Rare earth elements and Sr, S isotope compositions of hydrothermal deposits from the Okinawa Trough: Insight into mineralization condition and metal sources

Abstract As an incipient back-are basin, the mineralization characteristic in the Okinawa Trough (OT) is quite unique. Many studies have been conducted in OT hydrothermal deposits, however the mineralization condition and metal source differences between the middle (MOT) and southern Okinawa Trough (SOT) are still unclear. The rare earth elements (REEs) and Sr, S isotope compositions of the OT hydrothermal deposits were determined in this paper in order to solve these problems. The Eu anomalies of the hydrothermal sulfide samples show large variation, with δEu values range from 0.62 to 21.38, which reflects the differences in the redox condition of their parental hydrothermal fluids. The 87Sr/86Sr ratios of the hydrothermal deposit samples are abnormally high, indicating the fluid/sediment interaction during the fluid circulation. The δ34S values of sulfide samples from the MOT fall between those of basement volcanic rocks and seawater, which can be attributed to the mixing of volcanic and seawater-derived sulfur. While samples from the SOT have abnormally low δ34S values, remobilization of biogenic sulfur in sediments is the major cause of the sulfur isotope anomaly. Both of S and Pb isotopes indicate that the proportions of sediment component in the SOT hydrothermal deposits are higher than those of the MOT. Whereas the Sr isotope compositions of the hydrothermal deposit samples show no systematic difference between the MOT and SOT. We propose that the decoupling of Sr and S, Pb isotopes may be caused by the fluid mobility differences between these three elements during the fluid/sediment interaction.