Effect of Water on the Dislocation Creep of Enstatite Aggregates at 300 MPa

Abstract

To investigate the effect of water on the rheological properties of enstatite, we have conducted triaxial compressive creep experiments on enstatite aggregates using a gas medium apparatus at a confining pressure of 300 MPa and temperatures of 1373–1473 K under both water‐saturated and anhydrous conditions. Samples were mainly deformed in the dislocation creep regime; analyses of mechanical data yield activation energies of 603 and 567 kJ/mol for hydrous and anhydrous conditions, respectively. Under similar differential stress and temperature, the creep rate of enstatite under water‐saturated conditions is a factor of ~50 greater than that under anhydrous conditions. Based on a comparison of creep strength between olivine and enstatite in the dislocation creep regime, our results suggest that, at least under limited pressure and temperature conditions, enstatite is weaker than olivine. The results from this study provide a solid database for modeling geological processes occurring within Earth's interior.

Microstructural Analysis of a Mylonitic Mantle Xenolith Sheared at Laboratory-like Strain Rates from the Edge of the Wyoming Craton

“…The influence of water content is included in the A term of the flow law listed in Table S1 (Supplementary Materials). The opx water content of our sample (3,504 H/10 6 Si) is within the range of the value determined for the opx in Zhang et al's [78] experiments (2760-5120 H/10 6 Si). We calculated trends for a range of activation volumes (V* = 10, 20, and 30 m 3 10 6 /mol).…”

“…Relying on the microstructure and, in particular, the observation that the opx porphyroclasts show a significantly lower recrystallized grain fraction than olivine, we interpreted that the opx porphyroclast grains were deformed to lower strain [81] and that the coarse-grained opx was stronger than olivine at the relevant stresses. Therefore, using opx dislocation creep flow laws from Zhang et al [78] and the comparison between calculated strain rates of opx and olivine porphyroclasts (Figure 11a), we recognized that an activation volume of V* > 20 m 3 10 6 /mol provided the best fit to our interpretation of relatively strong opx porphyroclasts.…”

“…Using existing flow laws for wet olivine and opx under the P-T-COH conditions inferred for the shearing of sample H69-15F, we compared the predicted strain rates for the porphyroclasts grain size and for the recrystallized grain size using a standard flow law [21], e.g.,: = ( * ) , where is the strain rate in units of s −1 , A is the preexponential parameter, is the stress in MPa, d is grain size in μm, COH is the water content in H/10 6 Si, E is the activation energy in J/mol, P is pressure in GPa, V* is the activation volume in m 3 /mol, T is the temperature in K, and R is the gas constant with units of J/K mol. The flow law parameters for olivine from [21] and opx from [78,79] are listed in Table Figure 10. (a) Recrystallized grain size versus stress relations (piezometers) for olivine and opx from Van der Wal et al [74] and Linckens et al [75], respectively.…”

“…ε is the strain rate in units of s −1 , A is the pre-exponential parameter, σ is the stress in MPa, d is grain size in µm, C OH is the water content in H/10 6 Si, E is the activation energy in J/mol, P is pressure in GPa, V* is the activation volume in m 3 /mol, T is the temperature in K, and R is the gas constant with units of J/K mol. The flow law parameters for olivine from [21] and opx from [78,79] are listed in Table S1 (Supplementary Materials). Since the dislocation flow law for wet opx [78] was constructed using experiments at a confining pressure of 0.3 GPa, we included an activation volume to extrapolate from the pressure in the experiments to the 4.6 GPa pressure constrained for H69-15F.…”

“…The flow law parameters for olivine from [21] and opx from [78,79] are listed in Table S1 (Supplementary Materials). Since the dislocation flow law for wet opx [78] was constructed using experiments at a confining pressure of 0.3 GPa, we included an activation volume to extrapolate from the pressure in the experiments to the 4.6 GPa pressure constrained for H69-15F. The influence of water content is included in the A term of the flow law listed in Table S1 (Supplementary Materials).…”