Petrological Characteristics of the Upper Mantle Peridotites Beneath the Japan Island Arcs Petrogenesis of Spinel Peridotites

Relationships between the Fo content of olivine and the Cr/(Cr+Al) atomic ratio (= Cr* ratio) of chrome spinel are useful to evaluate petrological characteristics of mantle derived spinel peridotites. Residual spinel peridotites (lherzolite and harzburgite) have respective Fo-Cr* variation trends (= an olivine-spinel mantle array) depending on the conditions of partial fusion (pressure, H2O, etc.). Low-pressure or hydrous residual peridotites have more Fo-enriched olivine than high-pressure or anhydrous ones if compared on a fixed Cr* ratio. Lherzolite-harzburgite xenoliths from the Japan Island Arcs have lower-Fo olivine than those from other tectonic settings (oceanic hot spots and continental rift zones), which indicates that the Japan-Arc mantle peridotites are hydrous and/or low-pressure restites. The chromite harzburgite xenoliths in kimberlites have the most Fo-enriched olivine on a fixed Cr* ratio of spinel of all spinel peridotites and are high-pressure restites within a garnet lherzolite stability field. Chrome spinel in the Japan-Arc peridotites has a wide spread of the Cr* ratio, from 0.1 to 0.7, which is almost coincident with that in nearly primary Quaternary island-arc magmas from the Northeast Japan Arc. This fact, combined with their Fo-Cr* relationships, indicates that the Japan Arc mantle peridotites could have been formed as restites of islandarc magmas under hydrous and/or low-pressure conditions. Dunite pyroxenite xenoliths associated with the lherzolite-harzburgite xenoliths from the Japan Island Arcs could be early cumulates from island-arc magmas in terms of the Fo-Cr* relationship.