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. Plain Language Summary: The Pleiades volcanic field is made up of some 20 monogenetic scoria and spatter cones, which erupted in the last 900 ka close to the head of the Mariner Glacier in northern Victoria Land, Antarctica. The erupted products are very unusual for alkali basaltic volcanism: indeed, whereas few samples show clear within‐plate subcontinental lithospheric characteristics and were directly derived from the mantle source, most of the products formed after extensive fractional crystallization matched by significant crustal assimilation, implying that primitive magma volumes are 10 times larger than outcropping products in an unusually large magma plumbing system. These peculiar features coincided with a climax of activity during the last glacial maximum (30–25 ka). Therefore, we speculate that a thick ice cap favored high rates of crystal fractionation coupled with crustal assimilation and was responsible for increasing magma residence times in chambers at crustal depths and suppressing the eruptive potential of magmas. Key Points: The Pleiades complex (NVL, Antarctica) is made up of some 20 monogenetic cones aged 900–0 ka, defining a complete Na‐alkaline trendFractionation models show much larger volumes of primitive magmas, indicating the occurrence of an unusually large magma plumbing systemA climax of activity occurred during the last glacial maximum (30 ka). Thickness variation of the ice cap may have influenced volcanic activity [ABSTRACT FROM AUTHOR]