THE NATURE AND EVOLUTION OF MANTLE UPWELLING AT ROSS ISLAND, ANTARCTICA

Ross Island Antarctica comprises four silica-undersaturated alkaline volcanic centers, including the active Erebus volcano (1.3 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula (~4.6 to 0.3 Ma) surround Erebus on the periphery of Ross Island, each approximately 30 km from Erebus. The geochemical compositions and the HIMU isotopic signatures of the phonolitic Erebus lavas are well characterized. The mostly basanitic lavas of the three peripheral volcanic centers were, until now, poorly characterized. Here we detail the major and trace element concentrations and the Sr, Nd, Pb, and Hf isotope compositions of the peripheral volcanic centers and compare these results to previous isotopic studies of Erebus volcano (e.g., Sims et al., 2008, J. Volcanol. Geotherm. Res. 177). These new data provide a means to further investigate the sources and nature of small-scale mantle upwelling across Ross Island. Fifty-seven samples have ranges of ⁸⁷Sr/⁸⁶Sr from 0.702907 to 0.703147, ε_Nd from +4.3 to +6.3, ε_Hf from +5.6 to +8.6, and ²⁰⁶Pb/²⁰⁴Pb from 19.282 to 20.241. All of these ranges are greater than those of Erebus volcano (excluding the Erebus enriched iron series, in which the trachytes show signs of crustal assimilation), suggesting greater mixing of lithospheric or asthenospheric sources on the periphery of Ross Island. ²⁰⁶Pb/²⁰⁴Pb for lavas and tephras from Mt. Terror and Mt. Bird decreases with increasing distance from Erebus volcano, which could indicate dilution of a HIMU hotspot signature and increased mixing with surrounding lithosphere and asthenosphere with increasing distance from the center of the thermal anomaly, or hotspot, beneath Erebus volcano. Additionally, recent tomographic imaging indicates that low shear wave velocity anomalies are present at depths greater than 1000 km beneath Ross Island. The HIMU signature has been hypothesized to arise from melting of lithosphere that was metasomatized at <500 Ma; however, our calculations show that this scenario is unlikely to produce the isotopic signatures measured at Ross Island. The radial symmetry of Ross Island’s peripheral volcanic centers, Ross Island’s high eruptive volumes, and the presence of kaersutite in peripheral lavas suggest that a radially-symmetric asthenospheric sourced thermal anomaly is associated with Ross Island magmatism.