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

Abstract

The sources and petrogenetic processes that generated some of the Earth’s 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. 207Pb/ 206Pb 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 18O/ 16O 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 ( δ 18O Ol = 6.0 ± 0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ 18O measurements from the six felsic rocks ( n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ 18O 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 δ 18O WR compositions (6.7–6.9‰) resemble those of modern adakites. LA-MC-ICPMS zircon 176Hf/ 177Hf analyses were obtained from six samples ( n = 122). Five samples record weighted mean initial ε Hf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ 176Lu = 1.867 × 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 ∼80% unaltered gabbro ( δ 18O WR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt ( δ 18O WR = 4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-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 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.