Chlorine isotopic compositions of high temperature hydrothermal vent fluids over ridge axes

Abstract

We present chlorine stable isotope compositions of high temperature hydrothermal vent fluids from two mid-oceanic ridges. The six hydrothermal fluids come from sites along the slow-spreading Mid-Atlantic Ridge (Menez Gwen at 37°50′ N, Lucky Strike at 37°17′ N and Rainbow at 36°14′ N) and the ultrafast-spreading East Pacific Rise (from three contiguous segments centered, respectively, at 17°25′ S, 18°15′ S and 18°26′ S). These samples exhibit a large range in temperature, pH, chlorinity and Mg content reflecting various geological settings and substrata nature (basaltic or ultramafic). Although showing a large range of chlorinity from 251 to 821 mmol/kg, the fluids display homogeneous chlorine isotope signatures: δ 37Cl SMOC = − 0.03 ± 0.06‰, indistinguishable from that of seawater. This homogeneity contrasts with the large range of δ 37Cl previously observed in continental or off-axis oceanic fluids. This study explores possible processes that may change Cl concentration during fluid genesis and discusses their theoretical isotopic effects. The δ 37Cl homogeneity implies that, at the P– T conditions of the ridge axis, the process of phase separation (at sub- or super-critical conditions) that explains most of chlorinity variations, does not significantly fractionate chlorine isotopes. This study shows that the physical and chemical processes that likely alter Cl concentrations in ridge axis systems (i.e. transport processes, mixing or water–rock interaction) contribute to minimize the range of δ 37Cl around the seawater δ 37Cl value.