Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland: The role of early silicate differentiation and missing late veneer. (15th July 2019)

• Record Type: Journal Article
• Authors: Tusch, J.; Sprung, P.; van de Löcht, J.; Hoffmann, J.E.; Boyd, A.J.; Rosing, M.T.; Münker, C.
• Is Part Of: Geochimica et cosmochimica acta. Volume 257(2019)
• Page Start: 284
• Page End: 310
• Publication Date: 2019-07-15
• Subjects: Archean -- Isua -- 182W -- Early Earth -- Late veneer -- Early silicate differentiation -- late accretion -- Nuclear field shift; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.05.012
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Eoarchean rocks from the Isua region in southern West Greenland are known to exhibit isotope anomalies of 182 W and 142 Nd. The excess 182 W in many rocks from the Isua region have been explained by two endmember models: (1) missing late veneer or (2) silicate differentiation shortly after Earth's formation. Furthermore, it has been proposed that pristine W isotope systematics in rocks from the Isua region (herein the Isua supracrustal belt and adjacent area) have been obscured by metamorphic disturbance. To address these issues, we present a comprehensive dataset, combining high precision 182 W isotope data with trace element data, including high precision elemental W-Th-U-Ta abundance data. We present an improved analytical protocol that allows processing gram-sized samples with W abundances in the lower ng/g range. This protocol also results in markedly improved ion exchange column yields and cleaner W fractions, thereby minimizing nuclear volume effects on 183 W. To compare with previous studies, our dataset includes some samples from previously investigated units in the Isua region (Isua Supracrustal Belt (ISB) mafic–ultramafic assemblages and Ameralik dikes). In addition, several Eoarchean key units in the Isukasia terrane (Isua region) and the Færingehavn terrane (Nuuk region) have been examined for their W isotope composition in this study for the first time. These newly investigated units include recently recognized mantle-like peridotites from both terranes that display PGE abundances and patterns similar to modern depleted mantle peridotites, felsic lithologies from the ISB, as well as key amphibolite and TTG localities from low-strain domains south of the ISB. Virtually all rocks from the Isua region show significant W enrichment with W/Th of up to 160 in mafic to ultramafic samples. None of the samples from SW Greenland that were object of 182 W isotope analysis in this study and virtually all previous studies appear to have preserved near canonical W/Th ratios (i.e., between 0.09 and 0.24). Independent of W enrichment, however, it can now be shown that there is a uniform 182 W isotope excess in the different rock types from the Itsaq Gneiss Complex (IGC) (average 12.8 ppm ± 1 ppm, 95% confidence interval). Importantly, none of the rock suites investigated exhibit modern mantle-like 182 W isotope signatures of µ 182 W = 0. By combining 182 W signatures in the different lithologies with elemental W systematics, we therefore can infer that the 182 W excess in Eoarchean rocks from the Isua region is widespread, and independent of W enrichment. Hence, we regard the 182 W excess as an intrinsic feature of the Eoarchean assemblages in the Isua region. Notably, mantle-like peridotites from both the Isukasia and Færingehavn terranes display the same 182 W excess, as all other units, although they have been shown to display the full inventory of Highly Siderophile Elements (HSE) found in Phanerozoic mantle peridotites. Evidently, the W isotope budget in these rocks is clearly decoupled from HSE systematics, which hampers a straightforward explanation for 182 W isotope excesses in terms of the missing late veneer model. As Platinum Group Element (PGE) patterns in mantle-like Eoarchean peridotites from the Isua region are similar to those in Phanerozoic rocks, we rather propose that to a large extent the 182 W excesses are a vestige of early silicate differentiation processes, in line with positive 142 Nd anomalies found in rocks from the Isua region, suggesting initial silicate differentiation prior to 4.50 Ga.