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Shockwave data on mineral‐forming compounds such as Mg2SiO4 are essential for understanding the interiors of Earth and other planets, but correct interpretation of these data depends on knowing the phase assemblage being probed at high pressure. Hence, direct observations of the phase or phases making up the measured states along the forsterite Hugoniot are essential to assess whether kinetic factors inhibit the achievement of the expected equilibrium, phase‐separated assemblage. Previous shock recovery experiments on forsterite, which has orthorhombic space group Pbnm, show discrepant results as to whether forsterite undergoes segregation into its equilibrium phase assemblage of compositionally distinct structures upon shock compression. Here we present the results of plate impact experiments on polycrystalline forsterite conducted at the Dynamic Compression Sector of the Advanced Photon Source. In situ X‐ray diffraction measurements were used to probe the crystal structure(s) in the shock state and to investigate potential decomposition into periclase and bridgmanite. In contrast to previous interpretations of the forsterite shock Hugoniot, we find that forsterite does not decompose but instead reaches the forsterite III structure, which is a metastable structure of Mg2SiO4 with orthorhombic space group Cmc21.

We performed a series of extrusion experiments on partially molten samples of forsterite plus 10 vol% of an anorthite‐rich melt to investigate melt segregation in a pipe‐extrusion geometry and test the predictions of two‐phase flow theory with viscous anisotropy. The employed flow geometry has not been experimentally investigated for partially molten rocks; however, numerical solutions for a similar, pipe‐Poiseuille geometry are available. Samples were extruded from a 6‐mm diameter reservoir into a 2‐mm diameter channel under a fixed normal stress at 1350°C and 0.1 MPa. The melt distribution in the channel was subsequently mapped with optical and backscattered electron microscopy and analyzed via quantitative image analysis. Melt segregated from the center toward the outer radius of the channel. The melt fraction at the wall increased with increasing extrusion duration and with increasing shear stress. The melt fraction profiles are parabolic with the melt fraction at the wall reaching 0.17–0.66, values 2 to 16 times higher than at the channel center. Segregation of melt toward the wall of the channel is consistent with base‐state melt segregation as predicted by two‐phase flow theory with viscous anisotropy. However, melt‐rich sheets inclined at a low angle to the wall, which are anticipated from two‐phase flow theory, were not observed, indicating that the compaction length is larger than the channel diameter. The results of our experiments are a test of two‐phase flow theory that includes viscous anisotropy, an essential theoretical frame work needed for modeling large‐scale melt migration and segregation in the upper mantle.

We examine the hard X-ray (HXR) footpoint sources of three flares, as observed by RHESSI, in combination with the topology given by the extrapolation of line-of-sight magnetograms into the corona. Assuming the HXR footpoint sources are chromospheric consequences of magnetic reconnection that takes place on separators, we further assume a relationship between the buildup of energy in stressed coronal magnetic fields and the measurement of the change in separator flux per unit length. We find that the value of this quantity is larger for the separators that connect the HXR footpoint sources than the quantity for the separators that do not. Therefore, we conclude that we are able to understand the location of HXR sources observed in flares in terms of a physical and mathematical model of the topology of the active region.

<p>Transient creep controls the behavior of Earth’s mantle at human timescales. Transient creep occurs during postseismic creep, glacial isostatic adjustment, and tidal deformation of planetary interiors experiencing large tidal stresses, such as Jupiter’s moon Io or various identified exoplanets. Unfortunately, laboratory data of transient creep of olivine, the most abundant mineral in Earth’s upper mantle, remain limited, and at present we lack the microphysical understanding of transient creep required to extrapolate experimental data to geological grain sizes and time scales. Several mechanisms for transient creep have been proposed, both intergranular mechanisms such as plastic anisotropy and elastically or diffusionally-assisted grain boundary sliding, and intragranular mechanisms including long-range dislocation interactions and various other dislocation damping mechanisms. Each mechanism produces distinct rheological behavior, presenting a hurdle for modeling geodynamic processes occurring on timescales of hours to years.</p> <p>To distinguish among the various proposed microphysical mechanisms for transient creep, we performed compressional load-reduction experiments on cylindrical, isostatically hot-pressed aggregates of San Carlos olivine in a gas-medium Paterson apparatus at confining pressures of 300 MPa and temperatures of 1200°C. Samples were subjected to a constant differential stress of 200 MPa, which resulted in a steady-state strain rate of ~10<sup>-5</sup> s<sup>-1</sup>. After steady state was achieved, the samples were subjected to a near-instantaneous load reduction of 10–70% of the original load. </p> <p>For load reductions exceeding ~50%, the samples exhibited a period of transient anelastic relaxation with zero or negative strain rate before continuing to strain at a positive strain rate lower than the previous steady state. The duration of relaxation increased with the magnitude of the load reduction. Multiple load reductions from the same steady-state strain rate were performed during a single experiment to test for reproducibility. </p> <p>We interpret our results to indicate that, at these conditions, the backstress stored in polycrystalline olivine is approximately half of the differential stress applied to the material. The magnitude of backstress is compatible with long-range dislocation interactions on the [100](010) and [100](001) or [001](100) slip systems previously observed for single crystals of olivine. If transient creep is controlled by such dislocation interactions then it may be inappropriate to apply the traditional Burgers rheology based exclusively on intergranular dissipation processes or power-law flow laws calibrated for steady-state creep to model transient creep and transient viscosity evolution in the upper mantle.</p>