Abstract
Ilmenite in alkali feldspar quartz syenite from Cape Ashizuri contains up to 4.4 wt.% Nb2O5. Niobium substitutes for Ti in the octahedral site of the ilmenite structure. Substitution of Nb for Ti may involve a coupled exchange to maintain charge balance, and an exchange of 2Nb + 2Fe3+ = 3Ti + 2Fe2+ is advocated. An Fe-Ti oxide geothermometer obtained from mineral pairs of granular and lamellar intergrowths indicates a subsolidus re-equilibration temperature of 510–640 °C andfO2 between the FMQ and MW-buffers, implying that it is very undersaturated with respect to water. On the other hand, Zn-ilmenite, containing up to 5.4 wt.% ZnO, occurs in miarolitic cavities in peralkaline rhyolite which cuts the quartz syenite. The Zn-ilmenite is one of the last crystalline phases of the Ashizuri magmatic activities under volatile-rich conditions. Nb-oxides, such as fergusonite, samarskite, columbite and a pyrochlore-like mineral, are Ta- and Mn-poor, which corresponds to those of less-fractionated rocks of anorogenic alkali granite and pegmatitic granite in the continental situation. Ta- and Mn-poor Nb-oxides in F- and Li-rich alkaline felsic magmas such as the Ashizuri syenites are unusual; this may be related to a rapid emplacement and cooling of mantle-derived small-volume magma in the island are situation.
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