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

从弧火山中排出二氧化碳和硫对全球气候、喷发预测和俯冲带挥发物循环至关重要。特别是，火山气体中CO2/SO2比例的变化可能是火山爆发的潜在前兆。典型地，高温火山气体中的CO2/SO2比率用于推断脱气压力，这是由于CO2和S物种在蒸气饱和的岩浆液体中的不同溶解度和分配。然而，有显着的差异，在预测S的行为和脱气压力之间的不同脱气模型，主要是由于缺乏高压实验约束S平衡脱气。此外，这种对火山气体的解释依赖于玄武岩熔体在平衡状态下发生脱气的传统假设。然而，硫和二氧化碳的浓度梯度已普遍观察到在橄榄石托管海湾和基质玻璃，表明扩散脱气的S和CO2。一旦不平衡脱气被接受，由于潜在的不同的扩散系数的CO2和S的玄武岩熔体，高温火山气体中的CO2/SO2比的解释也需要重新审视。该项目旨在研究弧火山气体中CO2/SO2比率的平衡与动力学控制，并评估动力学脱气导致喷发前CO2/SO2比率升高的假设。本研究采用活塞柱和内热式压力容器的平衡实验和减压实验，对三种主要的岩浆挥发分进行了定量的定量分析，结果表明，三种主要的挥发分在火山岩中的分布规律是一致的。CO2、H2O和S，在平衡和动力学脱气过程中。首先，1000 MPa和50 MPa之间的平衡实验与控制的fO 2和玄武安山岩将被用来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。第二，减压实验与缓冲fO 2相同的散装组合物从300 MPa到各种最终压力的缓慢和快速的减压速率将被用来调查CO2-S在熔体中，H2O/CO2，和CO2/SO2的比例在共存的蒸汽动力学脱气过程中。实验结果可用于检验和改进现有的动力学脱气和气泡生长模型。随着更新的平衡和动力脱气模型，这项研究将构建区域图，展示动态与平衡脱气对熔体和共存蒸汽中CO2/S和H2O/CO2演变的控制，作为不断变化的减压率。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.