Universität zu Köln

Tusch, J., Sprung, P., van de Loecht, J., Hoffmann, J. E., Boyd, A. J., Rosing, M. T. and Muenker, C. (2019). Uniform W-182 isotope compositions in Eoarchean rocks from the Isua region, SW Greenland: The role of early silicate differentiation and missing late veneer. Geochim. Cosmochim. Acta, 257. S. 284 - 311. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

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Abstract

Eoarchean rocks from the Isua region in southern West Greenland are known to exhibit isotope anomalies of W-182 and Nd-142. The excess W-182 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 W-182 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 W-182. 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 Faeringehavn 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 W-182 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 W-182 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 W-182 isotope signatures of mu W-182 = 0. By combining W-182 signatures in the different lithologies with elemental W systematics, we therefore can infer that the W-182 excess in Eoarchean rocks from the Isua region is widespread, and independent of W enrichment. Hence, we regard the W-182 excess as an intrinsic feature of the Eoarchean assemblages in the Isua region. Notably, mantle-like peridotites from both the Isukasia and Faeringehavn terranes display the same W-182 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 W-182 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 W-182 excesses are a vestige of early silicate differentiation processes, in line with positive Nd-142 anomalies found in rocks from the Isua region, suggesting initial silicate differentiation prior to 4.50 Ga. (C) 2019 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Tusch, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Sprung, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED van de Loecht, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Hoffmann, J. E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Boyd, A. J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rosing, M. T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Muenker, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-135155
DOI:	10.1016/j.gca.2019.05.012
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	257
Page Range:	S. 284 - 311
Date:	2019
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Place of Publication:	OXFORD
ISSN:	1872-9533
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language SOUTHERN WEST GREENLAND; HIGHLY SIDEROPHILE ELEMENT; ITSAQ GNEISS COMPLEX; EARLY MANTLE DIFFERENTIATION; FIELD STRENGTH ELEMENTS; EARLY PRECAMBRIAN ROCKS; SM-ND SYSTEMATICS; SUPRACRUSTAL BELT; GREENSTONE-BELT; GEOCHEMICAL EVIDENCE Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/13515