Abstract. We have performed an experimental study to quantify the high-temperature creep behavior of natural diabase rocks under dry deformation conditions. Samples of both Maryland diabase and Columbia diabase were investigated to measure the effects of temperature, oxygen fugacity, and plagioclase-to-pyroxene ratio on creep strength. Flow laws determined for creep of these diabases were characterized by an activation energy of Q = 485_+30 kJ/mol and a stress exponent of n = 4.7+0.6, indicative of deformation dominated by dislocation creep processes. Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is significantly stronger than the Columbia diabase. Thus the modal abundance of the various minerals plays an important role in defining rock strength. Within the sample-to-sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock. The dry creep strengths of both rocks are significantly greater than values previously measured on diabase under "as-received" or wet conditions [Shelton and Tullis, 1981; Caristan, 1982]. Application of these results to the present conditions in the lithosphere on Venus predicts a high viscosity crust with strong dynamic coupling between mantle convection and crustal deformation, consistent with measurements of topography and gravity for that planet.

[1] We performed triaxial compressive creep experiments on aggregates of San Carlos olivine to develop a flow law and to examine microstructural development in the dislocationaccommodated grain boundary sliding regime (GBS). Each experiment included load and temperature steps to determine both the stress exponent and the activation energy. Grain boundary maps, created with electron backscatter diffraction data, were used to quantify grain size distributions for each sample. Inversion of the resulting data produced the following flow law for GBS: _ " GBS = 10 4.8 ± 0.8 (s (010)[100] slip system contributes significantly to deformation. We propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations. Extrapolation of our flow laws to mantle conditions suggests that GBS is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow, and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep.

One of the principal means of understanding upper mantle dynamics involves inferring mantle flow directions from seismic anisotropy under the assumption that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment of a axes and (ii) when melt segregates and forms networks of weak shear zones, strain partitions between weak and strong zones, resulting in an alignment of a axes 90 degrees from the shear direction in three-dimensional deformation. This orientation of a axes provides a new means of interpreting mantle flow from seismic anisotropy in partially molten deforming regions of Earth.

[1] We demonstrate that deformation of partially molten ductile rocks can produce melt segregation by two-phase flow. In simple shear experiments on several melt-rock systems at high temperature and pressure, melt segregates into distinct melt-rich layers oriented 20°to the shear plane. Melt segregates in samples in which pressure gradients can develop at length scales less than the sample thickness. A simple scaling argument combined with a comparison of length scale data suggests that such pressure gradients can develop in the samples with compaction lengths less than or on the order of the sample thickness. In nature, stress-driven melt segregation may produce both high-permeability pathways that contribute to rapid extraction of melt and localization of deformation that increases the anisotropy in viscosity of partially molten regions of the upper mantle and lower crust.

[1] Although microstructural evolution is critical to strain-dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low-strain experiments. Therefore, we conducted a series of high-strain torsion experiments on thin-walled cylinders of iron-rich olivine aggregates. Experiments were performed in a gas-medium apparatus at 1200 C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain-size exponent of 0.73, values which agree well with those from previous small-strain experiments conducted on olivine in the dislocation-accommodated grain-boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010)[100] dislocations. The observed strain weakening is not entirely explained by grain-size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high-strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow-law parameters and microstructural observations from low-strain experiments and provide a means for incorporating strain weakening into geodynamic simulations.Citation: Hansen, L. N., M. E. Zimmerman, and D. L. Kohlstedt (2012), The influence of microstructure on deformation of olivine in the grain-boundary sliding regime,

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The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit.Results show large variations in 7 Li/ 6 Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average ␦ 7 Li for single thin sections along the traverse reveal a low in ␦ 7 Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ϳ10 per mil in the low ␦ 7 Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7 Li/ 6 Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction.We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in ␦ 7 Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.

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