Calcium isotope (δ44/40Ca) fractionation along hydrothermal pathways, Logatchev field (Mid-Atlantic Ridge, 14°45'N)

We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14 degrees 45'N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca](sw)) increases from about 10 mM to about 32 mM in the hydrothermal fluid endmember ([Ca](HydEnd)) and thereby adopts a delta Ca-44/40(HydEnd) of -0.95 +/- 0.07 parts per thousand relative to seawater (SW) and a Sr-87/Sr-86 isotope ratio of 0.7034(4). We demonstrate that delta Ca-44/40(HydEnd) is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a delta Ca-44/40 of -1.17 +/- 0.04 parts per thousand (SW) for the host-rocks in the reaction zone and -1.45 +/- 0.057 parts per thousand (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed delta Ca-44/40 for Bulk Earth of -0.92 +/- 0.187 parts per thousand (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta 61,(12) 2505-2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust (Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic Calcium cycle. Geochim. Cosmochim. Acta 62,(10) 1691-1698: Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1-16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Delta(44/4C) Ca = -0.57 parts per thousand relative to their assumed parental solutions, Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average delta Ca-44/40 of -1.54 +/- 0.08 parts per thousand (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration. (C) 2008 Elsevier Ltd. All rights reserved.