On the Stability of Talc in Subduction Zones: A Possible Control on the Maximum Depth of Decoupling Between the Subducting Plate and Mantle Wedge

Geophysical observations including surface heat flow data indicate the subducting slab becomes fully coupled to overlying mantle wedge at ?70–80 km depth. This maximum depth of decoupling (MDD) separates cool, stagnant forearc from warmer, convecting capable generating arc magmas. Thermodynamic calculations demonstrate that talc is stable in H2O-undersaturated parts where its stability controlled by pressure-dependent, fluid-absent reaction: + forsterite = antigorite enstatite, which occurs pressures ?2–2.5 GPa (?70–80 depth) and temperatures <650°C. At shallower depths, portions basal contain talc, experimental studies show dramatically weakens rocks. greater restricted silica-rich wedge. The common MDD subduction zones may reflect downdip transition a talc-present decoupled shear zone talc-absent interface along base