Temporal evolution of melt composition during the 2021 Cumbre Vieja eruption

The 2021 eruption of Cumbre Vieja volcano (La Palma, Canary Islands) produced sustained Strombolian to violent Strombolian explosive activity, resulting in widespread tephra fall deposits in addition to lava flows. Frequent sampling of rapidly quenched volcanic ash provides the rare opportunity to document the compositional evolution of fragmenting magma at a high temporal resolution. Here we present preliminary textural observations and electron microprobe measurements of matrix glass from dated ash samples spanning the first four weeks of the eruption. Ash shards show two broad types of groundmass texture: Type 1 groundmass comprises abundant glass with microlites of plagioclase, clinopyroxene, and Fe-Ti oxides ± olivine, whereas Type 2 groundmass is microcrystalline (plagioclase, clinopyroxene, Fe-Ti oxides) and contains little to no glass. Type 1 and Type 2 groundmasses are sometimes observed mingling together at the ash shard scale. The glass composition of Type 1 groundmass is consistently tephritic, but displays significant variations over time. Glass from the earliest sample collected on 19 September is among the most primitive of the sequence, with 46.4 wt.% SiO₂ and 4.0 wt.% MgO. In contrast, a sample erupted on 21–22 September records a shift to higher silica content (48.2 wt.%) and lower MgO (3.6 wt.%). Over the following five days (until 27 September), glasses return to lower silica contents, down to 45.9 wt.%, and then continue to evolve more subtly towards more primitive compositions for the next three weeks. Overall, from 21 September to 16 October, SiO₂ decreases from 48.2 to 45.1 wt.%, while FeO_t and MgO increase from 9.6 to 11.8 wt.% and from 3.6 to 4.1 wt.%, respectively. Chlorine concentrations also decrease from 1300 to 830 ppm. We interpret Type 1 groundmass to represent the main magma batch feeding the 2021 eruption. The observed temporal trends may be related to variable extents of microlite crystallization, particularly Fe-Ti oxides, as suggested by the association of high SiO₂, low FeO_t and high Fe-Ti oxide crystal fractions for the 21–27 September samples. We note that these samples coincide with a phase of the eruption characterized by highest volcanic tremor amplitudes and lowest eruption column heights (≤3 km). The origin of microcrystalline Type 2 groundmass is more ambiguous, but it may represent Type 1 magma that has undergone a more protracted cooling history, a remobilized mushy magma intersected by Type 1 magma, or lithic material. Further textural and chemical analyses of Type 1 and Type 2 groundmasses are underway to tell these scenarios apart.