Plagioclase antecrysts record syn-eruptive incorporation of evolved mush during the 2021 Tajogaite eruption (La Palma, Spain)

Abstract

Magma mixing in mush zones constitutes a common eruption trigger and modulates the composition of erupted lavas and their hazard potential. Plagioclase is an excellent archive of mixing processes, however in ocean island basalts, the mineral crystallizes late and is typically restricted to matrix microcrysts. The 2021 Tajogaite eruption (La Palma, Canary Islands) included rare macrocrysts of plagioclase with disequilibrium textures that afford investigation of mush interactions during volcanism. The 85-day-long eruption produced initial clinopyroxene-amphibole-bearing tephrites followed by dominant clinopyroxene-olivine-rich basanites. From days 13–35, some basanite lavas contained macrocrysts of plagioclase with strong compositional contrasts between cores (evolved andesine antecrysts; An_22–52) and rims (labradorite; An_50–64). The An-rich rims are chemically similar to microcrysts in the matrix (labradorite; An_54–66), cogenetic with the erupted basanite liquid. Trace element compositions change abruptly between antecryst cores (3352–5139 ppm Sr) and rims and matrix microlites (1624–3097 ppm Sr). ⁸⁷Sr/⁸⁶Sr ratios are similar for antecryst cores and rims (0.70313–0.70322), more radiogenic than the host matrix (0.70308–0.70314) in several samples. Such contrasts suggest recycling of distinctly evolved, Sr-radiogenic plagioclase-bearing mush by recharging basanite magma prior to, or during the eruption, as new inputs of basanite melt led to eruption of more fluidal and rapid lavas. Evolved plagioclase compositions have been reported in tephriphonolites from previous eruptions, interpreted to form in magma accumulation zones in the lower oceanic crust beneath La Palma. Our work provides new evidence of evolved magma mush developing upon ponding of basanite melts around Moho depths below ocean island basalts.