Melt inclusion constraints on volatile systematics and degassing history of the 2014–2015 Holuhraun eruption, Iceland

The mass of volatiles emitted during volcanic eruptions is often estimated by comparing the volatile contents of undegassed melt inclusions, trapped in crystals at an early stage of magmatic evolution, with that of the degassed matrix glass. Here we present detailed characterisation of magmatic volatiles (H 2 O, CO 2 , S, Fl and Cl) of crystal-hosted melt and fluid inclusions from the 2014–2015 Holuhraun eruption of the Bárðarbunga volcanic system, Iceland. Based on the ratios of magmatic volatiles to similarly incompatible trace elements, the undegassed primary volatile contents of the Holuhraun parental melt are estimated at 1500–1700 ppm CO 2 , 0.13–0.16 wt% H 2 O, 60–80 ppm Cl, 130–240 ppm F and 500–800 ppm S. High-density fluid inclusions indicate onset of crystallisation at pressures ≥ 0.4 GPa (~ 12 km depth) promoting deep degassing of CO 2 . Prior to the onset of degassing, the melt CO 2 content may have reached 3000–4000 ppm, with the total magmatic CO 2 budget estimated at 23–55 Mt. SO 2 release commenced at 0.12 GPa (~ 3.6 km depth), eventually leading to entrapment of SO 2 vapour in low-density fluid inclusions. We calculate the syn-eruptive volatile release as 22.2 Mt of magmatic H 2 O, 5.9–7.7 Mt CO 2 , and 11.3 Mt of SO 2 over the course of the eruption; F and Cl release were insignificant. Melt inclusion constraints on syn-eruptive volatile release are similar to estimates made during in situ field monitoring, with the exception of H 2 O, where field measurements may be heavily biased by the incorporation of meteoric water.