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Hadean Geology : From the lunar impact to the creation of the first continents

Since the discovery of isotopic anomalies in 142Nd in Archean rocks, we know that the primary crust was differentiated at approximately 4.45 Gy. It is also known that this proto-crust was preserved from remixing in the mantle during approximately 1 Gy. It is not known, however, whether this longevity reflects the average life of the proto-crust on the surface or its storage in the deep mantle. Nor is it clear whether this early differentiation is related to the crystallisation of a magma ocean or the establishment of the first continents. The difficulties in the reconstruction of this early period of Earth’s history are well known : rocks older than 3.8 Gy are rare, and their dating is often controversial. Thus, while we can date a zircon with an accuracy of a few million years, the dating of metamorphosed mafic and ultramafic rocks can present real difficulties. One of our primary goals is the development of methods suitable for the geochronological dating of ancient mafic rock. This work includes, in particular, the short-lived radioactivity 146Sm-142Nd and the development of "new" chronometers, such as the 40K-40Ca couple. To these technical developments will be added the geological exploration of the oldest cratons, which could potentially contain Hadean rocks. Our research is currently focused on the North Atlantic craton (Quebec, Labrador, and Greenland), where old supracrustal formations have been identified but whose age could not be determined precisely. We will also continue the geological exploration of the Nuvvuagittuq belt and attempt to resolve the current controversies concerning the age of the metabasalts.

Pillow lavas in the belt of Nuvvuagittuq (Quebec) : a seafloor that is 4.3 Gy old ? Photo : E. Thomassot