Magma Redox Geochemistry

Roberto Moretti, Daniel R. Neuville, Roberto Moretti, Daniel R. Neuville

• Publisher: American Geophysical Union
• Year: 2021
• Print ISBN: 9781119473251
• eBook ISBN: 9781119473244
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geophysics

Explores the many facets of redox exchanges that drive magma's behavior and evolution, from the origin of the Earth until today

The redox state is one of the master variables behind the Earth's forming processes, which at depth concern magma as the major transport agent. Understanding redox exchanges in magmas is pivotal for reconstructing the history and compositional make-up of our planet, for exploring its mineral resources, and for monitoring and forecasting volcanic activity.

Magma Redox Geochemistry describes the multiple facets of redox reactions in the magmatic realm and presents experimental results, theoretical approaches, and unconventional and novel techniques.

Volume highlights include:

• Redox state and oxygen fugacity: so close, so far
• Redox processes from Earth's accretion to global geodynamics
• Redox evolution from the magma source to volcanic emissions
• Redox characterization of elements and their isotopes

The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

1
Redox Equilibria: From Basic Concepts to the Magmatic Realm

Roberto Moretti^1,2 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 of metals. Transition metals and main group elements (e.g., N, halogens, O, S, C) have multiple oxidation states and important redox chemistry, which affect element distribution within the geochemical shells on Earth but also through the boundaries between such shells (e.g., Moretti et al., 2020a). For instance, it is the redox state of metals and ligands that complex them, which then determines (i) their “unlocking” from pristine reservoirs (e.g. minerals in which they occur at trace level); (ii) their mobility on Earth through carriers such as magma, water, or vapor; and eventually (iii) their accumulation and precipitations in new phases making up ore deposits.

Redox reactions involve a coupled transfer of electrons, so for any oxidation (loss of electrons) a recip...

Table of contents

• Cover
• Table of Contents
• Series Page
• Title Page
• Copyright Page
• List of Contributors
• Preface
• 1 Redox Equilibria
• Part I: Redox from the Earth’s Accretion to Global Geodynamics
• Part II: Redox at Work: From Magma Sources to Volcanic Phenomena
• Part III: Tools and Techniques to Characterize the Redox and its Effect on Isotope Partitioning
• Index
• End User License Agreement