Volatiles in Tonga Arc magmas and their role in unraveling subduction zone processes

Subduction zones are locations where volatile species are recycled. Volatiles, especially H2O, contained within the subducted oceanic crust and overlying sediment are expelled into the mantle wedge via dehydration reactions, where they drive mantle melting. The resulting hydrous primary magmas are erupted, fueled by volatile degassing, through overlying volcanoes. To learn about subduction processes, undegassed primary H2O contents were estimated for seven submarine volcanoes within the Tonga Arc by measuring volatiles in melt inclusions (in addition to major and trace elements in melt inclusions, whole rocks, glasses, and phenocrysts). These are the first volatile measurements for Tonga.;Tonga is an end-member subduction zone. Located in the southern Pacific, active spreading within the adjacent Lau Backarc Basin causes convergence rates at the trench to increase three-fold from south to north, where it is the fastest in the world. Mantle H2O contents are coupled indirectly to convergence rate, while the extents of mantle melting beneath the arc front are relatively constant over the length of the arc. The dominant signal is a strong variability in mantle source composition along the arc strike, due to prior melting in the Lau Basin. In support of this is the first documented occurrence of an active arc volcano erupting a rare rock-type called a boninite; this was discovered in the central portion of the arc, where extents of prior melting are the greatest. A backarc-arc evolutionary model is developed to explain the petrogenesis of this volcano. The model results, as well as geochemical arguments, suggest that many of the surrounding Tonga Arc volcanoes may also have a boninitic origin. Lastly, a new H2O/Ce fluid thermometer is used to estimate slab fluid temperatures within several global subduction zones to infer the temperature of the slab surface directly beneath the arc front. While temperatures range significantly between subductions zones, with Tonga being the coldest, all temperatures are greater than the wet solidus, implying that slab-derived fluids may be better described as hydrous melts than aqueous fluids. These estimates may be used to ground-truth geodynamic thermal models and provide constraints for the efficiency of volatile recycling within subduction zones.