Rheological behavior and electrical conductivity of San Carlos olivine at low melt fraction
Résumé
To determine the rheological behavior of partially molten mantle rocks, we performed deformation experiments at constant shear strain rate on aggregates of San Carlos olivine containing 2% melt. Samples were hot pressed in Ni cans with central Ni rods at high temperature (T) and high pressure (P). With this thin-walled cylindrical sample geometry, we also conducted in-situ electrical conductivity measurements during two of the deformation experiments. Electrical conductivity was measured radially through the sample, that is, perpendicular to the shear direction, by coupling an impedance spectrometer to the deformation apparatus. The mechanical data demonstrate that our partially molten samples deform outside of the diffusion creep regime with a stress exponent, n > 1, and a grain size exponent, p > 2. With increasing strain, stress generally increased sharply before reaching a steady-state regime in which the stress remained roughly constant. This behavior is consistent with previous studies for melt-bearing samples with ≤ 4% melt. The mechanical data also indicate that our melt-bearing samples are 100 MPa weaker than melt-free samples at the same T/P/strain rate conditions. With 2% melt, our samples were more electrically conductive by 1.5 orders of magnitude than melt-free olivine samples. Electrical conductivity did not change significantly during torsion experiments performed to outer radius strains of 4.5, consistent with previous experimental observations on partially molten aggregates. Even though the in-situ measurements of the mechanical and electrical properties of the samples did not evolve with strain, scanning electron microscopy (SEM) observations of the tangential sections of the deformed samples did reveal an evolution from an initially homogeneous distribution of melt in the starting material either to aligned, interconnected melt pockets or to narrow bands in the twisted samples. The aligned melt phase is oriented 15-20° counterclockwise to the shear direction, in agreement with previously published observations on similar aggregates with higher melt contents. SEM imaging of the radial sections of the samples does not exhibit any fully developed planar melt features, consistent with the unchanging electrical conductivity values with increasing strain.