Initial Pb of the Amı̂tsoq gneiss revisited: implication for the timing of early Archaean crustal evolution in West Greenland

The rapid change in 207Pb/206Pb ratio during early Earth history provides a powerful constraint for temporal resolution of Archaean crustal evolution. New and published data for Amı̂tsoq gneiss feldspars and whole rocks define the Earthʹs least radiogenic silicate Pb known until now. The most important feature of the data is that independent of the exact slope of the regression in a common Pb diagram, a very well-defined intercept with plausible terrestrial Pb evolution lines is obtained between 3.63 and 3.66 Ga, identical to a combined Pb/Pb regression age of 3654±73 Ma (MSWD=17.6). We conclude that the magmatic precursors of the Amı̂tsoq orthogneisses separated from a depleted mantle-like source at ca. 3.65 Ga. This contrasts sharply with claims based on ion-probe U–Pb zircon dating that the Amı̂tsoq gneisses had a prolonged, episodic, magmatic evolution extending from ca. 3.87 to 3.60 Ga. Pb isotopes offer no evidence that any Amı̂tsoq gneiss analysed until now began its evolution in the crust as long ago as 3.85 Ga. We propose that zircons significantly older than 3.65 Ga were inherited from chemically evolved rocks as old as ca. 3.85 Ga. Geochemical arguments are presented to support this proposal. New feldspar and whole rock Pb isotope data are also reported for sheets of quartz-dioritic gneiss on Akilia Island that yield the oldest zircon U–Pb age clusters in the range 3.84–3.87 Ga. These sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose C-isotope ratios are regarded as biogenic in origin. As above, Pb-isotopes in the sheets are consistent with a magmatic age of 3.65 Ga, which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers. Additional Pb-isotope data, as well as limited published Sm–Nd and U–Pb zircon data on the Akilia enclaves themselves, suggest an age between ca. 3.65 and 3.70 Ga for their original deposition, some 150–200 Ma younger than proposed previously. Thus the question of overlap of earliest life with a bolide impact scenario terminating ≥3.80 Ga (as on the moon) becomes irrelevant.