Barite geochemistry from hydrothermal chimneys of the Okinawa Trough : insight into chimney formation and fluid/sediment interaction

We present Pb-210/Ra-226 and Sr-87/Sr-86 data of barite separates from mineralized hydrothermal sulfate-sulfide chimneys collected from two young and active back-arc basins (i.e., Okinawa Trough and southern Mariana Trough), to examine the age of chimney formation and the relationship between the fluid chemistry and the field occurrence. Precipitation ages of purified barites in the mineralized chimneys are 24.8 (+/- 1.3)-73.9 (+/- 13.6) years BP2005 for chimney #220 from the Hakurei site in the Izena Cauldron (middle Okinawa Trough), 14.1 (+/- 0.6)-53.4 (+/- 2.8) years BP2005 for chimney #562-R13 from the Yaeyama graben (southern Okinawa Trough), and 29.8 (+/- 1.4)-39.2 (+/- 2.9) years BP2005 for chimney #903-R7 from the Archaean site in the southern Mariana Trough. Therefore, the time required for hydrothermal fluid venting to form these chimneys is of the order of decades. Sr-87/Sr-86 values for barites from chimney #220 are similar to 0.7094; these values are higher than those of the JADE site (0.7091) at the slope of the Izena Cauldron and ambient seawater (0.70918). Given the relatively high Ag and Sb contents in chimney #220 bulk samples, we suggest that interactions have occurred between the hydrothermal fluid and the sediment cover. In contrast, barites from the southern Mariana Trough have relatively low Sr-87/Sr-86 values (similar to 0.7046), implying that magmatic fluids play a significant role in mineralization in this sediment-starved setting.