Li isotope compositions of hydrothermally altered sediments in the Iheya North Knoll of the middle Okinawa Trough

We reported the Li isotopic compositions (δ7Li) of background and hydrothermally altered sediments from IODP331 drilling cores in the Iheya North Knoll of the middle Okinawa Trough. The sediments, primarily altered from volcanic clasts, exhibits a pristine δ7Li value (5.2 ± 0.7 ‰) similar to that of MORB (3.7 ± 1 ‰). Intense hydrothermal activity led to the complete conversion of primary feldspars into phyllosilicates, predominantly Mg-chlorite, at site C0013 and in the middle and deep parts of C0014. The altered samples showed a depletion of 7Li (ranging from -10.0 ‰ to 4.2 ‰) compared to background sediment, indicating the preferential incorporation of 6Li during the formation of secondary minerals. Furthermore, hydrothermally impacted drilling sites exhibited increasing trends in δ7Li values with depth, with the deepest samples approaching the pristine δ7Li values. These trends were attributed to the significant thermal gradients present at both sites. Our data underscores the potential of Li isotopes as a novel geothermometer tool, although further calibration work is required.