Abstract
The depth of crystallization differentiation was investigated for tholeiite basalts from Hakkoda and Hachimantai. Olivine-hosted melt inclusions contain aluminous clinopyroxene, spinel, plagioclase, and rare garnet, suggesting melt-entrapment under garnet-granulite facies conditions. Removal of the observed phenocrysts cannot account for the chemical variations, although the variations are typical of a tholeiitic differentiation trend. The trace element modelling indicates fractionation of cpx, pl, and spinel. The assemblage is consistent with that in the melt inclusions except for garnet. Fractional crystallization modelling closely approximates the major element variations at 10 kbar under anhydrous conditions. The modelling is consistent both with the melt-inclusion mineralogy and with the trace element modelling. An estimated depth of crystallization of 34 km (10 kbar) corresponds to the seismic Moho in the region (35-36 km). Our data also suggest that the relatively anhydrous magmas that were emplaced at Moho depths became hydrous concurrent with differentiation, implying incorporation of water from the surrounding crustal rocks.
