Geochemical evolution of rift magmas by progressive tapping of a stratified mantle source beneath the Ross Sea Rift, Northern Victoria Land, Antarctica
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Source compositions of Neogene-Quaternary volcanic rocks from the McMurdo Volcanic Group of the Ross Sea Rift in Northern Victoria Land, Antarctica are constrained by Nd-Sr-Pb isotopes and trace element ratios in near-primary basalts. The rocks erupted along the western rift margin (Victoria Land Basin) and the western rift shoulder (Transantarctic Mountains). Near-primary basalts show no evidence of crustal contamination, suggesting that their initial Nd-Sr-Pb isotopes reflect the composition of their mantle sources. The initial isotope ratios of near-primary basalts range from about Sr-87/Sr-86 = 0.70281 to 0.70504 and Nd-143/Nd-144 = 0.51269 to 0.51291 (epsilon(Nd(t)) = 1.3-5.5). The Pb-206/Pb-204 ratios vary between 19.3 and 20.1. Our results, in conjunction with data from the literature [1-3], suggest the involvement of three mantle source components (or their mixtures) during the formation of Ross Sea magmas: depleted MORB-type mantle, an enriched mantle component (EM), and a component akin to HIMU. The involvement of these mantle components during magma genesis correlates with tectonic setting: whereas MORB-type compositions are restricted to Recent within-rift basalts, EM and HIMU isotope signatures dominate off-rift magmas from the western rift shoulder and Marie Byrd Land respectively. Basalts from the western rift shoulder show temporal isotopic and trace element variations from EM towards HIMU-type signatures between 15 and 5 m.y. On the other hand, within-rift and Marie Byrd Land basalts changed from HIMU-type towards MORB-type compositions through time. These temporal geochemical variations together with the respective tectonic settings of magmatism suggest that the mantle beneath the Ross Sea Rift is stratified in the order MORB- to HIMU- to EM-type sources. It appears that during rift evolution the EM- and HIMU-type sources are consumed and depleted asthenospheric MORB-type mantle rises progressively into the melting region. This suggests that EM resides in the mantle lithosphere. The HIMU-type component may be related to the head of an active or, alternatively, ‘fossilized’ mantle plume attached to the base of the lithosphere beneath the Ross Sea area.