
204
pages
12 pages
publisher
Elsevier
external identifiers
  • scopus:85013835338
ISSN
0016-7037
DOI
10.1016/j.gca.2017.01.048
language
English
LU publication?
no
id
7372eb3d-1381-44fe-9531-37589e5def8f
date added to LUP
2017-06-16 15:37:26
date last changed
2017-08-01 09:51:14
@article{7372eb3d-1381-44fe-9531-37589e5def8f,
  abstract     = {<p>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 δ<sup>18</sup>O of ∼32–36‰ (VSMOW), and Δ<sup>17</sup>O 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 δ<sup>18</sup>O of ∼21–24‰ (VSMOW) and a Δ<sup>17</sup>O 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 δ<sup>18</sup>O and δ<sup>17</sup>O 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 δ<sup>18</sup>O (∼25–29‰ VSMOW), with Δ<sup>17</sup>O ∼−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.</p>},
  author       = {Lindgren, Paula and Lee, M. R. and Starkey, N. A. and Franchi, I. A.},
  issn         = {0016-7037},
  keyword      = {Aqueous alteration,Calcite,CM carbonaceous chondrites,Oxygen isotopes},
  language     = {eng},
  month        = {05},
  pages        = {240--251},
  publisher    = {Elsevier},
  series       = {Geochimica et Cosmochimica Acta},
  title        = {Fluid evolution in CM carbonaceous chondrites tracked through the oxygen isotopic compositions of carbonates},
  url          = {http://dx.doi.org/10.1016/j.gca.2017.01.048},
  volume       = {204},
  year         = {2017},
}