Experimental constraints on pre-eruptive conditions at Nabro volcano, Eritrea, 2011, and implications for sulphur emissions.

UNLABELLED: We present new experimental constraints on the pre-eruptive magma storage conditions at Nabro Volcano, Eritrea, prior to its 2011 eruption, and discuss the implications for sulphur emissions. We find that the magma was likely stored in the shallow crust prior to eruption, following an intrusion in March 2011. This reservoir was located at ~ 7 km depth, and was recharged immediately prior to the eruption, from a deeper reservoir located at 20 km. The temperature of the magma was 1030 °C with 3 wt% dissolved H2O, at redox conditions around the Quartz-Fayalite-Magnetite buffer (QFM). Our experimental results show that most melt inclusions from previous studies record earlier stages of magmatic evolution. Based on the determined pre-eruptive conditions, we calculate the composition and amount of pre-eruptive excess fluid and find that 3-8 wt% of an excess fluid with 1-3 wt% S is needed to explain the amount of SO2 measured by satellite remote sensing. The abundance of this excess fluid may have contributed to the explosive character of the eruption, whose plume reached the lower stratosphere, assisted by convective transport linked to the Asian monsoon. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at https://doi.org/10.1007/s00410-026-02357-2.