Volatile contents of Kermadec Arc–Havre Trough pillow glasses: Fingerprinting slab-derived aqueous fluids in the mantle sources of arc and back-arc lavas

Aqueous fluids and sediment melts from subducting oceanic lithosphere are the driving force for the refertilisation of the mantle overlying subduction zones. Volatile elements in particular play an important role in the generation of aqueous fluids and the transport of fluid mobile elements. Here we report volatile contents of quenched glasses from pillow-lava rims in the Kermadec Arc–Havre Trough subduction system to provide constraints on the source, generation, and composition of slab-derived aqueous fluids. Water (1.5 wt.% in glasses, 2.5 wt.% in two melt inclusions), carbon (up to 16 ppm in glasses, 180 ppm in one inclusion) and sulphur (< 700 ppm) contents of all glasses are consistent with degassing of volatiles, which migrated from the melt, through growing vesicles, and into the water column, even in samples collected at depths of up to 3000 m. By contrast, halogen contents have not been affected by degassing or any other secondary processes such as assimilation of seawater. Fluorine contents (250–520 ppm) can be obtained almost entirely from melting of depleted mantle similar to that which generates mid-ocean ridge basalt. However, Cl (650–3000 ppm) is enriched tenfold compared to depleted mantle-derived lavas. A minimum of 70% H2O, > 80% Ba and Pb, and > 98% of Cl is required to be derived from aqueous slab-derived fluids. We propose a single homogenous reservoir for the aqueous fluid source. This reservoir is most likely to be hydrated serpentinite formed above the subducting slab by fluids derived from altered oceanic crust and subducting sediment. Dehydration of the serpentinite occurs as it descends into the mantle, transporting fluid mobile elements into the overlying mantle and triggering partial melting. The similarity of aqueous fluid compositions derived for other oceanic arcs suggests that this process may occur in arc systems worldwide.