Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology

Zircon U-Pb geochronology has become a keystone tool across Earth science, arguably providing the gold standard in resolving deep geological time. The development of rapid in situ analysis of zircon （via laser ablation and secondary ionization mass spectrometry） has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable （e.g. detrital zircons as a sedi- ment fingerprinting method）. The ease of acquisition, while bringing benefit to the Earth science com- munity, has also led to diverse interpretations of geochronological data. In this work we seek to refocus U -Pb zircon geochronology toward best practice by providing a robust statistically coherent workflow. We discuss a range of data filtering approaches and their inherent limitations （e.g. discordance and the reduced chi-squared; MSWD）. We evaluate appropriate mechanisms to calculate the most geologically appropriate age from both 238U/206pb and 207pb/206pb ratios and demonstrate the cross over position when chronometric power swaps between these ratios. As our in situ analytical techniques become progressively more precise, appropriate statistical handing of U-Pb datasets will become increasingly pertinent.

Source to sink zircon grain shape:Constraints on selective preservation and significance for Western Australian Proterozoic basin provenance

The effect of selective preservation during transportation of zircon grains on the detrital age spectrum is difficult to quantify and could potentially lead to systematic bias in provenance analysis. Here we investigate whether the shape of detrital zircon grains holds provenance information and if the grain shape can assist in understanding preservation. We applied multiple linear regression analysis to identify significant shape properties in detrital zircons from Proterozoic metasediments of the Capricorn and Amadeus basins and their Archean and Proterozoic sources in the Yilgarn Craton and the Musgrave Province in Western Australia. Digital images and isotopic data from 819 SIMS U-Pb dated zircons were examined for correlation between grain shape, age, U and Th content. Out of twelve shape descriptors measured, Minor Axis, the width of zircon grains perpendicular to the crystallographic c-axis, consistently shows the most significant correlation with isotopic age. In the studied population Archean grains are narrower than Proterozoic grains: the probability that grains wider than 75 μm are Archean is less than30%.Calculations of the proportions of source material in sedimentary rocks relative to the proportions of source material in the overall catchment area(erosion parameter 'K' calculated based on age spectra)produced values typical for mature river systems, with K = 6 for the Yilgarn-Capricorn and K = 5.5 for the Musgrave-Amadeus source-sink system. For the Yilgarn-Capricorn system, we also calculated 'K' based on Minor Axis, to determine whether grain width can be linked to age populations. Results of the shapebased K of 5.3 suggest a similarity between age-based and shape-based 'K' values, demonstrating that zircon grain width may be a useful discriminator of provenance. Contrary to commonly applied qualitative shape classifications, we found no consistent correlations between shape descriptors of magmatic zircons and the composition of their host rock. While metamict zircons were preferentially removed during transport, the similarities in grain shape and age distribution of magmatic and detrital populations suggest that hydraulic sorting did not have a significant effect. We conclude that transport of zircon grains from magmatic source to sedimentary sink affects their width less than their length.

The complexity of sediment recycling as revealed by common Pb isotopes in K-feldspar

Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mineral for provenance studies, is also a hindrance as zircon can be recycled through numerous sedimentary basins, thus obscuring the first cycle source to sink relationship. An elegant approach to addressing this potential issue is to compare the Pb isotope composition of detrital K-feldspar, a mineral which is unlikely to survive more than one erosion-transport-deposition cycle, with that of magmatic K-feldspar from potential basement source terranes. Here we present new in situ Pb isotope data on detrital K-feldspar from two Proterozoic arkosic sandstones from Western Australia, and magmatic K-feldspar grains from potential igneous source rocks, as inferred by the age and Hf isotope composition of detrital zircon grains. The data indicate that the detrital zircon and K-feldspar grains could not have been liberated from the same source rocks, and that the zircon has most likely been recycled through older sedimentary basins. These results provide a more complete understanding of apparently simple source to sink relationships in this part of Proterozoic Western Australia.

Strategies towards robust interpretations of in situ zircon Lu-Hf isotope analyses

The combination of U-Pb and Lu-Hf compositions measured in zircon crystals is a remarkably powerful isotopic couplet that provides measures on both the timing of mineral growth and the radiogenic enrichment of the source from which the zircon grew.The U-Pb age documents the timing of zircon crystallization/recrystallization and Hf isotopes inform on the degree to which the host melt was derived from a radiogenic reservoir(e.g.depleted mantle) versus an unradiogenic reservoir(e.g.ancient continental crust),or some mixture of these sources.The ease of generating large quantities of zircon U-Pb and Lu-Hf data has been in large part facilitated by instrument advances.However,the dramatic increase in time constrained zircon Lu-Hf analyses in the Earth science community has brought to the fore the importance of careful data collection and reduction workflows,onto which robust geological interpretations may be based.In this work,we discuss the fundamentals of Lu-Hf isotopes in zircon,which then allows us to provide a robust,accessible,methodology for the assessment of data quality.Additionally,we discuss some novel techniques for:data visualization—that facilitates better transparency of data interpretation;integration of geographic information—that may reveal spatial trends where temporal trends were only apparent before;and some novel statistical evaluation tools—that may provide more rigorous interand intra-sample comparisons.

Provenance bias between detrital zircons from sandstones and river sands:A quantification approach using 3-D grain shape,composition and age

Preservation bias may significantly impact the application of detrital zircon geochronology in reconstructing Earth surface processes.Here we compare detrital zircons from the actively eroding Murchison River channel in Western Australia with Ordovician fluvial sediments that have drained similar source rocks along the western margin of the West Australian Craton.In addition to standard analysis of detrital zircon age spectra we apply multivariate statistics to test the relation between 3-D grain shape,U-content and U-Pb ages,with the objective to quantify differences between both sample groups and track preservation along the transport pathway of the Murchison River.Our results show that zircon grains in modern river sands display an upstream trend toward larger surface areas,volume equivalent diameters and grain widths,as well as toward higher U-contents and lower apparent grain densities.3-D grain shape,size and age spectra of Murchison River zircons evolve consistently downstream,but even at the river outlet remain distinct from the Ordovician samples,as a less mature representation of source.We interpret Ordovician river zircons to represent a significantly depleted subset from which up to 22% of the zircon population may have been lost compared to the actively transported detrital load.This discrepancy between the characteristics of detrital zircons in modern active rivers and ancient fluvial Ordovician sandstones demonstrates a bias that could be relevant for other source-sink detrital transport systems throughout Earth history.