EFFECTS OF MELT-ROCK INTERACTIONS ON PERIDOTITE DEFORMATION TEXTURES AND FABRICS IN THE SHALLOW MANTLE LITHOSPHERE

Melt-rock interactions in the shallow mantle lithosphere modify the rock composition and may affect the microstructures and physical properties by recrystallization processes. To investigate how melt-rock interactions change the grain-scale fabrics in the mantle lithosphere, we analyzed mantle xenoliths sourced from the Holocene Kostal Lake volcanic center, which is located in southern British Columbia at the western margin of the ancestral North American craton. Kostal Lake peridotites can be separated into two groups based on distinct olivine magnesium numbers (Mg#). Group 1 samples with Mg# ranging from ~0.88 to 0.91 and generally low Al content are interpreted to represent the residues of partial melting of a relatively fertile mantle source. Group 2 samples have lower Mg# (0.79-0.86) and are interpreted to result from melt-rock reactions between peridotite and percolating mafic melts. Group 1 samples have protogranular and equigranular textures, strong crystallographic preferred orientations (CPOs), and consistently display fabrics indicative of syn-kinematic deformation of olivine and pyroxene. Group 2 samples have porphyroclastic or transitional tablet textures. Compared to the Group 1 samples, Group 2 samples contain smaller olivine grains, more complex olivine and pyroxene grain shapes, mixing of olivine and pyroxenes by recrystallization, and ubiquitously exhibit weaker CPOs. We interpret Group 1 xenoliths to correspond to mantle that was deformed during Cordilleran orogenisis, while Group 2 xenoliths represent mantle through which recent reactive melt transport has occurred. Based on the Group 2 xenolith characteristics, we infer that melt-rock interactions in the mantle reduce the average olivine grain size and decrease olivine CPO. By changing grain sizes, modifying textures, and redistributing mineral phases, melt-rock interactions may also significantly change the anisotropy of the elastic properties of the rock and the rheology of the mantle lithosphere.