Sulfur, chlorine and fluorine degassing and atmospheric loading by the Roza eruption, Columbia River Basalt Group, Washington, USA

In this study we attempt to quantify the amount of S, Cl and F released by the 1300 km3 Roza member (~ 14.7 Ma) of the Columbia River Basalt Group, which was produced by a moderate-size flood basalt eruption in the mid-Miocene. Our results are the first indication of the potential atmospheric SO2 yield from a flood basalt eruption, and indicate the mechanism by which flood basalt eruptions may have seriously affected the environment. Glass inclusions in phenocrysts and quenched glass in products from various stages of the eruption were analyzed for concentrations of S, Cl and F and major elements. Glass inclusions contain 1965 ± 110 ppm S, 295 ± 65 ppm Cl and 1310 ± 110 ppm F. Groundmass glass of Roza dike selvages contains considerably lower concentrations: 1110 ± 90 ppm S, 245 ± 30 ppm Cl and 1020 ± 25 ppm F. Scoria clasts from near vent deposits contain 665 ± 75 ppm S, 175 ± 5 ppm Cl and 950 ± 20 ppm F, and the groundmass glass of lava selvages contains 520 ± 30 ppm S, 190 ± 30 ppm Cl and 890 ± 55 ppm F. In crystalline lava, the concentrations are 195 ppm S, 100 ppm Cl and 830 ppm F. Volatile element concentrations in these samples represent the progress of degassing through the eruption and can be used to estimate the potential amount of the volatiles S, Cl and F released by the magma into the atmosphere, as well as to evaluate the amount liberated by various phases of the eruption. The total amount of volatiles released by the Roza eruption is estimated to have been ~ 12,420 Mt SO2, ~ 710 Mt HCl and ~ 1780 Mt HF. The Roza magma liberated ~ 9620 Mt SO2 (77% of the total volatile mass released), ~ 400 Mt HCl (56%) and ~ 1450 Mt HF (81%) at the vents and lofted by the eruption columns to altitudes of 7–13 km. Degassing of the lava is estimated to have released an additional ~ 2810 Mt SO2, ~ 310 Mt HCl and ~ 330 Mt HF. za eruption is likely to have lasted for ~ 10 years, indicating an annual H2SO4-mass loading of ~ 1800 Mt. Thus, the atmospheric perturbations associated with the Roza eruption may have been of the magnitude predicted for a severe “nuclear” or “volcanic” winter, but lasting up to a decade or more.