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

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 (δ^98/95Mo) in a Mn crust collected from the Ryukyu arc system, and assessed the possible use of δ^98/95Mo 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. δ^98/95Mo values in the sample ranged from -0.56‰ to -0.66‰, which are ~2.7‰ lighter than the present-day seawater δ^98/95Mo 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 δ^98/95Mo 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 δ^98/95Mo data are useful for constraining the source of Mo in hydrothermal Mn crusts.