Inherited 142Nd anomalies in Eoarchean protoliths

Geological records of the earliest history of the Earth are rare; rocks older than 3700 Ma 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 4500 Ma during magmatic and metamorphic perturbations associated with the emplacement of the NSB at ca. 3750 Ma (Cates and Mojzsis, 2009). Our model further predicts a whole-rock 147Sm–143Nd age of 3800 Ma 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 and Rizo et al., 2011).

► We report new 147,146Sm-143,142Nd data for the Nuvvuagittuq Supracrustal Belt. ► A new model for 147,146Sm-143,142Nd systematics explains these data. ► The coupled 147,146Sm-143,142Nd systematics are shown to be disturbed. ► We demonstrate that it is impossible for these rocks to have crystallized ca. 4362 Ma ago. ► The 147,146Sm-143,142Nd systematics in Eoarchean protoliths worldwide can now be reconciled.