David L. Kohlstedt

Naturally derived, fine-grained olivine ceramics were synthesized by an evacuated hot-pressing method that yielded samples with porosities of < 0.1%, a marked reduction compared to samples fabricated by conventional hot pressing with porosities of ≳1%. Evacuated hot pressing yields bright-green olivine samples that are transparent at the mm-scale, resembling the appearance of the single crystals from which they were synthesized, while conventional hot pressing produces milky-green aggregates that are opaque. This contrast in macro-scale transparency reflects the difference in micro-scale porosity. Annealing experiments on samples synthesized by the two different methods, some at 1 atm and others at 300 MPa confining pressure, reveal contrasting styles of grain and pore growth. During high-temperature annealing at both low and high pressures, evacuated hot-pressed samples underwent rapid, abnormal grain growth, while conventionally hot-pressed samples remained fine grained over long annealing times with very limited abnormal grain growth. During annealing at 1 atm, evacuated hot-pressed samples exhibited very little pore growth compared to conventionally hot-pressed samples that disaggregated due to pervasive pore growth. These experiments demonstrate the primary influence that porosity plays in grain growth in olivine aggregates. Further, the methods presented in this study provide a means to produce low-porosity olivine aggregates from naturally derived powders that can be used for high-temperature experiments at low pressures, as well as a method to make dense, coarse-grained olivine aggregates for laboratory studies.

The crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b ‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.

We investigated the co‐evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs, and CPOs) in experimentally deformed partially molten rocks, from which we calculated the influence of MPO and CPO on seismic anisotropy. Olivine‐basalt aggregates containing 2 to 4 wt% melt were deformed in general shear at a temperature of 1,250°C under a confining pressure of 300 MPa at shear stresses of τ ≤ 175 MPa to shear strains of γ ≤ 2.3. Grain‐scale melt pockets developed a MPO parallel to the loading direction by γ < 0.4. At higher strains, the grain‐scale MPO remained parallel to the loading direction, while incipient sample‐scale melt bands formed at ∼20° to the grain‐scale MPO. An initial SPO and CPO were induced during sample preparation, with [100] and [001] axes girdled perpendicular to the long axis of the starting material. At the highest explored strain, a strong SPO was established subperpendicular to the loading direction, and the [100] axes of the CPO clustered nearly parallel to the shear plane. Our results demonstrate that grain‐scale and sample‐scale alignments of melt pockets are distinct. Furthermore, the melt and the solid microstructures evolve on different timescales: in planetary bodies, changes in the stress field will drive a relatively fast reorientation of the melt network and a relatively slow realignment of the crystallographic axes. Rapid changes to seismic anisotropy in a deforming partially molten aggregate are thus caused by MPO rather than CPO.

As partially molten rocks deform, they develop melt preferred orientations, shape preferred orientations, and crystallographic preferred orientations (MPOs, SPOs and CPOs). We investigated the co-evolution of these preferred orientations in experimentally deformed partially molten rocks, then calculated the influence of MPO and CPO on seismic anisotropy. Olivine-basalt aggregates containing 2 to 4 wt% melt were deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa at shear stresses of τ = 0 to 175 MPa to shear strains of γ = 0 to 2.3. Grain-scale melt pockets developed a MPO parallel to the maximum principal stress, s1, at γ < 0.4. At higher strains, the grain-scale MPO remained parallel to s1, but incipient, sample-scale melt bands formed at ~20° to s1. An initial SPO and CPO were induced during sample preparation, with [100] and [001] axes girdled perpendicular to the long axis of the sample. At the highest explored strain, a strong SPO was established, and the [100] axes of the CPO clustered nearly parallel to the shear plane. Our results demonstrate that grain-scale and sample-scale alignments of melt pockets are distinct. Furthermore, the melt and the solid microstructures evolve on different timescales: in planetary bodies, changes in the stress field will first drive a relatively rapid reorientation of the melt network, followed by a relatively slow realignment of the crystallographic axes. Rapid changes to seismic anisotropy in a deforming partially molten aggregate are thus caused by MPO rather than CPO.

When a confined packing of sand grains is sheared, the shear strain generates a compressive normal stress [1]. If the sand is unconfined, the shear leads to a volume expansion of the pore space between grains, low fluid pressure, and imbibition of fluid [1]. This physics is known as dilatancy [2]. We hypothesise that dilatancy occurs within a deforming, basalt-saturated aggregate of olivine grains. We extend a theory for the dynamics of partially molten rock [3,4] to describe this. We analyse the theory in the geometry of laboratory experiments and show that the dilatancy hypothesis can explain a variety of robust, non-trivial features of experiments. These include the angle of melt bands and the inward melt segregation in torsional and Poiseuille flows [5,6].One mechanism by which partially molten rock can deform is grain-boundary sliding with geometric incompatibility between grains accommodated by mass diffusion. Dilatancy would also accommodate granular incompatibility. The balance of diffusive and dilatant accommodation of compatibility might depend on the ratio of shear stress to confining stress. Rock sheared by a larger stress would strain faster and potentially undergo more dilatant accommodation. Moreover, shear strain could be associated with an anisotropy in the generated normal stress. At smaller melt fractions, partially molten rock might create greater dilatancy stress, but would also have a smaller resistance to (de)compaction. Our theory addresses these issues.A theory of anisotropic viscosity [7,8] has previously been proposed to explain the features of deformation experiments on olivine aggregates. We compare and contrast its physical basis and predictions with those of dilatancy.[1] Guazzelli, and Pouliquen, Rheology of dense granular suspensions, J Fluid Mech, 2018.[2] Reynolds, LVII. On the dilatancy of media composed of rigid particles in contact. With experimental illustrations. The London, Edinburgh, and Dublin Phil Mag and J Sci, 1885.[3] McKenzie, The generation and compaction of partially molten rock, J Pet, 1984.[4] Katz, The Dynamics of Partially Molten Rock, Princeton University Press, 2022.[5] King, Zimmerman, & Kohlstedt. Stress-driven melt segregation in partially molten olivine-rich rocks deformed in torsion. J Petrology, 2010.[6] Quintanilla-Terminel, Dillman, Pec, Diedrich, & Kohlstedt. Radial melt segregation during extrusion of partially molten rocks. Geochem. Geophys. Geosys., 2019.[7] Takei & Holtzman. Viscous constitutive relations of solid–liquid composites in terms of grain boundary contiguity: 1. Grain boundary diffusion control model. JGR: Solid Earth, 2009.[8] Takei & Katz. Consequences of viscous anisotropy in a deforming, two-phase aggregate. Part 1. Governing equations and linearized analysis. J Fluid Mech, 2013.

Partial melting often occurs alongside sites of rapid deformation in the Earth’s mantle and crust. Microstructural molten and crystalline components align in response to deformation, leading to anisotropies in mechanical, transport, and seismic properties detectable by remote sensing. Here, we investigate the co-evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs & CPOs) at early stages of experimental shear deformation, constraining their contribution to observable signatures. We characterized the microstructures of partially molten (2-4 wt% melt) olivine-basalt aggregates deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa, at shear stresses of τ = 0-175 MPa and shear strains of γ = 0-2.3. We then used the Gassman poroelastic differential effective medium method to calculate resultant seismic anisotropy.The grain-scale network of melt pockets developed a strong preferred orientation parallel to the maximum principal stress at γ < 0.4. At higher strains, the orientation of the grain-scale melt pockets remained parallel to the maximum principal stress, but incipient, sample-scale melt bands formed at ~25° antithetic to the direction of shear. While the orientation of individual melt pockets evolved quickly, grain SPOs and CPOs required larger strains (γ > 2) to strengthen and change. A weak SPO and CPO were induced during sample preparation, with grain long axes oriented perpendicular to the direction of maximum principal stress and a- and c-axes girdled perpendicular to the long axis of the sample. At the highest explored shear strain, a strong SPO was established and the girdled a-axes of the CPO rotated to align nearly parallel to the shear plane, developing clusters parallel to the shearing direction. These results yield two key conclusions about the orientation of melt networks in deforming partially molten rocks. First, the grain-scale and sample-scale alignments of melt pockets are distinct. At the grain scale, melt pockets align approximately parallel to the maximum principal stress, but the en echelon arrangement of melt pockets yields a sample-scale MPO at ~25o to the maximum principal stress (20o to the shear plane). Second, the relative timescales of melt and solid microstructural evolution are different, and are directly reflected in changes to seismic anisotropy. The grain-scale MPO reacts to a change in the orientation of the maximum principal stress after only a small amount of strain; in contrast, CPOs and SPOs require much higher strains before responding to a change in stress conditions. Seismic anisotropy is greatest when olivine a-axes and the grain-scale orientation of melt pockets are in relatively close alignment, so anisotropy will quickly decrease with any change in the orientation of the stress field that results in a rotation of the MPO away from the orientation of olivine a-axes. Perturbations to a local stress field can thus be observed almost immediately due to a rapidly reorienting melt network, making MPOs a more valuable predictor of instantaneous change than CPO-driven anisotropies.

To study the microstructural evolution of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (f_Per) between 0.1 and 0.8. Additionally, single‐phase samples of both Ol and Per were deformed under the same experimental conditions to facilitate comparison of the microstructures in two‐phase and single‐phase materials. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate up to a final shear strain of γ = 6 to 7. Microstructural developments, analyzed via electron backscatter diffraction (EBSD), indicate differences in both grain size and crystalline texture between single‐ and two‐phase samples. During deformation, grain size approximately doubled in our single‐phase samples of Ol and Per but remained unchanged or decreased in two‐phase samples. Zener‐pinning relationships fit to the mean grain sizes in each phase for samples with 0.1 ≤ f_Per ≤ 0.5 and for those with 0.8 ≥ f_Per ≥ 0.5 demonstrate that the grain size of the primary phase is controlled by phase‐boundary pinning. Crystallographic preferred orientations, determined for both phases from EBSD data, are significantly weaker in the two‐phase materials than in the single‐phase materials.

To study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (f_Per) between 0.1 and 0.8. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate to a final shear strain of γ = 6 to 7. The stress‐strain data and calculated values of the stress exponent, n, indicate that Ol in our samples deformed by dislocation‐accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains of γ < 1, the strengths of samples with f_Per > 0.5 match model predictions for both phases deforming at the same stress, the lower‐strength bound for two‐phase materials, while the strengths of samples with f_Per < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper‐strength bound. These observations suggest a transition from a weak‐phase supported to a strong‐phase supported regime with decreasing f_Per. Above γ = 4, however, the strength of all two‐phase samples is greater than those predicted by either the uniform‐stress or the uniform‐strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two‐phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries.