Temporal evolution of 142Nd signatures in SW Greenland from high precision MC-ICP-MS measurements

Abstract

Measurements of 142Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ142Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143Nd/144Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390−0.060+0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142Nd/144Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ142Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ142Nd = +9.8 ± 1.0). Even lower but homogeneous μ142Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε143Nd(i) and εHf(i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ142Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147Smsingle bond143Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules out a significant disturbance by younger alteration events. Based on isotope and trace element modelling, we argue that the temporal evolution of coupled 142,143Nd/144Nd isotope compositions in the ambient mantle beneath the Isua rocks is best explained by the progressive admixture of material to the Isua mantle source that must have had present-day-like μ142Nd compositions. In contrast, Mesoarchean mafic rocks from the ∼3.08 Ga Ivisaartoq greenstone belt and the 2.97 Ga inner Ameralik Fjord region as well as a 2.0 Ga Proterozoic dike within that region all have higher μ142Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ142Nd compositions.