to be between ~–10 and –7 log units as normalized to the oxygen buffer (Fig. 2.1) (Allende Prieto et al., 2002). At these low fO2, metallic Fe stably alloys with Ni over a wide temperature range. However, the occurrence of fayalitic (i.e., Fe as Fe2+) olivine inclusions in chondrules of ordinary chondrites (Fe# of 0.15) implies the presence of oxidized regions in the Solar Nebula. This heterogeneity in fO2 of the Solar Nebula has been explained by the input of oxidized materials from beyond the snow line. This resulted in a local increase of the bulk oxygen content accounting for a fO2 of IW‐4.5, although an additional contribution of oxidized chondritic material cannot be discounted (Grossman et al., 2012; Rubie et al., 2015). A further fO2 increase by 5–6 log units is recorded by rims of CAI inclusions from some CV3 meteorites (e.g., Allende & Leoville meteorites; Wark & Lovering, 1977) showing a refractory assemblage (i.e. spinel, perovskite, hibonite, melilite, Al‐rich cpx, and forsterite) from which fO2 can be estimated through the application of Mg isotopes (i.e. 25Mg/24Mg) as a cosmo‐barometer, and the Ti3+/Ti4+ ratio in pyroxene. Such an fO2 increase was estimated to occur over 100–300k years and marks the formation of disks consisting of chondritic dust from which proto‐planets formed (Wark & Lovering 1977).
Figure 2.1 Range of oxygen fugacity estimates for Solar Nebula, achondrites, chondrites, and terrestrial planets (modified from Righter et al., 2016). The range of fO2 of the Martian mantle refers to the intrinsic measurements of fO2 in Martian basalts, while the fO2 of Earth’s mantle includes data for continental mantle xenoliths, abyssal peridotites, MORBs, arc lavas, and lamprophyres (Frost and McCammon, 2008).
The fO2 of Planetesimals and the Role of Volatiles.
Planetesimals of km‐size diameter formed by condensation and melting from the Solar Nebula. Thus, their chemical composition and oxidation state likely resemble those of the portion of Solar Nebula where they formed, taking into consideration the proposed correlation between heliocentric distance and H2/H2O and C/O ratios reflected in the oxidized nature of the planetesimals upon melting (Grossman et al., 2008). On the basis of N‐body simulations of particle interaction and planetary accretion, the bulk composition of planetesimals and their redox state change with respect to the heliocentric distances in the Solar Nebula. The bodies formed at a distance less than 1.1–1.7 AU exposed to higher temperatures were highly reduced at IW‐4 log units (high C/O ratio) at which all Fe would be stable as metal (plus some Si) whereas the bodies formed beyond 1.1–1.7 AU were more oxidized (low C/O and H2/H2O ratios) with a proportion of Fe as FeO dissolved in silicates (Rubie et al., 2015). Melting of the interior of planetesimals enhanced by C‐O‐H species might also contribute to variations of their redox state with consequent volatile depletion and oxidation (or reduction) of the surrounding mineral matrix before magma can reach the surface and degas (Fu et al., 2017). The variations in oxidation state of the planetesimals are inferred from the intrinsic oxygen fugacity estimated for various meteorite classes (Brett and Sato, 1984), as discussed in Section 2.2.1.3.
The Oxidation State of Earth’s Interior, Other Planets, and Meteorites.
Fig. 2.1 summarizes the estimated fO2 of the Solar Nebula, achondrites, and chondrites along with estimates of the fO2 for the interiors of planets including Mars, Mercury, Venus, Earth, and the Moon (Righter et al., 2016). Aubrites and achondrites are the most reduced materials, followed by ordinary chondrites and the oxidized carbonaceous chondrites. These partially overlap with the fO2 determined for Mercury, Martian basalts, and Earth’s mantle rocks; the latter, however, displays a uniquely broad range of values. Although there is geochemical evidence supporting bulk Earth’s origin by accretion from enstatite chondrite material (Javoy, 1995), the fO2 displayed by both appears quite different. Indeed, considerations of the Si isotopic variation and Mg/Si ratio of enstatite chondrites with respect to fertile mantle rocks (Fitoussi & Bourdon, 2012), as well as initial volatile content, are in favor of a multi‐component mixing model involving more than one type of material, with a dominating enstatite chondrite (EH or EL) component, ~30% of carbonaceous chondrites, ~24% achondrites (angrites or eucrites), and a minor amount of ordinary L‐type chondrites (Liebske & Khan, 2019). Importantly, it has been pointed out that Si isotopes are fractionated during core formation (e.g., Georg et al., 2007), nebular processes (Savage & Moynier, 2013), and evaporation during accretion (Pringle et al., 2014). Computational models suggest that the resulting fO2 after accretion must have been low enough (IW‐2) to allow core–mantle separation (Rubie et al., 2015; Schaefer & Fegley, 2017). Early Earth differentiation set a stratified redox interior, with a metallic core below and an FeO‐bearing silicate mantle above the IW buffer. Core‐mantle separation followed by magma ocean crystallization, meteorite impact processes (e.g., Late Heavy Bombardment) involving more oxidized impactors (“heterogeneous accretion”; e.g., O’Neill, 1991) and loss of primordial atmospheres are all likely to have played a major role in the gradula and stepwise oxidation of Earth’s interior (Armstrong et al., 2019), although tight quantitative constraints remain elusive. Recent models suggest that the escape of H2 from the early atmosphere must have been limited being the H2/H2O ratio < 0.3 in the gas released from the magma ocean, and this implies more oxidizing conditions (> IW+1) than those established during equilibrium with the core (Pahlevan et al. 2019).
2.3. MANTLE OXIDATION STATE OVER TIME AND ITS EFFECT ON THE C–O–H VOLATILE SPECIATION
The present‐day large oxygen fugacity range in Earth’s mantle (Fig. 2.1) reflects the evolution of the mantle through the pressure‐driven crystallization of the magma ocean to form Fe‐bearing minerals potentially able to set the fO2 locally (e.g., bridgmanite, ferropericlase, majorite, garnet; McCammon, 2005), trigger volatile outgassing and ingassing by recycling. Given that there is controversy regarding the dominant oxygen‐buffering species (e.g., Fe vs. C) even in the modern convecting mantle (e.g., Ballhaus & Frost, 1994; Cottrell & Kelley, 2013; Eguchi & Dasgupta, 2018), it is not surprising that to date it remains unclear how the redox state of the terrestrial mantle has evolved over time, and how this might have influenced volcanism, and ultimately, the atmospheric composition on Earth. Whether the chemical interaction between mantle rocks and volatile species like C played the most important role in determining conditions for the geological activity and habitability of our planet depends on the abundance of Fe (Fe0, Fe2+, and Fe3+) species compared to the abundance of C in its different forms (C0, CO, CO2, CH4) relevant in processes such as partial melting of rocks, melt migration, and diamond formation. However, three important variables influence the circulation of volatiles, among them carbon, in a planet: (i) the primordial volatile budget; (ii) the bulk oxygen fugacity of the early accreted terrestrial body; and (iii) the mineralogy of the planet’s interior. The deep volatile cycle in any planet is inextricably linked to changes of these variables through time.
2.3.1. The Redox State of the Upper Mantle and the Mobilization of Deep C
Carbon can occur in the Earth’s interior in different forms as elemental species (graphite and diamond), linked with iron as metal carbides (Fe3C, Fe7C3, etc.), in association with hydrogen (e.g., methane CH4 and a variety of aqueous organic and inorganic species) or in oxidized forms such as CO2 carbonates