Abstract Distinct mantle compositions recorded in primitive West Antarctic magmatic rocks vary by tectonic setting and time. Deep asthenospheric mantle-plume sources or shallow metasomatized mantle sources may operate either coincidently or independently to supply melts for magmatism. For example, contemporaneous subduction and plume dynamics produced the Ferrar–Karoo Large Igneous Province; subduction-related melting followed by slab-rollback or melting of slab-hosted pyroxenite explains Antarctic Peninsula volcanism through time; Marie Byrd Land magmatism results from plume materials variably mixed with subduction-modified mantle; while magmatism in Victoria Land and western Ross Sea is best explained by plate dynamics and melting of asthenospheric and metasomatized lithospheric sources, and not by an upwelling plume. Element and isotopic ratios show a fundamental change between Marie Byrd Land and Victoria Land mantle domains. Specifically, Pb isotopes indicate that Victoria Land magmatism sources have a stronger focal zone (FOZO) mantle component, while Marie Byrd Land magmatism possesses more of the high μ = high 238 U/ 204 Pb (HIMU) mantle component that leads to high 206 Pb/ 204 Pb over time. The chemical and isotopic heterogeneity of relatively unfractionated igneous rocks in West Antarctica reflects fundamental differences in mantle domains and melting conditions. This mantle variability coincides with changes in crustal structure and composition, and has a geophysical signature that is manifest in seismic data and tomographic models.

Abstract Two small monogenetic volcanoes are exposed at Mount Early and Sheridan Bluff, in the upper reaches of Scott Glacier. In addition, the presence of abundant fresh volcanic detritus in moraines at two other localities suggests further associated volcanism, now obscured by the modern Antarctic ice sheet. One of those occurrences has been attributed to a small subglacial volcano only c. 200 km from South Pole, making it the southernmost volcano in the world. All of the volcanic outcrops in the Scott Glacier region are grouped in a newly defined Upper Scott Glacier Volcanic Field, which is part of the McMurdo Volcanic Group (Western Ross Supergroup). The volcanism is early Miocene in age ( c. 25–16 Ma), and the combination of tholeiitic and alkaline mafic compositions differs from the more voluminous alkaline volcanism in the West Antarctic Rift System. The Mount Early volcano was erupted subglacially, when the contemporary ice was considerably thicker than present. By contrast, lithologies associated with the southernmost volcano, currently covered by 1.5 km of modern ice, indicate that it was erupted when any associated ice was either much thinner or absent. The eruptive setting for Sheridan Bluff is uncertain and is still being investigated.

Abstract This study discusses the petrological and geochemical features of two monogenetic Miocene volcanoes, Mount Early and Sheridan Bluff, which are the above-ice expressions of Earth's southernmost volcanic field located at c. 87° S on the East Antarctic Craton. Their geochemistry is compared to basalts from the West Antarctic Rift System to test affiliation and resolve mantle sources and cause of melting beneath East Antarctica. Basaltic lavas and dykes are olivine-phyric and comprise alkaline (hawaiite and mugearite) and subalkaline (tholeiite) types. Trace element abundances and ratios (e.g. La/Yb, Nb/Y, Zr/Y) of alkaline compositions resemble basalts from the West Antarctic rift and ocean islands (OIB), while tholeiites are relatively depleted and approach the concentrations levels of enriched mid-ocean ridge basalt (E-MORB). The magmas evolved by fractional crystallization with contamination by crust; however, neither process can adequately explain the contemporaneous eruption of hawaiite and tholeiite at Sheridan Bluff. Our preferred scenario is that primary magmas of each type were produced by different degrees of partial melting from a compositionally similar mantle source. The nearly simultaneous generation of lower degrees of melting to produce alkaline types and higher degrees of melting forming tholeiite was most likely to have been facilitated by the detachment and dehydration of metasomatized mantle lithosphere.