In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland: Implications for Hf and Nd isotope decoupling in Earth's ancient rocks

Abstract

Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, HfNd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent HfNd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.