Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXII, No.3


Cr-rich and Al-rich Spinels in the Kibi-kogen Alkali Basalts, Southwest Japan

Fujibayashi, Norie

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Cr-rich spinel (Cr2O3 11~33 wt%, Al2O3 18~39 wt%) and Al-rich spinel (Cr2O3<2 wt%, Al2O3 48~65 wt%) occur in the Kibi-kogen alkali basalts. The latter is extremely high in Al2O3 and plots near the Al end of the Fe+3-Al side of Cr-Fe+3-Al diagram. The occurrence and chemistry of the spinels indicate that the Cr-rich spinel crystallized as a near-liquidus phase, whereas the Al-rich one formed subsequently. The calculation in terms of the Rayleigh fractional crystallization model resulted in notable depletion of Cr in the residual liquid with progress of crystallization of the Kibi-kogen alkali basalt magma. Thus, when the Cr content of the magma became less than the solubility limit of Cr, the Cr-rich spinel may have ceased from crystallization and the Al-rich spinel in turn crystallized as an aluminous phase prior to the appearance of plagioclase. The composition of the near-liquidus Cr-rich spinel depends mainly on the Cr content of host magma rather than the Al content and Si/Al ratio.