Time series petrological insights into magmatic evolution during the 2021 Cumbre Vieja eruption, La Palma, Canary Islands, Spain

On 19 September 2021, Cumbre Vieja volcano, La Palma, Canary Islands erupted after 50 years of quiescence. The eruption lasted 85 days, ending on 13 December. At present, whilst geophysical data may be used to estimate the scale of magma reservoirs (and when combined with the magma output rate can provide a guide to eruption longevity), experience shows that using such techniques to see through activity at crustal levels and quantify deeper magmatic processes during an eruption is not always productive or possible. Success of geophysical techniques is dependent on both the level of instrumentation and the degree to which local magmatic and tectonic environments are understood. Thus, deep magma supply and crustal interactions may be intractable or even invisible, even if they are seismogenic. Simple on-site compositional information (e.g. from handheld XRF) can indicate broad-scale changes in erupted compositions and reflect, for example,  changes in crystallinity or melt composition. However, such bulk data can be ambiguous and therefore insufficiently robust to be useful for decision-makers. In contrast, petrological observations of mineral textures and compositions can provide direct, quantifiable evidence of deep and shallow magmatic processes that, in tandem with upper crustal stress states, ultimately drive magma ascent and eruption. Advancements in the use of precise and automated sample preparation techniques, rapid and high-resolution textural and compositional characterisation, and increasing computing capacity now allows samples to be collected, analysed and interpreted within days rather than months. Measurements of volcanic products include: textures, mineralogy, mineral chemistry (and profiles), whole-rock geochemistry, volatiles, isotope geochemistry and rheology. Petrology combines these data into interpretations of the magmatic system state and evolution, which can inform understanding of the dynamic processes driving eruptions and physical behaviours of tephra and lava. Hence, forecasts of volcanic behaviour underpinned by petrological characterization and trends are more robust. Here we present textural and chemical data from time-resolved samples of lavas and tephras from the eruptive sequence, marking the initiation, duration and cessation of volcanism. These data are used to constrain and trace temperature(s) and pressure(s) of mineral growth and magma storage; mineral-melt equilibrium dynamics; and timescales of magmatic processes through diffusion chronometry. Initial petrographic study has shown the lavas to be hypocrystalline, porphyritic and vesicular. Clinopyroxene is the most common coarse mineral, with olivine and amphibole also present; however, these mineral abundances are not constant through time. This study highlights the importance of time-resolved sampling and shows how both rapid qualitative observations and in situ petrological characterisation can be used to couple volcanic behaviour with subsurface magma dynamics.