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Kinetics of iron-oxidation-reduction in hydrous silicic melts.

机译:含水硅熔体中铁氧化还原的动力学。

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摘要

The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800 ∞C, 2kb, for melt water contents from ~5 wt% to saturation and fO2 between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeOT) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeOT) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the Fe3+/Fe2+ ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of H2 nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of H2 in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, H2O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/ reduction explains why melt Fe3+/Fe2+ can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to H2 exchange with wall rocks is performed using these new kinetics laws and two DH2 values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of Fe3+/Fe2+ during short processes such as ascent prior to Plinian-style eruptions.
机译:在800∞C,2kb的条件下研究了两种含水流变质熔体(一种金属和另一种碱金属)中Fe的氧化还原动力学,其熔融水含量从〜5 wt%到饱和,fO2在NNO-2和NNO +之间。 3(NNO =镍-镍氧化物氧化还原缓冲液)。 10小时后,金属熔体(〜1 wt%FeOT)达到氧化还原平衡,而3小时后,过碱性碱(〜3 wt%FeOT)达到氧化还原平衡。 Fe氧化和还原的动力学是相似的,不受含水液相的存在与否的影响。随着Fe3 + / Fe2 +比值的变化,玻璃中没有观察到氧化还原前沿,这意味着水合硅质熔体中的Fe氧化还原动力学不受速率的限制,既不受H2扩散的限制,也不受二价阳离子迁移率的限制,正如无水玄武质熔体所观察到的那样。 。我们提出了一个两步反应机制,该机制涉及:(1)样品中H2的瞬时扩散,然后(2)围绕Fe原子的结构/化学重组变慢。因此,贫铁,富H2O的熔体中涉及铁和氢的整个氧化还原过程受到反应的限制,并服从一阶对数速率定律。相对缓慢的氧化/还原动力学解释了为何熔融Fe3 + / Fe2 +可以在实验室实验中轻松淬灭。使用这些新的动力学定律和从文献中提取的两个DH2值,进行了因与壁岩交换H2而引起的岩浆氧化的模拟。我们证明,金属元素的组成没有显着改变,而过碱性组成在短过程中(如在普利尼式喷发之前上升)经历了Fe3 + / Fe2 +的重要且快速的变化。

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