Petrology, Geochemistry and Coupled ¹⁴²Nd-¹⁴³Nd Isotopes in Archean Rocks from NE Brazil: Investigating a Hadean Enriched Reservoir

Abstract

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 couple of decades, the use of the short-lived ¹⁴⁶Sm-¹⁴²Nd isotopic system has opened 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. Evidence for an early depleted mantle has been recognized in multiple occurrences of Eo- to Paleoarchean rocks (e.g., SW Greenland, NE Canada, NE China, NE India) exhibiting excesses in ¹⁴²Nd compared to the modern mantle. A complementary enriched reservoir is, however, more elusive, with only rare Paleoarchean rocks from South Africa hinting at its existence and displaying slight deficits in ¹⁴²Nd. This thesis focuses on some of the oldest rocks from South America, more specifically from NE Brazil, which are still poorly studied regarding their isotopic compositions from short-lived systems. A complex crustal history has been recorded by the northeastern Brazil Precambrian crust, from the northern end of the São Francisco Archean Craton to the northern domain of the Neoproterozoic Borborema Province. Archean rocks from this region mostly span from 3.5 Ga to 2.5 Ga, and three of these localities, including Paleoarchean mantle-derived rocks have been investigated: The São José do Campestre massif and the Seridó belt on the NE end of the Borborema Province, as well as the Sobradinho block on the northern extent of the São Francisco Craton. The studied Paleoarchean mantle-derived samples include 1) amphibolite xenoliths hosted in felsic rocks in two areas of the São José do Campestre massif, which are typically enriched in rare-earth-elements and show high concentrations in most incompatible trace elements; and 2) mafic and ultramafic cumulate rocks from the São Tomé layered intrusion in the Seridó belt. Most of these mantle-derived rocks yielded negative µ¹⁴²Nd resolved from the Nd JNdi-1 terrestrial standard, averaging a µ¹⁴²Nd value of ~-10. This implies their sourcing from a Hadean enriched mantle reservoir, which existence is clearly confirmed for the first time. Although the mantle-derived rocks from the São José do Campestre massif show evidence of disturbance of the long-lived ¹⁴⁷Sm-¹⁴³Nd system, most mafic-ultramafic rocks from the São Tomé intrusion appear to be relatively undisturbed regarding their initial ε¹⁴³Nd, which can be coupled with their µ¹⁴²Nd to suggest differentiation of their mantle source at ~4.44 Ga, with a ¹⁴⁷Sm/¹⁴⁴Nd ratio of ~0.18. This Hadean enriched source has been recorded at the scale of the NE Borborema Province and perhaps in the São Francisco craton, mostly by ~3.5 Ga Paleoarchean mantle-derived rocks, but also possibly until ~3.1 Ga. This suggests preservation of the Hadean enriched reservoir for at least 1 billion years. Felsic crustal rocks ranging in age from 3.5 Ga to 0.5 Ga have also recorded negative µ¹⁴²Nd, likely resulting from protracted crustal reworking of the low-µ¹⁴²Nd mafic crust. The well-resolved negative µ¹⁴²Nd values measured in rocks of distinct nature and covering a regional extent of Precambrian crust provide strong evidence for the widespread involvement of an early enriched mantle source in the formation and evolution of the of the NE Brazilian Archean crust.