Contrasting messages from sulfur and oxygen isotopes on the origin of Eoarchean crust

Mantle-like oxygen isotope compositions in zircons from Eo- and Paleoarchean tonalite-trondhjemite-granodiorites (TTG) suites have been taken as evidence for the formation of early continental crust from juvenile hydrated basalts that formed in a plateau-like setting [1,2]. Other studies found elevated oxygen isotopes in zircons from >3.6 Ga TTGs in line with recycled crustal components in the sources (e.g., 3). Recent studies have shown that Eoarchean TTGs, amphibolites and mantle peridotites from the Itsaq Gneiss Complex (IGC) of SW Greenland bear small but significant mass independent sulfur isotope (MIF-S) anomalies in cap33S and cap36S (e.g., 4). These can only be explained by the incorporation of surface-derived sulfur of Archean age into the mantle and crustal sources. For the same suite of samples, oxygen isotope compositions in zircon and olivine overlap with mantle values, providing no indication for recycled material in the sources and therefore no evidence of early subduction processes. MIF-S anomalies have been observed in almost all Eoarchean magmatic rocks analyzed so far, which is a strong indication of a wide distribution of subduction processes in operation in the IGC. We argue that the oxygen isotope composition of zircon in TTGs may represent a mixed signal from different mafic source rocks and may not have the same resolution for surface-derived material compared to sulfur isotopes.

[1] Smithies, H.S. et al. (2021) Nature 592, 70-75. [2] Næraa, T. et al. (2012) Nature 485, 627-630. [3] Vezinet, A., et al. (2019) Geology 47(7), 605-608. [4] Lewis et al. (2021) EPSL 576, 117218.