Inherited 142Nd anomalies in Eoarchean protoliths

Abstract

Geological records of the earliest history of the Earth are rare; rocks older than 3700Ma comprise only a few percent of continental surfaces. Evidence is mounting, however, that vestiges of primordial planetary differentiation continued to influence the compositions of the oldest rocks during the Hadean and into the Archean. Here, we report new whole-rock 147,146Sm–143,142Nd data for the ancient Nuvvuagittuq Supracrustal Belt (NSB) in Québec (Canada) and confirm the 142Nd deficits reported by O’Neil et al. (2008). We show that the assigned (O’Neil et al., 2008) and recently revised (Kinoshita et al., 2012) 142Nd “age” of 4362−54+35 Ma claimed for NSB amphibolites is at odds with the younger 147Sm–143Nd record. This discrepancy can be reconciled by partial Nd isotope equilibration of rocks with Hadean model ages of up to 4500Ma during magmatic and metamorphic perturbations associated with the emplacement of the NSB at ca. 3750Ma (Cates and Mojzsis, 2009). Our model further predicts a whole-rock 147Sm–143Nd age of 3800Ma for other NSB lithologies in agreement with U-Pb zircon chronology (Cates and Mojzsis, 2007). Hence, 146Sm–142Nd systematics for the Eoarchean NSB rocks represent inheritance of a Hadean signature that was stored either in pre-existing crust or in early-enriched mantle sources. The decoupled 147,146Sm–143,142Nd systematics of the NSB is similar but complementary to the Hadean mantle isochron preserved in Eoarchean rocks from West Greenland (Bennett et al., 2007; Rizo et al., 2011).