Thesis advisor: Seth C. Kruckenberg / The West Antarctic rift system is one of the most expansive regions of extended continental crust on Earth, but relatively little is known about the structure of the mantle lithosphere in this region. This research aims to examine a suite of ultramafic mantle xenoliths from several volcanic centers located throughout Marie Byrd Land, West Antarctica. Through the use of several complementary analytical methods, the deformational and compositional heterogeneity of the lithospheric mantle in this region is characterized. The Marie Byrd Land xenoliths have equilibration temperatures between 779 and 1198°C, which is a range that corresponds to extraction depths between 39 and 72 km. These samples preserve significant mineralogical and microstructural heterogeneities that document both lateral and vertical heterogeneities within the Marie Byrd Land mantle lithosphere. The modal mineralogy of spinel peridotites varies between 40 – 99% olivine, 0 – 42% diopside, 0 – 45% enstatite and 0 – 5% chromite. Minimum olivine grain sizes range from 60 to 110 µm and maximum olivine grain sizes range from 2.5 to 10.0 mm. The geometric mean grain size of olivine in these samples ranges from 100 µm to 2 mm and has an average of 694 µm. The geometric mean grain size of diopside ranges from 90 to 865 µm and has an average of 325 µm, whereas that of enstatite ranges from 120 µm to 1.2 mm and has an average of 625 µm. Comparatively, the pyroxenites contain 0 – 29% olivine, 29 – 95% diopside, 1 – 36% enstatite and 1 – 11% chromite. Deformation mechanism maps suggest that the olivine within the MBL peridotite xenoliths primarily accommodate strain through the operation of dislocation-accommodated grain-boundary sliding at strain rates between 10-19/s and 10-11/s. This is consistent with microstructural observations of the suite made using optical microscopy (e.g., deformation bands and subgrains in olivine; aligned grain boundaries between contrasting phases). Application of the olivine grain size piezometer indicates that the suite preserves differential stresses ranging from 0.5 MPa to 50 MPa, with mean differential stresses ranging from 4 to 30 MPa. Values of mean differential stress only vary slightly throughout the field area, but generally decrease in magnitude towards the east with maximum values migrating upwards in the lithospheric mantle along this transect. The samples from some volcanic centers are highly homogenous with respect to their microstructural characteristics (e.g., Mount Avers – Bird Bluff), whereas others display heterogeneities on the sub-five-kilometer-scale (e.g., Demas Bluff). Comparatively, mineralogical heterogeneities are more consistent throughout the sample suite with variations generally being observed between the sub-five-kilometer-scale and the sub-ten-kilometer-scale. Most samples within the MBL peridotite suite display axial-[010] or A-type olivine textures. Although less dominant, axial-[100], B-type and random olivine textures are also documented within the suite. Axial-[010] textures have J-indices and M-indices ranging from 1.7 – 4.1 and 0.08 – 0.21, respectively. The average value of the J-index for axial-[010] textures is 2.9, whereas the average M-index of these samples is equal to 0.15. Overall, A-type textures tend to be stronger with J- and M-indices ranging from 1.4 – 9.0 and 0.07 – 0.37, respectively. The olivine crystallographic textures of the MBL xenolith suite are heterogeneous on scales that are smaller than the highest resolution that is attainable using contemporary geophysical methods, which implies that patterns of mantle flow and deformation are far more complex than these studies suggest. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Glacial isostatic adjustment (GIA) is the process by which the solid Earth responds to past and present-day changes in glaciers, ice caps, and ice sheets. This thesis focuses on vertical crustal motion of the Earth caused by GIA, which is influenced by several factors including lithosphere thickness, mantle viscosity profile, and changes to the thickness and extent of surface ice. The viscosity of the mantle beneath Antarctica is a poorly constrained quantity due to the rarity of relative sea-level and heat flow observations. Other methods for obtaining a better-constrained mantle viscosity model must be investigated to obtain more accurate GIA model predictions. The first section of this study uses seismic wave tomography to determine mantle viscosity. By calculating the deviation of the P- and S-wave velocities relative to a reference Earth model (PREM), the viscosity can be determined. For Antarctica mantle viscosities obtained from S20A (Ekstrom and Dziewonski, 1998) seismic tomography in the asthenosphere range from 1016 Pa∙s to 1023 Pa∙s, with smaller viscosities beneath West Antarctica and higher viscosities beneath East Antarctica. This agrees with viscosity expectations based on findings from the Basin and Range area of North America, which is an analogue to the West Antarctic Rift System. Section two compares bedrock elevations in Antarctica to crustal thicknesses, to infer mantle temperatures and draw conclusions about mantle viscosity. Data from CRUST 2.0 (Bassin et al., 2000), BEDMAP (Lythe and Vaughan, 2001) and specific studies of crustal thickness in Antarctica were examined. It was found that the regions of Antarctica that are expected to have low viscosities agree with the hot mantle trend found by Hyndman (2010) while the regions expected to have high viscosity are in better agreement with the trend for cold mantle. Bevis et al. (2009) described new GPS observations of crustal uplift in Antarctica and compared the results to GIA model predictions, including IJ05 (Ivins and James, 2005). Here, we have generated IJ05 predictions for a three layered mantle (viscosities ranging over more than four orders of magnitude) and compared them to the GPS observations using a χ2 measure of goodness-of-fit. The IJ05 predictions that agree best with the Bevis et al. observations have a χ2 of 16, less than the null hypothesis value of 42. These large values for the best-fit model indicate the need for model revisions and/or that uncertainties are too optimistic. Equally important, the mantle viscosities of the best-fit models are much higher than expected for West Antarctica. The smallest χ2 values are found for an asthenosphere viscosity of 1021 Pa•s, transition zone viscosity of 1023 Pa∙s and lower mantle viscosity of 2 x 1023 Pa∙s, whereas the expected viscosity of the asthenosphere beneath West Antarctica is probably less than 1020 Pa∙s. This suggests that revisions to the IJ05 ice sheet history are required. Simulated annealing was performed on the ice sheet history and it was found that changes to the recent ice load history have the strongest effect on GIA predictions. / Graduate