A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt

Abstract

We found fine-grained Fe-rich orthopyrox- ene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite vein- lets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite for- mation, indicating its high SiO2 activity. The secondary orthopyroxene of this type is characterized by low Mg# (= Mg/(Mg + total Fe) atomic ratio) (down to 0.73) and high Al2O3 contents (5-6 wt%). It is differ- ent in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicat- ing involvement of similar alkali basaltic melts. The Fe- rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersat- urated in silica might have evolved to silica-oversatu- rated compositions by fractional crystallization at high- pressure conditions. The Fe-rich orthopyroxenites oc- cur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.