1 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet2 Universite Blaise Pascal3 Centre National de la Recherche Scientifique, Aix-Marseille Université4 Ecole Normale Supérieure de Lyon5 Universite Blaise Pascal6 Natural History Museum of Denmark, Natural History Museum of Denmark, Faculty of Science, Københavns Universitet
The composition and evolution of the silicate Earth during Hadean/Eoarchean times are widely debated and largely unknown due to the sparse geological record preserved from Earth's infancy. The short-lived Sm-146-Nd-142 chronometer applied to 3.8-3.7 Ga old mantle-derived amphibolites from the Isua Supracrustal Belt (ISB) in southwest Greenland has revealed ubiquitous Nd-142 excesses in these rocks compared to modern samples and terrestrial Nd standards. Because the parent isotope, Sm-146, was extant only during the first few hundred million years of Solar System history, this implies derivation of the Greenland samples from a source formed in the Hadean. This mantle source is the oldest yet identified on Earth and therefore provides key information about the nature and evolution of early-differentiated reservoirs. In contrast, modern mantle-derived rocks from around the world do not have Nd-142 anomalies, suggesting that the primordial heterogeneities detected in Earth's early mantle have been erased over time. In order to better constrain the rate at which early mantle heterogeneities have been re-homogenized, we produced new Sm-146-Nd-142 data for both 3.8 and 3.3 Ga old mafic rocks from different tectonic domains of the ISB, accompanied by their corresponding Sm-147-Nd-143 and Lu-176-Hf-176 systematics. The 3.8 Ga suite yields Nd-142 excesses comparable to those detected previously in 3.7 Ga old ISB amphibolites, indicating that Eoarchean mafic ISB Iavas originated from sources with similar differentiation histories despite being from different juxtaposed tectonic segments. Conversely, 3.3 Ga old amphibolites from the ISB do not show resolvable Nd-142 anomalies compared to terrestrial Nd standards. Since Rizo et al. (2012) reported Nd-142 anomalies in 3.4 Ga old ISB samples, the present data suggest that the primordial Nd-142 heterogeneities in the Isua mantle disappeared between 3.4 and 3.3 Ga. The present data set consists of samples from a unique location where 500 million years of history of the early terrestrial mantle have been preserved, hence offering an exceptional opportunity to gain new insight into the compositional evolution and dynamic workings of Earth's primordial mantle. (C) 2013 Elsevier B.V. All rights reserved.
Earth and Planetary Science Letters, 2013, Vol 377-378, p. 324-335
Nd-142; Hadean; Archean; mantle dynamics; Isua Supracrustal Belt