How and why magmatic systems reactivate and evolve is a critical question for monitoring and hazard mitigation efforts during initial response and ongoing volcanic crisis management. Here we report the first integrated petrological results and interpretation provided to monitoring authorities during the ongoing eruption of Cumbre Vieja, La Palma, Canary Islands, Spain. The first eruptive products comprised simultaneous Strombolian fountain-fed lava flows and tephra fall from near-continuous eruption plumes. From combined field, petrographic and geochemical analyses conducted in the 10 days following sample collection, we infer low percentage mantle melts with a variably equilibrated multimineralic crystal-cargo and compositional fractionation by winnowing during eruptive processes. Hence ‘rapid response’ petrology can untangle complex magmatic and volcanic processes for this eruption, which combined with further study and methodological improvement can increasingly assist in active decision making.

A comparative study of three main igneous rock associations that plot in the K₂O–SiO₂ diagram shoshonite field: shoshonite series absarokites–shoshonites–banakites (henceforth referred to as shoshonites s.l.), vaugnerites, and potassic lamprophyres, reveals that similarities between the associations are superficial. Vaugnerites and lamprophyres are more magnesian, richer in large ion lithophile and high field strength elements and have higher light rare earth/heavy rare earth ratios than shoshonites. Furthermore, shoshonites have low radiogenic heat production, typical of subduction-related rocks, but most vaugnerites and some lamprophyres are highly radioactive. Relative to bulk-Earth, shoshonites have depleted, asthenospheric mantle-like Sr and Nd isotope signatures, whereas vaugnerites and potassic lamprophyres have enriched, crust or lithospheric mantle-like compositions. Though vaugnerites and some lamprophyres show evidence of crustal contamination, the contaminated magma was not originally shoshonitic. Their composition is consistent with derivation from a metasomatised upper mantle source enriched long before melting, thus precluding an active subduction setting. In conclusion, the term shoshonite, implying late-stage arc magmas, cannot be applied to a rock series simply because it plots into the K₂O–SiO₂ diagram shoshonite field. Shoshonites with a subduction-related source may, however, be identified by discriminant function analysis.

Major element variations in North Atlantic Tertiary Province primitive, early erupted, alkaline-transitional-tholeiite basalts, recalculated to a restricted value of MgO, give insights into the process of plume-related magmatism. Basalts primitive enough to be crystallizing only olivine were recalculated to a proposed primary magma composition of 15 wt % MgO. The recalculated dataset shows clear inter-element correlations including a strong, significant, negative correlation between Fe and Si indicating polybaric melt segregation. Overlap between basalt compositions and experimental melts from a fertile, Fe-rich, low mg-number (85.5) peridotite suggests that, relative to normal peridotite with mg-number > 89, the North Atlantic basalt source was Fe rich. Linear regression of the experimental data gives apparent pressures of magma segregation of 17·5–37 kbar, with intra-region variability in the depth derivation from the melt column for each sample, thus suggesting that lithospheric thickness ‘lid-effect’ control on magma generation may have been overemphasized in recent studies. Comparable source composition, magma segregation depth and calculated mantle potential temperature (1440–1460°C) throughout the Province supports the previously suggested plume impact model, arriving below East Greenland, derived from a variably enriched and depleted lower-mantle source. Given the good agreement between conclusions drawn from major element data and previously published results we suggest that restricted-MgO recalculated datasets may be usefully applied to study other large igneous provinces.