Origin of two types of olivine from the Ogi Picritic Dolerite Sill, northeast Japan

Abstract

A Middle Miocene picritic dolerite sill is exposed to the southwest of the Sado Island, the largest island in the Sea of Japan. This picrite has two types of phenocrystic olivine: high-Fo and -Ca normally zoned olivine with abundant Cr-spinel inclusions (type-1) and low-Fo and -Ca reverse-zoned olivine without Cr-spinel inclusions (type-2). Type-1 olivine and type-2 olivine mantle are in equilibrium with the chilled margin of the sill, but the type-2 olivine core is clearly in disequilibrium with that. This suggests that the types-1 and -2 olivine crystals may be autocrysts and antecrysts, respectively. These two types of olivines contain multiphase solid inclusions in their cores. These inclusions are also classified into the following two types based on the mineral assemblage and average bulk composition: plagioclase + clinopyroxene + hornblende assemblage with basaltic composition (O1-type) and orthopyroxene + clinopyroxene + hornblende (± plagioclase) assemblage with high-magnesian andesitic composition (O2c-type). The composition of glasses coexisting with plagioclase in multiphase solid inclusions indicates that plagioclase crystallized at the early stage of crystallization in the O1-type inclusions but did not crystallize until the last stage in the O2c-type. The mineral assemblages suggest that the O1- and O2c-types multiphase solid inclusions represent the trapped basaltic and high-Mg andesitic melts, respectively. Our results suggest that the type-1 olivine is autocrysts crystallized from a primitive basaltic magma and the type-2 olivine core is antecrysts crystallized from an H₂O-rich high-Mg andesitic magma. After the type-2 olivine core crystallized at the deeper parts (>130-190 MPa), it was then incorporated into the primitive basaltic magma. Consequently, the high-Fo and -Ca olivine mantles grew around the type-2 olivine cores under the condition of moderate undercooling at a shallower depth. The high-Mg andesitic magmatism inferred from the O2c-type multiphase solid inclusions in the picrite suggest that the highly depleted oceanic mantle infiltrated by slab-derived melts/fluids may have existed ubiquitously under the back-arc basin during the Miocene back-arc opening of the Japan Sea.