DOI: 10.1016/j.epsl.2020.116101
论文题名: High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems
作者: Brown S.T. ; Turchyn A.V. ; Bickle M.J. ; Davis A.C. ; Alt J.C. ; Bédard J.H. ; Skulski T. ; DePaolo D.J.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2020
卷: 535 语种: 英语
中文关键词: calcium isotopes
; epidote
; hydrothermal mineralization
; kinetic isotope fractionation
; ophiolite
; reactive transport
英文关键词: Calcium
; Hot springs
; Isotopes
; Kinetics
; Seawater
; Silicate minerals
; Strontium compounds
; Sulfur compounds
; Temperature
; Underwater mineral resources
; Calcium isotopes
; epidote
; Hydrothermal mineralization
; Kinetic isotope fractionation
; ophiolite
; Reactive transport
; Transport properties
; calcium
; epidote
; high temperature
; hydrothermal deposit
; hydrothermal fluid
; hydrothermal system
; isotopic fractionation
; kinetics
; mineralization
; ophiolite
; reactive transport
; seafloor
英文摘要: 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 δ44Ca of likely modern hydrothermal fluids to a narrow range at δ44Ca=−0.05±0.1. A reactive-transport model is used to evaluate whether the δ44Ca 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 δ44Ca 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 Δ44Ca=−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 Δ44Caeq≈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. © 2020 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/164891
Appears in Collections: 气候变化与战略
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作者单位: Dept of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720, United States; Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 947203, United States; Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom; Dept of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, United States; Geological Survey of Canada, 490 de la Couronne, Québec, QC G1K 9A9, Canada; Geological Survey of Canada, Natural Resources Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
Recommended Citation:
Brown S.T.,Turchyn A.V.,Bickle M.J.,et al. High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems[J]. Earth and Planetary Science Letters,2020-01-01,535