Iron oxidation state and environment in magmas affect their phase diagram and their properties, including viscosity and density, which determine magma mobility and eruptive potential. In turn, magma composition, pressure, temperature and oxygen fugacity affect iron oxidation state and coordination, potentially leading to complex feedbacks associated with magma ascent, degassing and eruption. While equilibrium experiments and models have led to a deep understanding of the role of iron in melts, our knowledge of the effects of disequilibrium processes on iron oxidation state and its structural role in lavas and magmas remains limited. Accordingly, we performed a series of dynamic disequilibrium experiments on a natural melt composition (a phonolite lava from Erebus volcano, Antarctica) at atmospheric pressure, in which oxygen fugacity and temperature were controlled and varied. During the experiments, we continuously measured iron oxidation and coordination using Fe K-edge dispersive X-ray Absorption Spectroscopy (XAS). We found that iron oxidation state changes in the phonolite melt are reversible and well reproduced by existing models. Changes in iron oxidation state are driven by joint diffusion of alkali cations and oxygen anions at magmatic temperatures (~ 1000 °C for Erebus phonolite). However, redox diffusion timescales are too slow for any significant oxygen exchange with the atmosphere at the lava/air interface or via air entrainment. Turning to iron coordination, while Fe 2+ and Fe 3+ are present mostly in an average five-fold coordination, complex coordination variations decoupled from redox changes were detected. The data suggest transitions between Fe 3+ in four-fold and six-fold coordination prior to reduction or as a consequence of oxidation. This questions the possible implication of Fe coordination changes in triggering crystallisation of magnetite nanolites upon magma ascent, and, through such crystallisation events, in promoting magma explosivity.

中文翻译：

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不同温度和氧逸度对音质熔体中铁氧化态和配位影响的原位 XANES 研究

岩浆中的铁氧化态和环境影响它们的相图和它们的性质，包括粘度和密度，这决定了岩浆的流动性和喷发潜力。反过来，岩浆成分、压力、温度和氧逸度会影响铁的氧化态和配位，可能导致与岩浆上升、脱气和喷发相关的复杂反馈。虽然平衡实验和模型使人们深入了解了铁在熔体中的作用，但我们对不平衡过程对铁氧化态的影响及其在熔岩和岩浆中的结构作用的了解仍然有限。因此，我们在大气压下对天然熔体成分（来自南极洲埃里伯斯火山的音岩熔岩）进行了一系列动态不平衡实验，其中氧逸度和温度受到控制和变化。在实验过程中，我们使用 Fe K 边缘色散 X 射线吸收光谱 (XAS) 连续测量铁的氧化和配位。我们发现，音石熔体中铁氧化态的变化是可逆的，并且可以被现有模型很好地再现。铁氧化态的变化是由碱金属阳离子和氧阴离子在岩浆温度下的联合扩散驱动的（对于 Erebus 音沸石，约为 1000 °C）。然而，氧化还原扩散时间尺度太慢，无法在熔岩/空气界面或通过空气夹带与大气进行任何显着的氧气交换。转向铁配位，虽然 Fe 2+ 和 Fe 3+ 主要以平均五倍配位存在，但检测到与氧化还原变化解耦的复杂配位变化。数据表明Fe 3+ 在还原之前或作为氧化的结果以四倍和六倍配位之间的转变。这质疑 Fe 配位变化在岩浆上升时触发磁铁矿纳米岩结晶的可能含义，并通过这种结晶事件促进岩浆爆炸。