Calcium isotope systematics at hydrothermal conditions: Mid-ocean ridge vent fluids and experiments in the CaSO4-NaCl-H2O system

Abstract

Two sets of hydrothermal experiments were performed to explore Ca isotope fractionation and exchange rates at hydrothermal conditions (410-450°C, 31.0-50.0 MPa). The first set of experiments determined the magnitude of vaporliquid Ca isotope fractionation and anhydrite solubility in the CaSO 4 -NaCl-H 2 O system. The data indicate no statistical difference between the Ca isotopic composition of coexisting vapor and liquid. The second set of experiments utilized an anomalous 43 Ca spike to determine the rate of Ca exchange between fluid and anhydrite as a function of total dissolved Ca concentration. Results show that the rate of exchange increases with dissolved Ca concentrations (12-23 mM/kg), but no change in exchange rate is observed when the Ca concentration increases from 23 to 44 mM/kg Ca. 74-142 days are required to achieve 90% anhydrite-fluid Ca isotope exchange at the conditions investigated, while only several hours are necessary for vapor-liquid isotopic equilibrium. The lack of vapor-liquid Ca isotope fractionation in our experiments is consistent with d 44 Ca of mid-ocean ridge hydrothermal vent fluids that remain constant, regardless of chlorinity. Moreover, the narrow range of end member fluid d 44 Ca, À0.98 to À1.13‰ (SW), is largely indistinguishable from MORB d 44 Ca, suggesting that neither phase separation nor fluid-rock interactions at depth significantly fractionate Ca isotopes in modern high-temperature midocean ridge hydrothermal systems.

Barium isotopes in mid-ocean ridge hydrothermal vent fluids: A source of isotopically heavy Ba to the ocean

“…4b). As expected, water-rock interaction at elevated temperatures and pressures appears to occur without Ba isotope fractionation, and a similar observation has been discovered for Ca isotopes (Scheuermann et al, 2018). Therefore, source rocks may be used to constrain the endmember vent fluid δ 138/134 Ba values.…”

“…Campbell et al, 1988;Butterfield et al, 1990;Von Damm, 1990;Von Damm, 2000;Charlou et al, 2002;Douville et al, 2002;Seyfried et al 2003;Seyfried et al, 2011;Fornari et al, 2012). Most of the fluid samples provided for this study have been analyzed and discussed for other trace elements and isotopes in previous studies (Foustoukos and Seyfried, 2007;Foustoukos et al, 2009;Pester et al, 2011;Seyfried et al, 2011;Pester et al, 2014;Syverson et al, 2017;Scheuermann et al, 2018).…”

Stable and radiogenic strontium isotope fractionation during hydrothermal seawater-basalt interaction

“…Furthermore, it is possible that the fast precipitation of anhydrite at the beginning of the experiments lead to the incorporation of higher amounts of Sr than at equilibrium due to kinetic effects, so that Sr was released from anhydrite during the later stages due to its recrystallization (Syverson et al, 2016;Scheuermann et al, 2018), contributing to the release of radiogenic Sr.…”

Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems

“…However, as demonstrated by TAG, Logatchev, and EPR 17-19°S end-member vent fluids, the degree of anhydrite formation in the subseafloor is not sufficient to significantly shift the Ca isotope composition of endmember fluids relative to MORB. This lack of Ca isotope compositional variability between end-member vent fluids and MORB is very likely due to the significant flux of hydrothermally derived-Ca leached from MORB, effectively overwhelming any reservoir effects caused by anhydrite precipitation in the subseafloor (Scheuermann et al, 2018). Overall, a range of À0.30 to À0.60‰ is appropriate to describe the magnitude of fractionation between anhydrite and fluid in MOR hydrothermal systems.…”