16:30 〜 16:45
[SCG59-11] Origin of Lithium in Pore Fluids Near Offshore Fault Zone, Northern Noto Peninsula, Two Months After the 2024 Noto Peninsula Earthquake (M7.6) During the R/V Hakuho-maru cruise
キーワード：2024 Noto Peninsula Earthquake , Fluid Migration , Lithium Isotope , Geochemical Analysis
The Noto Peninsula earthquake swarm, which intensified in 2020, has been linked to fluid migration beneath the northern part of the Noto Peninsula based on geophysical results. Yet, the origin of these fluids remains debated, with hypotheses proposing either deep-seated mantle-derived sources or dehydration of the subducting slab. Geochemical analyses of pore water collected from seafloor and sub-seafloor sediments in and around fault zones have been widely used to investigate the potential upwelling of deep-seated fluids through faults. To explore the presence and characteristics of such fluids, we analyzed porewater samples from near the active fault associated with the 2024 Noto Peninsula earthquake (M7.6). These samples were collected during the R/V Hakuho-maru cruise (KH-24-E1) in March 2024 from two sites in the Sea of Japan: NW-1 and TCW-2.
Traditional hydrogen and oxygen isotopic analyses for fluid origin determination are complicated by seawater mixing. Instead, lithium (Li) and strontium (Sr) isotopic ratios, along with major and trace element concentrations, provide a more reliable approach for identifying fluid sources and assessing the possible existence of deep-seated fluids. Our Li and Sr isotopic results did not provide definitive evidence of deep-seated fluids at the two investigated sites, as the Li-Sr isotopic ratios closely resembled those of present-day seawater, unlike those observed in forearc mud volcanoes. However, at the TCW-2 site, a notable shift in Li concentration, was observed in the core samples. The Li depletion may be influenced by volcanic material alteration and possibly calcite precipitation. While no direct signatures of deep-seated fluid involvement were detected in Li and Sr isotope analyses at TCW-2, the Cl/Li ratio at this site was comparable to that of onshore samples from the northern Noto Peninsula.
Traditional hydrogen and oxygen isotopic analyses for fluid origin determination are complicated by seawater mixing. Instead, lithium (Li) and strontium (Sr) isotopic ratios, along with major and trace element concentrations, provide a more reliable approach for identifying fluid sources and assessing the possible existence of deep-seated fluids. Our Li and Sr isotopic results did not provide definitive evidence of deep-seated fluids at the two investigated sites, as the Li-Sr isotopic ratios closely resembled those of present-day seawater, unlike those observed in forearc mud volcanoes. However, at the TCW-2 site, a notable shift in Li concentration, was observed in the core samples. The Li depletion may be influenced by volcanic material alteration and possibly calcite precipitation. While no direct signatures of deep-seated fluid involvement were detected in Li and Sr isotope analyses at TCW-2, the Cl/Li ratio at this site was comparable to that of onshore samples from the northern Noto Peninsula.