Effects of Pressure on Diffusion Creep of Hydrous Polycrystaline Olivine

Abstract:

Diffusion and dislocation creep are two dominant mechanisms of deformation in the Earth's mantle. The relative importance of these two mechanisms may change with depth, temperature and water content. While the effects of pressure are well understood for dislocation creep, the pressure effects on diffusion creep at the upper mantle conditions are poorly constrained due to experimental challenges. The uncertainties in effects of pressure, especially on diffusion creep of hydrous olivine present serious problems in constraining the viscosity of the upper mantle as well as nature of seismic anisotropy.

In this study, we report the effects of pressure on diffusion creep of hydrous synthetic and natural polycrystalline olivine samples. We used Deformation-DIA (D-DIA) coupled with synchrotron X-ray facility (6-BM,B beamline at APS), to perform in-situ high pressure (3 – 10 GPa) deformation experiments of ultra-fine-grained samples in the temperature range 973 – 1123 K.

The operation of diffusion creep in our samples was inferred from the absence of (hkl) dependence of lattice strain as well as the much smaller strength of our samples compared to the strength expected for dislocation creep. Combining our new results at high pressures (3-10 GPa) with previously published results at low pressures (<0.45 GPa) by Mei and Kohsltedt (2000), we determined the activation volume (V*, representing the pressure effect) and the water content exponent ("r").

The normalized data were analyzed using a generalized least-squares method which yields activation volume V* = 5.47 ± 0.51 cm³ and water exponent r = 1.15 ± 0.08. We compare our results with the results on dislocation creep to construct deformation mechanism maps. We discuss some implications of these results including the dominant mechanisms of deformation and mechanisms of seismic anisotropy.