Disequilibrium iron isotopic fractionation during the high-temperature magmatic differentiation of the Baima Fe–Ti oxide-bearing mafic intrusion, SW China

Abstract Iron isotopiccompositions of olivine, clinopyroxene and titanomagnetiteof the Baima Fe–Ti oxide-bearing layered mafic intrusion, SW China, are used to investigate Fe isotopic fractionationduring the formation of the Fe–Ti oxide ore bodies and to constrain the origin of silicate mineralsand Fe–Ti oxides. The Baima intrusion comprises the Lower Zone of interlayered troctolite, clinopyroxenite and oxide ores and the Upper Zone of gabbros. Significant differences of Fe isotopevalues between olivine( δ Fe 56 = − 0.01 to + 0.11 ‰ ), clinopyroxene ( δ Fe 56 = + 0.11 to + 0.22 ‰ ), and titanomagnetite( δ Fe 56 = + 0.20 to + 0.31 ‰ ) were observed in rocks and ores of the Lower Zone. Iron isotopic fractionationbetween olivineand clinopyroxene, olivineand titanomagnetiteand clinopyroxene and titanomagnetiteexhibits large variations of 0.06‰ to 0.22‰, 0.12‰ to 0.27‰, and 0.00‰ to 0.20‰, respectively, suggesting disequilibriumfractionation. This disequilibriumcannot be explained by thermal or chemical gradient-induced kinetic fractionationor by possible subsolidus exsolution of granular ilmenitefrom titanomagnetite. Instead, it could be attributed to crystallization of silicates and titanomagnetitefrom two immiscible Si-rich and Fe-rich liquids. Continuous segregation of the Fe-rich liquid changed the Fe isotopiccomposition of the Si-rich liquid and thus the Fe isotopiccompositions of olivineand clinopyroxene crystallized from it, resulting in disequilibriumFe isotopic fractionationbetween them. The effect of oxygen fugacityon the crystallization order of titanomagnetiteand ilmenitefrom the Fe-rich melts, on the other hand, gave rise to the disequilibriumFe isotope fractionationbetween titanomagnetiteand olivine/clinopyroxene.