Redox evolution of a degassing magma rising to the surface
Redox evolution of a degassing magma rising to the surface
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DOI：
10.1038/nature05509
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发表时间：
2007-01-11
期刊：
影响因子：
64.8
通讯作者：
Scaillet, Bruno
中科院分区：
文献类型：
--
作者：
Burgisser, Alain;Scaillet, Bruno
岩浆所携带的挥发物，无论是溶解还是出溶，对各种地质现象都有根本性的影响，例如岩浆动力学（1-5）和地球大气的组成（6）。特别是，人们普遍认为，地球表面火山气体的氧化还原状态反映了岩浆源的氧化还原状态，并被认为对大气氧气的长期演化产生了一级控制（6，7）。然而，根据熔岩或相关气体化学估算的氧逸度（f（O2））可能相差一个对数单位（8-10），造成这种差异的原因仍然不清楚。在这里，我们使用一个耦合的化学-物理模型的导管流表明，上升的岩浆的氧化还原状态的演变，从而其共存的气相，是强烈依赖于在储层中的气体的组成和数量。不含硫的岩浆在上升过程中表现出系统的f（O2）增加，增加幅度高达2个对数单位。含硫岩浆在上升过程中也表现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）。我们的计算密切再现的H2S/SO2比的火山气体在收敛的设置，但f（O2）在水库和火山管道的出口之间的差异可能是多达1.5个对数单位。因此，喷发岩浆的氧化还原状态不一定是它们释放的气体的氧化还原状态的良好代表。我们的研究结果可能需要重新评估模型，旨在量化岩浆挥发分在地质过程中的作用。
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.