Differentiation of the Early Silicate Earth: Isotopic Constraints from the Hadean Rock Record – DESIR
Differentiation of the early silicate Earth
Exploring the Hadean Earth through an isotopic lens
From the Moon forming impact to the first continents
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 (Canup 2004), which likely created a global magma ocean (Benz & Cameron 1990, Melosh 1990), 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 (Mojzsis et al 2001), and the growth of the first continents in a geodynamic context perhaps similar to modern subduction zones (Harrison et al 2008, Hopkins et al 2008, Harrison 2009). It is also entirely plausible that none of these events took place: high temperatures in the Hadean mantle may have prevented the operation of plate tectonics (e.g. Rollinson 2007), and favored instead the emplacement of a static lithosphere (van Thienen et al 2004). Late-stage impacts may have also played an important role, by vaporizing the early oceans, and causing local or global resurfacing events that may have delayed the inception of the modern rock cycle.
The main challenge in testing these vastly different scenarios is that rocks formed during the first 800 Ma of Earth's history are almost entirely missing from the geological record (Condie 2000, Harrison 2009). Furthermore, recent studies in the Nuvvuagittuq supracrustal belt (Quebec, >3.75 Ga) suggests that ca. 4.36 Ga-old crustal components were locally preserved witin ancient cratons. However, in absence of a reliable chronometer for dating metamorphosed rocks that do not contain zircons, Hadean mafics and ultramafics are difficult to identify by conventional methods. By applying more selective tools such as the short-lived 146Sm-142Nd chronometer, we are now able to selectively identify rocks formed during the first 500 Ma of our planet, whether felsic, mafic or ultramafic. Our work builds on this recent progress, in order to address questions related to the early evolution of the mantle-crust system, the onset of the sedimentary cycle, and the chemical and isotopic composition of the Earth's primitive mantle.
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In this proposal, we outline a series of projects whose goal is to obtain a better understanding of the transitional epoch ranging from the Moon-forming impact to the formation of the first Archean cratons. This period, known as the Hadean Eon, has long remained a dark age due to the apparent lack of rocks older than 3.8 Ga. However, we are starting to find evidence, mainly on the basis of 146Sm-142Nd studies, for the presence of Hadean crustal components within ancient terranes. This proposal builds 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 terrestrial proto-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. These questions will be 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. Specifically, we will investigate using the 146Sm-142Nd system whether Hadean rocks made any contribution to the Archean sedimentary cycle, and/or were involved in the genesis of the most ancient cratons. We will also perform extensive chronological studies in the Nuvvuagittuq Greenstone belt using a combination 146Sm-142Nd, 40K-40Ca, U-Pb methods with the aim of establishing the timing of alteration and sedimentation in the belt. Finally, we will attempt to revisit the 87Rb-87Sr isotopic evolution of the Earth's mantle and define the initial 87Sr/86Sr composition of the Earth, in order to better constrain the provenance of the Moon-forming material. The proposed projects are based on our previous analytical developments (146Sm-142Nd, 40K-40Ca) and fieldwork (Nuvvuagittuq, Isua), and aim at extending these investigations to a new range of case studies, including the Zimbabwe and Slave cratons, the Hebron mafic/ultramafic complex, as well as newly discovered greenstone belts from South Greenland and Labrador.
Project coordination
Guillaume CARO (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE CENTRE-EST)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
CRPG CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE CENTRE-EST
Help of the ANR 220,428 euros
Beginning and duration of the scientific project:
September 2011
- 48 Months
