Lattice-preferred Orientation and Volatile Content of Olivine in Arc Mantle Lithosphere, Based on Xenoliths from the Sierra Nevada Batholith, California

Abstract:

Peridotite xenoliths from the Mesozoic Sierra Nevada Batholith, California, USA record the origin and growth of arc mantle lithosphere. The peridotites vary from coarse-grained, clinopyroxene-poor spinel peridotite to fine-grained, recrystallized, clinopyroxene-rich garnet peridotite. Garnet coronas around spinel and garnet exsolution indicate cooling and/or increasing pressure. Previous thermobarometry (Chin et al., 2012) interpreted these peridotites as originally shallow (<3 GPa), melt-depleted residues that were subsequently refertilized by basaltic melt, transported to final pressures ~3 GPa, and cooled to 650 – 845 C. Here, we investigate deformation microstructures and volatile content of Sierran peridotites. Owing to the dependence of olivine lattice preferred orientation (LPO) on stress, temperature, and water content, we can gain new constraints on the relative chronology of deformation and introduction of volatiles, which may be related to the refertilization event. Olivine LPOs were determined using the electron backscatter diffraction (EBSD) technique with the HKL Channel 5 software. Measurements were done manually on a grain-by-grain basis along 3 mm-spacing profiles parallel to the long axis of each thin section. We analyzed three garnet peridotites and one mylonitic spinel peridotite. Of the garnet peridotites, one sample shows alignment of [001] parallel to lineation with [010] aligned normal to the foliation (B-type fabric), while the other samples display a girdle distribution of [100] and [001] parallel to lineation (possibly suggesting axial compression). The mylonitic spinel peridotite is also characterized by a B-type fabric. The B-type fabric is associated with low temperatures, high stress, and hydrous conditions, and could be the dominant fabric type in the cold part of the mantle wedge. The presence of B-type fabric in the Sierran peridotites is consistent with their equilibration at cold temperatures and high pressures, conditions that could be associated with refrigeration by the Farallon slab. We are currently obtaining water and fluorine concentrations in olivine, pyroxenes, and garnet in the Sierran peridotites. These data will be presented at the meeting.