Oxygen fugacity of gases and rocks from Momotombo Volcano, Nicaragua: Application to volcanological monitoring
Oxygen fugacity of gases and rocks from Momotombo Volcano, Nicaragua: Application to volcanological monitoring
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尼加拉瓜莫莫托博火山气体和岩石的氧逸度：在火山监测中的应用
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发表时间：
1988
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通讯作者：
A. Creusot
中科院分区：
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作者：
G. Benhamou;P. Allard;J. Sabroux;G. Vitter;D. Dajlevic;A. Creusot
通过对1978年至1985年收集的气体样品进行现场电化学测量（电解池组装）和热力学计算，确定了尼加拉瓜莫莫通博火山高温（600°-860 ° C）气体排放的氧逸度（fO 2）和fO 2与T°C的关系。然后将其与上一次喷发（1905年）的新鲜和蚀变熔岩的固有fO 2进行比较，在实验室中测量的温度为500° C至1100°C。电化学结果表明，在相同温度下，Momotombo火山岩的氧逸度远高于1905年新鲜熔岩（CIMFMQ缓冲区），而更接近于其蚀变围岩（CIMFMQ缓冲区）。从气体样品的化学计算的平衡O2逸度证实了这种模式。然而，他们认为气体混合物保留了（可变的）记忆，即在温度和fO 2分别高达1050°C和10−9.0 atm的条件下，在深度达到更高的热平衡，这对应于fO 2-T气体和熔岩趋势之间的交叉。因此，这些数据支持这样一种观点，即莫莫托姆博火山气体在活动增加的时期释放，在上升过程中通过无缓冲冷却和与环境流体和岩石的反应而遭受可变氧化之前，从浅岩浆体中逃逸。这种晚期氧化作用在中央气孔处比在周边气孔处弱。虽然在1978年至1985年之间，最热的热分子的温度从750° C上升到865° C-900 ° C，但气体的平衡fO 2下降了近一个数量级（在可比的平衡温度下）。这种演化可能反映了地表喷气作用与沿着深部岩浆脱气作用之间的联系。这项工作强调了通过连续记录热火山孔中氧逸度和温度的变化来监测火山喷发前岩浆上升和气体-岩浆分离过程的可能性。
The oxygen fugacity (fO2) and the fO2 versus T°C relationship of high-temperature (600°–860°C) gas emissions from Momotombo volcano, Nicaragua, was determined from both field electrochemical measurements (electrolytic cell assembly) and thermodynamic computations on gas samples collected between 1978 and 1985. It was then compared with the intrinsic fO2 of fresh and altered lavas from the last eruption (1905), as measured between 500° and 1100°C in laboratory. The electrochemical results show that the oxygen fugacity of Momotombo fumaroles, at equivalent temperature, is much higher than that of the fresh 1905 lava (∼FMQ buffer) and closer to that of their altered wall rocks (∼FMQ buffer). The equilibrium O2 fugacities calculated from the chemistry of gas samples confirm this pattern. However, they suggest that the gas mixtures preserve the (variable) memory of a higher thermal equilibrium achieved at depth, under temperature and fO2 conditions of up to 1050°C and 10−9.0 atm, respectively, which correspond to the cross over between the fO2-T gas and lava trends. These data thus support the idea that Momotombo volcanic gases, released in a period of increasing activity, escape from a shallow magma body before suffering a variable oxidation during their ascent through both unbuffered cooling and reactions with environmental fluids and rocks. This late oxidation is weaker at central fumaroles than at peripherical ones. While between 1978 and 1985 the temperature of the hottest fumarole increased from 750° to 865°–900°C, the equilibrium fO2 of the gas decreased by nearly one order of magnitude (at comparable equilibrium temperature). Such an evolution presumably reflects an increasing connection between the surface exhalations and the magma degassing at depth along with time. This work underlines the possibility of monitoring the processes of magma ascent and gas-magma separation within a volcano before an eruption by continuously recording the changes of both oxygen fugacity and temperature in hot fumaroles.