In situ XANES study of the influence of varying temperature and oxygen fugacity on iron oxidation state and coordination in a phonolitic melt
In situ XANES study of the influence of varying temperature and oxygen fugacity on iron oxidation state and coordination in a phonolitic melt
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DOI：
10.1007/s00410-020-01701-4
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发表时间：
2020-06
影响因子：
3.5
通讯作者：
C. Le Losq;R. Moretti;C. Oppenheimer;F. Baudelet;D. Neuville
中科院分区：
文献类型：
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作者：
C. Le Losq;R. Moretti;C. Oppenheimer;F. Baudelet;D. Neuville
岩浆中铁的氧化状态和环境影响着岩浆的相图及其性质，包括粘度和密度，从而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已经使我们对铁在熔体中的作用有了深入的了解，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，我们在大气压力下对自然熔体成分（来自南极洲埃里伯斯火山的phonolite熔岩）进行了一系列动态不平衡实验，其中氧气逸度和温度被控制和改变。在实验过程中，我们使用Fe K-edge色散x射线吸收光谱（XAS）连续测量了铁的氧化和配位。我们发现，铁氧化态的变化在phonolite熔体是可逆的，并被现有的模型很好地再现。在岩浆温度（~ 1000℃）下，铁的氧化态变化是由碱性阳离子和氧阴离子的联合扩散驱动的。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重要的氧气交换。至于铁配位，虽然Fe2+和Fe3+主要以平均五重配位形式存在，但可以检测到与氧化还原变化解耦的复杂配位变化。数据表明，在还原或氧化之前，Fe3+在四倍和六倍配位之间发生转变。这就质疑了铁配位变化在岩浆上升过程中触发磁铁矿纳米石结晶，并通过这种结晶事件促进岩浆爆发的可能含义。
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 Fe2+and Fe3+are present mostly in an average five-fold coordination, complex coordination variations decoupled from redox changes were detected. The data suggest transitions between Fe3+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.