Eoarchean low δ18O zircon indicates emergent land at 3.73 Ga

Abstract

The question as to whether there was emergent land in the Eoarchean (4.0–3.6 Ga) is fundamental not only for understanding Earth’s evolution but also for life itself. Magmatic rocks with isotopically light oxygen can indicate interaction of magmas or their sources with surface water (e.g. meteoric water). Consequently, the presence of such rocks in the geological record of the early Earth can provide an indicator for the emergence of land. Zircon from two Eoarchean orthogneisses in the Tula Mountains, Napier Complex, East Antarctica, show such light isotopic signatures. A ca. 3.75 Ga trondhjemitic gneiss and a ca. 3.55 Ga dioritic gneiss, both with high Y-HREE-Nb-Ta that can be ascribed to the melting of shallow sources at < 1.0 GPa, contain zircon with exceptionally low δ18O values of 1.0–2.7 ‰ (normalized to Vienna Standard Mean Ocean Water). The lowest δ18O value in zircon previously reported from Paleoarchean orthogneisses is 3.7 ‰ at 3.56 Ga. We show that although the generation of such isotopically light signatures can be achieved through mixing with seawater during a hydrothermal event, the proportion of oxygen from seawater would have to be exceptionally high. In contrast, only a small and more realistic proportion of water with much lighter δ18O, such as that characteristic of meteoric water would be required. These new results, therefore, are consistent with the presence of shallow magmatic or hydrothermal systems involving meteoric water at 3.73 Ga, providing the earliest known evidence for the emergence of land on Earth.