Author name: Pascal Richet

Magma is the most important chemical transport agent throughout our planet. This paper provides an overview of the interplay between magma redox, major element chemistry, and crystal and volatile content, and of the influence of redox on the factors that drive igneous system dynamics. Given the almost infinite combinations of temperature, pressure, and chemical compositions relevant to igneous petrology, we focus on the concepts and methods that redox geochemistry provides to understand magma formation, ascent, evolution and crystallization. Particular attention is paid to the strong and complex interplay between melt structure and chemistry, and to the influence that redox conditions have on melt properties, crystallization mechanisms and the solubility of volatile components.

The oxidation–reduction (‘redox’) state is an important intensive property of any geologic system and is typically measured (and reported) as either the redox potential (Eh) or the oxygen fugacity (fO2). These two concepts cover the whole spectrum of geologic systems: from low-temperature aqueous and sedimentary systems to high-temperature rock-forming environments. The redox state determines the speciation of a fluid phase and exercises fundamental controls on phase relations and geochemical evolution. Here, we review the concepts that underpin the redox state and outline a framework for describing and quantifying the concept of the oxidation state.