Carbon isotopic evolution of aqueous fluids in CM chondrites: Clues from in-situ isotope analyses within calcite grains in Yamato-791198

1Wataru Fujiya,1Yuto Aoki,2Takayuki Ushikubo,1Ko Hashizume,3Akira Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.003]
1Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512 Ibaraki, Japan
2Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe-otsu, Nankoku, Kochi 783-8502, Japan
3National Institute for Polar Research, Midoricho10-3, Tachikawa, Tokyo 190-8518, Japan
Copyright Elsevier

We report the variability in carbon and oxygen isotopic compositions, chemical compositions, and cathodoluminescence intensities within calcite grains in the Yamato-791198 CM chondrite measured by secondary ion mass spectrometry. To understand the change in carbon isotopic compositions during calcite formation, the carbon isotope-analyses were performed on a series of crystal growth bands of each calcite grain. The crystal growth of calcite grains was inferred from comprehensive analyses of oxygen isotopes, chemical compositions, and cathodoluminescence characteristics.

The observed δ18O variations within individual grains are as large as 13‰. The oxygen-isotope data plot on a single straight line with a slope of 0.61 ± 0.06 (2σ) in an oxygen three-isotope diagram. This slope steeper than that of the terrestrial fractionation line indicates that the oxygen isotopic compositions of aqueous fluids evolved from higher δ18O and Δ17O to lower δ18O and Δ17O compositions due to the oxygen-isotope exchange between water and anhydrous silicates in the parent body. Thus, calcite crystals grew from higher Δ17O to lower Δ17O areas. The crystal growth inferred from oxygen isotopic compositions is corroborated by the morphology and cathodoluminescence characteristics of the calcite grains. The minor element concentrations of the calcite grains did not increase/decrease monotonically during calcite formation.

The δ13C variations within individual grains are no more than 4‰ except for one grain. The intra-grain δ13C variations observed here are much smaller than inter-grain δ13C variations of ∼80‰ previously reported. These observations indicate that the carbon isotopic compositions of dissolved carbon species did not change during calcite formation and that they were locally heterogeneous which reflects variable proportions of carbon reservoirs with different isotopic compositions.

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