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Apatite halogen and hydrogen isotope constraints on the conditions of hydrothermal alteration in carbonaceous chondrites
Author(s) -
Piralla Maxime,
Tartèse Romain,
Marrocchi Yves,
Joy Katherine H.
Publication year - 2021
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13639
Subject(s) - apatite , hydroxylapatite , chondrite , geology , hydrothermal circulation , meteorite , mineralogy , geochemistry , breccia , impact crater , chemistry , astrobiology , biochemistry , physics , seismology , enzyme
Apatite has been widely used for assessing the volatile inventory and hydrothermal fluid compositions of asteroidal and planetary bodies. We report the OH, F, and Cl abundances, as well as the hydrogen isotope composition, of apatite in the CM1‐2 chondrite Boriskino and in the C1‐ungrouped Bench Crater meteorite. Apatite in both meteorites is halogen‐poor, close to the hydroxylapatite endmember composition, and characterized by average δ D SMOW values of −226 ± 59% and 233 ± 92%, respectively. Compared to apatite, the matrix in Bench Crater is depleted in D with a δ D SMOW value of −16 ± 119‰. Comparing apatite and water H isotope compositions yields similar apatite‐water D / H fractionation Δ D Apatite‐Water of approximately 120–150‰ for both chondrites, suggesting that apatite formed at similar temperatures. Combining a lattice strain partitioning model with apatite F and Cl abundances in Boriskino and Bench Crater yields low F and Cl abundances <300 μg g −1 in apatite‐forming fluids, and fluid F/Cl ratios that are roughly consistent with the bulk F/Cl ratios of other CI and CM chondrites. This suggests that hydrothermal alteration on these meteorite parent bodies took place under closed‐system conditions. Based on the OH abundance estimates for the apatite‐forming fluids, we estimated the pH values of alteration fluids to be of approximately 10–13. Such alkaline fluid compositions are consistent with previous modeling and suggest that apatite formed late, toward the end of completion of hydrothermal alteration processes on the Boriskino and Bench Crater parent bodies.