The recognition of variations in extinct nuclide signatures persistent in ancient terrestrial rocks provided a new window into early Earth processes from accretion to the start of the rock record. Additionally, these Hadean formed signatures are powerful tracers of subsequent chemical reservoir mixing and in combination with other geochemical data, illuminate crustal differentiation processes. Variations in 142Nd where both the parent (146Sm; half-life = 103myr) and daughter are lithophile elements record silicate differentiation events occurring in the first ca. 300 ma of Earth history. Unlike for siderophile/chalcophile element signatures, e.g. 182W, mass balance considerations preclude modifying 142Nd compositions by late veneer additions or core-mantle interactions. Previous studies have demonstrated the existence of positive (up to +20 ppm) anomalies compared with modern rocks throughout the Eoarchean Itsaq gneiss complex (IGC) of Southwest Greenland. Here we extend this work in both space and time and present new, high precision 142Nd data for dykes cutting the IGC and from samples dated by U-Pb zircon SHRIMP from presently adjacent Mesoarchean terranes including Kapisilik, Tuno, Akia, and Tasiusarsuaq [1]. The youngest dated dyke (3.27 Ga) with +6 ppm 142Nd documents the persistence, but diminishment of the Hadean anomaly within the IGC. As most previously measured 142Nd anomalies have been confined to >3.6 Ga rocks, a significant result is that the analysed Mesoarchean rocks from adjacent to IGC terranes are characterised by positive 142Nd anomalies (+4 to +8 ppm) for both felsic samples and mafic samples and including all samples from the 3.07 Ga Ivisaartoq mafic/ultramafic pillow basalt complex. This suggests that the source is a long-lived, mantle feature rather than reworking of Eoarchean crust. As the Mesoarchean terranes did not amalgamate and share a common history until later in the Archean [1] the high 142Nd domain was likely originally a much larger, regional, chemical feature. Initial ε176Hf is not simply correlated with 142Nd. For example, some Mesoarchean rocks from Ivisaartoq that have positive 142Nd, also have positive ε176Hf and positive ε143Nd, and with the ε176Hf- ε143Nd array similar to the IGC and distinct from the traditional Hf-Nd isotopic array. Also, at 3.72 Ga the Isua supracrustal belt ‘garbenschiefer’ unit of boninitic-like chemistry and units of the same age with island arc tholeiite – like chemistry, show positive and chondritic initial ε176Hf respectively, but have identical +13 ppm 142Nd anomalies. This requires that the Lu/Hf fractionation event post-dated and was thus unrelated to Hadean Sm/Nd fractionation, which generated the 142Nd signatures. This isotopic pattern likely reflects residual garnet storage, rather than magma ocean scenarios to generate high Lu/Hf domains. Contrasting isotopic patterns for SW Greenland terranes as compared with other Archean regions emphasises that Hadean formed and persistent (>1.5 byr) chemical heterogeneity may be a key characteristic of early Earth and reflect different co-existing thermal and geodynamic regimes.

Reference(s)

Friend, CRL and Nutman AP, 2019, Tectono-stratigraphic terranes in Archean gneiss complexes as evidence for plate tectonics: The Nuuk region, southern West Greenland. Gondwana Research 72, 213-237.