Experimental constraints on the behaviour of sulphur in the 2021 Cumbre Vieja (La Palma) basanite

We performed experiments to constrain the effects of sulphur and oxygen fugacities on magma chamber and outgassing conditions of the La Palma 2021 eruption. Based on a series of controlled experiments on basanitic products carried out at 1040 °C and 200 MPa, we show that sulphur addition affects the stabilities of amphibole and olivine, in particular at high fO₂ and elevated S contents which together inhibit amphibole crystallization. We also show that the overriding control on S systematics is oxygen fugacity, with melts capable of dissolving from 1000 up to 8000 ppm S, depending on fO₂. Increasing the bulk S content increases the S content of the silicate melt up to ∼2000 ppm for fO₂ < NNO + 2, and 7000–8000 ppm at higher fO₂. Further increase in dissolved S is prevented by the buffering effects of either sulphide at low fO₂ or anhydrite at high fO₂. Modelling shows that the observed CO₂/SO₂ and H₂O/SO₂ ratios of volcanic gas emissions during the eruption imply a pre-existing >5 wt% exsolved fluid in the reservoir, with fS₂ at ∼0.1 MPa at fO₂ above NNO. Our work confirms that basaltic magmas may coexist with a significant amount of excess fluid which in turn holds an important part of the sulphur budget emitted to the atmosphere.