Origin of Hadean 146Sm-142Nd signatures in supracrustal enclaves of the Innuksuac complex (Quebec)

Abstract:

The discovery of negative ¹⁴²Nd anomalies produced by the decay of now extinct ¹⁴⁶Sm (T_1/2=103 Ma) in mafic rocks from the Nuvvuagittuq supracrustal belt (NSB) has fueled a debate on the presence of a 4.2-4.3 Ga old component within the Archean Innuksuac complex (Northeastern Superior Province, Canada). Despite the uniqueness of these Hadean signatures, the NSB remains so far the only studied supracrustal belt in an otherwise largely unexplored terrane. This study investigates the ^146,147Sm-^142,143Nd record of newly discovered supracrustals located ca. 2 km northeast of Nuvvuagittuq. Petrologically, the Ukalik supracrustal belt (USB) comprises mafic and ultramafic rocks associated with chemical sediments (BIFs, chert). The metabasalts yield near-concordant ¹⁴⁷Sm-¹⁴³Nd and ¹⁴⁶Sm-¹⁴²Nd ages at 4.07±0.22 Ga and 4.22±0.04 Ga, respectively. The negative ¹⁴²Nd effects, however, are restricted to Low-Ti ("boninite-like") metabasalts, whereas High-Ti ("MORB-like") metabasalts have μ¹⁴²Nd=0. Neither group shows any internal relationship between Sm/Nd and μ¹⁴²Nd. Furthermore, chemical sediments interleaved with the metabasalts display no resolvable ¹⁴²Nd effect despite their very low Sm/Nd ratios. These observations suggest that Hadean ^146,147Sm-^142,143Nd signatures characterizing Ukalik mafic lithologies were generated, possibly in an Eoarchean supra-subduction zone environment, by addition of a metasomatic fluid with μ¹⁴²Nd<-15 ppm to a normal mantle with μ¹⁴²Nd=0.