Rare earth element geochemistry of hydrothermal deposits and Calyptogena shell from the Iheya Ridge vent field, Okinawa Trough

Abstract

Although the rare earth element (REE) geochemistry of hydrothermal systems have intensively been investigated at mid-oceanic ridges, studies are relatively few at the converging plate zones like the Okinawa Trough. The "chimney" deposits obtained by using submersible "SHINKAI 2000" from the Iheya Ridge site. Okinawa Trough are largely composed of massive carbonates and silicates with minor amounts of sulfides, and appear black and white depending upon their composition. The concentrations of REEs are 2-3 orders of magnitude higher in the sulfide-rich black portion than in the white portion. Both deposits show a pronounced positive Eu anomaly and the light REE enrichment which are characteristic of the high-temperature fluids venting from sediment-free oceanic ridges. Whilst the Okinawa Trough REE data are unique in the hydrothermal vent systems in having a flat chondrite-normalized pattern for the REEs heavier than Tb. The Eu-anomaly is greater in the white deposit than in the black deposit, suggesting that Eu2+ is more mobile (resistant to scavenging by sulfides) than stricktly trivalent REEs in the hydrothermal solution and may be incorporated into the former by substitution of Sr2+ in the calcite lattice. The Calyptogena shell shows the REE pattern typical for seawater without any positive Eu anomaly, suggesting that the influence of hydrothermal solution is negligible. Although the light REE concentrations are comparable, the heavy REEs are significantly higher in the shell than in the white deposit. This suggests that the concentrations of the heavy REEs in the carbonate-forming hydrothermal solution may be lower than those of ambient seawater. Thus, the hydrothermal system in the Okinawa Trough may not be a source but a sink for dissolved REEs in seawater.