Equilibrium vs. Kinetic control of the CO2/SO2 ratio in the arc volcanic gases

从弧火山中脱除二氧化碳和硫是全球气候、火山喷发预测和通过俯冲带的挥发性物质循环的基础。特别是，火山气体中CO2/SO2比值的变化可能是火山爆发的潜在前兆。通常，高温火山气体中的CO2/SO2比值被用来推断脱气压力，因为在蒸汽饱和的岩浆液体中，CO2和S的溶解度和分配不同。然而，不同脱气模型在预测S行为和脱气压力方面存在显著差异，这主要是由于缺乏约束S平衡脱气的高压实验。此外，这种对火山气体的解释依赖于传统的假设，即脱气发生在玄武岩熔体的平衡状态。然而，硫和二氧化碳的浓度梯度在含橄榄石的矿体和基质玻璃中普遍观察到，表明S和CO2的扩散脱气。一旦不平衡脱气被接受，由于玄武岩熔体中CO2和S的扩散率可能不同，高温火山气体中CO2/SO2比值的解释也需要重新考虑。本项目旨在研究弧火山气体中CO2/SO2比值的平衡与动力学控制，并评估动力学脱气导致火山爆发前CO2/SO2比值升高的假设。研究结果将显著提高对玄武岩-安山岩火山系统硫行为的认识，指导火山气体数据的解释，并对利用挥发性扩散和总S放气的岩石学估计来限制上升速率具有重要意义。本研究采用活塞缸装置和内加热压力容器进行平衡和减压实验，对平衡和动力学脱气过程中三种主要岩浆挥发物CO2、H2O和S的行为进行了约束。首先，在控制fO2和玄武岩安山岩条件下，在1000MPa和50mpa之间进行平衡实验，以约束平衡脱气过程中的硫行为。结果将用于测试和改进现有的脱气模型，并作为与减压运行进行比较的基线。其次，在相同体积成分下，以缓冲的fO2进行减压实验，从300MPa到不同的最终压力，以慢速和快速减压速率进行减压实验，以研究动力学脱气过程中熔体中的CO2- s，共存蒸汽中的H2O/CO2和CO2/SO2比。实验结果可用于验证和改进现有的动力学脱气和气泡生长模型。利用最新的平衡脱气和动力学脱气模型，本研究将构建区域图，展示随着减压速率的变化，动力学脱气和平衡脱气对熔体和共存蒸汽中CO2/S和H2O/CO2演化的控制。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。

Degassing CO2 and S from arc volcanoes is fundamental to global climate, eruption forecasting, and cycling of volatiles through subduction zones. Particularly, changes in CO2/SO2 ratio in the volcanic gases could be a potential precursor to eruptions. Typically, the CO2/SO2 ratios in high-temperature volcanic gases are used to infer the pressure of degassing due to the different solubility and partitioning of CO2 and S species in vapor-saturated magmatic liquid. However, there are significant discrepancies in predicting S behavior and degassing pressures among different degassing models, primarily due to the scarcity of high-pressure experiments constraining S equilibrium degassing. Moreover, such an interpretation of volcanic gases relies on the conventional assumption that degassing occurs at equilibrium in basaltic melt. However, concentration gradients of sulfur and carbon dioxides have been commonly observed in olivine-hosted embayments and matrix glasses, indicating diffusive degassing of both S and CO2. Once disequilibrium degassing is accepted, due to potentially different diffusivities of CO2 and S in the basaltic melt, the interpretation of the CO2/SO2 ratios in high-temperature volcanic gases also requires a revisit. This project aims at investigating the equilibrium vs. kinetic control of the CO2/SO2 ratio in the arc volcanic gases and evaluating the hypothesis that kinetic degassing leads to elevated CO2/SO2 ratios prior to eruptions. The results will significantly improve understanding of sulfur behavior in basaltic-andesitic volcanic systems, instruct the interpretation of volcanic gas data and bear significance in constraining the ascent rates using volatile diffusion and petrological estimates of total S outgassing.This study employs both equilibrium and decompression experiments with piston cylinder apparatus and internally-heated pressure vessel to constrain the behavior of three major magmatic volatiles, CO2, H2O, and S, during equilibrium and kinetic degassing. First, equilibrium experiments between 1000MPa and 50 MPa with controlled fO2 and a basaltic andesite will be used to constrain the sulfur behavior during equilibrium degassing. The results will be used to test and improve existing degassing models and serve as a baseline to compare to decompression runs. Second, decompression experiments with buffered fO2 on the same bulk composition from 300MPa to various final pressures with slow and fast decompression rates will be used to investigate CO2-S in the melt, H2O/CO2, and CO2/SO2 ratio in the co-existing vapor during kinetic degassing. The experimental results can be used to test and improve existing kinetic degassing and bubble growth model. With updated equilibrium and kinetic degassing models, this study will construct region diagrams demonstrating the control of kinetic vs. equilibrium degassing on evolution of CO2/S and H2O/CO2 in the melt and in the co-existing vapor as changing decompression rates.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.