Molybdenum isotopes in hydrothermal manganese crust from the Ryukyu arc system: Implications for the source of molybdenum

Abstract

A high concentration of molybdenum (Mo) is a striking feature of modern marine hydrothermal manganese (Mn) crusts from both low- and high-temperature hydrothermal systems; however, the origin of that Mo is poorly constrained. In this study, we analyzed Mo isotopic composition (δ98/95Mo) in a Mn crust collected from the Ryukyu arc system, and assessed the possible use of δ98/95Mo to constrain the origin of Mo in hydrothermal Mn crusts. Along with Mo isotope analysis, we also measured the abundance of major and trace elements and the Re–Os isotopic composition. The Mn crust sample had a high Mn content (42%–47%) and was low in Fe (0.2%–0.5%). Among the manganophile elements (Co, Ni, Cu, Zn, and Mo), only Mo showed high concentrations (>550ppm). We also found low contents of Os (~35ppt) with an isotopic composition of 187Os/188Os=~1.00. These results are consistent with previously reported chemical compositions of hydrothermal Mn crusts. δ98/95Mo values in the sample ranged from −0.56‰ to −0.66‰, which are ~2.7‰ lighter than the present-day seawater δ98/95Mo but similar to those in modern hydrogenous Fe–Mn crusts and Mn nodules. Although current data do not preclude the possible contribution of hydrothermally derived Mo, the light δ98/95Mo values can be explained by isotope fractionation associated with a change in coordination number during the adsorption of Mo from seawater onto Mn oxides. We suggest that the δ98/95Mo data are useful for constraining the source of Mo in hydrothermal Mn crusts.