EFFECTS OF SHEAR-INDUCED MELT SEGREGATION AND STRAIN PARTITIONING ON OLIVINE MICROSTRUCTURES

We investigated the effects of melt segregation and strain partitioning on rock microstructures in chromite-bearing olivine + MORB rocks deformed in direct shear. Previous studies on deformed melt-bearing peridotites report development of inclined, ‘back-rotated’ (relative to the shear direction and bulk shear plane) patterns of olivine crystallographic preferred orientations (CPOs). These CPOs are characterized by well-developed clusters of [010] axes and girdles of [100] and [001] axes, often referred to as an ‘axial [010]’ pattern. The back-rotation of CPOs observed in melt-bearing rocks is not typically observed in melt-free dunites, and the prevailing model to explain the rotation is that the kinematics of deformation are locally partitioned between melt-rich bands and melt-depleted lenses. We analyze a sample that developed large melt-rich bands during deformation that permit local microstructural analyses within the bands and within melt-depleted lenses. We measured olivine and chromite grain-shape preferred orientations (SPOs) and olivine CPOs to quantify their angular relationships to deformation kinematics. Olivine CPOs in the melt-depleted lenses exhibit the characteristic back-rotated patterns described in previous studies; however, CPOs in the melt-rich domains are notably less back-rotated (~7°) than those measured in the melt-depleted lenses (~23°). The orientation of olivine SPOs are also domain-dependent, with rotations of 42° and 60° from the shear plane in the melt-rich bands and melt-depleted lenses, respectively. Conversely, chromite SPOs are nearly identical in both domains (~75° from the shear plane). Our results provide new evidence to confirm previous hypotheses that strain/flow is partitioned if melt segregates into bands during deformation. Additionally, the insensitivity of chromite SPO to the inferred flow partitioning and overall strain geometry suggests that spinel grain shapes are not reliable proxies for deformation kinematics or strain geometry. In conclusion, strain partitioning due to melt-segregation causes local reorientation of olivine CPO and SPO, which are commonly used structures in microtectonic and seismic studies.