Lithosphere-melt interactions, evidence from basalt and xenolith cargo, Terror Rift, Ross Sea, Antarctica

It remains a persistent point of contention among igneous petrologists as to how alkaline magmas generated by small degrees of melting within the upper-most convecting mantle can traverse through relatively cool continental lithosphere without freezing and being trapped at depth. Melting experiments demonstrate that volatile-rich, silica-undersaturated liquid react with peridotite under P-T conditions equivalent to the base of lithosphere and can form hydrous cumulates consisting of clinopyroxene, amphibole and phlogopite^1-3. Furthermore, this metasomatic process enriches the mantle in incompatible elements in amounts similar to basalt (i.e. nephelinite, basanite) and may be a melt source for alkaline lavas⁴. The experiments and theoretical models provide important clues as to the cause and source of alkaline volcanism that occur within plates but demand evidence from natural settings. Here we present major and trace elements and ⁴⁰Ar/³⁹Ar ages from Pliocene-Pleistocene, olivine-phyric alkaline basalt erupted through extended continental lithosphere within and bordering the Terror Rift, southwestern Ross Sea, Antarctica. Subaerial and submarine basaltic tephra and lava from this region contain mantle xenoliths that include hydrous-phases that also display melt-solid reaction textures^5,6. We compare basalt erupted across the central portion of the rift with basalt erupted at the rift shoulder along the base of the Transantarctic Mountains^7,8. Our comparison shows that silica-undersaturation (i.e. nepheline-normative content) and highly incompatible trace element concentrations decrease with decreasing degree east longitude within the rift and on average are at their lowest on the rift shoulder. Variable degrees of partial melting of a common mantle source are modelled to match the trace element trends but require an unrealistic range of values: F = <3% beneath rift and as much as 15% beneath the rift shoulder. The models are also not consistent given the greater depth to the lithosphere-asthenosphere boundary (LAB) beneath the rift shoulder (>95 km) relative to the central rift (<85 km)⁹. We propose that the compositional variability may be explained by interaction-reaction of asthenospheric melt with mantle lithosphere manifest to a greater degree beneath the rift shoulder. But also, that that portions of the continental lithosphere that have been metasomatized by low-degree, volatile-rich, silica-undersaturated melt, evident in mantle xenoliths hosted by the basalt, are likely to be a contributing source for alkaline volcanism in this region.   

¹Foley 1992, Lithos 28; ²Pilet et al., 2008, Science 320; ³Pilet et al., 2010, Contrib. Mineral. Petrol. 159; ⁴Pilet et al., 2011, Jour. Petrol. 52; ⁵Martin et al., 2021, Geol. Soc. Lond. Mem. 55; ⁶Panter et al., 2025, AGU Fall Meet. Abst. 2025, OS51F-0475; ⁷Tominaga et al., 2025, Comm. Earth Environ. 6:921; ⁸Panter et al., unpubl.; ⁹An et al., 2015, Jour. Geophys. Res. 120:12.