Geochemistry and Nd isotopic characteristics of Earth's Hadean mantle and primitive crust

The Isua supracrustal belt (ISB) and the Nuvvuagittuq greenstone belt (NGB) are among the oldest suites of mafic volcanic rocks preserved on Earth and are the best candidates for representing its early crust. Despite the possible 500 Ma age difference between the belts, these mantle -derived rocks show compositional similarities, with features resembling rocks formed in subduction initiation environments. With the addition of new Nd-142 data for the Garbenschiefer unit of the ISB reported here, high precision Nd-142 data are now available for all the mafic lithologies from both belts. Mantle -derived rocks from both the ISB and NGB belts exhibit a range of Nd-142/Nd-144 ratios. The datasets for the two belts, however, are significantly different, suggesting a different origin for their Nd-142 anomalies. Nearly all ISB samples have excesses in Nd-142, including the newly analyzed Garbenschiefer boninitic amphibolites (mean of +12 ppm). Excesses in Nd-142/Nd-144 compared to the Nd standard for all the ISB rocks range between +8 and +20 ppm, with a near Gaussian distribution around +12 ppm. This distribution could simply reflect the analytical error ( 5 ppm) around a single Nd-142/Nd-144 ratio indicating that the samples formed after the extinction of Sm-146 from a source with a nearly uniform Nd-142/Nd-144 ratio. In contrast, the NGB shows a range of Nd-142/Nd-144 ratios from +8 to 18 ppm relative to the modern Nd standard and displays a flat distribution of Nd-142/Nd-144 ratios, The ISB samples show no significant correlation between their Nd-142/144Nd and Sm/Nd ratios, consistent with their formation in the Eoarchean via melting of a Hadean depleted mantle. In contrast, all NGB samples display a Nd-142/144Nd vs. Sm/Nd correlation, consistent with their crystallization in the Hadean. The mantle sources for both the ISB and NGB mantle derived rocks have a similar Nd-142/144Nd ratio at the possible age of formation of the NGB (similar to 43 Ga) suggesting the derivation of ISB and NGB rocks from a common early-formed depleted mantle source formed between 4.47 and 4.42 Ga with a Sm-147/Nd-144 ratio similar to 0.218. This mantle appears to have been an important source component involved in the formation of the primitive crust during most of the Hadean and Eoarchean eons. (C )2016 Elsevier B.V. All rights reserved.