The search for the terrestrial protocrust

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Unlike the Moon and Mars, where heavily cratered original (primordial) crust is preserved, there is an apparent complete absence of rocks older than 4.05 billion years on Earth. Yet minerals as old as 4.35 billion years have been discovered, testifying to the former existence of a terrestrial protocrust. The formation and destruction of this crust are the most cryptic steps of the Earth's differentiation. There are, however, radiogenic isotope clues preserved in the oldest known rocks that were inherited, either via mantle depletion or crustal recycling, from the ancient but lost crust. Learning to read these clues will decipher the history of the primordial crust. Here I propose the first systematic study of the significance of one of these, the so called high mu (U/Pb) signature, which is expressed in the lead isotope systematics of Precambrian rocks in several shields, including Canada. Significantly, the high mu provinces (Slave, Yilgarn, Zimbabwe, SW Greenland, Wyoming, Winnipeg River Terrane) are those places on Earth where researchers also find the oldest known minerals, which in all cases significantly exceed the ages of the rocks. The rationale behind the proposed approach is two-fold. First, I propose to test whether there exists a relationship between the strength of the high mu character of a rock and the amount of ancient zircon in the rocks that carry the signature. The implication being that both were inherited from the no longer preserved protocrust. The research will employ rapid, in situ U/Pb zircon dating in a CFI-funded state-of-the-art facility on minerals selected from the highest mu late Archean gneisses and metasediments. Second, regardless of the zircon yield, the proposal also aims at studying whether there are any chemical variations that accompany the lead isotopic fingerprint, like on the Moon (also with a high mu crust). This will be studied along systematic transects across isotopic gradients, exposed e.g. in the Winnipeg River Terrane. A junior postdoctoral researcher and graduate students will be trained in this program aimed at strengthening Canada's position in early Earth research.