Initial 87Sr/86Sr as a sensitive tracer of Archaean crust-mantle evolution: Constraints from igneous and sedimentary rocks in the western Dharwar Craton, India

Abstract

Among the rocky planets of the solar system only the Earth has “granitic” continental crust. The timing and processes involved in the formation of Earth’s first extensive crust is still enigmatic. The chemical and isotope compositions of ancient crustal rocks preserve a record of their genesis. The Rb-Sr system proves to be an efficient proxy for the reconstruction of crust-mantle evolution since it can bring together information from seawater as preserved in chemical sedimentary rocks and information from magmatic rocks that can trace the time and extent of crust formation and concomitant mantle depletion during the Archaean eon. The Dharwar Craton in India preserves a suite of metamorphosed igneous and sedimentary rocks that record its early crustal evolution. To overcome the susceptibility for resetting and the difficulty in determining initial 87Sr/86Sr, the minerals barite and apatite are used to obtain precise and accurate 87Sr/86Sr, because these minerals preferentially incorporate Sr and exclude Rb and preserve the initial Sr isotope compositions at the time of their formation. Initial 87Sr/86Sr of apatite were obtained in situ using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The robustness and only minor dispersion of the isotope ratios demonstrate the capability of matrix apatites in preserving initial Sr ratios. The least radiogenic value is used as the best estimate for the initial 87Sr/86Sr. The 87Sr/86Sr ratios of apatite from igneous rocks that formed from 3.5 Ga to 2.6 Ga constrain the Rb/Sr of the source over the whole time-span. A comparison of the Sr isotopes between seawater-derived barite and initial Sr isotope ratios in apatite from igneous rocks reveals that significant mafic to intermediate crust had formed by 3.2 Ga. Studying the entire Archaean time window, a dominantly mafic crust was the main source for the granitoid rocks in the Dharwar Craton from 3.5 to 3.1 Ga, whereas the rocks from 2.9 to 2.7 Ga were extracted dominantly from the depleted mantle.