Magnetite-rutile symplectite in ilmenite records magma hydration in layered intrusions

Abstract

Abstract The textures and geochemical characteristics of the rocks in layered intrusions potentially provide insights into the physicochemical processes that have taken place in mafic magma chambers. Diverse exsolution textures of Fe-Ti oxides in layered intrusions may record the variation of subsolidus temperature and oxygen fugacity (fO2) of cooling magma chambers. Here we investigated ilmenitehematite solid solution (Ilmss) relationships evident in preserved intergrowths of magnetite-rutile and ilmenite-hematite in the gabbro of the Xinjie layered intrusion. The crystallographic orientation and 3D morphology of the two intergrowth types constrain the transformation mechanism of the exsolution textures from Ilmss. The results reveal that the interface of the ilmenite-hematite intergrowth is more energetically favorable than that of the magnetite-rutile symplectite when they are transformed from Ilmss on cooling. The QUILF equilibria suggests that the magnetite-rutile symplectite can be transformed from Ti-rich ilmenite with Ilm≥0.85 above 550 °C when the subsolidus T-fO2 trend is buffered by the biotite-ilmenite-feldspar-ulvöspinel (KUIlB) mineral assemblages crystallized from hydrated mafic magmas. The magnetite-rutile symplectite may be used as a unique texture indicator of magma hydration in the evolution history of terrestrial, martian, and lunar magmas.