Emerging geothermometers for estimating slab surface temperatures

A great variety of volatile-rich material is fed into subduction zones, including all sorts of marine sediment, altered oceanic crust and some serpentinized mantle lithosphere1. Subduction delivers these materials to depth, but their subsequent history depends on how pressure and temperature increase together. In the cool shallow subduction zone, !uids are liberated at subsolidus temperatures, whereas at higher temperatures some volatile components can be entrained directly into slab-derived melts. "ese !uids and melts percolate into the overlying mantle, ultimately causing it to melt, thereby generating the magma that forms arc volcanoes. Water is by far the dominant volatile species evolved during subduction of everything but calcium carbonate sediments and veins, and so in most subduction zones the #rst !uids are dilute aqueous solutions (for example, >90% water)2. Such !uids may drive melting in portions of the subducting plate that exceed 700–800 °C (refs 3, 4). At higher pressures, solute-rich !uids and water-rich melts may become miscible above a second critical endpoint, generating ‘supercritical liquids’ of intermediate composition5. "is much is known about the potential variety of !uids in subduction zones, but little is yet known about the actual pressure and temperature of !uid generation or delivery to the arc mantle source. Most constraints on the pressure and temperature of slab !uids come from two complementary approaches used for active subduction zones: the trace element signatures in arc magmas and numerical simulations on the thermal structure of subducting plates. (A third important approach involves pressure–temperature estimates from exhumed metamorphic rocks from past subduction zones. "e focus here is instead on products and processes that can be related to known subduction conditions.) "e magmas that erupt from arc volcanoes carry within them vestiges of the slab !uids responsible for their genesis. "e problem is that magmas degas almost all of their H2O before and during eruption, so most methods of inferring magmatic H2O content have been indirect. For over 40 years, it has been recognized that arc magmas have unique trace element signatures that must originate from the slab. Emerging geothermometers for estimating slab surface temperatures