Isotopic systems hold the key to understanding the nature of surface environments in the Eoarchean (4.0-3.6 Ga). At that time, the Earth was mostly covered by oceans, with little evidence of emergent land (Rosas and Korenanga 2021). However, the emergence of land may have been a necessary condition for prebiotic chemical evolution and development of life (e.g. Van Kranondonk et al. 2021), as it controls the global climate feedbacks and supply of nutrients to the oceans. Sub-mantle δ18O (<4.7‰) in magmas provide evidence for interaction with meteoric water rather than seawater (Troch et al. 2020), because the δ18O of the latter is only ~5‰ lighter than typical mantle values, and thus would require high volumes to induce significant variations. Since δ18O in magma can be preserved without significant fractionation in magmatic zircon, the latter can be used as a proxy for magmatic δ18O. We investigated three >3.5 Ga orthogneisses with Eoarchean magmatic protoliths from the eastern Tula Mountains, Napier Complex, East Antarctica. Oxygen isotope compositions were analyzed on zircon domains previously dated by Król et al. (2020). Two gneisses were found to have high Y-HREE-Nb-Ta, consistent with the melting of crustal sources at pressures below 1.0 GPa. These rocks contain magmatic zircon with sub-mantle δ18O values of ca. +2 ‰. Mixture modelling suggests that to produce the observed signatures, either a large amount of seawater or a small and realistic amount of light δ18O water, such as meteoric water, needs to be provided. Similar low δ18O signatures have been found in hot-spot or extensional environments, including Yellowstone and Iceland, where meteoric water can interact with magmatic systems at shallow depths (Troch et al. 2020). The discovery of isotopically light δ18O values in >3.5 Ga zircon from the Napier Complex can therefore be attributed to the existence of shallow magmatic-hydrothermal systems driven by meteoric water in the early Archean. This, in turn, provides evidence for exposure of land to meteoric systems, indicating it must have emerged above sea level prior to 3.5 Ga.

Reference(s)

Król, P., Kusiak, M.A., Dunkley, D.J., Wilde, S.A., Yi, K., Lee, S. and Kocjan, I. (2020). Diversity of Archean crust in the eastern Tula Mountains, Napier Complex, East Antarctica. Gondwana Research, v. 82, p. 151-170. 

Rosas, J.C. and Korenanga, J. (2021). Archean seafloors shallowed with age due to radiogenic heating in the mantle. Nature, v. 14, p. 51-56. Troch, J., Ellis, B.S., Harris, C., Bachmann, O. and Bindeman, I.N. (2020). Low-δ18O silicic magmas on Earth: A review. Earth-Science Reviews, v. 208, p. 103299.

Van Kranendonk, M. J., Baumgartner, R., Djokic, T., Ota, T., Steller, L., Garbe, U. and Nakamura, E. (2021). Elements for the origin of life on land: a deep-time perspective from the Pilbara Craton of Western Australia. Astrobiology, v. 21(1), p. 39-59.