Radiogenic isotopes provide an important means towards elucidating Archean crustal evolution. The global Hf and Nd isotope record of Archean crustal fragments has been instrumental to unveiling the history of ancient crustal growth and differentiation. The Rb-Sr system could provide valuable complementary constraints in this regard, as this system is particularly sensitive to magmatic fractionation processes, and the chemical and isotopic evolution of magma sources. Application of this system has so far been complicated, however, by its susceptibility to isotope re-equilibration or alteration of the Rb/Sr parent-daughter ratio. In-situ Sr isotope analysis of primary igneous minerals with very low Rb/Sr, such as apatite, provides a new means to determine the initial 87Sr/86Sr (87Sr/86Sri) values for igneous rocks directly. In this study, we apply in-situ Sr isotope analysis of apatite by LA-MC-ICPMS to tonalite-trondhjemite-granodiorite (TTG) rocks and end-member sanukitoids from Archean cratons worldwide. The 87Sr/86Sri values of sanukitoids are relatively radiogenic, supporting the model in which such rocks are formed by flux melting of a mantle strongly enriched by metasomatism, possibly by slab-derived fluids. The 87Sr/86Sri values for TTGs formed between 3.72 and 3.45 Ga are generally radiogenic, indicating aged amphibolite sources. The 87Sr/86Sri values of younger TTGs are systematically lower and were derived from mafic sources that had an average age of ≤0.2 Gyr. This evolution matches with observations from Hf isotopes for TTGs of similar age and indicates a systematic change in the nature or efficiency of TTG crust formation during the Paleoarchean. In-situ Sr isotope analysis of apatite provides a useful method to uncover the Sr record of the early continental crust, and enables constraints on local source evolution and the general two-step evolutionary process of Archean crust formation.
|Original language||English (US)|
|State||Published - Aug 9 2022|
Bibliographical noteKAUST Repository Item: Exported on 2022-12-12
Acknowledgements: We thank research scientists at PCIGR (V. Lai and M. Amini) for analytical support, D. Weis and J.S. Scoates for fruitful discussions, D. Chew and an anonymous reviewer for constructive comments and suggestions, and D.-C. Zhu for editorial handling. This research was financially supported by the University of British Columbia (International Doctoral Student fellowship to S.C.), the Natural Sciences and Engineering Research Council of Canada (Discovery Grants RGPIN-2015-04080 and RGPIN-2019-0 1506) and Canadian Foundation for Innovation and British Columbia Knowledge Development Fund (Joint Project 229814 to M.A.S.). J.E.H. acknowledges the German Science Foundation (Grants HO4697/1-1 and HO4697/1-2), K.M. the Swiss National Science Foundation (Grant 51NF40-141881) and E.K. the Swedish Research Council (VR) for financial support to the NordSIMS-Vegacenter national research infrastructure (Grant 2021-00276).