In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions, or equivalently the oxygen fugacities, at which a rock forms and evolves is important for interpreting its history. Iron, sulfur and manganese are relatively abundant crustal elements that occur in more than one oxidation state and are therefore widely used to reconstruct redox conditions.
Iron, the fourth most abundant element in the crust, exists as native iron, ferrous iron and ferric iron. The redox state of a rock affects the relative proportions of these oxidation states and hence determines both the minerals present and their compositions. If a rock contains pure minerals that constitute a redox buffer, the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity-temperature diagram.
Redox buffers were developed in part to control oxygen fugacities in laboratory experiments investigating mineral stabilities and rock histories. Each curve in the fugacity-temperature diagram corresponds to an oxidation reaction in a buffer. The buffers are listed here in order of decreasing oxygen fugacity at a given temperature, that is, from more oxidising to more reducing conditions in the plotted temperature range. As long as all the pure minerals or compounds are present in a buffer assemblage, the oxidising conditions are fixed on the curve for that buffer. Pressure has only a minor influence on these curves under crustal conditions.
The coefficients and products in these reactions depend on temperature and pressure, and calibrated thermodynamic data are normally used to calculate the curves rather than the simplified stoichiometric equations alone.
The ratio of ferrous to ferric iron in a rock partly determines its silicate and oxide mineral assemblage. In a rock of given bulk composition, iron enters minerals according to bulk chemistry and the phases stable at that temperature and pressure.
At conditions more oxidising than the MH buffer, at least much of the iron is likely to be ferric and hematite is a likely mineral in iron-bearing rocks. Iron may enter minerals such as olivine only as ferrous iron, ferric iron cannot enter the fayalite lattice. Magnesium and other elements in olivine, however, stabilise ferrous iron-bearing olivine to conditions more oxidising than those required for fayalite stability. Solid solution between magnetite and the titanium-bearing end member ulvöspinel enlarges the stability field of magnetite. Likewise, at conditions more reducing than the IW buffer, minerals such as pyroxene can still contain ferric iron. Redox buffers therefore provide only approximate guides to ferrous and ferric iron proportions in minerals and rocks.
Terrestrial igneous rocks commonly record crystallisation at oxygen fugacities more oxidising than the WM buffer and more reducing than about one log unit above the NiNiO buffer. Their oxidising conditions are thus not far from those of the FMQ buffer.
There are systematic differences that correlate with tectonic setting. Igneous rocks emplaced and erupted in island arcs typically record oxygen fugacities one or more log units more oxidising than the NiNiO buffer. In contrast, basalt and gabbro in non-arc settings typically record oxygen fugacities from about the FMQ buffer to about one log unit more reducing.
These differences reflect the oxidising character of subduction-zone fluids, the nature of magma sources and crystallisation-differentiation processes. High oxygen fugacity in arc magmas favours magnetite, amphibole and anhydrite and influences the speciation of sulfur and metal mineralisation.
Oxidising conditions are common in some depositional and diagenetic environments. The oxygen fugacity of the MH buffer at 25 °C is only about 10⁻⁷⁰, whereas the oxygen partial pressure of the Earth's atmosphere is about 0.2 atmospheres, so some sedimentary environments are far more oxidising than magmatic environments.
Other sedimentary environments, such as those forming black shale, are relatively reducing. Black shales are rich in organic matter and pyrite and record anoxic, organic-rich and sulfate-reducing conditions. Redox state affects organic-matter preservation, metal enrichment and early diagenetic mineral formation.
Oxygen fugacities during metamorphism can extend to higher values than in magmatic environments because some sedimentary rocks inherit more oxidising compositions. Nearly pure hematite occurs in some metamorphosed banded iron formations. In contrast, native nickel-iron occurs in some serpentinites and records strongly reducing conditions.
Metamorphic fluid composition, protolith redox state and metamorphic reactions all affect oxygen fugacity. Equilibria involving graphite, methane and carbonate are commonly used to estimate metamorphic fluid oxygen fugacity, while buffers such as magnetite-hematite and fayalite-magnetite-quartz constrain mineral-assemblage stability.
In meteorites, the iron-wüstite buffer may be more appropriate for describing oxygen fugacity. Meteorites formed in the reducing environments of the solar nebula and parent asteroids, and their assemblages include metallic iron, troilite, olivine and pyroxene, recording conditions from extremely reducing to relatively oxidising.
Sulfide minerals such as pyrite and pyrrhotite occur in many ore deposits. Pyrite and its polymorph marcasite are also important in many coal deposits and shales. These sulfides form in environments more reducing than the Earth's surface. When they contact oxidising surface waters, sulfides react, sulfate forms and the water becomes acidic and charged with a variety of elements, some potentially toxic. The consequences can be environmentally harmful, as in acid mine drainage.
Sulfur oxidation to sulfate or sulfur dioxide is also important in generating sulfur-rich volcanic eruptions, such as Pinatubo in 1991 and El Chichón in 1982. These eruptions contributed unusually large quantities of sulfur dioxide to the atmosphere, with effects on atmospheric quality and climate. The magmas were unusually oxidising, almost two log units more so than the NiNiO buffer, and anhydrite phenocrysts were present in the erupted tephra. In contrast, sulfides contain most of the sulfur in magmas more reducing than the FMQ buffer.
Redox buffers are important tools for understanding mineralisation, magmatic evolution and environmental geochemistry. The transport and precipitation of gold, copper, uranium and vanadium depend strongly on redox conditions, and acid mine drainage, radioactive-waste disposal and contaminant migration all require a redox framework. Constraining oxygen fugacity through mineral assemblages, fluid inclusions and isotopes allows geological processes to be reconstructed more accurately and environmental risks to be predicted.