Magma Differentiation, Contamination/Mixing and Eruption Modulated by Glacial Load—The Volcanic Complex of The Pleiades, Antarctica

The Pleiades Volcanic Field is made up of some 20 monogenetic, partly overlapping scoria and spatter cones, erupted in the last 900 ka, cropping out from the ice close to the head of the Mariner Glacier in northern Victoria Land, Antarctica. Erupted products vary from hawaiite to trachyte, defining a complete mild Na-alkaline differentiation trend. Mafic samples are characterized by multi-elemental patterns typical of OIB magmas, moderately low ⁸⁷Sr/⁸⁶Sr (0.7037) and high ¹⁴³Nd/¹⁴⁴Nd (0.51284), with a clear within-plate affinity, indicating a subcontinental lithospheric source. With increasing SiO₂, ⁸⁷Sr/⁸⁶Sr ratios increase up to 0.7052 and ¹⁴³Nd/¹⁴⁴Nd decrease to 0.51277, supporting the hypothesis of open-system evolution, with significant crustal assimilation during fractional crystallization. The erupted volume of most evolved products (∼7 km³), according to fractionation models, suggests that primitive magmas should have been more than 10 times larger, indicating the occurrence of a large magma plumbing system, unexpected for a volcanic field of monogenetic scoria cones. The occurrence of a complete fractionation trend with large magma chambers and large assimilation rate is unusual, if not unique, among the alkali basaltic volcanic fields and it is matched by a climax of activity during the last glacial maximum (30 ka), as indicated by new ⁴⁰Ar-³⁹Ar ages (30 ± 3 ka and 25 ± 2 ka) for samples from the two most prominent edifices. Therefore, we hypothesize a role of a thick ice cap in suppressing eruptions and ultimately leading to prolonged magma residence time in the subsurface, favoring significant fractionation coupled with unusual high rates of crustal assimilation.