Abstract

Recycling of supracrustal materials, and in particular hydrated rocks, has profound impacts on mantle composition and thus on the formation of continental crust because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain.

Here, we use zircon O isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal craton, southern Africa. The mean δ¹⁸O values of zircon from TTG (tonalite–trondhjemite–granodiorite) rocks abruptly increase at the Paleo-Mesoarchean boundary (ca. 3230 million years ago; Ma), from mantle zircon values of 5–6‰ to approaching 7.1‰, and this increase occurs in the ≤ 3230 Ma rocks with elevated Dy/Yb ratios.

The ¹⁸O enrichment is a unique signature of low temperature water-rock interaction on the Earth's surface. Because the later phase was emplaced into the same crustal level as the older one and TTG magmas would derive from melting processes in the garnet stability field (>40 km depth), we suggest that this evident shift in TTG zircon O isotopic compositions records the onset of recycling of the mafic oceanic crust that underwent the seawater hydrothermal alteration at low temperature.

The onset of enhanced recycling of the supracrustal materials may also develop elsewhere in other Archean cratons and reflects a significant change in tectonic realm during craton formation and stabilization, which may be important processes for the operation of plate tectonics on early Earth.