HETEROGENEITY OF THE LITHOSPHERIC MANTLE BENEATH MARIE BYRD LAND, WEST ANTARCTICA

Mantle deformation processes, which are recorded in rock microstructures (i.e. fabric) and patterns of crystallographic preferred orientations (i.e. CPO texture), exert a fundamental control on the rheology of the lithosphere. Textural transitions develop in response to to the activation of different slip systems due to variations in lithology, conditions of deformation, volatile content, and strain. In order to assess the compositional and textural heterogeneity of actively deforming mantle lithosphere, forty-five xenolith samples were collected from recently erupted lavas (ca. 1.4 Ma) from seven volcanic centers in Marie Byrd Land (MBL), West Antarctica – a region noted for its high heat flow and ongoing extension since ca. 30 Ma. MBL volcanic centers are known from previous studies to be derived from contrasting mantle sources. Similarly, recent petrologic characterization indicates that the xenolith suite was sourced from depths of 35 to 85 km, which places them within the uppermost mantle above a geophysically imaged seismic low velocity zone.

Coarse-grained spinel lherzolites comprise the majority of the xenolith sample suite, but lesser amounts of harzburgite, dunite, and pyroxenites are also observed. Patterns of CPO (i.e. texture) for all constituent mineral phases are quantified through the acquisition of full thin section orientation maps using electron backscatter diffraction (EBSD). These data are used to generate textural maps and pole figures, which are compared with known CPO distributions and used to assess mantle deformation processes. The observed CPOs are interpreted in the context of the spinel fabric, which has been determined using X-ray computed tomography. Olivine CPOs are dominated by the activation of [100](010) “A-type”, axial-010 “AG-type”, and [100]{0kl} “D-type” slip systems, however some samples are characterized by the activation of [001](010) “B-type” and [100](001) “E-type” slip systems. The intensity of crystallographic textures is also highly variable within the xenolith suite. These observations suggest that the mantle underlying west Antarctica is laterally and vertically heterogeneous on a range of spatial scales, as reflected by the compositional and textural variations documented within and between individual volcanic centers.