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 142 Nd data for the Garbenschiefer unit of the ISB reported here, high precision 142 Nd 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 142 Nd/ 144 Nd ratios. The datasets for the two belts, however, are significantly different, suggesting a different origin for their 142 Nd anomalies. Nearly all ISB samples have excesses in 142 Nd, including the newly analyzed Garbenschiefer boninitic amphibolites (mean of + 12 ppm ). Excesses in 142 Nd/ 144 Nd 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 142 Nd/ 144 Nd ratio indicating that the samples formed after the extinction of 146 Sm from a source with a nearly uniform 142 Nd/ 144 Nd ratio. In contrast, the NGB shows a range of 142 Nd/ 144 Nd ratios from +8 to − 18 ppm relative to the modern Nd standard and displays a flat distribution of 142 Nd/ 144 Nd ratios. The ISB samples show no significant correlation between their 142 Nd/ 144 Nd 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 142 Nd/ 144 Nd 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 142 Nd/ 144 Nd ratio at the possible age of formation of the NGB (∼4.3 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 147 Sm/ 144 Nd ratio ∼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. [ABSTRACT FROM AUTHOR]