What Controls the CO2/SO2 Ratio in Arc Volcanic Gas?

弧火山二氧化碳和硫的脱气对于喷发预测和挥发物通过俯冲带的循环至关重要。测量高温火山气体中的二氧化碳/二氧化硫已成为量化弧火山二氧化碳和硫向大气流出量的主要方法。对几座火山气体排放的新的高速率、长时间序列测量现在已经显示出火山爆发前几周二氧化碳/二氧化硫上升的异常信号，这通常被认为反映了深层高压气体进入系统。另一方面，最近的研究表明，具有固有的高二氧化碳/二氧化硫气体比例的火山似乎与全球碳酸盐俯冲地区相关。这种相关性导致了二氧化碳/二氧化硫反映母岩浆来源的建议。由此对岩浆气二氧化碳/二氧化硫变化的来源进行了讨论。该项目的目的是通过提供关于火山融化阶段的新数据和已公布的气体记录，解决压力与来源对二氧化碳/二氧化硫的影响。该项目直接关系到对火山爆发的二氧化碳/硫气体前兆的解释，关系到为当地居民制定及时的预报和警报。该项目将成为拉蒙特-多尔蒂地球观测站一名职业生涯早期女科学家博士后项目的核心。它还将使本科生通过NSF资助的拉蒙特-多尔蒂大学本科生项目的研究经验参与拟议的研究。矿物熔融包裹体中的水、二氧化碳和硫的微分析可以成为二氧化碳和硫脱气研究的有力工具。然而，蒸汽“收缩”气泡存在于熔体包裹体中可能会导致显着低估的散装熔体包裹体中的二氧化碳含量，并导致脱气压力和二氧化碳流出量的错误估计。已发表的熔体包裹体数据库中充斥着这个问题。该项目将通过实验，在沿着哥斯达黎加-尼加拉瓜火山弧的五座火山（圣克里斯托瓦尔、泰利卡、马萨亚、波阿斯和图里亚尔瓦）的喷发产物中回收橄榄石为寄主的熔融包裹体，限制母火山和脱气弧岩浆中的二氧化碳和硫含量。重新均质化的熔融包裹体将用于1）推断母岩浆的二氧化碳和硫含量（来自玄武岩端元）和包封压力（来自水-二氧化碳气压），以及2）计算共存的流体/蒸汽成分，以与火山气体数据进行比较，并为现有的热力学模型提供信息。通过这种方式，我们的数据将测试脱气压力与母岩浆驱动弧火山气体中二氧化碳/二氧化硫变化的程度，并量化喷发气体前体的起源。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The degassing of carbon dioxide and sulfur from arc volcanoes is fundamentally important to eruption forecasting and cycling of volatiles through subduction zones. Measurements of carbon dioxide/sulfur dioxide in high-temperature volcanic gases have become the main approach to quantifying the arc volcanic carbon dioxide and sulfur outflux to the atmosphere. New high-rate, long-time series measurements of gas emissions from several volcanoes have now shown precursory signals in the rise of carbon dioxide/sulfur dioxide weeks before the eruption, which is typically considered to reflect deep, high pressure gas moving into the system. On the other hand, recent work has shown that volcanoes with intrinsically high carbon dioxide/sulfur dioxide gas ratios seem to correlate globally with regions of carbonate subduction. Such correlations have led to the suggestions that carbon dioxide/sulfur dioxide reflects the source of the parental magma. The source of variation in carbon dioxide/sulfur dioxide of magmatic gas is thus debated. This project aims at resolving the pressure vs. source effects on carbon dioxide/sulfur dioxide by providing novel data on the melt phase for volcanoes with published gas records. This project bears directly on the interpretation of carbon dioxide/sulfur gas precursors to volcanic eruptions, of relevancy to developing timely forecasts and warnings to local populations. This project will form the core of the postdoctoral project of an early-career female scientist at Lamont-Doherty Earth Observatory. It will also enable an undergraduate student to participate in the proposed research via an NSF-funded research experience for undergraduates program at Lamont-Doherty.Microanalysis of water, carbon dioxide, and sulfur in mineral-hosted melt inclusions can be a powerful tool for the study of carbon dioxide and sulfur degassing. However, vapor “shrinkage” bubbles present in melt inclusions can cause a significant underestimate of carbon dioxide contents in the bulk melt inclusion and lead to false estimates of degassing pressures and carbon dioxide outflux. The published melt inclusion database is rife with this issue. This project will constrain the carbon dioxide and sulfur contents in parental and degassing arc magmas by experimentally rehomogenizing olivine-hosted melt inclusions in the eruptive products from five volcanoes (San Cristóbal, Telica, Masaya, Poás and Turrialba) along the Costa Rica-Nicaragua volcanic arc. The rehomogenized melt inclusions will be used 1) to infer the carbon dioxide and sulfur contents of parental magmas (from basaltic end-members) and entrapment pressures (from water-carbon dioxide barometry), and 2) to calculate the coexisting fluid/vapor composition to compare with the volcanic gas data and to inform existing thermodynamic models. In this way, our data will test the extent to which the pressure of degassing versus the parental magma drive carbon dioxide/sulfur dioxide variations in arc volcanic gas, and quantify the origin of eruptive gas precursors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.