Abstract

Phase relations of a tephritic lava erupted at La Palma, Canary Islands, the 2nd of October 2021, during the eruption of Cumbre Vieja volcano were determined experimentally in the temperature/water content space, T (°C)/H2O (wt%). The crystallisation experiments were carried out at a fixed pressure of 275 MPa for temperatures varying from 1000 to 1200 °C and with H2O added contents ranging from 0 to 5 wt%. Results from the synthetic products are compared with those obtained from thermodynamic simulations and with the natural phases to constrain the pre-eruptive conditions of the tephritic magma stored in the shallow reservoir. The major phases (clinopyroxene + olivine + oxides) occurring in the natural products were reproduced. The experimental results combined with thermodynamic modelling allow us to constrain the pre-eruptive temperature to ∼1100 °C considering H2O to 3 wt% and to restrict the crystallisation of plagioclase at low pressures (< 20 MPa). The results indicate that, under the presence of a pure H2O fluid, amphibole is stable at pressures higher than 300 MPa and temperatures ∼1000 °C. Our results on pre-eruptive conditions of the 2021 Tajogaite eruption provide important constraints on mechanisms of storage and transport in primitive alkaline magmas.