Relationships between the microstructural evolution and the rheology of talc at elevated pressures and temperatures

Details Relationships between the microstructural evolution and the rheology of talc at elevated pressures and temperatures Relationships between the microstructural evolution and the rheology of talc at elevated pressures and temperatures

Apr 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008e%26psl.268..463e&link_type=abstract

Earth and Planetary Science Letters, Volume 268, Issue 3-4, p. 463-475.

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17

Scientific paper

We conducted triaxial compression experiments to investigate the effects of pressure, temperature and strain rate on the rheology of talc rocks. The tests were carried out at T from 25 to 860 °C, P from 0.1 to 300 MPa, and ɛ˙ from 0.3 to 30 × 10- 5 s- 1. In addition to being very weak, there are other important differences between the mechanical behavior of talc and that of most other silicates. Deformation at all temperatures up to dehydration was accommodated by a combination of crystal plasticity, frictional sliding, and cataclasis. A transition from localized to distributed deformation occurred at P = 300 MPa and T = 400 °C, but this transition is ill-defined as both distributed and localized deformation were observed at P = 300 MPa and T = 600 °C. Unlike most silicates, both the coefficient of internal friction and the coefficient of (sliding) friction are very low and nearly equal to each other. Temperature enhances plastic deformation (kinking), and inter- and intra-granular microcracks are observed parallel to the (001) planes at all conditions. Voids are created by delamination along (001) planes at kink-band boundaries. Full crystal plasticity was not achieved under any of the conditions tested, i.e., the von Mises criterion was never satisfied. However, the strength of the aggregates remained much less than the confining pressure. These unusual properties are likely a manifestation of the pronounced mechanical anisotropy of talc at the grain scale. At T ≥ 750 °C dehydration is observed, but only along shear zones. These results suggest that feedbacks between deformation and reaction kinetics could control fluid flow in fault zones. The presence of talc can promote significant weakening and strain localization in the oceanic lithosphere, the subducting plate and the overlying mantle wedge.

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