Devolatilization during subduction

Subducted crust refertilizes the subarc mantle wedge as well as the deep convecting mantle. In the subarc region, mass transfer occurs mainly through fluids, melts, or supercritical liquids. Experiments show that devolatilization reactions may occur at almost any depth in the chemically complex subducting lithosphere. Extreme temperature gradients across the slab lithosphere, the subduction channel, and the mantle hanging wall promote diachronous fluid production. Fluids are dominated by H2O, and the temperature of melting is controlled by the availability of H2O and CO2. The exact nature of the transfer agent and its composition depends on the source lithologies and the temperature regime, which vary significantly between different subduction zones. Nevertheless, P–T paths calculated from mechanical subduction zone models suggest that, in most subduction zones, the subducted crust remains at subsolidus conditions in the subarc regime. In a few, the pelitic solidus is overstepped, and fluid-saturated melting may be induced by flushing of H2O derived from more deep-seated metamorphic reactions. A steady volatile flow into the subarc wedge does not imply a steady supply of most trace elements, because low temperatures prevent diffusive equilibration, and, thus, the reactive volumes, in combination with the residual minerals formed through the devolatilization reactions, determine the efficiency of trace element transfer to the mantle wedge. Three main lithologies contribute to the devolatilization signal: peridotites, the mafic oceanic crust, and pelitic sediments + eroded continental crust, all three of which may contain carbonates that were added through hydrothermal or sedimentary processes. Most of the subducted CO2 survives the subarc regimen, while most of the H2O is lost to the subarc mantle, making CO2 the dominant volatile that is transported into the deeper mantle. At > 200–250 km depth, a fairly high temperature stability of hydrous phases relative to a typical subduction P–T path, in combination with relatively low melting temperatures for carbonated lithologies, leads to the possible formation of carbonatites within the subducting crust. Finally, examination of volcanic arcs worldwide reveals that their position is controlled not by devolatilization from the slab, but mainly by convection in the mantle wedge (which generally does not reach steady state) and to a minor extent by the structure of the overriding plate.