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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. 

Constraints on the state of stress in the lithosphere are fundamental to understanding a breadth of geological phenomena. Paleo-stresses are generally estimated using microstructural elements for which there are experimentally calibrated relationships with applied stress, with an emphasis on recrystallised grain-size piezometers. However, it is often difficult to clearly distinguish newly recrystallised grains from the relict matrix. Furthermore, these grain-size piezometers are only applicable to rocks consisting of a single mineral. An alternative proxy for paleo-stress in polymineralic rocks is the average subgrain size. Unfortunately, estimates of subgrain size differ significantly among different measurement methods, and therefore, piezometers must be individually calibrated for the method used. Existing subgrain-size piezometers are based on calibrations using optical or transmission electron microscopy. We use electron backscatter diffraction (EBSD), a common method of subgrain-boundary characterisation, to calibrate subgrain-size piezometers for both olivine and quartz. To test the application of our olivine subgrain-size piezometer to polymineralic rocks, we deformed synthetic mixtures of olivine and orthopyroxene. Experiments were conducted using a Deformation-DIA apparatus at beamline 6BM-B Advanced Photon Source, Argonne National Laboratory. These experiments offer the unique possibility of simultaneously deforming the sample and measuring the average stresses within each phase using X-ray diffraction, before applying subgrain-size piezometry to the recovered samples. The results provide tests of (1) the manner in which stress is partitioned between phases, (2) whether the stresses measured in each phase by X-ray diffraction are comparable to those estimated by subgrain-size piezometry, and (3) whether stresses from subgrain piezometry can be used to estimate the macroscopic average applied stress. Stresses estimated from X-ray diffraction agree well with those made from subgrain-size piezometry in both monomineralic and polymineralic samples. In harzburgites, average stresses are similar in both phases and indicate that in this system, subgrain-size piezometric measurements from a single phase can be used to estimate the bulk stress. 

Abstract Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.