Differentiation of the Early Silicate Earth

PI : Guillaume Caro

Participants : Emilie Thomassot, Precillia Morino, Mathieu Lallement, Steve Mojzsis, Nicole Cates, Béatrice Luais, Yumi Kitayama

There is little question that the first 800 million years of Earth evolution, a period known as the Hadean Eon, was the most geodynamically active in our planet’s history. During this time, the Earth experienced a collision with a Mars-sized embryo, which likely created a global magma ocean, gave birth to the Moon, and provided the energy required for differentiating the first terrestrial crust. It has also been speculated that this period saw the emergence of life, the onset of the modern sedimentary cycle, and the growth of the first continents in a geodynamic context perhaps similar to modern subduction zones. While the Hadean Eon has long remained a dark age due to the apparent lack of rocks older than 3.8 Ga, we are starting to find evidence, mainly on the basis of 146Sm-142Nd studies, for the presence of Hadean crustal components within ancient terranes of Northern Canada. Our current projects build on this recent technical progress in order to address fundamental questions related to the very early evolution of our planet. These include : (1) The fate of the first terrestrial crust, its composition, and its involvement in the Archean rock cycle, (2) The onset of sedimentation and alteration processes on the young Earth, and (3) The composition of the Earth’s primitive mantle and the Earth-Moon system. These questions are addressed by fieldwork and exploration in localities known to contain Hadean rocks, combined with novel analytical developments aimed at extracting specific chronological and compositional information from this Hadean record.

Hadean ultramafic rocks near Inukjuak (Nunavik, Quebec, Canada)

Hadean metabasalts and tonalites in the Nain Province (Nunatsiavut, Labrador, Canada)

A summary of 142Nd data in early Archean rocks (Figure from Caro (2011)). Positive anomalies in early Archean rocks are inherited from a mantle reservoir that experienced differentiation 4.4 Ga ago. NGB amphibolites with negative anomalies, when plotted against their Sm/Nd ratio, define an isochron age of 4.28 Ga. Only three localities have been investigated outside the West Greenland craton. Two of these yielded measurable 142Nd anomalies, both positive and negative, suggesting long-term preservation of Hadean mantle and crustal components.