An assessment of heterogeneity within the lithospheric mantle, Marie Byrd Land, West Antarctica

Abstract

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.