Abstract
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.
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Acknowledgements
We thank Dominique de Ligny and Yves Moussallam for assistance with XANES experiments. We also thank Hans Keppler, Margaret Hartley and two anonymous referees for their constructive comments that led to significant improvements of the manuscript.
Funding
We acknowledge SOLEIL (Gif sur Yvette, France) for provision of synchrotron radiation facilities (proposal 20101038). CLL acknowledges support received from the Australian Research Council Laureate Fellowship (FL130100066) of Hugh St. C. O’Neill as well as from the Chaire d’Excellence of the University of Paris during data processing and manuscript preparation. CO acknowledges support from the Natural Environment Research Council (Grant NE/N009312/1).
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CO collected the samples for analysis. CLL, RM, CO and DN designed the study. CLL, RM, FB and DN performed the XANES experiments. CLL processed the data and drafted the manuscript. All authors contributed to the final version of the manuscript.
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Le Losq, C., Moretti, R., Oppenheimer, C. et al. In situ XANES study of the influence of varying temperature and oxygen fugacity on iron oxidation state and coordination in a phonolitic melt. Contrib Mineral Petrol 175, 64 (2020). https://doi.org/10.1007/s00410-020-01701-4
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DOI: https://doi.org/10.1007/s00410-020-01701-4