Geochemical records of hydrothermal activities in the sediment fillings within pumice's vesicles in the vicinity of a seafloor hydrothermal field in the Okinawa Trough

The pumices samples from HOBAB2-T3 and HOBAB2-T6 near the Iheya North hydrothermal field, Okinawa Trough (OT) are divided into two groups. One group (T3-C and T6-C) was carefully cleaned to remove any sediment filled in vesicles, while the second group (T3-D and T6-D) remained unchanged. To qualitatively understand the mineral and chemical compositions of the sediment fillings in pumice's vesicles, each group of pumices was examined by scanning electron microscope (SEM) and their mineralogy and trace elemental compositions were analyzed and compared. Results show that the sediments in pumices are mainly from OT clastic deposits. Vesicles in T3 pumice are smaller and they are mainly filled by illite and quartz in terms of mineral compositions. However, sediments in larger vesicles of T6 pumice are dominated by quartz and feldspar, same as the minerals in OT in general. The sediments in T3 pumice, adjacent to the hydrothermal field, however, contain extremely high concentrations of Pb (>101 μg/g), Zn (>196 μg/g), and Cu (>47 μg/g), indicating strong hydrothermal activities. For T6 pumice that is slightly farther away from the hydrothermal field, the sediment fillings also exhibit anomalies in Pb and Cu, which might stand for relatively weaker hydrothermal signal. The above indicates that sediments in pumice's vesicle near the hydrothermal field may record information of hydrothermal activity, which has great significance in exploring new hydrothermal filed in the region with pumiceous volcanic rocks.