Diffusion Creep of Enstatite at High Pressures Under Hydrous Conditions

Abstract

Mantle convection and large‐scale plate motion depend critically on the nature of the lithosphere‐asthenosphere boundary and thus on the viscosity structure of Earth's upper mantle, which is determined by the rheological properties of its constituent minerals. To constrain the flow behavior of orthopyroxene, the second most abundant constituent of the upper mantle, deformation experiments were carried out in triaxial compressive creep on fine‐grained (~6 μm) samples of enstatite at high pressures (3.8–6.3 GPa) and high temperatures (1323–1573 K) using a deformation‐DIA apparatus. Based on results from this study, the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress, water fugacity, temperature, and pressure. Specifically, the creep rate depends approximately linearly on stress, indicating deformation in the diffusion creep regime. A least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of ~200 kJ/mol and an activation volume of ~14 × 10−6 m3/mol. The magnitude of the water‐weakening effect is similar to that for olivine with a water fugacity exponent of r ≈ 0.7. This strong dependence of viscosity on water fugacity (concentration) indicates that the viscosity of an orthopyroxene‐bearing mantle varies from one geological setting to another, depending on the large‐scale water distribution. Based on the rheology contrast between olivine and enstatite, we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime.

How Does Viscosity Contrast Influence Phase Mixing and Strain Localization?

“…To briefly examine the conditions favoring mechanical phase mixing in natural systems, we calculate viscosity contrast as a function of stress, grain size, and temperature for quartz‐feldspar (i.e., crustal) and olivine‐pyroxene (i.e., mantle) lithologies under both wet and dry conditions (Figures 9 and S9). Viscosities are calculated using laboratory flow laws for quartz (Rutter & Brodie, 2004; Tokle et al, 2019), plagioclase (Rybacki & Dresen, 2004), Mg‐rich olivine (Hansen et al, 2011; Hirth & Kohlstedt, 2003; Ohuchi et al, 2015), and enstatite (Bystricky et al, 2016; Ross & Nielsen, 1978; Zhang et al, 2017). Water fugacities are calculated as a function of temperature and pressure following Shinevar et al (2015).…”

“…Journal of Geophysical Research: Solid Earth dry conditions (Figures 9 and S9). Viscosities are calculated using laboratory flow laws for quartz (Rutter & Brodie, 2004;Tokle et al, 2019), plagioclase (Rybacki & Dresen, 2004), Mg-rich olivine (Hansen et al, 2011;Hirth & Kohlstedt, 2003;Ohuchi et al, 2015), and enstatite (Bystricky et al, 2016;Ross & Nielsen, 1978;Zhang et al, 2017). Water fugacities are calculated as a function of temperature and pressure following Shinevar et al (2015).…”

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

“…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.…”

“…We next considered the predicted strain rates for recrystallized opx during dislocation creep [78] and diffusion creep (based on experiments by Zhang et al [79], which were conducted at comparable pressures to our natural sample, and, thus, require no extrapolation in pressure). Dislocation creep is predicted to be the dominant deformation mechanism when the activation volume is V* <~20 (m 3 10 6 /mol) for dislocation creep.…”