Does the stability of talc in the mantle wedge control the maximum depth of slab-wedge decoupling in subduction zones?

Abstract:

Thermal models of subduction zones, combined with surface heat flow data, demonstrate that the subducting slab becomes fully coupled to the overlying mantle wedge at a relatively uniform depth of ~70-80 km. The maximum depth of decoupling (MDD) is a fundamental feature of subduction zones, separating (i) shallow, cool, relatively stagnant forearc mantle from (ii) deeper, hotter, convecting mantle capable of generating arc magmas. Updip of the MDD, the subduction shear zone is rheologically weaker than the overlying mantle and underlying oceanic slab, and most strain is localized within the shear zone. Downdip of the MDD, the subduction shear zone has the same strength as the overlying mantle wedge and full coupling between the subducting slab and overlying mantle induces corner flow in the mantle wedge. What controls the MDD is a fundamental question in subduction zone dynamics.

In the subducting slab and down-dragged mantle wedge, hydrous minerals break down as a result of metamorphic dehydration reactions that depend strongly on temperature (T) and less so on pressure (P). Talc, an unusually weak hydrous mineral, is stable in H₂O-saturated ultramafic rocks, but only under restricted conditions (T ~ 600 C and P < 1.5 GPa) making it a seemingly unlikely candidate for controlling the MDD. However, talc is stable in H₂O-undersaturatedultramafic rocks where it breaks down via the fluid-absent, P-dependent reaction: talc + forsterite = antigorite + enstatite at P ~ 2 GPa (~70 km depth) and T < 650 °C. Field and petrographic observations of mantle rocks now exposed at the surface demonstrate hydration is spatially heterogeneous at all scales, from the meter to kilometer spacing of major fractures along which fluids infiltrate the mantle wedge to the millimeter scale of individual olivine and pyroxene grains. At depths < 70 km, H₂O-undersaturated portions of the mantle wedge along the plate boundary are predicted to contain minor amounts of talc, which has been shown by experimental studies to dramatically weaken rocks. At depths > 70 km, talc is not stable in the ultramafic mantle wedge regardless of H₂O content except for rare pyroxene-rich and Si-rich (metasomatic) lithologies. Thus, the uniform MDD inferred for subduction zones may reflect the breakdown of talc in H₂O-undersaturated mantle rocks along the plate interface.