| Abstrakt: |
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. Plain Language Summary: We studied the influence of melt alignment and crystal alignment on the properties of partially melted regions in planetary bodies. Molten and crystalline elements within the rocks in these layers can deform and reorient in response to stress, but it is difficult to predict how the effect of realignment of each phase affects the seismic properties of the rocks. Reorientation of melt networks during the deformation of partially molten rocks is not well constrained as experiments and computational models disagree on the most favorable alignment of melt pockets. Here, we measured the angles and shapes of melt and crystals in experimentally deformed partially molten rocks, then calculated the seismic properties of the deformed rocks. We found that melt pockets change orientation and shape quickly, but crystallographic axes take longer to reorient. This observation indicates that immediate changes to seismic properties after a sudden change in stress field are caused by melt rather than crystals. Our results show that when stress fields abruptly change in Earth and other planetary bodies, melt pocket orientation controls seismic properties and is the best instantaneous indicator of stress changes. Key Points: When a partially molten rock is stressed, its microstructural melt pockets reorient much more quickly than its crystallographic axesIndividual melt pockets orient parallel to the loading direction at the onset of deformationRapid changes to seismic anisotropy in a deforming partially molten rock can be attributed to the reorientation of melt pockets [ABSTRACT FROM AUTHOR] |
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