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Small continents on the Hadean Earth: New Isotopic Evidence from combined Hf and 142Nd Isotopic Signatures

Vickie C. Bennett1, Joe M. Hiess1, Alan D. Brandon2 and Allen P. Nutman3

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
2 NASA Johnson Space Center, Houston, TX 77058, USA
3 Chinese Academy of Geological Sciences, Beijing 100037, P.R. China

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Questions of the antiquity and volumes of Earth's ancient continental crust have been actively debated for more than 100 years and remain controversial. One view is that massive amounts of continental crust formed very early in Earth's history and persisted to the present, with new crustal growth balanced by recycling into the mantle. In direct contrast are models calling for progressive growth of the continental mass over geologic time, with small volume early continents. These debates take on added significance when considered in conjunction with potential early life habitats and the timing and mechanisms of formation of Earth's major chemical domains after the initial stages of planetary accretion.

We have taken a new perspective on Earth’s early history by combining information from two isotopic systems (176Hf/176Hf formed from decay of 176Lu; half-life=37.1 Gy) and 142Nd/144Nd (formed by decay of now extinct 146Sm; half life =103 My), whose signatures are preserved in the oldest rocks.  The analysed samples include 3.63 -3.87 Ga (billion years old) rare, early crustal relicts from southwest Greenland, Western Australia, and China. Hafnium isotopic data from well-characterised and U-Pb dated zircons extracted from these rocks show that the oldest rocks have initial Hf isotopic compositions that are the same as bulk Earth and primitive meteorite compositions (Hiess et al, in review), indicating that the source of these rocks did not experience early Lu/Hf modification. In contrast the 142Nd isotopic compositions measured from the same rocks as yielded the zircons, are distinct from both modern rocks and from primitive meteorites (Bennett et al., 2007) requiring Hadean Sm/Nd fractionation.

Modelling of the trace element pattern of the Hadean (>4.0 Ga) mantle, as defined from the combined isotopic data, shows that it could not have been generated by extraction of average low Sm/Nd, low Lu/Hf continental crust (Figure 1). This is in contrast to the modern Earth, where the continental crust and the upper mantle have complementary isotopic and trace element characteristics. Thus, continent formation could not have been the primary mechanism of differentiation on the Hadean Earth. Furthermore, owing to the effectiveness of crust formation at changing Lu/Hf ratios, the near-chondritic Hf isotopic data require that the preserved earliest continents were of very limited extent, likely less than 5% of the present day continental mass. The chemistry of the Hadean mantle as recorded by the short and long half-life isotopic systematics of the oldest rocks points towards models of early silicate differentiation in a global terrestrial magma ocean.

Bennett, VC, Brandon AD and Nutman AP (2007) Coupled 142Nd-143Nd Isotopic Evidence for Hadean Mantle Dynamics. Science 318: 1907-1910.
Hiess J, Bennett VC, Nutman AP. and Williams IS (In revision) In situ U-Pb, O and Hf isotopic compositions of zircon from Eoarchaean rocks, southern West Greenland: New Views of old Crust. Geochimica et Cosmochimica Acta.