Peridotite xenoliths from the Shiribeshi Seamount, Japan Sea: insights into mantle processes in a back-arc basin

Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-O shima) in the Japan Sea suggests that the Oshima-O shima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-O shima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-O shima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.