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Redox evolution of a degassing magma rising to the surface

机译：脱气岩浆上升到表面的氧化还原演化

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Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma dynamics(1-5) and the composition of the Earth's atmosphere(6). In particular, the redox state of volcanic gases emanating at the Earth's surface is widely believed to mirror that of the magma source, and is thought to have exerted a first-order control on the secular evolution of atmospheric oxygen(6,7). Oxygen fugacity (f(O2)) estimated from lava or related gas chemistry, however, may vary by as much as one log unit(8-10), and the reason for such differences remains obscure. Here we use a coupled chemical - physical model of conduit flow to show that the redox state evolution of an ascending magma, and thus of its coexisting gas phase, is strongly dependent on both the composition and the amount of gas in the reservoir. Magmas with no sulphur show a systematic f(O2) increase during ascent, by as much as 2 log units. Magmas with sulphur show also a change of redox state during ascent, but the direction of change depends on the initial f(O2) in the reservoir. Our calculations closely reproduce the H2S/SO2 ratios of volcanic gases observed at convergent settings, yet the difference between f(O2) in the reservoir and that at the exit of the volcanic conduit may be as much as 1.5 log units. Thus, the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Our findings may require re-evaluation of models aimed at quantifying the role of magmatic volatiles in geological processes.
机译：溶解或溶解的岩浆携带的挥发物对多种地质现象具有根本影响，例如岩浆动力学（1-5）和地球大气的组成（6）。特别是，人们广泛认为地球表面的火山气体的氧化还原状态与岩浆源的氧化还原状态一致，并被认为对大气中氧气的长期演化起了一级控制作用（6,7）。然而，根据熔岩或相关气体化学方法估算的氧气逸度（f（O2））可能相差一个对数单位（8-10），而造成这种差异的原因仍然不清楚。在这里，我们使用管道流动的化学-物理耦合模型来表明，岩浆上升的氧化还原态演化以及因此与之共存的气相强烈依赖于储层中的气体组成和数量。不含硫的岩浆在上升过程中显示出系统性的f（O2）增加，多达2个对数单位。含硫的岩浆在上升过程中也显示出氧化还原状态的变化，但变化的方向取决于储层中的初始f（O2）。我们的计算紧密地再现了在收敛位置观察到的火山气体的H2S / SO2比，但是储层中f（O2）与火山管道出口处的f（O2）之间的差异可能高达1.5 log单位。因此，喷出的岩浆的氧化还原状态不一定是它们排放气体的氧化还原状态的良好替代。我们的发现可能需要对旨在量化岩浆挥发物在地质过程中的作用的模型进行重新评估。

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