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

该项目由岩石学和地球化学项目（地球科学部）和非洲、近东和南亚项目（国际科学与工程办公室）提供支持。硫是火山气体中最丰富的成分之一，是岩浆的重要组成部分。就气候影响而言，硫是最重要的火山气体，因为硫在大气中氧化成硫酸盐气溶胶。像1991年菲律宾皮纳图博火山这样的大型火山爆发，导致全球气温在两年内下降了2度。硫最近受到了很多关注，因为地球工程的想法，科学家们建议向平流层注入大量的硫来抵消全球变暖。因此，了解火山岩浆中硫自然脱气的方式和数量，是评估全球硫循环对气候影响的关键一步，也是评估人类对地球硫收支影响的潜在后果的关键一步。在这项提议的工作中，研究小组的目标是研究一个持续活跃的熔岩湖的硫脱气。最适合这个项目的熔岩湖是埃塞俄比亚的Erta Ale火山。这个湖是地球上最长寿的熔岩湖，已经持续活跃了100多年。它从玄武岩岩浆中除去大量的硫。他们将收集活动熔岩湖、脱气硫和大气中形成的硫酸盐气溶胶的样本。将分析熔岩、气体和气溶胶中硫的同位素组成，以了解硫循环的本质。这项研究的最终目标是建立一个一致的模型，说明硫磺是如何从世界各地的火山岩浆中脱气的。二氧化硫的通量是可以测量的，因为二氧化硫对紫外光的吸收使其在大气中的浓度很容易被检测到。原生岩浆气体的H2S/SO2比值与压力有关，因此可以作为脱气深度的代表。因此，S的脱气为火山脱气过程提供了有用的见解，可作为火山活动增加或减少的指示，并常规用于火山监测。硫同位素为研究岩浆的硫源、脱气过程和喷发前挥发性成分提供了有力的工具。该研究是对火山脱气过程中硫同位素分馏的定量、严格约束评估，将允许评估目前使用的同位素分馏模型和分馏因子。该研究将测量气体和熔体中的S丰度，熔体和气相中S的形态，所有S的同位素组成，并使用测量的脱气条件约束，如温度和氧的逸度（fO2）。该研究还将评估脱气过程中的不平衡分馏。所有数据将从埃塞俄比亚埃尔塔阿莱持续脱气的玄武岩熔岩湖获得。Erta Ale岩浆相对还原（fO2 ~ QFM）， SO2通量高且稳定，气体中S含量高，是本研究的理想选择。Erta Ale为陆地S脱气提供了一个简化的端元。本研究结果将与位于俯冲带背景的Masaya火山为代表的氧化端元（QFM +1.6，高S通量）进行比较。

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.