Magma Redox Geochemistry. Группа авторов

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of redox exchanges in higher temperature geological systems, geoscientists turned their attention to molecular oxygen transfer among molecular components such as oxides or mineral‐like macromolecular entities.

      The practice between geoscientists becomes to assess criteria for fO2 (or aO2) estimations disconnected from the formal description of the acid–base character of magmas. In particular, techniques were established involving mineral phases coexisting in igneous rock to establish thermodynamic or empiric laws and trends from quenched glasses via indirect measurements, most often of spectroscopic nature (e.g., Neuville et al., 2020). This change of perspective reflects the obvious consideration that geoscientists deal with samples (solidified rocks) made accessible at Earth’s surface and which represent the final snapshots at the end of a long thermal and chemical evolution, whose a posteriori reconstruction is the objective of the geochemical (lato sensu) investigation.

      We may then say that for practical reasons geoscientists remained anchored to the original Lavoisier‐like definition of oxidation occurring in combustion processes, related to the exchange of oxygen molecules. The fact that most of the chemical analyses were from techniques in which oxygen was not directly determined but allowed to give oxides has also further favored these approaches.

      In this framework, a mutual exchange of knowledge has always characterized the field of geochemistry and petrology on one side and that of metal extraction in metallurgy in the other. Relations of the type

      (1.46)equation

      with R the universal gas constant and A and B constants.

      To compare the relative stabilities of the various oxides, the Ellingham diagram is prepared for oxidation reactions involving one mole of oxygen. For the oxidation of a metal, ΔG0 represents the chemical affinity of the metal for oxygen. When the magnitude of ΔG0 is negative, the oxide phase is stable over the metal and oxygen gas. Furthermore, the more negative the value, the more stable the oxide is. The Ellingham diagram also indicates which element will reduce which metal oxide. The similarity between the electromotive force series (E0) and the Ellingham diagram, which rates the tendency of metals to oxidize, should be easily recognized.

      When both Me and Meν+2/νO in Reaction 1.45 are in their standard states, the equilibrium constant, K45, corresponding to this reaction can be expressed as:

      Schematic illustration of (a) Ellingham diagram for the main components of the slag (solid lines) and possible reducing agents (dotted lines). Schematic illustrations of (b) the full Ellingham diagram for some relevant oxides including CO2 equilibria and normographic scales for oxygen fugacity and related quantities via CO/CO2 and H2/H2O ratios at Ptot equals 1 bar.

      Modified from Hasegawa (2014). All reactions’ components are considered in their pure stable phase t 1 bar and T of interest.

      Reaction 1.45 illustrates in fact how pairs of metals and their oxides, both having unitary activity, can be used as redox buffers, such that fO2 values can be easily fixed at any temperature. Even in the presence of a third phase, such as silicate melts or any other liquid, gas–solid assemblages allow a straightforward application of Equation 1.47 to impose fO2, unless solid phases are not refractory, and dissolve other components exchanges with the coexisting liquid. Reaction 1.1, involving metal iron and wustite, is a typical example (so‐called IW buffer) of one of these gas–solid equilibria fixing fO2.

      In order to illustrate the effect of the fluid phase, refined versions of the Ellingham diagrams

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