Rheology of Talc at High P-T Conditions with Implications for Subduction-zone Interface

Abstract:

Subduction-zone dynamics, kinematics, and seismicity are strongly affected by the rheology of hydrous phyllosilicates. Specifically, talc is stable to depths of ≈100 km and has been found in fault rocks and subduction-zones mélanges, and interpreted to prevail on the interface of flat subduction-zones. The rheology of talc was studied under low to intermediate pressures (up to ~400 MPa); however, there is a lack of data for pressures of P > 0.5 GPa. Here we present the first rheological and microstructural analysis of experimentally deformed talc under pressure and temperature conditions relevant for the rheology of a subducted slab or mantle wedge.

We analyzed the mechanical and microstructural evolution of 15 samples of natural talc cylinders (>98 % talc) deformed using a high P-T deformation ‘Griggs’ type apparatus. The experiments were performed at confining pressures from 0.5 to 2 GPa and temperatures of 25 to 700 °C; all within the talc stability field. Microstructural and chemical analysis were performed using optical microscopy, scanning electron microscopy, and electron microprobe. Results show that the strength of talc at 25 °C or 400 °C is pressure dependent up to the highest pressure tested (2 GPa). This behavior is attributed to brittle/semi-brittle mechanisms. At higher temperatures (500-700 °C) and above a pressure threshold the strength becomes independent of pressure (e.g., when P > 1 GPa at T = 600 °C), indicating that dilatant cracking is suppressed at these pressures. Interestingly, samples deformed at higher temperatures (>600 °C) show more localized deformation. A synthesis of results from this study and previously published studies demonstrates that the strength of talc only becomes temperature-dependent at higher pressures. Along an increasing P-T geotherm of a subducted slab, the presence of talc along the slab-wedge interface is likely to induce weakening and localization within talc-rich layers. Possible implications of high P-T talc rheology on the dynamics and mechanics of the flat subduction-zone beneath Mexico will be illustrated.