[1] Abstract: New multianvil experimental data are reported for the anhydrous solidus for peridotite KLB-1 at 5 to 9.7 GPa. The solidus is unaffected by variable experimental run durations, but it is lowered in temperature when rhenium capsules are replaced with graphite. Using data reported previously, the anhydrous solidus for KLB-1 can be described by T = 1086 À 5.7P + 390 ln P (688C 2s) at pressures that range from 2.7 to 22.5 GPa. At lower pressures the effect of composition on the temperature of the spinel peridotite solidus is small and dominated by alkali abundances; variations in Mg number have almost no effect on solidus temperature.

Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg 2 SiO 4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373-1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (ε . ) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones.These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the "puzzling" seismic-anisotropy anomalies commonly observed in subduction zones.

Synthesized polycrystalline enstatite samples were deformed in a Paterson gas‐medium apparatus at 1200–1300°C, oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 MPa (protoenstatite field) or 450 MPa (orthoenstatite field). At both confining pressures, the mechanical data display a progressive increase of the stress exponent from n = 1 to n~3 with increasing differential stress, suggesting a transition from diffusional to dislocation creep. Nonlinear least squares fits to the high‐stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kJ/mol for orthoenstatite and protoenstatite, respectively. Deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy. Microstructures show undulatory extinction and kink bands, evidence of dislocation processes. Crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on (100)[001]. Most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae. While many lamellae may have formed during quenching from run conditions, those in samples deformed in the orthoenstatite field are often bordered by partial [001] dislocations, suggesting transformation due to glide of partial [001] dislocations in (100) planes. Comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions. However, as orthoenstatite has a higher activation energy and smaller stress exponent than olivine, this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle.

Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented San CarlosB B B olivine single crystals, at pressure (P) ranging from 3.5 to 8.5 GPa, temperature (T) from 1373 to 1673 K, and in poor water These results may explain the discrepancy between olivine low-P and high-P deformation data which has been debated in the literature for more than a decade.