Whether Earth’s early continental crust was formed by subduction of oceanic plates or other processes (e.g., volcanic resurfacing, greenstone sagduction and giant impact) in a stagnant-lid tectonic regime is a fundamental question to understanding Earth’s dynamic evolution and habitability. Phanerozoic arc magmas formed by subduction are characterized by high oxygen fugacity (fO2) and water (H2O) content, but these variables are difficult to quantify for Archean rocks due to ubiquitous metamorphism. We propose a magma oxybarometer-hygrometer based on a combination of the zircon Ce4+/Ce3+ oxybarometer (Smythe and Brenan, 2016) and the zircon Ce-U-Ti oxybarometer (Loucks et al., 2020) to simultaneously determine magmatic fO2 and H2O content of zircon-bearing igneous rocks. The zircon Ce4+/Ce3+ oxybarometer is very sensitive to equilibrium magma composition, especially H2O content, so that the fO2 obtained from the zircon Ce-U-Ti oxybarometer can be used to retrieve the H2O content. Our calibration using magmas with well-known H2O content indicates that this zircon oxybarometer-hygrometer is able to retrieve the median magmatic H2O content within ~1% uncertainty. We apply this oxybarometer-hygrometer to Archean granitoids (dominantly TTGs) worldwide (Acasta, Greenland, Barberton, Superior, Yilgarn and Tarim). Our results show that most Archean granitoid magmas have fO2 around the FMQ buffer (median ΔFMQ+0.1 ± 1.1, 2SD), which are significantly more reducing than Phanerozoic arc magmas. However, because the Archean ambient mantle were likely more reducing than the Phanerozoic counterpart (e.g., Aulbach and Stagno, 2016; Gao et al., 2022), there is at least one log unit increase in fO2 from the Archean ambient mantle-derived magmas to Archean granitoid magmas. More importantly, most Archean granitoids had high magmatic H2O contents (≥6 – 10 wt%) and high H2O/Ce ratios (≥1000), similar to Phanerozoic arc magmas. We find that magmatic fO2, H2O contents and H2O/Ce ratios of Archean granitoids positively correlate with geochemical proxies of magma formation depth (e.g., Sr/Y and La/Yb ratios), indicating a deep origin of the H2O and presence of excess H2O in the magma sources. These observations are difficult to reconcile with non-subduction models of crustal formation, but can be readily explained by subduction of hydrated oceanic slabs. We note a rise in magmatic fO2 and H2O content between 4.0 – 3.6 Ga, which coincides with a positive shift in zircon Hf isotopic compositions (Bauer et al., 2020), indicating onset of large-scale subduction during the Eoarchean.

Reference(s)

Aulbach, S., Stagno, V., 2016. Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle. Geology 44, 751-754 

Bauer, A.M., Reimink, J.R., Chacko, T., Foley, B.J., Shirey, S.B., Pearson, D.G., 2020. Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonics. Geochemical Perspectives Letters 14, 1-6 

Gao, L., Liu, S., Cawood, P.A., Hu, F., Wang, J., Sun, G., Hu, Y., 2022. Oxidation of Archean upper mantle caused by crustal recycling. Nature Communications 13, 3283 

Loucks, R.R., Fiorentini, M.L., Henríquez, G.J., 2020. New magmatic oxybarometer using trace elements in zircon. Journal of Petrology 61, egaa034. 

Smythe, D.J., Brenan, J.M., 2016. Magmatic oxygen fugacity estimated using zircon-melt partitioning of cerium. Earth and Planetary Science Letters 453, 260-266.