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 the overlying mantle wedge at ∼70–80 km depth. This maximum depth of decoupling (MDD) separates cool, stagnant forearc mantle from warmer, convecting mantle capable of generating arc magmas. Thermodynamic calculations demonstrate that talc is stable in H2O‐undersaturated parts of the mantle wedge where its stability is controlled by the pressure‐dependent, fluid‐absent reaction: talc + forsterite = antigorite + enstatite, which occurs at pressures ∼2–2.5 GPa (∼70–80 km depth) and temperatures <650°C. At shallower depths, H2O‐undersaturated portions of the basal mantle wedge contain talc, which experimental studies show dramatically weakens rocks. At greater depths, talc is restricted to silica‐rich portions of the mantle wedge. The common MDD in subduction zones may reflect the downdip transition from a talc‐present decoupled shear zone to a talc‐absent fully coupled interface along the base of the mantle wedge. Plain Language Summary: In many subduction zones, the relative strength of the plate interface compared to the overlying mantle changes dramatically at 70–80 km depth. At shallower levels, the interface acts as a weak shear zone decoupling the subducting plate from the overlying cool, stagnant mantle. At deeper levels, the strength of the shear zone increases such that subducting plate drives convection in the overlying mantle creating the high temperatures needed for arc magmatism. We propose the change in interface strength may be related to the stability of talc, a very weak mineral, in the forearc mantle. At shallower levels, talc is stable in a wide range of H2O‐undersaturated mantle compositions. At deeper levels, the stability field of talc is dramatically reduced and restricted to silica‐rich regions of the mantle. Key Points: Talc, a very weak mineral, is stable in H2O‐undersaturated ultramafic (mantle) rocks at P < ∼2–2.5 GPa (70–80 km depth) and T < ∼650°CAt pressures greater than ∼2–2.5 GPa, talc breaks down via the P‐dependent reaction: talc + forsterite = antigorite + enstatiteThe breakdown of talc may explain the change in subduction‐interface rheology at 70–80 km depth where full slab‐mantle coupling begins [ABSTRACT FROM AUTHOR]