Slab dehydration beneath forearcs: Insights from the southern Mariana and Matthew-Hunter rifts

Abstract

The water flux delivered into the forearc mantle of currently active subduction zones remains poorly constrained. Estimates, which mostly derive from numerical modelling, have so far been untested, as shallow subduction processes are hindered by the serpentinised forearc mantle. Here, I examine the composition of near trench magmas from the southern Mariana and Matthew-Hunter rifts, which provides unique glimpses into slab dehydration underneath the forearcs of two modern subduction zones. The near trench magmas captured the water-rich slab fluids that usually serpentinise the cold forearc mantle. The near trench magmas possess higher markers in slab dehydration (Rb/Th = 3-141, Cs/Th = 0.04-17.79, H 2 O/Ce = 436-23,531) than do arc and back-arc magmas, implying that the subducted plates might dehydrate efficiently within 80 km from the trench.