Volcanic S emissions in Iceland

This work is (1) an effort to establish best practises for using petrological means to estimate emissions of sulfuric gases from volcanic eruptions (2) calculate and catalogue S emission estimates from available petrological data in Iceland, and (3) understand more fundamentally what igneous processes affect sulfur contents in Iceland. Results of this work are used by the Iceland Meteorological Office to inform SO2 dispersal models and hazard evaluation prior to available observational data.

Rationale

Sulfur dissolved in silicate melts drives volcanic SO2 emissions—one of the principal hazards of eruptions—and partakes in many ore-forming processes. However, magmatic S systematics remain incompletely understood because the record of true melt S contents in both extrusive and intrusive rocks is largely erased by degassing. Here, we use a new compilation of published and new data from mineral-hosted melt inclusions—which preserve pre-eruptive melt S contents—from the Iceland hotspot to investigate the behavior of S during mantle melting, crustal magma evolution and eruptive degassing.

Results

Modelling of partial mantle melting indicates that lower-degree melting, prevalent at off-rift zones, likely occurs under sulfide-saturated conditions and leads to decoupling of S from incompatible volatiles like Cl seen in natural melt inclusion data. In chemical terms, this effect is expressed as an anticorrelation between S/Cl and La/Yb, which further requires that the enriched mantle source preferentially tapped at off-rift settings has a low S/Cl ratio (5-10). By contrast, higher S/Cl and low La/Yb of rift zone melt inclusions reflect higher-degree and sulfide-undersaturated melting of a more depleted source with high S/Cl (~200-300). 

Pre-eruptive sulfur concentrations are strongly regulated by the sulfide solubility limit. Most melts appear to reach sulfide saturation at MgO contents of ~6 wt.% during crustal magma evolution. A peak in modelled sulfide solubility limit coincides with highest S values (500–2600 ppm, average 1400 ppm) seen in evolved basalts at 4-6 wt.% MgO. The S concentrations are moderate (400–1500 ppm, average 1100 ppm) in Mg-rich basaltic eruptions (>8 wt.% MgO) and low (0–400 ppm, average 100 ppm) in andesitic and rhyolitic melts (SiO2 > 57 w%).

Iceland S emission catalog

We used a modified ‘petrological method’ (Devine et al. 1984) to estimate S emission potentials for 73 eruptions from 22 of Iceland’s 33 presently active volcanic systems. Highest S emission potentials (2100–2600 ppm) are found in the Hekla 1913 CE, Eldgjá 939 CE and Surtsey 1963-67 CE eruptions, all evolved basaltic eruptions in the propagating rift South Iceland Volcanic Zone. Notably, we think these emission estimates are relatively accurate, because similar methods yield an excellent match with direct SO2 emission measurements available for recent Icelandic eruptions (e.g., Bali et al. 2018, Caracciolo et al. 2024).

Applications

Our results can be used to assess volcanic gas hazards at Icelandic volcanoes where no direct measurements are available. More generally, the results underline the governing role of sulfide solubility in decoupling S from the other volatile elements during both melting and magma differentiation, and in ultimately controlling the eruptible S contents of hotspot magmas.

References

Devine JD, Sigurdsson H, Davis AN, Self S (1984). Estimates of sulfur and chlorine yield to the atmosphere from volcanic eruptions and potential climatic effects. Journal of Geophysical Research: Solid Earth 89, 6309-6325.

Bali E, Hartley ME, Halldórsson SA, Guðfinnsson GH, Jakobsson S (2018). Melt inclusion constraints on volatile systematics and degassing history of the 2014–2015 Holuhraun eruption, Iceland. Contributions to Mineralogy and Petrology 173, 1-21.