Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Island arc magmatism along convergent plate margins is one of the major processes of continental crust generation. The net flux from mantle to crust at convergent plate margins is considered to be basaltic (Ellam and Hawkesworth, 1988), since a basaltic parental magma is preferred for island arc magmatism (Ringwood, 1974; Arculus, 1981; Gill, 1981). It is inconsistent with the estimation of andesitic average composition of continental crust (e.g. Taylor and McLennan, 1985). This inconsistency can be explained by the hypothesis of partial melting of subducted oceanic crust (slab melting model: Martin, 1986; Drummond and Defant, 1990), however, silicic slab melts may reacted with mantle peridotite. High Mg andesites (HMA) may be produced by reaction between ascending silicic melts and mantle peridotite (Kelemen et al., 1993; Yogodzinski et al., 1995; Uematsu et al., 1995; Shimoda et al., 1998), and give us an important information on the ascending silicic melt through mantle.
The Eocene HMA in Kitakami carries phenocrystic olivine with remarkable high NiO contents (maximum 0.58 wt%), and are plotted beyond the olivine mantle array after Takahashi (1986). These Ni-rich olivine phenocrysts are consisitent with Fe-Mg exchange partitioning between olivine and the bulk rock. Moreover, since equilibrium crystallization trend for olivine phenocrysts obtained from MELTS calculations (Ghiorso and Sack, 1995) is consistent with the observed chemical compositions of olivine, high-Ni contents in olivine phenocrysts result from high-Ni contents in melt.
The petrochemical features of the HMA in Kitakami can be explained by the modelling calculation of mantle AFC, indicating derivation from slab melt/mantle reaction. When a slab melt is reacted with mantle peridotite, it produces orthopyroxene-rich rocks as a reaction of olivine + SiO2 (in liquid) = Opx. Then, Ni content in melts increases as a result of olivine dissolution, because Ni is enriched in olivine relative to orthopyroxene. Therefore, particularly Ni-rich nature of olivine phenocrysts in the Kitakami HMA may be an evidence for slab melt/mantle reaction.
