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Abstract We have measured the sound velocities and elasticity of synthetic polycrystalline β-Mg2SiO4 containing 1.2 wt% H2O to 10 GPa and 600 K using ultrasonic interferometry with synchrotron X-radiation. We determined sample length at high pressure and temperature using the sample’s X-radiographic image and applied travel times bond corrections appropriate to the experimental cell assembly configuration. Fitting the entire moduli data to third-order finite strain equations yields the adiabatic bulk [KS0 = 153.3(4) GPa] and shear [G0 = 101.8(2) GPa] moduli, their pressure derivatives (∂KS/∂P)T = 5.15(6) and (∂G/∂P)T = 1.68(3) and temperature derivatives (∂KS/∂T)P = −0.0179(9) GPa/K and (∂G/∂T)P = −0.0151(7) GPa/K. Comparing the bulk sound velocity contrast between the new hydrous wadsleyite data and olivine (0.38 wt% H2O) with seismic bulk sound velocity contrasts of 3.5% and 4.0% yields 53% and 60% olivine content, respectively, assuming an iso-chemical mantle model of the Earth. The results suggest that a hydrous mantle transition zone with a pyrolite model composition could explain the 410 km seismic velocity jump. 

Phase transformations are widely invoked as a source of rheological weakening during subduction, continental collision, mantle convection and various other geodynamic phenomena. However, despite more than half a century of research, the likelihood and magnitude of such weakening in nature remain poorly constrained. Here we use experiments performed on a synchrotron beamline to reveal transient weakening of up to three orders of magnitude during the polymorphic quartz to coesite (SiO2) and olivine to ringwoodite (Fe2SiO4) phase transitions. Weakening becomes increasingly prominent as the transformation outpaces deformation. We suggest that this behaviour is broadly applicable among silicate minerals undergoing first-order phase transitions and examine the likelihood of weakening due to the olivine-spinel, (Mg,Fe)2SiO4, transformation during subduction. Modelling suggests that cold, wet slabs are most susceptible to transformational weakening, consistent with geophysical observations of slab stagnation in the mantle transition zone beneath the western Pacific. Our study highlights the importance of incorporating transformational weakening into geodynamic simulations and provides a quantitative basis for doing so. 

Abstract Dislocation‐based dissipation mechanisms potentially control the viscoelastic response of Earth's upper mantle across a variety of geodynamic contexts, including glacial isostatic adjustment, postseismic creep, and seismic‐wave attenuation. However, there is no consensus on which dislocation‐based, microphysical process controls the viscoelastic behavior of the upper mantle. Although both intergranular (plastic anisotropy) and intragranular (backstress) mechanisms have been proposed, there is currently insufficient laboratory data to discriminate between those mechanisms. Here, we present the results of forced‐oscillation experiments in a deformation‐DIA apparatus at confining pressures of 3–7 GPa and temperatures of 298–1370 K. Our experiments tested the viscoelastic response of polycrystalline olivine—the main constituent of the upper mantle—at stress amplitudes from 70 to 2,800 MPa. Mechanical data are complemented by microstructural analyses of grain size, crystallographic preferred orientation, and dislocation density. We observe amplitude‐ and frequency‐dependent attenuation and modulus relaxation and find that numerical solutions of the backstress model match our results well. Therefore, we argue that interactions among dislocations, rather than intergranular processes (e.g., plastic anisotropy or grain boundary sliding), control the viscoelastic behavior of polycrystalline olivine in our experiments. In addition, we present a linearized version of the constitutive equations of the backstress model and extrapolate it to conditions typical of seismic‐wave propagation in the upper mantle. Our extrapolation demonstrates that the backstress model can explain the magnitude of seismic‐wave attenuation in the upper mantle, although some modification is required to explain the weak frequency dependence of attenuation observed in nature and in previous experimental work.