Magnesium isotope behavior during titanomagnetite fractionation in basaltic lavas

Knowledge of the behavior of magnesium (Mg) isotopes during magmatic differentiation is a prerequisite for applying Mg isotopes as a tracer of crustal recycling and mantle heterogeneity. Crystal fractionation of mafic silicate minerals leads to limited Mg isotope fractionation; hence, Mg isotopic variations in basalts largely result from contributions of subducted crustal materials into their mantle sources. However, recent studies have revealed that the segregation of iron oxides, such as chromite during early-stage basaltic differentiation and titanomagnetite and ilmenite during late-stage differentiation, can potentially modify Mg isotopic compositions of mantle-derived lavas. Titanomagnetite is much more commonly observed in terrestrial basaltic lavas than chromite and ilmenite, and can also form during early-stage basaltic differentiation. To constrain the effects of titanomagnetite crystallization on Mg isotopic systematics of basaltic magma, we report Mg isotopic data for a suite of well-characterized alkaline basaltic lavas (MgO wt% = 4.99 to 6.52) from Gerba Guracha, western Ethiopian Plateau, which have undergone extensive titanomagnetite fractionation. Their δ26Mg values range from −0.29‰ to −0.17‰ and do not correlate with indicators of titanomagnetite fractionation such as TiO2 and V. These observations suggest that the segregation of titanomagnetite produced limited Mg isotope fractionation, possibly because of its composition-dependent, highly variable Δ26MgTi-Mgt−melt (Δ26MgTi-Mgt−melt = δ26MgTi-Mgt − δ26Mgmelt) values. Considering that most iron oxides occur as complex solid solutions that are controlled by the oxygen fugacity of magmas, the effects of oxide segregation on Mg isotopic systematics of basalts require further evaluation.