Mg isotope fractionation during partial melting of garnet-bearing sources: An adakite perspective

Garnet that has a distinctly light Mg isotopic composition compared to other mafic minerals is commonly present in deep crustal and mantle sources. Whether partial melting of these garnet-bearing sources can lead to significant Mg isotope fractionation or not remains poorly understood. In order to investigate Mg isotope fractionation during partial melting of garnet-bearing sources, we report Mg isotopic data of 17 well-characterized low-Mg adakitic rocks (LMA) from the Dabie orogen, central China. These LMA are known for derivation from mafic sources leaving eclogitic residua with variable abundance of garnet. Our data together with previously published results show that fresh Dabie LMA have homogeneous Mg isotopic compositions, with an average δ26Mg of −0.22 ± 0.08‰ (2SD, N = 18), except that two samples display higher δ26Mg (−0.05‰ to +0.01‰) due to alteration during sericitization and chloritization of biotite. The lack of correlation between δ26Mg and (Dy/Yb)N of the Dabie LMA indicates an unresolvable residual garnet effect. Given that Mg isotopic compositions of the Dabie LMA are identical with local mafic basement within the current precision, Δ26Mgmelt-cpx is estimated to be around −0.02‰. Limited isotope fractionation of the Dabie LMA could be attributed to the dilution effect of residual clinopyroxene co-existing with garnet. The estimated Δ26Mgmelt-cpx is then applied to predict the possible isotope fractionation that may occur during hydrous and dehydration melting of subducted oceanic crust. With a lower melting temperature and more abundant garnet in the residua, slab-derived melts may have δ26Mg higher than their sources by up to +0.36‰, which, however, can be easily erased by later melt-mantle interaction. In this regard, Mg isotopes can serve as a powerful tracer of recycled supracrustal materials.