A kaersutite-bearing dunite xenolith from Ichinomegata, northeastern Japan

Abstract

A xenolith of dunite was found in the Ichinomegata ejecta, northeastern Japan. It consists of olivine (80%), clinopyroxene (7.4%), spinel-pyroxene symplectite (5.4%), chromian spinel (4.7%), plagioclase (1.3%), amphiboles (0.6%) and orthopyroxene (0.3%). Olivine is Fo86.8 in average composition, and Cr/(Cr+Al) atomic ratio of spinel is around 0.2. Al2O3 and TiO2 contents of clinopyroxene exceed 5 wt.% and 1 wt.%, respectively. Coexistence of relatively magnesian olivine with Al-rich spinel and Al- and Ti-rich clinopyroxene indicates a cumulative origin from some low-degree partial melt (e.g. alkali basalt) of a pyrolitic material. The dunite has suffered from metasomatic alteration which led to the subsolidus crystallization of titanian pargasite and kaersutite and the entrapment of fluid inculsions into olivine and spinel. Amphiboles in this rock are far more enriched in Ti than those in any ultramafic xenoliths ever documented from Ichinomegata. The metasomatizing fluid may have been enriched in Ti and Na over K, implying the consequence of some intra-plate magmatism.