also from the preservation of anomalous compositions of the short‐lived isotopes, which must date back to processes occurring during the first several 100 Ma, in magmas erupted throughout the Archean and even to this day (e.g., Rizo et al., 2016; Horan et al., 2018). Factors possibly contributing to retarded mantle homogenization include grain size (Foley & Rizo, 2017) and the strength of bridgmanite under lower mantle conditions (Girard et al., 2016). Vertical mixing of oxidized material left after core formation would eventually have been aided by the establishment of plate tectonics (Andrault et al., 2018), bearing in mind that the transition to sustained plate tectonics occurred late (3.2–2.5 Ga; Cawood et al., 2019) relative to the onset of upward mixing, sometime between the post‐core formation establishment of fO2 in the uppermost mantle of ~ΔFMQ‐4.5 (Frost and McCammon, 2008) and the significantly more oxidizing source fO2 recorded by 3.5 Ga komatiites (Fig. 2.3b; Nicklas et al., 2018).
Superposed on the upward mixing of post‐core formation oxidized mantle regions would be heterogeneities due to late accretion and associated downward mixing of late‐accreted material, which are difficult to quantify. For example, the abundance and partitioning of moderately siderophile elements apparently requires addition of increasingly oxidized material (Wood et al., 2006), whereas late meteoritic bombardment with reduced components has been inferred based on the Ce‐in‐zircon redox proxy (Yang et al., 2014). Wang and Becker (2013) suggest that volatile‐rich objects were involved based on Se–Te systematics, whereas Fischer‐Gödde and Kleine (2017) call for a reduced inner solar system source based on Ru isotopes. Thus, more work is required to quantitatively model the effects of downward mixing of late‐accreted material on mantle fO2 and to accurately capture the rise of post‐core formation oxidized lower mantle regions in numerical models. Pinpointing this evolution is critical to constraining the composition of gases released from Hadaean and Archean magmas to the atmosphere, which has implications for a multitude of issues affecting habitability on Earth, from the production of prebiotic molecules to the Faint Young Sun paradox and finally, the transition to an oxygenated atmosphere.
ACKNOWLEDGMENTS
V.S. acknowledges financial support from Sapienza University of Rome through “Bandi di Ateneo 2016 and 2019.” The Deutsche Forschungsgemeinschaft is acknowledged for funding under grant AU356/10 to S.A. The authors are grateful to two anonymous reviewers for their constructive comments.
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