Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex, Hokkaido, Japan : variations of spinel unmixing texture and chemical composition

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Spinel ((Mg Fe^<2+>)(Cr Al Fe^<3+>)_2O_4 in formula) with inhomogeneous textures were observed in chromitite from the Iwanai-dake complex, in the Kamuikotan zone, Japan, which mainly consists of highly depleted alpine-type peridotite. The spinel inhomogeneous texture is different between in the chromitite layer and in the surrounding peridotites but the variation is gradual. Spinel has a speckled texture (Type A spinel) in the chromitite. Irregular mottling and symplectic textures (Type B spinel) are predominant in spinel-rich dunite abutting to the chromitite layer. Spinel in the surrounding dunite and harzburgite is homogeneous (Type C spinel), and it is similar in composition to spinel in ordinary peridotite from the Iwanai-dake complex. The inhomogeneous spinel comprises two phases of Mg-and Al-rich spinel and Fe^<2+>^<->and Fe^<3+>^<->rich spinel. The variations of texture and composition were achieved by an unmixing process due to the miscibility gap of spinel between the Al-rich and Fe^<3+>^<->rich phases below 900℃. The Iwanai-dake peridotite is almost free from serpentinization and lacks evidence of regional metamorphism after emplacement, which suggests that the unmixing of spinel took place during a cooling process of the mantle peridotite. The pre-unmixing compositions of Type A and B spinels are estimated as bulk spinel compositions, and they are systematically correlated with the unmixing texture. We propose that the variation of unmixing texture is controlled by pre-unmixing spinel compositions.
金沢大学の論文

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex, Hokkaido, Japan : variations of spinel unmixing texture and chemical composition

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