High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems

Brown, Shaun T. and Turchyn, Alexandra V. and Bickle, Mike J. and Davis, Amy C. and Alt, Jeffrey C. and Bédard, Jean H. and Skulski, Thomas and DePaolo, Donald J. (2020) High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems. Earth and Planetary Science Letters, 535. p. 116101. ISSN 0012-821X DOI https://doi.org/10.1016/j.epsl.2020.116101

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Abstract

Calcium 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‰ 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 Ca of likely modern hydrothermal fluids to a narrow range at . A reactive-transport model is used to evaluate whether the Ca 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 Ca 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 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 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.

Item Type:	Article
Uncontrolled Keywords:	AREP2019, IA76
Subjects:	01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions:	01 - Climate Change and Earth-Ocean Atmosphere Systems 08 - Green Open Access
Journal or Publication Title:	Earth and Planetary Science Letters
Volume:	535
Page Range:	p. 116101
Identification Number:	https://doi.org/10.1016/j.epsl.2020.116101
Depositing User:	Sarah Humbert
Date Deposited:	19 Feb 2020 17:45
Last Modified:	12 Feb 2021 01:01
URI:	http://eprints.esc.cam.ac.uk/id/eprint/4630

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