142Nd evidence for early Earth differentiation

Abstract

We measured 142Nd/ 144Nd in metabasalts and metagabbros from the western and southern parts of the Isua Supracrustal Belt, West Greenland. The samples were selected based on field and thin section criteria so as to minimize metamorphic effects on the isotope systematics. We developed a new multi-stage chemical separation scheme and a new isotope measurement technique using multiple-collector inductively coupled plasma mass spectrometry and, upon replication of measurements, achieved a precision better than 20 ppm. Out of eight samples, three show a positive 142Nd anomaly of 30 ppm consistent with a previous measurement by Harper and Jacobsen [Nature 360 (1992) 728–732]. These samples also define a 147Sm– 143Nd isochron age that agrees with the commonly accepted U–Pb zircon age of 3.78 Ga for Isua, while the non-anomalous samples show substantial dispersion. The latter lack of coherence is interpreted as indicating either isotopic heterogeneities in the Isua upper mantle or redistribution of Nd by fluids during late Archean metamorphic events. Scatter among the non-anomalous samples is also observed for the Lu–Hf isotope systematics and confirms the severity of metamorphic remobilization. If the 147Sm– 143Nd and 146Sm– 142Nd systems are explained by a two-stage model, it is required that the upper mantle went through a major differentiation event within a few tens of millions of years after planetary accretion. If early Archean convection decoupled the two Nd isotope systems, this necessitates that the episode of differentiation took place within no more than 150 Myr of planetary accretion. One interpretation of the high-Sm/Nd reservoir is that it represents the remnants of a stagnant lithospheric lid overlying a magma ocean. Because the lid is shattered by impacts, it is unable to sink. Alternative explanations, such as an ultra-deep magma ocean at the core–mantle boundary or massive subduction of early crust, call for the early burial of enriched material and the ubiquitous presence of a 142Nd anomaly in the early Archean mantle.