Global variations in H2O/Ce: 1. Slab surface temperatures beneath volcanic arcs
We have calculated slab fluid temperatures for 51 volcanoes in 10 subduction zones using the newly developed H2O/Ce thermometer. The slab fluid compositions were calculated from arc eruptives, using melt inclusion-based H2O contents, and were corrected for background mantle contributions. The temperatures, adjusted to h, the vertical depth to the slab beneath the volcanic arc, range from ~730 to 900°C and agree well (within 30°C on average for each arc) with sub-arc slab surface temperatures predicted by recent thermal models. The coherence between slab model and surface observation implies predominantly vertical transport of fluids within the mantle wedge. Slab surface temperatures are well reconciled with the thermal parameter (the product of slab age and vertical descent rate) and h. Arcs with shallow h (~80 to 100 km) yield a larger range in slab surface temperature (up to ~200°C between volcanoes) and more variable magma compositions than arcs with greater h (~120 to 180 km). This diversity is consistent with coupling of the subducting slab and mantle wedge, and subsequent rapid slab heating, at ~80 km. Slab surface temperatures at or warmer than the H2O-saturated solidus suggest that melting at the slab surface is common beneath volcanic arcs. Our results imply that hydrous melts or solute-rich supercritical fluids, and not H2O-rich aqueous fluids, are thus the agents of mass transport to the mantle wedge.
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