The magma ascent path during the 2021 eruption of Cumbre Vieja (La Palma Island, Canary archipelago) highlighted by fluid inclusions and seismicity

The recent eruption from the Cumbre Vieja volcanic system at La Palma Island (19 September to 14 December 2021) occurred through the impulsive emission of various batches of magma. The first emitted magma is a tephrite (clinopyroxene, amphibole and rare olivine phenocrysts). The following pulses erupted basanites (clinopyroxene and olivine phenocrysts).

Fluid inclusions and seismicity data of the first 40 days of activity are here merged to provide a snapshot of the magma ascent path.

Fluid inclusions form trails through the crystals or are more rarely in isolated clusters. They show evidence of partial density re-equilibration events. At room temperature are single phase (L) or may contain a vapour bubble (V+L).

Trapped fluid is pure CO₂ (Tm=-56.6 ±0.1 °C). Final inclusion homogenisation occurred to the liquid (Th_L) phase in all crystals and to the vapor (Th_v) in few olivines. The corresponding density values have been recalculated to account for max 10% water in the trapped fluid.

In amphiboles (N=60) Th_L=23.3-30.9 °C (ρ_r=546-768 kg·m³);

In clinopyroxenes (N=69) Th_L=27.2-31 °C (ρ_r=514-703 kg·m³);

In early olivines (N=241) Th_V=30.4-30.9 °C (ρ_r=382-464kg·m³); Th_L=-6.2-31 °C (ρ_r=492-963 kg·m³);

In late october olivines (N=180) Th_V=30.9 °C (ρ_r=464kg·m³); Th_L=20.6-30.9 °C (ρ_r=546-802 kg·m³).

The histograms of density data reveals fluid trapping and re-equilibration events. Pressures were obtained from isochore distribution in the P-T space at the trapping temperature of 1075 °C for the tephrite and 1150° C for the basanite.

The tephrite ascended from a depth of ~17.2 km (487 MPa) and partially re-equilibrated at ~14.2 km (392 MPa), ~11.5 km (307 MPa), ~10 km (264 MPa) and ~8.2 km (218 MPa).

Basanites ascended from (or through) a depth between ~25.8 and ~22.6 km (656-757 MPa). Multiple ponding stages are between ~19 and ~17 km deep (484-543 MPa), at ~12 km (336 MPa) and from ~8.7 to ~6.1 km (162-229 MPa).

This picture agrees with the spatial and temporal pattern of the seismicity recorded during the eruption. After a very rapid pre-eruptive phase, lasting about a week, in which hypocentres rapidly ascended from about 10 km depth up to the surface a few hours before the eruption, seismicity waned considerably in just a few days. Since 27 September, a progressive increase of the seismicity in a cluster located at about 8-12 km depth was observed. In the following days, we observed the appearance of another cluster of hypocentres at a depth of about 20-25 km. Seismicity increased progressively during the first weeks of October, with many events having magnitudes higher than 4 in both clusters. The seismicity started waning at the beginning of December, disappearing almost entirely at the end of the eruption. We interpret these syn-eruptive seismicity clusters at the effect of crustal readjustment following the rapid emptying of two magmatic reservoirs located respectively just beneath each seismicity cluster. This model agrees well with the bimodal depth range inferred from fluid inclusion, as well as with the observed variation in the composition during the eruption.