Introduction of isotopically heavy barium from back-arc basin hydrothermal systems into the ocean

Abstract

The concentrations of Ba in high-temperature hydrothermal fluids are several orders of magnitude higher than those in seawater and therefore may play an important role in marine Ba cycle. However, the ultimate hydrothermal inputs of Ba (isotopically light or heavy) into the oceans remain debated, and the Ba inputs from back-arc hydrothermal vents are poorly constrained. To address this issue, we have analyzed the Ba isotope compositions of hydrothermal barites from the Okinawa Tough. The δ¹³⁸Ba values of the studied barites show a limited variation (δ¹³⁸Ba = −0.16 to −0.05 ‰) compared to the natural range and is primarily controlled by the Rayleigh Ba isotope fractionation from the parental hydrothermal fluids during precipitation. About 33 to 51 % removal of Ba from the hydrothermal fluid will account for the observed δ¹³⁸Ba values. The δ¹³⁸Ba values of hydrothermal fluids after barite precipitation would be 0.19 to 0.30 ‰, which are isotopically heavier than the Ba sinks in the ocean. The new data thus suggest that back-arc hydrothermal systems may introduce isotopically heavy Ba into the oceans due to the non-conservative behavior of Ba during the fluid-seawater mixing in these systems. The marine Ba budget has been evaluated and the calculated Ba inputs to the ocean are isotopically heavier than sediment outputs, implying either isotopically light sources or isotopically heavy sinks of Ba are missing. One possibility is that the light Ba released from MORB during low-temperature alteration may play an important role in maintaining marine Ba isotope budget balance.