For decades radiogenic Nd isotopes have formed a cornerstone of studies seeking to understand crustal growth and evolution (Allègre and Rousseau, 1984; DePaolo, 1980). Recent analytical advances mean we can now simultaneously obtain radiogenic and stable Nd isotope compositions (McCoy-West et al., 2017; McCoy-West et al., 2020), allowing us to reconstruct not just the temporal history of a magmatic rock, but also the processes involved in its formation. Studies of the chemical and isotopic composition of the Archean crust remain hindered by sampling biases and the lack of reference suites that have been systematically measured for multiple geochemical tracers. To this end, we collected a large collection of 154 samples of various lithologies (but with a strong emphasis on granitoids) ranging in age from ca. 3.6 Ga to 2.7 Ga collected from across the Kaapvaal craton and Limpopo belt in southern Africa (the SWASA collection). The wide range of crystallisation ages and the progressive stages of craton assembly they record, make this an ideal sample set for accessing temporal changes in magmatic compositions related crustal evolution. Here we present Nd isotope data for 67 of these samples from across the region and age spectrum. The radiogenic Nd compositions are consistent with previous determinations with depleted mantle model ages mostly ranging from 3.0 to 3.7 Ga, and crustal residence times of up to 0.5 Ga. Stable Nd istope compositions (δ146/144Nd) range from -0.11‰ to +0.12‰ which is 3 times more variability than seen in modern basalts (McCoy-West et al., 2021). Above 70 wt % SiO2, the proportion of samples with δ146/144Nd resolvable heavier than the bulk silicate Earth increases substantially. After 2.8 Ga, post tectonic high-K granites with heavier δ146/144Nd dominate the sample suite. This is distinguishable from the older trondhjemite-tonalite-granodiorite samples which possess more normal δ146/144Nd and consistent with a temporal change in the source compositions of the granoitoids.

Reference(s)

Allègre, C.J. and Rousseau, D. (1984) The growth of the continent through geological time studied by Nd isotope analysis of shales. Earth and Planetary Science Letters v. 67, p. 19-34. 

DePaolo, D.J. (1980) Crustal growth and mantle evolution: inferences from models of element transport and Nd and Sr isotopes. Geochimica et Cosmochimica Acta v. 44, p. 1185-1196. 

McCoy-West, A.J., Burton, K.W., Millet, M.-A. and Cawood, P.A. (2021) The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts. Geochimica et Cosmochimica Acta v. 293, v. 575-597. 

McCoy-West, A.J., Millet, M.-A. and Burton, K.W. (2017) The neodymium stable isotope composition of the silicate Earth and chondrites. Earth and Planetary Science Letters v. 480, p. 121-132. 

McCoy-West, A.J., Millet, M.-A., Nowell, G.M., Nebel, O. and Burton, K.W. (2020) Simultaneous measurement of neodymium stable and radiogenic isotopes from a single aliquot using a double spike. Journal of Analytical Atomic Spectrometry v. 35, p. 388-402.