In situ U-Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: new insights to making old crust

摘要

The sources and petrogenetic processes that generated some of the Earth\u27s oldest continental crust have been more tightly constrained via an integrated, in situ (U-Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. (207)Pb/(106)Pb dating of oscillatory-zoned zircon using SHRIMP RG (n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga. Analyses of (18)O/(16)O compositions of olivine from a harzburgite/dunite (n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (delta(18)O(OI) = 6.0 +/- 0.4 parts per thousand) was slightly enriched in (18)O, but not significantly different from that of the modern mantle. Zircon delta(18)O measurements from the six felsic rocks (n = 93) record mean or weighted mean compositions ranging from 4.9 +/- 0.7 parts per thousand to 5.1 +/- 0.4 parts per thousand, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. delta(18)O(Zr) compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite delta(18)O(WR) compositions (6.7-6.9 parts per thousand) resemble those of modern adakites. LA-MC-ICPMS zircon (176)Hf/(177)Hf analyses were obtained from six samples (n = 122), Five samples record weighted mean initial epsilon(Hf) compositions ranging from to 0.5 +/- 0.6 to -0.1 +/- 0.7 (calculated using lambda(176)Lu = 1.867 x 10(-11) yr(-1)), while one sample records a composition of 1.3 +/- 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf. Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of similar to 80% unaltered gabbro (delta(18)O(WR) = 5.5%.) with similar to 20% hydrothermally altered gabbro/basalt (delta(18)O(WR) = 4.0 parts per thousand) would produce tonalite magmas within the average compositional range observed. (18)O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high (18)O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab. (C) 2009 Elsevier Ltd. All rights reserved.