The petrology and source composition of alkaline basalts from the northwest Ross Sea, Antarctica inferred from Re-Os isotope systematics

Study of magmatism in the northwest Ross Sea region, Antarctica, has quite recently expanded with the discovery of seamounts in the Adare Basin, which in addition to continental volcanoes, form a transect of Cenozoic volcanism from continent to ocean. New Re-Os isotopic data were collected on representative alkaline basalts from the ocean to continent transect to complement other petrogenetic tracers and to better characterize potential lithospheric contaminants. Osmium concentrations and isotopic ratios were measured using NTIMS while Re concentrations were measured using ICP-MS. In general, the trace element and Sr-Nd-Pb isotopic compositions of the samples analyzed resemble those of high μ (HIMU) ocean island basalts. The 30 basalts analyzed include ~10 oceanic and ~20 continental samples. The oceanic basalts exhibit large variation in ¹⁸⁷Os/¹⁸⁸Os (from <0.13 to ~0.35), but the continental samples exhibit even larger variation and are generally more radiogenic (¹⁸⁷Os/¹⁸⁸Os up to ~0.65). The basalts from the shallower depths of continental margin exhibit the highest ¹⁸⁷Os/¹⁸⁸Os ratios among the oceanic samples and, in general, ¹⁸⁷Os/¹⁸⁸Os increases with decreasing water depth toward land. The continental basalts, however, show greater though less systematic variation in osmium isotopes and concentrations. Thus, Os isotopes do not show as systematic pattern as do the Sr-Nd-Pb isotopes, which generally become more radiogenic landward. Most of the basalts analyzed have very low Os concentrations (<30ppt, with only a few ranging up to 250ppt) and this makes them particularly susceptible to contamination [Shirey and Walker, Annu. Rev. Earth Planet. Sci. 26, 423-500, 1998]. Data seem to indicate that the Os isotopic composition of basalts from different regions in the continent had been affected by variable types and amounts of crustal contaminants.