An experimental study into the rheology of synthetic polycrystalline coesite aggregates

Coesite has been found as a relic in ultrahigh pressure metamorphic (UHPM) crust worldwide and is expected to play a major role in the mechanical beha vior of continental crust at UHPM conditions. We performed triaxial compres sion tests on synthetic polycrystalline coesitite in a solid medium apparat us at confining pressures of 3.1 to 3.7 GPa, temperatures of 700 degrees to similar to 1160 degreesC, and strain rates betweeen 6x10(-7) and 1x10(-3) s(-1). The problem of the limited stress resolution of the solid medium app aratus was addressed by applying two extreme friction corrections that yiel d lower and upper bounds to the differential stress. The correlation betwee n the mechanical data and the microstructural record of the deformed sample s, as a function of temperature and imposed strain rate, is consistent with deformation by dislocation creep. We deduced parameters of a power law (ep silon over dot = A sigma (n) exp[-Q/RT]) as n approximate to 3 +/- 1 and Q approximate to 275 +/- 50 kJ mol(-1). Extrapolation of the experimental dat a to natural conditions cannot be constrained by comparison with natural mi crostructures, due to the lack of preserved coesite other than as single cr ystal inclusions. Nevertheless, the extrapolation indicates a low strength (of order 10 MPa) for natural strain rates at typical UHPM conditions. Abse nt deformation of the UHPM Brossasco granite (Dora Maira Massif, Western Al ps) thus implies low stresses; deformation must have been localized in very weak shear zones during burial and exhumation.