Assessing the reliability of detrital zircon in Early-Earth provenance studies

Abstract: Our understanding of the Early Earth and the processes that have shaped its evolution have spawned predominantly from the geochemical and isotopic signatures of a small number of zircon populations around the world. Studies of trace element distributions, Hf and O isotope systematics as well as mineral inclusion chemistry in detrital zircon are combined with U-Pb chronology in order to constrain source rock characteristics. However, previous research has highlighted the potential for primary isotopic and geochemical signatures to be modified after deposition, enhanced by self-induced radiation damage and crystal-plastic deformation. Further complications arise when an unknown number of source rocks contribute to the detrital sediments, or when the source rocks are absent as is the case for Hadean detrital zircon. In this thesis, a range of analytical methods are applied to relatively uncharted ancient detrital zircon populations, in addition to a systematic investigation into the behaviour of titanium, rare earth elements, U-Th-Pb, Lu-Hf and O isotopes in a magmatic source-rock analogue to such ancient detrital zircon suites. Three localities are studied: Mt. Alfred, within the Yilgarn Craton of Western Australia (detrital study); the Saglek Block, the western-most extent of the North Atlantic Craton in northern Labrador (detrital study); and a locality south of Isua in southern West Greenland (analogue study). We have utilised a CAMECA ims 1280 Secondary Ion Mass Spectrometer (SIMS) for its high spatial resolution and small volume sampling, except for the Lu-Hf analyses which were carried out using a Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICP-MS).The main outcomes of this study are threefold. 1) Heavily discordant, ancient detrital zircon populations require extensive data filtering in order to produce reliable data for age comparison. Further, age estimations of pervasive Pb loss even in metamict detrital zircon may be achieved using intragrain discordia intercept ages. Applying this in a regional sense reveals that detrital zircon from Mt. Alfred, Western Australia have a distinct provenance in comparison to other metasedimentary units of the Youanmi Terrane, and bear resemblance to the Mt. Narryer metasediments of the Narryer Terrane. 2) Detrital zircon from metasedimentary rocks exposed to high grade metamorphism in the Saglek Block, Northern Labrador yield predominantly Mesoarchaean age signatures, along with a minor Eoarchaean aged component. Lu-Hf isotope data from these zircon reveal up to five near-chondritic populations. U-Pb-Hf data from two samples of metapelite (L1407 and L1408) suggest that a previous re-assignment of deposition age for this unit to >3.95 Ga is unsubstantiated, undermining later studies based on that interpretation. 3) Geochemical complexities in zircon from an Eoarchaean meta-tonalite, taken as a source analogue to ancient detrital zircon, challenge the assumptions and interpretations drawn from detrital zircon studies. Lu-Hf and O isotope systems display mostly homogenous compositions, despite recognised U-Pb disturbance. Petrogenetic trace element proxies such as REE and Ti yield heterogeneous results, even within individual grains. Discerning magmatic signals from detrital zircon populations can therefore, without careful scrutiny, portray artificially complex results and consequently, lead to false interpretations.