2 Redox Processes Before, During, and After Earth’s Accretion Affecting the Deep Carbon Cycle
Vincenzo Stagno1 and Sonja Aulbach2
1 Department of Earth Sciences, Sapienza University of Rome, Rome, Italy
2 Institute of Geosciences, Goethe‐Universität, Frankfurt am Main, Germany
ABSTRACT The distribution of volatile elements between Earth’s core, mantle, and atmosphere over the last 4.57 billion years has been controlled through redox reactions involving multivalent elements like Fe in minerals and melts, with important implications for the speciation of elements like carbon, oxygen, and hydrogen as fluids or through dissolution in melts. The redox state of Earth’s interior was likely established during accretion of predominantly chondritic building blocks and late‐stage meteoritic impact and concurrent formation of the metallic core. In contrast to thermodynamic predictions of decreasing mantle redox state with depth, some melt and mineral inclusions trapped in diamonds down to the mantle transition zone and exhumed with carbonate‐rich magmas (kimberlites) reveal both the passage of oxidized CO2‐rich fluids and the occurrence of oxidized minerals. These are proof of the redox heterogeneities of the whole mantle that might have in part been inherited from early-stage processes but also testify to the recycling of surface-altered, oxidized materials, a process that became particularly efficient with Earth's transition to a stable plate tectonic regime. Here, we review the current knowledge of the (early‐)mantle oxidation state, how this might have changed through space and time, its effect on the genesis of CO2‐rich magmas, and also highlight unresolved issues requiring further work. 2.1. THE REDOX STATE OF PLANETARY INTERIORS AND THE SPECIATION OF CARBON IN THE EARTH The redox state of a system determines the oxidation state of multivalent elements contained therein (e.g., Fe2+ vs Fe3+) and is gauged as the partial pressure (fugacity) of oxygen (fO2). In the mantle, fO2 is buffered by the coexisting Fe‐bearing minerals mainly within peridotite rocks (Luth et al., 1990; Stagno et al., 2013). Oxygen fugacity also controls the speciation of multivalent volatiles like carbon and defines conditions under which reduced accessory minerals like carbide, graphite, and diamond can be oxidized to C4+. The consequent bonding with an O2 molecule causes lowering of the melting temperature of the locally surrounding rocks (i.e., redox melting; Taylor & Green, 1988; Stagno et al., 2013). Conversely, carbonate minerals and carbonated melts (carbonatites and kimberlites) can be reduced to elemental carbon as a consequence of the locally buffered fO2 by Fe‐bearing mineral assemblages (i.e., redox freezing; Rohrbach & Schmidt, 2011; Stagno et al., 2011; Jacob et al., 2016) that behave as an “oxygen pump” by reducing the surrounding environment. The redox state of the interior of Earth, as in other planets, is, therefore, expected to have modulated the speciation of volatile elements and set the conditions for the onset of melting of rocks at variable mantle depths to produce a large spectrum of magmas (Gudfinnsson & Presnall, 2005; Green, 2015). While some relatively cool and highly reactive small‐fraction carbonated melts may experience thermal and chemical “death” through interaction with the lithosphere (McKenzie, 1989), those that ultimately reach the surface and degas to the atmosphere set, in turn, the conditions for planetary climates and, ultimately, habitability of our planet (Williams et al., 1992; Martin et al., 2007; Gaillard et al., 2011; Brune et al., 2017; Lammer et al., 2018). To better understand the link between the redox state of planetary interiors and volatile cycles, it is worth recalling the astronomical observation of the super‐Earth 55 Cancri e (Madhusudhan et al., 2012), the interior of which was proposed to host mostly Fe, diamonds, and SiC with no evidence of free gaseous species, based on the measurements of astronomical parameters of mass and radius used to constrain the chemistry of the interior of exoplanets (Endl et al., 2012; Gillon et al., 2012). The abundance of reduced phases obviously inhibits the formation of oxidized gaseous phases, their circulation, and exsolution at any depth in the planetary interior, and also precludes the possibility of generation and migration of volatile‐rich magmas to the surface of a planet, due to the low solubility of reduced oxygen‐free volatile species in melts, such as elemental C. In contrast, the tectonic and volcanic activity on Earth through time appears to have been coupled to the speciation of oxidized volatiles like water and CO2 both in terms of chemical composition and erupted rate of mantle‐derived magmas (Tappe et al., 2018; Fuentes et al., 2019). So, while 55 Cancri e can be considered a reduced end member among the so‐far observed planetary bodies along the carbide‐carbonate joint, Earth’s mantle represents a relatively oxidized body that records a variable redox state in its interior, allowing the stability of volatiles in different forms. The oxidation state of planetary interiors, the volatile cycle, and, therefore, the role of carbon (and water) in the origin of magmas are all strictly linked with (i) the redox state and thermal conditions of the condensates where the planetesimals formed within the protoplanetary disks; (ii) the intrinsic oxygen fugacity (i.e., the oxygen fugacity inferred by the coexisting condensated minerals) of the building blocks; (iii) the chemical composition of the impactors during early and late accretion; and (iv) early‐planetary differentiation, in particular separation of a metallic core. These points are addressed in the following sections. 2.2. OXIDATION STATE OF EARTH’S BUILDING BLOCKS AND EARLY DIFFERENTIATION The redox state and its evolution in Earth’s interior are established as a result of (i) early accretion under the influence of variably oxidized building blocks and late‐stage meteoritic impact; (ii) magma ocean(s) formation, core segregation, and mantle crystallization; (iii) establishment of mantle convection exhuming early‐generated deep heterogeneities; and (iv) recycling of surface materials by (initially regional and episodic) subduction processes. 2.2.1. The Solar Nebula Redox State
The fO2 of Chondrites.
The redox condition of the early accretion was set by the Solar Nebula environment and the intrinsic fO2 of the building blocks. The in meteorites represent the first crystalline materials that condensated in equilibrium with the gaseous Solar Nebula (Bouvier & Wadhwa, 2010). These inclusions are made of refractory minerals like spinel, fassaite‐like, melilite, perovskite, and anorthite, formed according to their condensation temperature (Grossman, 1972). They record, therefore, the redox state of the Solar Nebula, which is defined as the fO2 at which the Solar Nebula vapor phase and the condensed mineral assemblage were equilibrated. Important fO2 indicators are represented by those elements with multiple oxidation state (e.g., Fe and Ti). The relative abundance of the species with different oxidation state (e.g., the Ti3+/Ti4+ ratio in pyroxene) correlates with the fO2 largely buffered by the relative abundances of carbon, oxygen, and hydrogen in the cosmic gases. The application of the Ti3+/Ti4+ ratio in pyroxenes is based on the experimental calibration of the following equilibria (Grossman et al., 2008) (2.1) and (2.2) These two oxy‐thermometers resemble the assemblage found in CV3 Allende meteorite and allow us to determine
