ABSTRACT

Ilmenite occurs as an intercumulus phase in most gabbroic rocks and some plagioclase wehrlites of the Asama igneous complex, an ultramafic-mafic layered intrusion in the Mikabu greenstone belt, central Japan. The Mg/(Mg + Fe²⁺) ratio of ilmenite reaches 0.35, but generally ranges from 0.01 to 0.22; the Fe³⁺ content varies from 0.04 to 0.26 pfu (per formula unit calculated on the basis of O = 3). Ilmenite crystallized together with intercumulus clinopyroxene in plagioclase wehrlite, but most of the calculated clinopyroxene-ilmenite temperatures are much lower than the magmatic temperature. The subsolidus Mg-Fe²⁺ exchange probably occurred between ilmenite and surrounding olivine and pyroxene during the cooling stage of the igneous body. Ilmenite, entirely surrounded by plagioclase, occurs in plagioclase wehrlite of a lower portion of the exposed layered sequence; it contains about 9 wt.% MgO, and is the most magnesian in the complex. The ilmenite shows clinopyroxene-ilmenite equilibration temperatures of 980-1100°C, and seems to preserve the magmatic composition. The TiO₂ solubility of melt and the MgO distribution coefficient ilmenite/melt suggest that the Asama magma crystallizing the ilmenite contained 3-5 wt.% TiO₂ and 6-7.5 wt.% MgO. The Asama magma was apparently similar to Hawaiian tholeiites, and different from mid-oceanic ridge basalts or island-arc tholeiites. The Asama complex is considered to have formed in an oceanic island.