Constraining Sulfur Isotope Fractionation During Volcanic Degassing Through the Study of an Open-system Basaltic Volcano

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This project is being supported by the Petrology and Geochemistry program (Division of Earth Sciences) and the Africa, Near East & South Asia Program (Office of International Science and Engineering).Sulfur (S) is one of the most abundant constituents of volcanic gases and is an important component of magmas. Sulfur is the most important volcanic gas in terms of climate impact due to the oxidation of sulfur to sulfate aerosols in the atmosphere. Large eruptions like the one at Pinatubo, Philippines in 1991, caused global temperatures to drop by two degrees over a period of two years. Sulfur has recently received a lot of attention due to ideas of geo-engineering where scientists propose to inject large quantities of sulfur into the stratosphere to counterbalance global warming. Therefore, understanding how and in what quantities sulfur degases naturally from magmas at volcanoes is a critical step in evaluating the global sulfur cycle with implications for climate impact as well as evaluating potential consequences of human impacts on the Earth's sulfur budget. In this proposed work, the team aims to investigate sulfur degassing from a persistently active lava lake. The lava lake that is best suited for this project is that of the Erta Ale volcano in Ethiopia. This lake is the Earth's longest-lived lava lake and has been persistently active for over 100 years. It degases large quantities of sulfur from a basaltic magma. They will collect samples of the active lava lake, the degassing sulfur, and the sulfate aerosol formed in the atmosphere. The isotopic composition of the sulfur in the lava, gas, and aerosols will be analyzed to understand the nature of the sulfur cycle. The ultimate goal of this study is to develop a consistent model for how sulfur degases from magmas in volcanoes worldwide. Fluxes of sulfur dioxide can be measured because their atmospheric concentrations are easily detectable due to its absorption of ultraviolet light. The H2S/SO2 ratio of primary magmatic gases is pressure dependent and therefore can be used as a proxy for depth of degassing. Therefore, degassing of S provides useful insights into volcanic degassing processes, can be used as an indication of increasing or decreasing volcanic activity, and is routinely used in volcano monitoring. Sulfur isotopes provide a powerful tool for studying sources of sulfur, degassing processes and pre-eruptive volatile contents of magmas. The proposed study is a quantitative, robustly constrained evaluation of sulfur isotope fractionation during volcanic degassing that will allow assessment of currently employed isotope fractionation models and fractionation factors. The study will measure S abundances in gas and melt, speciation of S in the melt and gas phases, isotope compositions of all S species, and use measured constraints on degassing conditions such as temperature and the fugacity of oxygen (fO2). The study will also assess disequilibrium fractionation during degassing. All data will be obtained from the consistently degassing basaltic lava lake of Erta Ale, Ethiopia. Erta Ale is ideal for this study because it has a relatively reduced magma (fO2 ~ QFM), high, consistent SO2 flux and high S content in the gases. Erta Ale provides a reduced end-member for terrestrial S degassing. The results from this study will be compared with an oxidizing end-member (QFM +1.6 and high S flux) represented by volcano Masaya that is located in a subduction zone setting.