Antarctic Rift and Hot Spot Volcanism

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This award supports a study of volcanic rocks in the Ross Sea area of Antarctica. Existing strontium (Sr), neodymium (Nd), and lead (Pb) isotopic work on more than 80 basalts shows that a HIMU (High U/Pb)-like isotopic signature (low 87/86Sr, high 206/204Pb) is ubiquitous along some 4500 km of the West Antarctic Rift System (WARS). Three oceanic hotspots have been defined in the region; Balleny and Scott Islands show the same HIMU-like signatupe as the rift volcanics. Peter I Island shows a hint of this component, though these rocks indicate, in general, a more enriched mantle hotspot source (refered to as an EM-2-like hotspot). Many of the WARS volcanics also show the presence of an enriched component, but one which is derived from the continental lithosphere (low Rb/Cs and Th/Pb). In Victoria Land, the presence or absence of the enriched component is directly related to location with respect to the boundary between the Ross Sea rift and the Transantarctic Mountain Front. Its minimal presence in Marie Byrd Land suggests a thinner and/or younger crust, relative to the Transantarctic Mountain area. The virtual absence of enriched component in the rift proper suggests a highly thinned crust, perhaps without significant underlying lithosphere. In this second phase of a longer term project, the work will focus on geochemical study three WARS volcanoes (Sidley, Overlord, Pleides) in detail, to constrain a crustal versus mantle lithospheric source for the enriched component. A number of cognate and mantle xenoliths will be studied as a further aid in this source discrimination issue. The geographic extent of the HIMU-WARS signature will be tested by analyzing Cenozoic basalts from sub-Antarctic islands on the Campbell Plateau (including cognate and mantle xenoliths). In all of this work we will, for the first time, employ osmium and oxygen isotopes in combination as a tool to discriminate crustal versus mantle lithosphere signatures.