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

通过开放系统玄武岩火山的研究限制火山脱气过程中的硫同位素分馏

• 批准号: 1049891
• 负责人: Tobias Fischer
• 金额: $ 10.13万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-03-01 至 2014-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1049891&HistoricalAwards=false
• 关键词: Constraining; Sulfur; Isotope; Fractionation; During

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.

该项目得到了岩石学和地球化学项目（地球科学部）以及非洲、近东和南亚项目（国际科学与工程办公室）的支持。硫（S）是火山气体中最丰富的成分之一是岩浆的重要组成部分。由于硫在大气中氧化成硫酸盐气溶胶，因此硫是对气候影响最重要的火山气体。像 1991 年菲律宾皮纳图博火山那样的大规模火山喷发导致全球气温在两年内下降了两度。由于地球工程的想法，硫最近受到了广泛关注，科学家提出将大量硫注入平流层以抵消全球变暖。因此，了解硫从火山岩浆中自然脱气的方式和数量是评估全球硫循环及其对气候影响的影响以及评估人类影响地球硫预算的潜在后果的关键一步。在这项拟议的工作中，该团队的目标是研究持续活跃的熔岩湖中的硫磺脱气。最适合该项目的熔岩湖是埃塞俄比亚的尔塔阿雷火山的熔岩湖。这个湖是地球上寿命最长的熔岩湖，已经持续活跃了 100 多年。它从玄武岩浆中脱除大量的硫。他们将收集活跃熔岩湖、脱气硫以及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，以了解硫循环的性质。 这项研究的最终目标是开发一个一致的模型来解释世界各地火山岩浆中的硫如何脱气。二氧化硫的通量可以测量，因为二氧化硫的大气浓度由于其吸收紫外线而很容易检测到。原生岩浆气体的 H2S/SO2 比率与压力有关，因此可以用作脱气深度的代表。因此，S 的脱气为火山脱气过程提供了有用的见解，可以用作火山活动增加或减少的指示，并且通常用于火山监测。 硫同位素为研究硫的来源、脱气过程和岩浆喷发前的挥发物含量提供了有力的工具。拟议的研究是对火山脱气过程中硫同位素分馏的定量、严格约束的评估，这将允许评估当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硫的丰度、熔体和气相中硫的形态、所有硫形态的同位素组成，并使用测量的脱气条件限制，例如温度和氧逸度 (fO2)。该研究还将评估脱气过程中的不平衡分馏。所有数据都将从埃塞俄比亚尔塔阿雷持续脱气的玄武岩熔岩湖中获得。 Erta Ale 是这项研究的理想选择，因为它的岩浆相对较少 (fO2 ~ QFM)、高且稳定的 SO2 通量以及气体中的高硫含量。 Erta Ale 为陆地 S 脱气提供了减少的端元。这项研究的结果将与以位于俯冲带环境中的马萨亚火山为代表的氧化端元（QFM +1.6 和高 S 通量）进行比较。