Subduction contribution to the magma source of the Okinawa Trough—Evidence from boron isotopes

Abstract

Boron (B) has been an excellent geochemical tracer for the investigation of magmatism in arcs and back‐arc basins, due to its high fluid mobility and distinct isotope compositions in the mantle wedge and subducting slabs. In this study, B isotope compositions of the volcanic rocks collected from the northern, middle, and southern Okinawa Trough were determined to discuss the sources of B in the magma and assess the contribution of subduction components to the magma source. The results show that B in the volcanic rocks from the Okinawa Trough are mainly from the underlying mantle wedge but influenced by the fluids derived from the subducted slab. Compared with volcanic rocks from the middle Okinawa Trough, samples from the southern Okinawa Trough are generated from the source with larger proportion of subduction components. For the volcanic rocks from the northern Okinawa Trough, the influence of the subduction components is the most significant and the proportion of slab‐derived B in the magma reaches up to 30%. It is estimated that the subduction components are composed of altered oceanic crust/serpentinite fluid and sediment fluid with the proportion of 94:6. The B‐rich fluid, loaded by cold and fast dipping slab, released into overlying mantle wedge resulting in partial melting of mantle is the genetic mechanism of B‐rich volcanic rocks in the Okinawa Trough.