Magma Redox Geochemistry. Группа авторов

mobility, solubility, and availability by redox state of metals and ligands that may complex them, and because of the widespread use of redox indicators for environmental hazards assessment, such as the volcanic one, which assessment greatly benefits from studies on volcanic gas speciation, which in turn is controlled by redox. Volatile components and their redox control on volcanic degassing and metal mobility offer good examples of redox exchanges in high‐temperature environments close to human experience. Knowledge of redox mechanisms acting in volcanism and hydrothermalism have a great impact on the socioeconomic development of human societies because of their key role in volcanic hazard assessment, geothermal energy exploration, and ore deposits formation.

      As shown in this monograph, redox state is also an image of the magma composition, and the understanding of magma (particularly melt) physicochemical nature is the basic prerequisite to understand how redox exchanges work in deep Earth systems and to understand “who controls what.” The latter is a difficult task, given the almost infinite conditions of temperature, pressure, and chemical composition relevant to igneous petrology. However, the study of redox state and related properties cannot be reduced to simple rule of thumbs or assumptions such as the existence of stoichiometric mineral assemblages buffering oxygen fugacity via solid‐gas equilibria.

      Knowledge of the redox potential (or alternatively, oxygen fugacity) at which a rock forms and evolves is relevant for interpreting the rock’s history. However, the approach inherited by mineral chemistry has avoided for too long to assess the role of the major phase making up the magma: the silicate melt. Silicate melts have been very often considered as a simple reservoir of elements almost chemically inert and fully controlled by other phases (mainly solid) able to impose their redox state, (i.e., fO₂), which in turn was treated like a Maxwell demon. However, magma is the most important transport agent throughout our planet, buffering entire planetary sectors both thermally and chemically.

      This volume shows the multiple concepts and approaches useful to the study of the complex interactions occurring between melts, crystals, and fluids that are behind magma formation, ascent, and evolution. By joining the description of magma physical chemistry with geological issues, the chapters of this book disclose the multifaceted implications that redox variables and their gradients have on magma evolution in time and on the dynamics of planet Earth, or in other words, it brings to the reader’s attention the power of redox geodynamics.

      This volume provide a comprehensive overview and a state‐of‐the‐art treatment of technological and scientific advances in our understanding of redox geochemistry. Given the almost infinite conditions of temperature, pressure, and chemical composition relevant to igneous petrology and volcanology, the chapters represent a selection of topics able to give a unique picture of the “redox” continuum of the Earth’s interiors.

Part I is composed of chapters about oxygen fugacity in Earth’s accretion and across geodynamic settings, with a focus on the redox boundaries associated with mantle melting. Part II deals with the role of redox in magma differentiation, from the magma source up to the surface throughout volcanic processes, particularly degassing. Part III gives an overview on the tools and experimental and theoretical techniques to measure the redox state in melts and glasses and estimate the role of redox state on element and isotope partitioning; the major recent advances in understanding redox mechanisms affecting multivalent elements other than iron and innovative oxybarometers are presented.

      This volume is the result of the sessions “Linking the Redox State of Silicate Melts to Magmatic Processes” at the Goldschmidt Conference in Paris in 2017 and “Oxygen fugacity and redox mechanisms in high‐ to low‐temperature geochemical processes” at the AGU Fall Meeting in San Francisco in 2019. In writing up their papers, the authors have taken into consideration the discussions had during the two sessions.

      The editors wish to thank all authors for their contributions and also acknowledge the assistance of the reviewers, whose conscientious efforts helped the authors to improve the quality of the chapters in this volume. The editors also wish to thank the Institut de Physique du Globe (Université de Paris) for its support, Aline Peltier (Institut de Physique du Globe de Paris – Observatoire Volcanologique du Piton de la Fournaise) for the cover image, the Volcanology, Geochemistry and Petrology (VGP) division of the American Geophysical Union (AGU), and the Commission on Physics of Mineral (CPM) of the International Mineralogical Association (IMA).

       Roberto MorettiDaniel R. Neuville

       Université de Paris, Institut de Physiquedu Globe de Paris, France

1 Redox Equilibria: From Basic Concepts to the Magmatic Realm

       Roberto Moretti1,2 and Daniel R. Neuville1

       1 Université de Paris, Institut de Physique du Globe de Paris, Paris, France

       2 Observatoire Volcanologique et Sismologique de Guadeloupe, Institut de Physique du Globe de Paris, Gourbeyre, France

      ABSTRACT

      The basic aspects of redox geochemistry are reviewed to provide a useful compendium of the redox connection between the aqueous‐hydrothermal and igneous realms of Earth. The redox description of a system is intimately coupled to the knowledge of acid‐base properties of the solvent in which redox exchanges take place. For magmas, and then silicate melts, approaches reporting the redox state were so far cantered around the sole concept of oxygen fugacity, fO₂. Mastering the concept of fO₂ in experimental and observational petrology was the key to constrain the processes behind the very large range of relative oxygen fugacity observed on Earth. Although current descriptions of silicate melts and magma thermodynamic properties are mainly based on oxides or mineral-like molecular components, disregarding the actual melt reactivity poses many limits in our understanding of the true chemical exchanges involving oxygen, iron and the other redox-sensitive elements. Because silicate melts, unlike aqueous solutions, lack of a full acid-base description, compositional dependencies are solved by means of empirical treatments based on oxides and their combinations. However, these can bias the interpretation of redox exchanges recorded in analyzed samples and used to identify the several processes (e.g., batch or fractional crystallization, elemental recycling, degassing, deep fluid infiltration) which characterize magma evolution and its geodynamic environment. This short compendium aims at stimulating the quest for a comprehensive and unifying picture of the acid-base and redox properties of melts from which we could extrinsic its reactivity in way similar to aqueous solutions and molten salts.

1.1. GENERAL ASPECTS AND RATIONALE

      1.1.1. Oxidation Number, Electron Transfer, and Half‐Reactions

Oxidation‐reduction (redox) geochemistry studies those natural reactions occurring on Earth in which the transfer of electrons determines a change in the oxidation number of participating chemical species. Oxidation state (or oxidation number) measures the degree of oxidation of an atom in a substance and it is the (hypothetical) charge of an atom if all bonds to atoms of different elements were 100% ionic, with no covalent bond fraction. The oxidation state of an atom is indicated with Roman numerals, whereas Arabic numbers are used for the charge on compound (e.g., in SiO₄^4–, silicon has oxidation number IV and oxygen has –II, whereas 4– is the formal charge of the whole silicate ion). Charge and oxidation number are the same for monoatomic ions. The oxidation number can be positive, negative, or zero, and it can have a fractional value as well, and although very similar, it does not correspond to valency, which is an atom property and represents the number of bonds the atom needs to become stable, i.e., to complete either duplet or octet rule.

      Many redox reactions are familiar to us, such as fire and combustion, rusting, and dissolution