Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions

Abstract

Stable isotopes of metals, such as magnesium (Mg) and iron (Fe), typically undergo minimal fractionation at magmatic temperatures. However, in Fe-Ti oxide-bearing intrusions, significant Mg isotope fractionation (Δ²⁶Mg values of up to 23 ‰) has been observed between silicates and Fe-Ti oxides. The mechanism responsible for this substantial fractionation remains unclear. This study presents Mg isotopic compositions for bulk rocks and mineral separates—including olivine, clinopyroxene, titanomagnetite, ilmenite—from sixteen Fe-Ti oxide ores in both the lower and the upper zones of the Baima layered mafic–ultramafic intrusion, as well as bulk rocks from three high-Ti basalts of the Emeishan large igneous province, SW China. Oxide ores from the lower zone and the lower section of the upper zone of the Baima intrusion are characterized by a low abundance of volatile-bearing minerals, such as apatite and hornblende. These ores exhibit δ²⁶Mg values ranging from −0.30 to −0.14 ‰ for olivine, −0.26 to −0.06 ‰ for clinopyroxene, 0.16 to 0.60 ‰ for titanomagnetite and −0.33 to −0.21 ‰ for ilmenite, indicative of near-equilibrium fractionation. In contrast, oxide ores from the upper section of the upper zone exhibit a high abundance of volatile-bearing minerals and significantly different δ²⁶Mg values, ranging from −0.19 to −0.05 ‰ for olivine, −0.25 to −0.10 ‰ for clinopyroxene, 0.57 to 0.60 ‰ for titanomagnetite and 0.86 to 2.34 ‰ for ilmenite, indicative of disequilibrium fractionation. A negative correlation between Fo content and δ²⁶Mg value in olivine suggests that Mg isotopes fractionate as olivine undergoes fractional crystallization. Trapped liquid fractions, calculated based on Ce contents in apatite, suggest that rocks with high volatile-bearing mineral content have undergone re-equilibration with trapped liquids during cooling. The large disequilibrium Mg isotopic fractionation between silicates and Fe-Ti oxides is likely due to sub-solidus re-equilibration between minerals and trapped liquids. We propose that the extent of sub-solidus, diffusion-driven kinetic Mg isotopic fractionation in Fe-Ti oxide-bearing intrusions likely correlates with the volume of trapped liquids, which may serve as a significant medium accelerating ion exchange between silicates and Fe-Ti oxides. Moreover, mass balance calculations indicate that data of Mg isotopes on their own are insufficient to determine the petrogenesis of Fe-Ti oxides in layered intrusions. This insufficiency is primarily due to the low MgO content in titanomagnetite and ilmenite, where their separation causes minimal Mg isotopic fractionation of the melts.