Iron isotope fractionation during crystallization and sub-solidus re-equilibration: Constraints from the Baima mafic layered intrusion, SW China

Abstract

To better understand Fe isotope fractionation between mafic minerals and oxides during crystallization of mafic magma and sub-solidus re-equilibration, the Fe isotopes of whole-rocks and separated minerals (olivine, clinopyroxene, magnetite and ilmenite) of the Baima mafic layered intrusion, SW China, have been investigated. The separated minerals show a systematical decrease in δ57Fe values, from magnetite (0.15 to 0.51‰) to olivine (−0.11 to 0.15‰) and clinopyroxene (−0.35 to 0.05‰) and then to ilmenite (−0.82 to −0.10‰), demonstrating regular fractionation between these minerals. Except for a few of samples, most of the olivine and clinopyroxene are similar to those of mantle xenoliths in Fe isotopes, indicating that Fe isotope equilibrium reached during magma crystallization was well preserved. By contrast, the Fe isotopes of the magnetite and ilmenite may be evidently modified by sub-solidus re-equilibration via the Fe3+ versus Ti4+ and Fe2+ exchange between the oxides. Furthermore, the sub-solidus re-equilibration in Fe isotope is strongly controlled by the proportions of magnetite and ilmenite in rocks. Therefore, although the δ57FeIlm of the Lower Zone rocks with magnetite/ilmenite ratios as high as 6–10 was reduced evidently by the sub-solidus re-equilibration, the magnetite preserved their original Fe isotope compositions. By contrast, the Fe isotopes of both most magnetite and ilmenite in the Middle Zone had been markedly modified by sub-solidus re-equilibration owing to the moderate magnetite/ilmenite ratios (4–7). The decreases of both δ57FeMt and δ57FeOl upwards in the cyclic units of the Lower Zone reveal that extensive early fractional crystallization of the magnetite resulted in depletion of heavier Fe isotopes in the magma. On the other hand, early crystallization of olivine and clinopyroxene gave rise to the slight elevation of δ57FeOl values upwards in the cyclic units of the Middle and Upper zones. The stratigraphic reversals in the δ57FeMt and δ57FeOl values suggest multiple magma recharges. Additionally, the δ57FeOl values in the base of the Lower Zone (0.10 to 0.15‰) indicate that the parental magma were heavy in Fe isotope due to extensive silicate mineral fractionation at depth. This study indicates fractionation in Fe isotope between silicates and oxides during magma crystallization and sub-solidus re-equilibration.