Sluggish Hadean geodynamics: Evidence from coupled 146,147Sm–142,143Nd systematics in Eoarchean supracrustal rocks of the Inukjuak domain (Québec)

Abstract

The discovery of deficits in 142Nd/144Nd in mafic rocks of the Nuvvuagittuq supracrustal belt (NSB) has triggered a debate about the possible preservation of Hadean (pre-3.85 Ga) crustal remnants in the little-known but areally extensive Innuksuac complex (3.6–3.8 Ga, Inukjuak domain, Northeast Superior Province, Canada). Geochronological investigations in the NSB, however, are hampered by the poor preservation and highly disturbed isotopic record of various mafic (amphibolite) lithologies that host the 142Nd anomalies. Here we present 146Sm–142Nd and 147Sm–143Nd data for rocks of extrusive magmatic and sedimentary protoliths from the Ukaliq supracrustal belt, a newly discovered volcano-sedimentary enclave enclosed in granitoid gneisses of the Inukjuak domain. Our study also includes the first 146Sm–142Nd data for quartz-magnetite rocks (banded iron-formation; BIF) of the NSB and the Eoarchean Isua supracrustal belt (ISB) in southern West Greenland. We show that Ukaliq amphibolites carry variably negative 142Nd anomalies, ranging from 0 to −10 ppm, which are positively correlated with their Sm/Nd ratio. If considered as an isochron relationship, the 146Sm–142Nd array yields an apparent Hadean emplacement age of 4215−76+50 Ma. The negative 142Nd anomalies, however, appear to be mainly restricted to amphibolites with boninitic affinities, likely reflecting inheritance from an enriched mantle source. In contrast, tholeiitic and ultramafic lavas have normal μ142Nd regardless of their Sm/Nd ratio. Furthermore, BIF from Ukaliq and Nuvvuagittuq lack the negative 142Nd anomalies that should have been produced by in situ decay of 146Sm had these sediments been deposited prior to ca. 4.1 Ga. Instead, they exhibit μ142Nd identical to that measured in Isua BIF. Collectively, our results suggest that the 146Sm–142Nd array characterizing mafic lithologies of Ukaliq and Nuvvuagittuq is an inherited signature with doubtful chronological significance. We interpret the volcanic protoliths of the Innuksuac complex to have been produced by metasomatically triggered melting of a variably enriched Eoarchean mantle, following addition of felsic melts and/or fluids derived from a foundering Hadean mafic crust. Application of coupled 146,147Sm–142,143Nd chronometry to Ukaliq lavas yields a model age of differentiation of 4.36−0.06+0.05 Ga for this Hadean precursor. This is similar to late-stage crystallization ages inferred for the lunar and terrestrial magma oceans. The long-term preservation of Earth's primordial crust points to subdued lithospheric recycling in the post-magma ocean Earth.