The chemical zoning of nickeliferous olivine phenocrysts in calc-alkaline andesites from the northern Yatsugatake volcanoes, Central Japan, is examined by a growth and diffusion model in the system Mg-Fe-Ni. Nickeliferous olivine phenocrysts are included in the high-alumina basaltic end-member magma which forms the mixed calc-alkaline andesites in the northern Yatsugatake volcanoes. The olivine phenocrysts can be divided into three types on the basis of chemical composition, zoning profile and occurrence. Type-1 olivine is rich in Ca and Mg, and the zoning profile of type-1 forms a slightly more Fe-rich trend than the maximum fractionation trend in the NiO versus forsterite content (Fo) diagram. The most Mg and Ni rich, and Mn poor core of type 1 (Fo 91.5 mol%, NiO 0.40 wt%, MnO 0.13 wt%) can be in equilibrium with peridotite in the upper mantle. By contrast, type-2 olivine is Ca poor, and rich in Fe and Ni compared to type-1. The zoning patterns of type-2 are more variable. The NiO content in Fe-rich type-2 phenocrysts decreases rapidly from core to rim within a narrow range of Fo. The maximum NiO content of type-2 is 0.53 wt%. The transitional-type olivine has an intermediate composition between type-1 and type-2 and a similar zoning profile as that of type-1. Type-1 olivine mainly occurs as isolated phenocrysts, while Fe-rich type-2 occurs as crystal clots. The growth and diffusion calculation in the system Mg-Fe-Ni revealed that the composition of normally zoned olivine is enriched in Fe by diffusion without a marked depression in the Ni content due to a larger Fe-Mg interdiffusion coefficient than the Ni tracer diffusion coefficient in olivine. The zoning patterns of both type-1 and type-2 can be reproduced by calculation. The residence time of type-1 olivine in the magma is estimated to be less than 10-0.5-1 y at 1150 - 1050 °C, whereas the residence time of Fe-enriched type-2 olivine is from ten to one hundred times longer than that of type-1. The long residence time and occurrence as crystal clots suggest that the Fe-enriched type-2 olivine phenocrysts were captured and crystallized in the crystal mush at the roof and bottom of a magma chamber.
ASJC Scopus subject areas
- Geochemistry and Petrology