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

弧火山的二氧化碳和硫的脱气对火山喷发预测和通过俯冲带的挥发物循环至关重要。高温火山气体中二氧化碳/二氧化硫的测量已成为量化弧火山向大气排放的二氧化碳和硫的主要方法。对几座火山的气体排放进行了新的高速率、长时间的系列测量，现在已经显示出在火山喷发前几周二氧化碳/二氧化硫上升的前兆信号，这通常被认为是深部高压气体进入系统的反映。另一方面，最近的研究表明，在全球范围内，具有高二氧化碳/二氧化硫气体比率的火山似乎与碳酸盐俯冲区域有关。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。因此，岩浆气体中二氧化碳/二氧化硫变化的来源是有争议的。该项目旨在通过提供火山融化阶段的新数据和已发表的气体记录，解决压力对二氧化碳/二氧化硫的影响。这一项目直接关系到火山爆发前二氧化碳/硫气体前兆的解释，关系到对当地居民作出及时的预报和警告。该项目将成为Lamont-Doherty地球观测站一位早期职业女性科学家博士后项目的核心。它还将使本科生能够通过美国国家科学基金会资助的Lamont-Doherty本科生项目的研究经验参与拟议的研究。矿物承载的熔体包裹体中水、二氧化碳和硫的微分析可以成为研究二氧化碳和硫脱气的有力工具。然而，熔体包裹体中存在的蒸汽“收缩”气泡会导致对整体熔体包裹体中二氧化碳含量的严重低估，并导致对脱气压力和二氧化碳流出量的错误估计。出版的熔体夹杂物数据库充斥着这个问题。该项目将通过实验对哥斯达黎加-尼加拉瓜火山弧上五座火山（San Cristóbal、Telica、Masaya、Poás和Turrialba）喷发产物中的含橄榄石熔融包裹体进行再均质化，从而限制母弧岩浆和脱气岩浆中的二氧化碳和硫含量。重新均质化的熔体包裹体将用于1)推断母岩浆的二氧化碳和硫含量（来自玄武岩端元）和圈闭压力（来自水-二氧化碳气压计），以及2)计算共存的流体/蒸汽组成，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对弧火山气体中二氧化碳/二氧化硫变化的驱动程度，并量化喷发气体前体的来源。该奖项反映了美国国家科学基金会的法定使命，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.