The end of the Hadean: the world turns over

A U-Pb/Hf-isotope study of >16,000 zircons from sources worldwide has demonstrated that at least 70% of Earth's continental crust probably formed in Archean time, and much probably is >3 Ga old. The model-age (T_RD and T_MA) distributions of ca 500 low-Re sulfides in mantle-derived peridotite xenoliths, mainly from the Kaapvaal, Siberian and Slave cratons, peak between 2.5-3.0 or 3.0-3.5 Ga, depending on locality. In detail, at each locality the oldest T_MA model ages from the mantle sulfides correlate well with the oldest U-Pb ages and Hf model ages from crustal zircons. Younger T_MA peaks commonly coincide with later major crustal events. The sulfides in mantle xenoliths are secondary phases, and Os model ages probably are biased toward young ages. Most of the studied sulfides are from garnet-bearing peridotites, and garnet is generally a secondary phase in these rocks. In Siberian xenoliths, model ages of sulfides included in garnet are younger on average than those of sulfides included in olivine. However, detailed searches of the most depleted peridotites available have thus far revealed very few sulfides with model ages >3.5 Ga, just as the oldest widespread crustal ages are around 3.5 Ga. These data suggest that the oldest crust and the oldest, highly depleted SCLM are broadly coeval, and are interpreted as forming during massive mantle overturns that produced the residual Archean SCLM, providing buoyant "life rafts" that ever since have supported and preserved the continental crust. The 3.5 Ga overturn event changed Earth's fundamental tectonic behaviour, and truly marks the end of the Hadean period.