Geochemical Constraints on Mantle Sources for Volcanic Rocks from Mt.Melbourne and the Western Ross Sea, Antarctica

Abstract

We report geochemical and isotope data (Sr, Nd, Pb) of submarine samples from the Terror Rift Region and subaerial lavas from Mt. Melbourne Volcanic Field (MMVF) in the western Ross Sea. The MMVF samples can be subdivided into Groups A and B based on their temporal and spatial distribution. All samples are alkaline, ranging from basanite to trachybasalt, and exhibit an HIMU-like isotopic signature (²⁰⁶Pb/²⁰⁴Pb = 18.510-19.683, ⁸⁷Sr/⁸⁶Sr = 0.70300-0.70398, ¹⁴³Nd/¹⁴⁴Nd = 0.51284-0.51297) and trace element affinities (Ce/Pb = 25-35, Nb/U = 45-60, Ba/Nb = 5-13, La/Nb = 0.5-0.9). The Terror Rift submarine lavas (0.46-0.57 Ma) display a distinct trend, with more primitive geochemical characteristics (higher MgO (7.2-9.8 wt.%) and CaO (9.9-11.9 wt%) and stronger HIMU signature than those of MMVF basalts. Results from a rare earth element (REE) model suggest that the Terror Rift submarine lavas are derived from small degrees (1-2%) of partial melting of an amphibole-bearing garnet peridotite mantle source. Incompatible trace element ratios (e.g., Ba/Nb = 6.4-13.2, La/YbN = 14.4-23.2, Dy/Yb = 2.2-3.0) and isotopic compositions of the MMVF Group A and B volcanics suggest derivation from higher degrees (2-5%) of partial melting of a garnet peridotite source and strong influence of an EMI-type mantle source. The stronger HIMU signature of the Terror Rift submarine lavas appears to be related to smaller degrees of partial melting, suggesting preferential sampling of the HIMU component in the less partially melted rocks from the Cenozoic NVL magmatism. In contrast, the higher degree of MMVF A and B magmas can be explained by greater interaction with heterogeneous lithospheric mantle, resulting in a diluted HIMU signature compared with that of the Terror Rift submarine lavas.