The tectono-magmatic formation and submarine geohazards of the Japan Sea: Constraints from in-situ trace elements and Sr isotopes of plagioclase and clinopyroxene

To address the nature of mantle-derived melts and submarine geohazards of the Japan Sea, we present new in-situ compositions for clinopyroxene (trace elements) and plagioclase (trace elements and Sr isotopes) from Sites 794 and 797 tholeiitic basalts. Plagioclase and clinopyroxene from Upper Site 797 and Lower Site 794 are characterized by relatively lower incompatible elements relative to those of Lower Site 797 and Upper Site 794. Lower Site 794 bulk-rocks and minerals have slight enrichment in Sr isotope compositions but depletion in trace element ratios. The observed zoning textures together with in-situ geochemical and isotopic diversity within individual plagioclase indicate fractional crystallization and the contribution of heterogeneous melt. In-situ Sr isotopic disequilibria between plagioclase phenocrysts and groundmass likely have important ramifications for the enriched compositional heterogeneity with the significant influence of recycled subduction-related oceanic fluid/sediment beneath the Japan Sea. The initial extension of the Japan Sea was relatively slow, resulting in the slow replenishment of depleted MORB-type melts derived from the upper mantle, and the extensive mixing of the migrating depleted melt with the preformed surrounding enriched melts. It thus forms the slight depletion of the highly incompatible elements but the enriched invariability of the isotopic compositions of Lower Site 794 bulk-rocks and minerals due to the relatively long half-life of the radiogenic elements. In summary, the expansion velocity of the Japan Sea varies from slow to rapid, which leads to time-progressive variations in the mineral and bulk-rock compositions beneath the Japan Sea. The maximum seismic depth of the earthquake decreases with the increase of the expansion velocity of the Japan Sea.