Mg-Fe isotopic constraint on the role of the lithosphere mantle in arc magmatism

Arc magmatism is commonly envisaged as generation through a two-endmember petrogenetic process involving the subducted components and asthenospheric mantle wedge. The contribution of sub-arc lithospheric mantle, as a key chemical reservoir in subduction zone, to arc magmas remains poorly constrained. Here we present, for the first time, a systematic dataset of MgFe isotopes on continental arc basalts (CABs) from the Sunda Arc for deciphering the role of sub-arc lithospheric mantle in arc magmatism. The Sunda CABs show significant variations in δ²⁶Mg (−0.28 ‰ to −0.05 ‰) and δ⁵⁶Fe values (−0.19 ‰ to 0.14 ‰), with mean values of −0.17 ‰ and 0.02 ‰, respectively, resembling those of global arc magmas. These samples mainly display low TiO₂/Yb (0.3–0.5) and Fe/Mn (30–60) ratios, indicating their derivation from peridotite melting within the depleted asthenospheric mantle wedge. The weak correlations of Mg and Fe isotopes with ε_Nd suggest that subduction-related metasomatism has not significantly influenced the Mg and Fe isotopic compositions of the asthenospheric mantle wedge. The majority of Sunda CABs exhibit a negative correlation between δ²⁶Mg and Fe/Mn, as well as a positive correlation between δ⁵⁶Fe and Fe/Mn, both also observed in global arc magmas. These significant variations in MgFe isotopes and Fe/Mn ratios within arc magmas likely result from their intensive interactions with the sub-arc lithospheric mantle through incorporations of the low Fe/Mn (30–50) pyroxenite and high Fe/Mn (50–70) peridotite in different proportions. The interactions may have also elevated the oxygen fugacity as evidenced by the co-variations of MgFe isotopes and V/Sc ratios for the Sunda CABs and global arc basalts. We therefore conclude the crucial role played by the sub-arc lithospheric mantle in global arc magmatism, which has long been overlooked in popular petrogenetic models for subduction zone magmatism.