50-11 Viscous Weakening of Talc at the Onset of Dehydration: Observations and Implications

Session: Latest Research Advances in Structural Geology and Tectonics

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Subduction zones are regions of highly varied mineralogy; the ranges of strength of the phases present having significant impacts on the dynamics of subduction. Especially important, though understudied, is the evolving rheology of the subduction melange. During subduction, fluid-rock, rock-rock, and dehydration reactions alter the composition and rheology of the slab and slab-mantle interface. Various slip phenomena (e.g. aseismic slip, deep earthquakes) have been attributed to these factors. However, the role of dehydration and reaction on rheology is not fully explored, especially at mantle conditions. As an example, while the dehydration of talc has been proposed to explain intermediate-depth seismicity due to fluid release, the rheological properties of talc during dehydration are not well documented. This study investigates the rheology of talc as it undergoes dehydration and phase decomposition to enstatite and quartz reaction products. We conducted load-controlled deformation experiments on talc and hydrated talc-quartz aggregates at conditions near and beyond the phase stability limit of talc (777 – 850 ℃, 1 GPa confining pressure) in the Griggs apparatus. X-ray diffraction analysis of samples post-deformation show evidence of reaction (presence of enstatite and quartz). Comparison of experimental data with plasticity flow laws for talc deformation derived at similar conditions show that near dehydration, the talc flow law underpredicts deformation rate by several orders of magnitude at low stress. Further, the experimental data show a viscous weakening relative to the flow law, not a brittle/frictional weakening, contrary to the expected effect of increased fluid pressure. Experiments on hydrated talc-quartz aggregates at low (500 ℃) and high (777 ℃) temperatures show little strength or stress dependence on fluid content, indicating that the observed weakening may not entirely be due to the release of OH from the crystal structure during dehydration. Mechanical data from all experiments observe a stress dependence of n = 2 for talc rheology during dehydration, suggesting contributions from grain (and/or phase) boundary sliding. Similar stress dependence (and misfit relative to published flow laws) has also been observed in antigorite deformation near dehydration, implying there may be a shared weakening mechanism near the phase stability limit of these phyllosilicates. We offer insights and perspectives on rheology during reaction, and applications to natural conditions in subduction zones.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10280

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