Deciphering the zircon Hf isotope systematics of Eoarchean gneisses from Greenland : Implications for ancient crust-mantle differentiation and Pb isotope controversies

• Main Authors: Kemp, Anthony I.S., Whitehouse, Martin J., Vervoort, Jeffrey D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Enheten för geovetenskap 2019
• Subjects: Eoarchean; Greenland; Zircon; Hf isotopes; Pb isotopes; Laser ablation; Geochemistry; Geokemi; Godthåbsfjord
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-3574

We report a Hf isotope investigation of zircons from four Eoarchaean orthogneisses from the Godthåbsfjord region of southern West Greenland by laser ablation MC-ICPMS, to elucidate crust-mantle differentiation processes in the early Earth. Zircon crystals of all samples record a complex, multi-stage growth and disturbance history, and these discrete growth phases also exhibit disparate Lu-Hf isotope systematics. The oldest (3.84–3.82 Ga) zircon cores have tightly clustered 176Hf/177Hf ratios that are consistent with derivation of their tonalitic precursors from chondritic mantle at this time, with no evidence of input from older crustal or depleted mantle sources. Younger (3.67–3.62 Ga) zircon overgrowths have subchondritic Hf and plausibly grew from small fraction partial melts of the tonalitic host, involving variable dissolution of the older zircon cores. The Neoarchean (ca. 2.7 Ga) zircon component in some samples extends to significantly higher 176Hf/177Hf than the >3.65 Ga zircon, a feature that is interpreted to reflect addition of radiogenic Hf from the rock matrix during metamorphic zircon growth and recrystallisation at 2.7 Ga. The strongly positive εHf (3.82 Ga) values obtained by dissolution of GGU110999 zircons are interpreted to be an artifact of calculating εHf values at ages that are too old, and also from the inclusion of radiogenic younger domains in the analysed multi-grain fractions, rather than to a contribution from depleted Eoarchean mantle. Such data – from zircon grains with multiple age and isotopic components – should not be used to define the evolution of crust-mantle reservoirs. A re-interpretation of the existing Pb isotope data, incorporating the new Hf isotope constraints, posits that the protoliths to the Godthåbsfjörd gneisses were influenced by radiogenic Pb introduced as a fluid mobile component during recycling of a high-μ stagnant basaltic lid at ≥3.8 Ga. The destruction of this mafic protocrust, with attendant fluid release into chondritic mantle, may have been instrumental for the generation of stable Eoarchean tonalitic crust from ca. 3.8 Ga.