Eemu Ranta, Sæmundur Ari Halldórsson, Bergrún A. Óladóttir, Melissa A. Pfeffer, Alberto Caracciolo , Eniko Bali, Guðmundur H. Guðfinnsson, Maren Kahl, Sara Barsotti

Outgassing of S (as SO2) is one of the principal hazards posed by volcanic eruptions. However, S emission potentials of most volcanoes globally are poorly constrained due to a short observational record and an incomplete understanding of the magmatic processes that influence pre-eruptive S concentrations. Here, we use a compilation of published and new data from melt inclusions—which preserve magmatic S concentrations prior to eruptive degassing—from the Iceland hotspot to evaluate the effects of mantle melting and crustal magmatic processes on the S budgets of Icelandic melts. We apply the petrological method to estimate S emission potentials (∆Smax) for 68 eruptions from 22 of the ~33 presently active volcanic systems in Iceland. We show that the S systematics of Icelandic melts are strongly regulated by the sulfide solubility limit. Sulfide-saturated conditions during lower-degree mantle melting, prevalent at off-rift zones, likely explains an observed decoupling between S and Cl. Modelled sulfide solubility peaks in evolved basalts (4-6 wt.% MgO), coinciding with highest melt inclusion S concentrations. Highest ∆Smax (2100–2600 ppm) are found in the Hekla 1913 CE, Eldgjá 934 CE and Surtsey 1963-67 CE eruptions in the South Iceland Volcanic Zone. Our results extend the record of volcanic sulfur emissions back in time and can be used to assess volcanic gas hazards at Icelandic volcanoes where no direct measurements are available. Broadly, the results underline the governing role of sulfide solubility during melting and magma differentiation in controlling the eruptible S contents of hotspot magmas.