Our understanding of the early evolution of the crust and the mantle is hindered by the scarcity of preserved Eoarchean/Hadean geological records. In the last decade, the use of the short-lived 146Sm-142Nd isotopic system has open a new window into the earliest crust-mantle differentiation history. With a half-life of ~103 Ma, radioactive 146Sm is considered extinct by ~4 Ga and thus variations in its daughter product, 142Nd, can only be produced by Sm-Nd fractionation in the Hadean. This short-lived tracer is therefore ideal to study the early evolution of silicate reservoirs. The Paleoarchean São Tomé layered mafic-ultramafic intrusion forms a small basement inlier located in the Neoproterozoic Seridó belt in north-eastern Brazil. Dated at 3506 ± 29 Ma (Ruiz et al., 2018), the São Tomé rocks are among the oldest mantle-derived rocks from NE Brazil, representing ideal candidates to study the early mantle evolution. The intrusion consists of a central ultramafic zone with upper and lower mafic zones, each metamorphosed to amphibolite facies and variably altered by metasomatic fluids. The ultramafic zone is mostly made of tremolite-bearing serpentinite interlayered with hornblendite, while the upper and lower mafic zones are dominated by intercalations of amphibolite and hornblendite, with localized layers of Fe-Ti oxides. Samples from each lithology and zone, as well as the host rock, were analysed for their 147,146Sm-143,142Nd isotopic compositions. Because some samples show evidence of isotopic disturbance of the long-lived 147Sm-143Nd system, a latent class regression using an iterative expectation-maximization (EM) algorithm (Davies et al., 2018) was applied to all samples to obtain the most pristine composition. The algorithm ran for 300 iterations repeated over 50 times, provide a best fit isochron with an age of 3582 Ma and initial ε143Nd = -0.23. This age is in agreement with the U-Pb age in zircon previously obtained for the São Tomé intrusion and thus the ε143Nd initial composition is taken as the best estimante of its mantle source. Additionally, the same samples were analysed for their 142Nd isotopic composition, revealing well-resolved negative anomalies and µ142Nd values as low as -14.6 with an average of -9.0 ± 6.0 ppm (n=13, 2SD). The existence of resolved negative µ142Nd anomalies is rare and suggest an enriched source that differentiated during the Hadean. Coupling the negative µ142Nd with the subchondritic ε143Nd hints towards an unique mantle source and potentially provide evidence for the existence of an enriched Hadean mantle reservoir.
 
Reference(s)

Ruiz, FV, Della Giustina, MES, de Oliveira, CG, Dantas, EL, Hollanda, MHB. 2018, The 3.5 Ga São Tomé layered mafic-ultramafic intrusion, NE Brazil: Insights into a Paleoarchean Fe-Ti-V oxide mineralization and its reworking during West Gondwana assembly: Precambrian Research, v. 326, p. 462-478.

Davies, JHFL, Sheldrake, TE, Reimink, JR, Wotzlaw, J-F, Moeck, C, Finlay, A. 2018, Investigating complex isochron data using mixture models.Geochemistry, Geophysics, Geosystems, v. 19, p. 4035–4047.