A legacy of Hadean silicate differentiation inferred from Hf isotopes in Eoarchean rocks of the Nuvvuagittuq supracrustal belt (Québec, Canada)

Abstract

New Lu–Hf isotopic data for mafic and felsic rocks from the Nuvvuagittuq supracrustal belt (NSB) in northern Québec (Canada) yield an Eoarchean age of 3864±70Ma consistent with both zircon U–Pb and whole-rock 147Sm–143Nd chronology, but in disagreement with ca. 4400Ma ages inferred from the 146Sm–142Nd chronometer (O'Neil et al., 2008, 2012). The Lu–Hf result is interpreted as the mean emplacement age of the different autochthonous units of the NSB. An observed alignment of the data along a Lu–Hf “scatterchron” precludes a Hadean age for the NSB because its isotopic characteristics appear to be controlled by long-term radiogenic ingrowth. Emplacement of the NSB in the Hadean (e.g., 4362−54+35 Ma if the decay constant of 146Sm of Kinoshita et al. (2012) is used with the O'Neil et al., 2008 data) should instead have caused age differences of hundreds of millions of years to manifest as strong deviations from the Lu–Hf scatterchron. Combined Lu–Hf and Sm–Nd data on the same NSB amphibolite samples (Ca-poor cummingtonite- and hornblende-bearing) define a mixing hyperbola at ca. 3800Ma with end-member compositions representative of the compositional groups identified for these lithologies (O'Neil et al., 2011). Anomalously low 142Nd/144Nd values relative to Bulk Silicate Earth are endemic to a group of rocks in the NSB termed “low-TiO2” amphibolites; this is attributable to an ancient multi-stage history of their mantle source. Modeling shows that the 142Nd/144Nd deficits could have developed in response to a re-fertilization episode within a previously fractionated mantle domain at 4510Ma.