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TitleHigh-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems
AuthorBrown, S T; Turchyn, A V; Bickle, M J; Davis, A C; Alt, J C; Bédard, J H; Skulski, TORCID logo; DePaolo, D J
SourceEarth and Planetary Science Letters vol. 535, 116101, 2020 p. 1-10, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20190199
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
Subjectsigneous and metamorphic petrology; mineralogy; geochemistry; economic geology; Science and Technology; Nature and Environment; hydrothermal systems; sea floor spreading; mid-ocean ridges; fluid dynamics; isotopic studies; isotope ratios; calcium; strontium strontium ratios; thermal analyses; temperature; crystal fractionation; epidote; peridotites; ophiolites; oceanic crust; models; sulphate; sea water geochemistry; precipitation; silicates; mineral deposits; ore mineral genesis; Methodology; Phanerozoic; Cenozoic; Quaternary; Mesozoic; Cretaceous; Paleozoic; Cambrian
Illustrationstables; plots; models
ProgramTargeted Geoscience Initiative (TGI-3), 2005-2010 Appalachian TGI-3
Released2020 02 12
AbstractCalcium isotope ratios in epidote from epidosites in ophiolites of varying Phanerozoic ages have 44Ca/40Ca ratios that are lower by 0.1 to 0.6 per mille relative to typical mid-ocean ridge hydrothermal fluids. Epidosites are inferred to form in high-temperature parts of seafloor hydrothermal systems at temperatures above 300°C and where fluid fluxes are high, so the Ca isotopic composition of the epidote is likely to reflect fractionation during growth of crystals from aqueous solution. Available Ca isotope data from MOR hydrothermal vent fluids and mantle peridotites constrain the delta-44Ca of likely modern hydrothermal fluids to a narrow range at delta-44Ca= -0.05 ± 0.1. A reactive-transport model is used to evaluate whether the delta-44Ca of hydrothermal fluids might have been higher during the Cretaceous and Late Cambrian, the ages of the Troodos, Oman, and Betts Cove ophiolites from which we have data. For these calculations we use the epidosite 87Sr/86Sr as a guide to the extent of Ca isotopic exchange that affected the ancient hydrothermal fluids, which were derived from seawater with higher Ca and Sr, and lower sulfate concentration, than modern seawater. The calculations suggest that the ancient hydrothermal fluid delta-44Ca values were not much different from modern values, with the possible exception of the Late Cambrian example. We infer that the epidote-fluid Ca isotope fractionation averaging delta-44Ca= -0.2 to -0.6, is most likely due to kinetic isotope fractionation during mineral precipitation. There is evidence from the literature that hydrothermal epidote may commonly form from oversaturated solutions, which makes the kinetic isotope interpretation plausible. The equilibrium epidote-fluid Ca isotope fractionation is estimated to be delta-44Caeq approximately equal to 0 based on recently reported DFT calculations. Our results suggest that kinetic calcium isotope fractionation can affect hydrothermal silicate minerals, and may be only slightly smaller in magnitude than the effects observed in Ca-bearing minerals at low temperature. Kinetic isotope effects during mineral growth could provide new insights into the formation mechanisms of hydrothermal silicate minerals.

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