Lu-Hf systematics of the ultra-high temperature Napier Complex, East Antarctica

Abstract

The Napier Complex comprises some of the oldest rocks on earth (~3.8 billion years old), overprinted by an ultra-high temperature (UHT) metamorphic event near the Archean&#61485

Proterozoic boundary. Garnet, orthopyroxene, sapphirine, osumilite, rutile and a whole rock representing a fully equilibrated assemblage from this UHT granulite belt have yielded a Lu&#61485

Hf isochron age of 2,403 +/- 43 Ma. Preservation of the UHT mineral assemblage in the rock analyzed suggests rapid cooling with closure likely to have occurred for the Lu-Hf system at post-peak UHT conditions near a temperature of ~800C. Zircon eHf values we have measured "see through" the UHT metamorphism and show that the source materials for the magmas that formed the Napier Complex were extremely depleted (> +5.6 eHf at 3.85 Ga) relative to the chondritic uniform reservoir (CHUR). These results also suggest significant depletion of the early Archean mantle, in agreement with the early differentiation of the earth that the latest core formation models require.