Magnesium isotopic composition of back-arc basin lavas and its implication for the recycling of serpentinite-derived fluids

Abstract

Dehydrated fluids expelled from the serpentinised mantle in subducted slabs contribute significantly to arc magmatism and element cycling on Earth. However, the recycling mechanism of serpentinite-derived fluids at various subduction zone depths remains unclear, as does their contribution to the genesis of back-arc basin lavas. In this study, we investigated magnesium (Mg) isotopic compositions of lavas from the Okinawa Trough (OT) and the Lau Basin, because Mg isotopes have shown great potential in tracing the dehydration of slab serpentinites in recent years. After excluding dubious samples affected by alteration, lavas from the OT and the Lau Basin have heavier Mg isotopic compositions than mid-ocean ridge basalt (MORB) mantle, which could be attributed to the involvement of slab serpentinite-derived fluids rather than crustal assimilation or subducted sediment inputs, as indicated by the isotopic modelling results. The δ26Mg values of the southern OT (SOT) and the southern Lau Basin lavas are generally higher than those in the middle OT (MOT) and the northern Lau Basin, respectively, with an average of −0.09 ± 0.02‰ (2SD, n = 6) for the SOT, −0.18 ± 0.10 ‰ (2SD, n = 9) for the MOT, −0.13 ± 0.08 ‰ for the southern Lau Basin (2SD, n = 5), and − 0.19 ± 0.06 ‰ (2SD, n = 10) for the northern Lau Basin. The binary modelling results show that various quantities of serpentinite-derived fluids could explain the variations in Mg isotopic compositions observed in the OT and the Lau Basin. We suggest that the thermal structure of the inter-subduction zone controls the signal of Mg-rich serpentinite-derived fluids, based on both reported δ26Mg values in subduction zones and our findings. In contrast, the contributions of these fluids to different segments in a specific subduction zone may depend on the slab depth beneath sites of magmatic activity.