科研之友
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摘要
New Lu-Hf isotopic data for mafic and felsic rocks from the Nuvvuagittuq supracrustal belt (NSB) in northern Quebec (Canada) yield an Eoarchean age of 3864 +/- 70 Ma consistent with both zircon U-Pb and whole-rock Sm-147- Nd-143 chronology, but in disagreement with ca. 4400 Ma ages inferred from the Sm-146-Nd-142 chronometer (O%26apos;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 %26quot;scatterchron%26quot; 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 Sm-146 of Kinoshita et al. (2012) is used with the O%26apos;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. 3800 Ma with end-member compositions representative of the compositional groups identified for these lithologies (O%26apos;Neil et al., 2011). Anomalously low Nd-142/Nd-144 values relative to Bulk Silicate Earth are endemic to a group of rocks in the NSB termed %26quot;low-TiO2%26quot; amphibolites; this is attributable to an ancient multi-stage history of their mantle source. Modeling shows that the Nd-142/Nd-144 deficits could have developed in response to a re-fertilization episode within a previously fractionated mantle domain at 4510 Ma.
- 出版日期2013-1-15
