Origin and evolution of the slab fluids since subduction inception in the Izu-Bonin-Mariana: A comparison with the southeast Mariana fore-arc rift

Abstract

Subduction zones have played a central role in exchanging volatiles (H2O, CO2, S, halogens) between the different Earth's reservoirs throughout its history. Fluids that are released as the subducted plates dehydrate are major agents that transfer these volatiles inside the Earth; but the origins and the compositional evolution of the slab fluids as plates begin to sink are yet to be understood. To explore processes that tool place during subduction infancy, here we examine the compositions of proto-arc magmas from the Izu-Bonin-Mariana (IBM) convergent margin that formed during subduction inception; and we compare these to a modern example of near-trench spreading in the southeast Mariana fore-arc rift (SEMFR). There is a temporal and spatial evolution in the slab fluid composition that is accompanied with a change in the fluid reservoirs, as subduction progresses. During the early stages of the subduction zone, dehydration of the serpentinized subducting mantle likely triggered dehydration and melting of the altered oceanic crust in the amphibolite facies to produce boninites. As the subduction zone matured, the volcanic arc front was displaced away from the trench. The arc magmas captured deeper slab fluids released from the subducted oceanic crust, the sediments and the underlying serpentinized mantle. Dehydration and melting of the subducted sediment became more prevalent with time and increasing slab depth (≥ 100 km) to produce arc magmas. This compositional evolution was associated with a deepening of magma generation, which is likely accompanied with the progressive serpentinization of fore-arc mantle and a slab rollback. Hence, fore-arc mantle serpentinization might have facilitated arc maturation and subduction stabilization throughout the IBM history.