Magnesium isotope fractionation during magmatic differentiation in the lower continental crust

The Mg isotopic compositions display a significant disparity between the lower continental crust and the mantle. However, due to the limited availability of representative samples from the lower crust, our understanding remains incomplete regarding whether Mg isotope fractionation occurred during its formation and its potential influence on the observed isotopic differences. This study addresses this gap by presenting Mg isotopic data for juvenile lower crustal mafic xenoliths from Daoxian, Southeast China, providing new insights into this poorly understood process. The xenoliths, classified as websterites and two-pyroxene granulites, equilibrated at 818–912 °C and 10.6–15.4 kbar. They exhibit high Mg# (77–85), Cr (154–1451 ppm), and Ni (143–312 ppm), as well as trace element patterns resembling those of island arcs and evolved Sr–Nd–Pb isotopic compositions (87Sr/86Sri = 0.7046–0.7071; εNd(t) = −2.44 to +4.62; 206Pb/204Pbi = 18.26–19.03), indicating their derivation primarily from an enriched mantle source. Despite this, these xenoliths display a diverse range of Mg isotopic compositions. Websterite xenoliths exhibit mantle-like δ26Mg values (−0.27‰ to −0.29‰), indicating no influence from hybridized mantle source or pyroxene fractional crystallization/accumulation on their Mg isotopes. In contrast, granulite xenoliths show lower and variable δ26Mg values (−0.26‰ to −0.49‰), which positively correlate with Mg#, Cr, and V, but negatively correlate with Na2O + K2O, indicating that magmatic processes likely influenced Mg isotope fractionation in these samples. Petrological features, coupled with correlations between MgO and CaO/Al2O3, Cr, Co, and Ni, indicate that fractional crystallization and/or accumulation of olivine, spinel, clinopyroxene, orthopyroxene, and plagioclase played crucial roles in the formation of the granulite protoliths. Furthermore, the segregation of spinel during magmatic differentiation could have enriched residual melts in light Mg isotopes inherited by the cumulate Cpx + Opx + Pl (i.e., the granulite protoliths). Overall, the weighted average δ26Mg value of the lower crust in South China is estimated to be −0.34 ± 0.04‰. This finding, together with previous studies on granulite xenoliths in North China and Australia, strongly indicates heterogeneous Mg isotopic compositions in the lower continental crust relative to the mantle. •Mg isotopes in lower crustal mafic xenoliths from Daoxian in SE China vary widely.•Spinel segregation ind