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.
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For each sample, an aliquot containing ~ 100 ng Li was initially digested in a 7 ml Teflon beaker using a 1:3 mixture of HNO3 and HF, followed by treatment with 4 ml of aqua regia and then 4 ml of concentrated HNO3. In order to ensure a clear digested solution, the aqua regia procedure was repeated for several times. The resulting digested solutions were subsequently dried and then taken up in 2 ml of 0.5M HNO3 for column separation. Li purification was accomplished through a single-step cation exchange chromatography method. Teflon microcolumns with a diameter of 6.4 mm and a length of 250 mm were packed with 8 mL of resin (Bio-rad® AG50W X-12, 100–200 mesh). The columns were conditioned with 12 ml of 0.5M HNO3, and then the samples were loaded in 2 ml of 0.5M HNO3. The matrix was eluted with 20 ml of 0.5 M HNO3, and the target Li was collected in the 20-48 ml fraction. The purity of the collected Li (Na/Li < 1) and the quantitative Li yield (>99%) were verified using ICP-MS. Li isotope values were determined using MC-ICP-MS with a sample-standard-sample bracketing method, and three replicates were performed for each sample. The data are reported in permil notation relative to LSEVC. The long-term reproducibility of the measurements, better than 0.9‰, was previously reported by Gou et al. (2019). To monitor data quality, reference materials BHVO-2 and AGV-2 were also purified and analyzed in parallel, which yields values of 3.9 ± 0.5‰ and 6.7 ± 0.5 ‰. Several samples were digested, purified, and analyzed in duplicates, which also yielded consistent values