Mineralogical, geochemical, and isotopic tools for the reconstruction of subglacial and marine eruptive paleoenvironments in Antarctica.

The Antarctic continent plays a key role in all the paleoclimatic reconstructions, and affects also the climatic evolution of lower latitudes. Therefore, knowledge of the past evolution of Antarctic ice cover is crucial for all the (paleo)climatic reconstruction. The study of Cenozoic eruptive sequences in northern Victoria Land is a significant source of information about the presence, extent, thickness and type of this ice cover. In combination with 40Ar/39Ar dating, the eruptive environment can be used to draw a picture of ice sheet dynamics in the past. This is particularly true for cold regions where there is little or no surviving evidence for ice cover. In the field, determination of the eruptive paleonvironment of a volcanic sequence (e.g. submarine vs. subglacial eruptions) could be a difficult task owing to occurrence of ambiguous sequences of volcanic lithofacies. We propose a new, helpful approach in the determination of eruptive environment, represented by the use of chemical and isotopic composition/evolution of glasses and authigenic minerals in water-cooled lavas to determine the original eruptive paleoenvironment (freshwater vs. seawater). The studied products are from Melbourne and Hallett volcanic provinces in northern Victoria Land and vary from hyaloclastite breccias to lapilli tuffs, characterized by variable degree of alteration. These products have been first characterized by petrographic investigations, mineralogical determinations (XRD) along with chemical analyses (SEM-EDS) on sideromelane, palagonitized glasses and secondary mineral phases such as zeolite, smectite. Then, analyses of specific indicator of eruptive environments have carried out, such as stable isotope systematics (D/H, δ18O) on handpicked glasses and plagioclase crystals separated from the juvenile lapilli along with altered hyaloclastite matrix and B contents on zeolite-rich matrix. (Na+K)/Ca of zeolites and glasses, B content and D/H isotopic ratios on altered matrix and δ18O on glasses and plagioclase, coupled with 40Ar/39Ar data, furnish fundamental information about the evolution of the ice cover during the last 10-12 Ma in northern Victoria Land.