Early-middle archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single zircon grains

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We report detailed high precision combined single grain U-Pb and Lu-Hf studies of early zircons to obtain more reliable indications of the extent of mantle depletion and crustal recycling in the Archean. Despite the possibility that MC-ICPMS affords for precise Hf isotopic measurement of single zircons, the complexity of grain populations adds uncertainty to initial isotopic ratios. Multiple episodes of zircon growth and ancient Pb loss, common in early Archaean rocks, result in 207Pb/ 206Pb ages, and in some cases 176Hf/ 176Hf ratios, that are variable between and within zircon grains. In order to evaluate the role of heterogeneity of zircon populations and to obtain the most reliable ε Hf(T), we have analysed several abraded zircon grains (from two to eleven) from each of several samples for both Lu-Hf and U-Pb. Hf isotopic analyses with precision better than 1 to 1.5 ε-units (2σ) were obtained from grains weighing between 3 and 10 micrograms. The observed internal variations in age, U-Pb discordance, and Hf isotopic composition have been tested against models of disturbance of isotopic systems in zircon. Application of the U-Pb and Lu-Hf methods to the same zircon grains and analysis of single grains appears to be crucial for finding closed geochemical systems and thereby obtaining reliable Hf isotope data from early Archaean rocks. The precision and accuracy of Hf isotopic data obtained with the approach presented here are limited mainly by the sensitivity of Hf isotopic analyses, and may be greatly improved with the progress of analytical techniques. Following our earlier study of the Jack Hill’s metaconglomerate in which we found no significant trace of depleted mantle-like Hf in >3.8 Ga zircons we have studied zircons from 15 rocks from four early-middle Archaean areas. Four ca. 3.6 Ga gneisses from the Acasta Gneiss Complex of Northwest Canada contain zircons with ε Hf(T) between +0.7 and −4.1, as well as xenocrystic zircon grains with unradiogenic Hf. Some older grains have reset U-Pb systems, but preserve their Hf isotopic composition. The 3.62 to 3.73 Ga Amı̂tsoq gneisses, West Greenland, have ε Hf(T) between +1.4 and +2.6, while 3.52 to 3.32 Ga gneisses and felsic metavolcanics from Barberton and Pilbara yielded ε Hf(T) between +1.9 and +4.2. These early-middle Archaean complexes were formed from crust derived from previously depleted mantle. These complexes grew over the first 100 to 200 m.y. of their evolution mainly by addition of juvenile crust, while reworking of older crust was less significant. The higher ε Hf(T) values in the Barberton and Pilbara rocks compared to the Amı̂tsoq gneisses are consistent with uniform or increasing mantle depletion during the early Archaean. The Acasta Gneiss Complex was probably formed from very old crust (3.8 to 4.0 Ga) that was extensively reworked during the Archaean. Both U-Pb and Hf isotopic data suggest the similarities between the evolution of the Acasta Gneiss Complex and the source of 4.2 to 3.4 Ga Jack Hills detrital zircons.