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TitleOn the stability of talc in subduction zones: a possible control on the maximum depth of decoupling between the subducting plate and mantle wedge
AuthorPeacock, S MORCID logo; Wang, KORCID logo
SourceGeophysical Research Letters vol. 48, issue 17, e2021GL094889, 2021 p. 1-8,
Alt SeriesNatural Resources Canada, Contribution Series 20210179
PublisherAmerican Geophysical Union
Mediapaper; on-line; digital
File formatpdf; html
Subjectstectonics; mineralogy; geophysics; Science and Technology; Nature and Environment; Health and Safety; tectonic environments; subduction zones; downgoing slab; mantle; talc; pressure-temperature conditions; rheology; heat flow; thermodynamics; shear zones; ultramafic rocks
Illustrationsschematic cross-sections; ternary diagrams; phase diagrams; plots
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2021 08 29
AbstractGeophysical 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.
Summary(Plain Language Summary, not published)
In many subduction zones, the relative strength of the 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 mantle compositions under H2O undersaturated conditions. At deeper levels, the stability field of talc is dramatically reduced and restricted to silica-rich regions of the mantle.

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