Université du Québec à Montréal (UQAM) / Geotop: U-Pb geochronologist
davies.joshua@uqam.ca
Origin and evolution of continental crust
Much of this work is done in collaboration with my friend and colleague Assistant Professor Jesse Reimink at Penn State. We did our PhD's together at the University of Alberta, and have continued to collaborate during our Postdocs at Geneva (me) and Carnegie (Jesse), before we took up faculty positions in Canada and the USA.
This photo (left) is Jesse and I standing on the Idiwhaa unit, the oldest known part of the Acasta Gneiss, which represents the Earliest known piece of continental (felsic) crust on earth.
High precision U-Pb dating of zircon from the Idiwhaa tonalite unit, part of the Acasta Gness, confirms that it is Hadean in age. Hf isotopes analysis of the dated zircon also suggests that it may have been formed by melting of much older mafic crust.
Using high precision U-Pb dating of zircon, in combination with Hf isotope analysis of the same zircon crystals allows us to gain insights into the origin of the oldest continental crust preserved on earth.
Click on picture for paper
A comparison between the chemistry of the zircon crystals from the Acasta gneiss and those from the Jack Hills conglomerate in Australia (which contains the oldest zircon crystals known), suggests that they may have been formed in similar environments. Our analysis suggests that zircon crystals from ancient gneisses may have distinct geochemical signatures and that their chemistry reflects that of their host rock. We suggest that the formation mechanisms for the rocks of the Acasta Gneiss could work perfectly to explain the formation of the host rocks from which the Jack Hills zircons were eroded.
Click on picture for paper
By investigating the published detrital zircon record, we have attempted to uncover when the continental crust became significantly exposed above sea level.
The exposure of the continents above sea level should result in the development of large riverine catchments and therefore a larger and more complex age distribution of zircon crystals found in the associated sediments. We used statistical tools to determine when the detrital zircon records became more complex, which we relate to the rise and exposure of the continents above sea level.
The time when the continents first became exposed is significant since continent exposure would have been coincident with the development of different types of erosion and weathering which would transport nutrients into the oceans facilitating the development of complex life.
