The oldest zircons of Africa—Their U–Pb–Hf–O isotope and trace element systematics, and implications for Hadean to Archean crust–mantle evolution

Abstract

More than 450 detrital zircon grains from two Limpopo Belt quartzite samples were investigated by a combination of scanning electron imaging, U–Pb dating, δ18O, Lu–Hf isotope and trace element analyses in order to get robust information about the early Earth's crust–mantle evolution. The detrital zircon grains have crystallization ages between 3.95Ga and 3.18Ga, show ɛHft between +1 to −15 (±1 ɛ-unit), and δ18OVSMOW mostly between +5.5 and +8.1‰ (±0.2‰). Pristine zircon domains reveal Ti-in-zircon temperatures between 700 and 865°C, and Th/U of 0.3–2.3. Trace elements point to zircon formation in predominately granitoid rocks. Metamorphic zircon rims have ages ≤2.65Ga, ɛHf2.65Ga∼−15, δ18O=7.0–8.1‰, and Th/U mostly <0.1. Nine zircon grains define an ɛHft-age array (I), which starts from a chondritic uniform reservoir (CHUR) at about 4.5Ga, and requires 176Lu/177Hf=0.020, indicative for mafic crust. Most zircon grains, however, plot on or above an ɛHft-age array (II), which runs parallel to array I, 176Lu/177Hf=0.021, and starts from CHUR at 4.01Ga. Oxygen isotope compositions of δ18O>5.5 indicate that the magmatic host rocks of the zircons have been formed either by melting of altered mafic crust, which interacted with cold water prior to granitoid formation, and/or that ancient sedimentary and/or magmatic rocks were involved in the melting process. The new U–Pb–Hf–δ18O datasets together with compiled data from worldwide sources indicate a significant gap of about 5 epsilon units between arrays I and II. Furthermore, they illustrate that many Hadean zircon analyses plot well below array I, and some above CHUR. These findings support an interpretation that the Hadean Earth was covered by a long-lived, mafic protocrust, perhaps forming a partially open “stagnant lid”. This protocrust was affected by internal reworking, but also injected and overlain by (ultra)mafic rocks derived from chondritic and (highly) depleted mantle sources. At <4.3Ga, the mafic protocrust was locally transformed into a TTG crust, perhaps caused by enhanced lower crust foundering, related to enhanced volcanic resurfacing and secular cooling of the Hadean lithosphere. Eventually, this heterogeneous Hadean protocrust became completely substituted by a new crust, which started to evolve from the mantle at <4.01Ga