Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

Abstract

Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu– 176Hf, 147Sm– 143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf (3720) values from −0.7 to +1.5 and initial εNd (3720) values from +1.6 to +2.1. Initial εHf (3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd (3720) values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO 2, Al 2O 3/TiO 2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs (3720) values for some of the samples. In addition to the positive εHf (3720) values, the least disturbed samples exhibit positive εNd (3720) values and a co-variation of εHf (3720) and εΝd (3720) values. Based on these observations, it is argued, that the most depleted samples with initial εHf (3720) values of up to +12.9 and high 176Lu/ 177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined 142Nd– 143Nd isotope data for early Archean rocks.