SO2 and HCl release during the 2024 March-May Sundhnúkur event

SO₂ and HCl release during the 2024 March-May Sundhnúkur event

Tephra samples were collected at the Sundhnúkur crater row shortly after the start of the eruption on the 17^th of March. The sulfur and chlorine contents of silicate melt inclusions and matrix glasses (Fig.1) from these samples were analyzed by electron microprobe at the Institute of Earth Sciences, University of Iceland. The melt inclusions preserve the S and Cl contents of magmas before magmas release SO₂ and HCl during eruption. The highest S content in melt inclusions (after correction for post-entrapment processes) reached 1600 ppm, whereas the average S content in matrix glasses was 250 ppm. The Cl content of melt inclusions and matrix glasses is 240±40 ppm. The difference between the melt inclusion and matrix glass compositions gives us information on the volatile release at the eruptive vents, whereas lava flow degassing can be estimated by the complete release of volatiles from the matrix glass. These values, combined with effusion rates, allow us to calculate the variation of SO₂ and HCl emissions during the eruption.

Figure 1: Backscattered electron image of a plagioclase+olivine glomerocryst containing silicate melt inclusions and surrounded by microlite-free, vesicular matrix glass. S contents in melt inclusions and matrix glass were analyzed by the electron microprobe at the Institute of Earth Sciences.

The variation of magma effusion rate during the March to May Sundhnúkur event in 2024 was published earlier on the website of the Institute of Earth Sciences.

Figure 2 shows the SO₂ and HCl flux we calculate throughout the eruption in kg/s. For better visualization, we report separately the SO₂ and HCl fluxes on the first day (Fig. 2a) and the whole eruption (Fig. 2b). Based on our calculations, in the first hours of the eruption the SO₂ flux reached over 10.5 t/s; however, it decreased quickly to ~100 kg/s. 84% of the SO₂ was released along the eruptive fissure (vents), while the rest was emitted from the lava flow. Our estimates fit well with the results of the SO₂ flux measurements in the field on the 4^th and 8^th of April reported by the Icelandic Meteorological Office.

Figure 2: Estimated SO₂ and HCl flux (in kg/s) during the 1^st day (a) and over the whole 2024 March-May Sundhnúkur event (b). Most SO₂ was released at the eruptive fissure (vents) whereas most HCl was released from the cooling and crystallizing lava flow. Note the good match between the field measurements conducted on the 4^th and 8^th of April and our estimates for SO₂ release.
 

At the start of the eruption the HCl flux might have reached over 800 kg/s; however, HCl emission decreased quickly to ~10 kg/s. As the Cl-content of melt inclusions and matrix glasses is similar, we suggest that most of the Cl was released from the cooling and crystallizing lava flow and only minimal HCl might have been released along the eruptive fissure. This is in agreement with field measurements, where the SO₂/HCl ratio measured at the eruptive vents is an order of magnitude higher than measured along lava flow.

The total SO₂ release during the 2024 March to May Sundhnúkur event is estimated to be 0.3 Mt. In comparison, the total SO₂ release was ~0.78-0.97 Mt during the 6-months-long Fagradalsfjall event and 10.5 Mt during the 6-months-long eruption at Holuhraun in 2014-15 (Caracciolo et al. 2024, Pfeffer et al. 2024, Bali et al. 2018). We calculate ~0.02 Mt total released HCl during the 2024 March to May Sundhnúkur event.

References:

Caracciolo et al. (2024): Medieval and recent SO2 budgets in the Reykjanes Peninsula: implication for future hazard. Geochemical Perspectives Letters https://doi.org/10.7185/geochemlet.2417

Pfeffer et al. (2024): SO₂ emission rates and incorporation into the air pollution dispersion forecast during the 2021 eruption of Fagradalsfjall, Iceland. Journal of Volcanology and Geothermal Research, doi: https://doi.org/10.1016/j.jvolgeores.2024.108064   

Bali et al. (2018): Melt inclusion constraints on volatile systematics and degassing history of the 2014–2015 Holuhraun eruption, Iceland. Contributions to Mineralogy and Petrology, doi: https://doi.org/10.1007/s00410-017-1434-1