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PREFACE

In chemistry, redox issues occupy a central role for our understanding of how chemical exchanges take place between substances. Redox reactions include all chemical reactions in which atoms have their oxidation state changed because of the transfer of electrons between chemical species. Such a transfer involves both a reduction process and a complementary oxidation process and the consequent rearranging of chemical bonds.

The word oxidation originally implied reaction with oxygen to form an oxide, since dioxygen (O₂) was historically the first recognized oxidizing agent. Later, the term was expanded to encompass oxygen‐like substances that accomplished parallel chemical reactions. Ultimately, the meaning was generalized to include all processes involving loss of electrons. The word reduction originally referred to the loss in weight upon heating a metallic ore, such as a metal oxide to extract the metal. In other words, ore was “reduced” to metal. Antoine Lavoisier (1743–1794) showed that this loss of weight was due to the loss of oxygen as a gas. This etymological premise also summarizes the significance that for a long time the world of geology has been attributed to the concept of “redox.” In high‐temperature geology, where aqueous solutions can no more be defined, redox has an upmost importance to both the formation of minerals and the mobilization of metals. However, it is often intended as a measure of oxygen fugacity (fO₂), anchored to an assemblage of minerals or compounds which constrains its variations as a simple function of temperature.

The redox state is one of the master variables driving Earth‐forming processes and since the dawn of geochemistry, knowledge of redox state has been essential to understanding the compositional makeup of our planet and the fundamental processes occurring from the core up to the atmosphere. Most of these processes involve the major transport agent of matter on Earth, i.e., magma. The social and economic impact of redox geochemistry is enormous, because of the control played on metal 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