Cryptic evolved melts beneath monotonous basaltic shield volcanoes in the Galápagos Archipelago

Abstract

Many volcanoes erupt compositionally homogeneous magmas over timescales ranging from decades to millennia. This monotonous activity is thought to reflect a high degree of chemical homogeneity in their magmatic systems, leading to predictable eruptive behaviour. We combine petrological analyses of erupted crystals with new thermodynamic models to characterise the diversity of melts in magmatic systems beneath monotonous shield volcanoes in the Galápagos Archipelago (Wolf and Fernandina). In contrast with the uniform basaltic magmas erupted at the surface over long timescales, we find that the sub-volcanic systems contain extreme heterogeneity, with melts extending to rhyolitic compositions. Evolved melts are in low abundance and large volumes of basalt flushing through the crust from depth overprint their chemical signatures. This process will only maintain monotonous activity while the volume of melt entering the crust is high, raising the possibility of transitions to more silicic activity given a decrease in the crustal melt flux.

Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

“…In particular, the most evolved crystals (Group 4; Fo~70-75) are in equilibrium with melts that are more evolved than the Floreana erupted basalts (likely basaltic andesites). This is consistent with a recent study which identified highly evolved (andesitic -dacitic) magmas beneath basaltic volcanoes in the western Galápagos Archipelago (Stock et al, 2020).…”

“…In contrast, our barometric data indicate that magmas beneath Floreana ascend directly from the upper mantle and undergo no detectable crustal storage. In addition, although mush-rich regions have been inferred beneath the western Galápagos shield volcanoes (based on whole-rock data and the presence of gabbroic glomerocrysts; Chadwick et al, 2011;Geist et al, 1995Geist et al, , 2014Stock et al, 2018), magmatic differentiation appears to be driven by simple fractional crystallisation and mixing of chemically diverse magmas (Geist et al, 1995;Naumann and Geist, 1999;Stock et al, 2020).…”

“…beneath Wolf volcano (northern Isabela) has been substantially greater than that beneath Floreana for several 100,000s of years (Geist et al, 2005). The high magma flux beneath Wolf maintains the average temperature of the mid-to-lower crust at ~1125 o C (ΔT ~22 o C), with only small-scale thermal and compositional heterogeneities present in the sub-volcanic plumbing system (Geist et al, 2014(Geist et al, , 2005Stock et al, 2020Stock et al, , 2018. In contrast, the flux of magma entering the magmatic system beneath Floreana is much lower and the temperature of the mid-crust is likely to be significantly cooler than the lowest temperature recorded by the Floreana xenoliths (that is, <<900 o C; Fig.…”

Machine learning thermobarometry and chemometry using amphibole and clinopyroxene: a window into the roots of an arc volcano (Mount Liamuiga, Saint Kitts)

“…This suggests that amphibole chemometry effectively reveals cryptic andesitic melts within the volcanic sub-system. Cryptic evolved melts are recorded in plagioclase via a similar process in the monotonous basaltic magmas of the Gálapagos (Stock et al, 2020). Upon amphibole crystallisation, the surrounding melts are chemically propelled from andesitic composition towards more silicic melts via two effects.…”

Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from Saint Kitts, Lesser Antilles

“…4 and 6). The disconnect between whole rock and mineral chemistry has been noted in a variety of systems (including the intrusive environment; Latypov, 2015) and for a range of mineral phases (Charlier et al, 2005;Kahl et al, 2013;Stock et al, 2020;Ubide et al, 2014). Such a decoupling is unsurprising in the case of phenocrysts (Fig.…”