The Nuvvuagittuq Supracrustal Belt (Nunavik, northern Québec) is the only current candidate for preserved Hadean (>4.03 Ga) crust on Earth, yet its age has been contested since 2008: Sm–Nd isotope correlations in its cummingtonite-rich amphibolites have been read as ¹⁴⁶Sm–¹⁴²Nd isochrons of ca. 4.4 Ga, while U–Pb zircon ages of ca. 3.75–3.8 Ga — including detrital zircons that limit deposition of the supracrustal sequence to younger than ca. 3.78 Ga — indicate an Eoarchean belt carrying inherited ¹⁴²Nd anomalies. This problem, drawn from the open questions left by Cates, Ziegler, Schmitt and Mojzsis (2013), asks for the nature and origin of the older component that imparted the negative ¹⁴²Nd anomalies (preserved Hadean rocks, a recycled Hadean crustal fragment, or an enriched mantle source), a question framed by the unresolved constraints on the belt: the sedimentary versus metasomatic origin of the fuchsitic quartzites and their contact relations to the amphibolites, the protolith of the cummingtonite amphibolites, the possible existence of >3.8 Ga detrital zircons, and the pressure–temperature–time record of the ca. 3.65 Ga thermal event. A 2025 Science study reporting ca. 4.2 Ga Sm–Nd ages for mafic intrusions in the belt, covered in the news as a contested claim, shows the controversy was unresolved as of 2026.

Background

The Nuvvuagittuq Supracrustal Belt (NSB) is an enclave of roughly 8 km² of metamorphosed volcanic and sedimentary rocks (a 'supracrustal' belt) enclosed in Archean granitoid gneisses of the Inukjuak domain, Northeast Superior Province, Nunavik, northern Québec. It is dominated by mafic rocks of the Ujaraaluk unit, including distinctive cummingtonite-rich amphibolites — metamorphic rocks whose pale Mg–Fe amphibole cummingtonite, rather than the dark hornblende typical of Archean amphibolites, dominates the mineral mode — together with banded iron formations (chemical seafloor precipitates), ultramafic and gabbroic intrusions, and rare metasedimentary units such as quartz–biotite schists and fuchsitic quartzites (quartz-rich rocks containing chromium-bearing green muscovite). The belt was deformed and metamorphosed to upper amphibolite facies, and many primary field relationships are obscured.

Whether any part of this belt is Hadean (older than about 4.03 Ga) or only Eoarchean (about 4.03–3.6 Ga) is one of the most consequential open questions in early-Earth geoscience, because the NSB is the only standing candidate for preserved Hadean crust. The dispute began when O'Neil, Carlson, Francis and Stevenson (2008) reported that some NSB mafic rocks have ¹⁴²Nd/¹⁴⁴Nd ratios below the terrestrial standard. ¹⁴⁶Sm decays to ¹⁴²Nd with a half-life of only tens of millions of years, so this radionuclide was extinct by about 4 Ga; ppm-level deviations in ¹⁴²Nd/¹⁴⁴Nd therefore fingerprint mantle and crust differentiation that occurred in the Hadean. Under an isochron interpretation, the observed correlation between ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴²Nd/¹⁴⁴Nd in NSB mafic whole rocks dates their crystallization; O'Neil and colleagues refined this to ages of ca. 4.39–4.41 Ga, which would make the belt by far the oldest known terrestrial rock suite and would require a low-Sm/Nd reservoir isolated within the first roughly 200 Myr of Earth history. The alternative interpretation is that the correlation is a mixing line (a 'scatterchron') between an Eoarchean rock suite and an older component carrying a ¹⁴²Nd deficit, so that the belt itself is Eoarchean and the anomalies are inherited.

Countering evidence comes from U–Pb dating of zircon (zirconium silicate, the most robust U–Pb geochronometer): zircons from trondhjemitic–tonalitic (felsic, sodic, TTG-type) bands intrusive into the belt, and detrital grains in its rare metasedimentary rocks, cluster tightly between about 3.75 and 3.8 Ga; Cates and Mojzsis (2007) used such ages to argue for an Eoarchean belt. The source study, Cates, Ziegler, Schmitt and Mojzsis (2013), sharpened the argument: detrital zircons from fuchsitic quartzites intercalated with the ¹⁴²Nd-anomalous cummingtonite amphibolites yielded an oldest near-concordant grain at 3780±22 Ma with concordia populations at ca. 3742 and 3733 Ma (dominant populations at 3649±10 and 3634±24 Ma reflect a ca. 3.65 Ga thermal event), so that — if the quartzites are truly terrigenous sediments deposited within the same succession as the amphibolites — the entire supracrustal sequence must be younger than about 3.78 Ga, and the ¹⁴²Nd deficits must be inherited from an older crustal component or an enriched mantle source rather than recording ca. 4.4 Ga formation. Supporting whole-rock geochemistry and isocon analysis (mass-balance of immobile elements) indicates the cummingtonite amphibolites are hydrothermally altered rather than pristine basalts, consistent with an altered mafic–ultramafic volcaniclastic protolith.

That is where the question now stands: the inheritance conclusion is contested — the sedimentary origin of the dated quartzites, the isochron-versus-mixing-line status of the Sm–Nd arrays, and the very existence of Hadean rocks in the belt all remain disputed — and in 2025 new Sm–Nd data on mafic intrusions in the belt revived the claim that Hadean (ca. 4.2 Ga) rocks are physically preserved at Nuvvuagittuq.

Problem Statement

Determine the nature and origin of the older component that imparted the negative ¹⁴²Nd/¹⁴⁴Nd anomalies to the mafic rocks of the Nuvvuagittuq Supracrustal Belt — specifically, determine the applicability of the following three candidate explanations, which are not mutually exclusive: (a) preservation of actual Hadean (ca. 4.2–4.4 Ga) rocks within the belt, whose Sm–Nd correlations are true isochrons; (b) inheritance by an Eoarchean (ca. 3.78–3.8 Ga) belt of a recycled Hadean crustal component incorporated into its volcanic and sedimentary protoliths; and (c) derivation of the anomalous rocks from an enriched mantle source that preserved a ¹⁴²Nd deficit. Each of these explanations applies to a specific unit or lithology, and where more than one applies they attach to distinct rocks of the belt. Establishing this settles the depositional and crystallization age of the belt's cummingtonite amphibolites and mafic intrusions — that is, whether Earth's oldest preserved crust is Hadean or Eoarchean. The determination concerns the anomalous component as a whole; the linked sub-questions set out below bear on it rather than standing as separate questions.

This determination bears directly on several linked open sub-questions for the belt's history: the sedimentary versus metasomatic origin of the fuchsitic quartzites and the contact relations that tie them to the ¹⁴²Nd-anomalous amphibolites; the protolith of the cummingtonite amphibolites (pristine basaltic–ultramafic volcanics versus hydrothermally altered mafic–ultramafic tuffs and volcaniclastics) and the extent of its alteration, which controls whether the Sm–Nd systematics date mantle extraction or a mixed source; whether any >3.8 Ga detrital zircons or inherited cores occur in the belt's sedimentary units, which would provide direct evidence of the older source; and the pressure–temperature conditions, timing, and tectonic cause of the ca. 3.65 Ga thermal event (tonalite intrusion and contact metamorphism) that overprinted the belt.

References

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