Arc magma heterogeneity induced by subslab mantle upwelling

Tomographic images of the Ryukyu subduction zone suggest upwelling of asthenospheric mantle through slab tears; this can affect the genesis of arc magma. However, geochemical evidence testing this hypothesis is lacking. Here, we analyzed magnesium (Mg) isotopes of volcanic rocks from various regions in the Ryukyu subduction zone and combined these with trace element and Sr-Nd isotopes to understand how heterogeneous mantle contributed to arc magmas. The δ26Mg values of Okinawa Trough samples (−0.30‰ to −0.09‰) are sometimes slightly higher than those of typical mantle; we think this reflects the role of slab-derived fluids. Kueishantao lavas exhibit radiogenic Sr-Nd isotopes and mantle-like δ26Mg values (−0.27‰ to −0.19‰), which are primarily contaminated by the continental crust. Notably, samples from the northern Taiwan volcanic zone display lower δ26Mg values in basalts (−0.45‰ to −0.28‰) and higher values in andesitic rocks (−0.18‰ to −0.14‰). We postulate that elevated δ26Mg values stem from the contribution of slab-derived fluids. Conversely, the heat and material transported by upwelling of subslab asthenosphere around the slab edge or tear may induce melting of carbonate sediment at subarc depths to yield low-δ26Mg melts, explaining the lower δ26Mg values in basalts. This study provides direct geochemical evidence underscoring the involvement of subslab asthenospheric material in the formation of arc magma and helps elucidate the causes of arc mantle heterogeneity.