June 2020

Download Article (PDF) June 2020 Issue Table of Contents The redox state of Earth’s atmosphere has undergone a dramatic shift over geologic time from reducing to strongly oxidizing, and this shift has been coupled with changes in ocean redox structure and the size and activity of Earth’s biosphere. Delineating this evolutionary trajectory remains a major…

Electron transfer in the critical zone is driven by biotic and abiotic mechanisms and controls the fate of inorganic and organic contaminants, whether redox-sensitive or not. In these environments, Fe- and Mn-bearing minerals, as well as organic matter, are key compounds. They interact with each other and constitute important electron shuttles. As a result, not only their solubility but also their structure controls the mobility of many essential and toxic elements. In addition, microorganisms that form hot spots and are widespread in environmental systems are also primordial players in electron transfer processes by acting as a catalyst between an electron donor and an acceptor, and through their contaminant detoxification metabolism.

The redox (reduction–oxidation) potential is an essential variable that controls the chemical reactions of fluids in magmatic and associated geothermal systems. However, the evolution of the redox potential is difficult to trace from a magma’s source at depth to the surface. The key is knowing that electron transfer is the twin face of the acid–base exchanges that drive charge transfer in the many reactions that occur in multiphase and chemically complex systems. The deduced redox reactivity can reveal many features about the evolution of a system’s composition and the external factors that control it. As such, redox potential analysis is an important geochemical tool by which to monitor volcanoes and to explore geothermal systems.

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 Earth is a unique rocky planet with liquid water at the surface and an oxygen-rich atmosphere, consequences of its particular accretion history. The earliest accreting bodies were small and could be either differentiated and undifferentiated; later larger bodies had formed cores and mantles with distinct properties. In addition, there may have been an overall trend of early reduced and later oxidized material accreting to form the Earth. This paper provides an overview—based on natural materials in our Earthbound sample collections, experimental studies on those samples, and calculations and numerical simulations of differentiation processes—of planetary accretion, core–mantle equilibration, mantle redox processes, and redox variations in Earth, Mars, and other terrestrial bodies.

The interior of the Earth is an important reservoir for elements that are chemically bound in minerals, melts, and gases. Analyses of the proportions of redox-sensitive elements in ancient and contemporary natural rocks provide information on the temporal redox evolution of our planet. Natural inclusions trapped in diamonds, xenoliths, and erupted magmas provide unique windows into the redox conditions of the deep Earth, and reveal evidence for heterogeneities in the mantle’s oxidation state. By examining the natural rock record, we assess how redox boundaries in the deep Earth have controlled elemental cycling and what effects these boundaries have had on the temporal and chemical evolution of oxygen fugacity in the Earth’s interior and atmosphere.

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.