Uniform W-182 isotope compositions in Eoarchean rocks from the Isua region, SW Greenland: The role of early silicate differentiation and missing late veneer

摘要

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).