Fe-Pb-Sr isotopic systematics of the hydrothermal chimney from the Minami-Ensei hydrothermal field, middle Okinawa Trough: Constraint on hydrothermal mineralization process in incipient back-arc basin

Abstract

The study of hydrothermal chimney from the Okinawa Trough (OT) is of important significance for uncovering the hydrothermal mineralization characteristics in incipient back-arc basins. Although many researches have been conducted on OT hydrothermal sulfides, the mineralization processes in this region are still unclear. In this paper, Sr-Pb-Fe isotope compositions of a hydrothermal chimney from the Minami-Ensei (ME) hydrothermal field, middle Okinawa Trough, are analyzed systematically in order to clarify the mineralization process in this field. The hydrothermal fluid has interacted with overlying sediment during the hydrothermal circulation. What is noteworthy is that the Sr and Pb isotope compositions of ME sulfides are less radiogenic than those of other OT fields, which is caused by the weaker fluid/sediment interaction and less leaching of sediment components under lower fluid temperature condition in ME. The δ56Fe values of ME pyrites range from −0.52 to −1.05 ‰, which is apparently lower than that of parental fluid (−0.10 to −0.30 ‰). During pyrites crystallization, strong Fe isotope kinetic fractionation occurs, resulting in the negative shifts in pyrite δ56Fe values. Although bacterial sulfate reduction occurred throughout the whole mineralization process, its influence on Fe isotope kinetic fractionation is negligible. This is because the production of bacterial H2S is limited in ME, which cannot influence the pyrite crystallization rates (i.e., the degrees of Fe isotope kinetic fractionation) remarkably. This research is helpful for shedding light on mineralization processes in incipient back-arc basin hydrothermal fields.