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.
Analysis of a database of ∼600,000 detrital zircon ages constrained by the depositional ages of the respective sedimentary units reveals a dynamic evolution of zircon age peaks through time. This ...analysis demonstrates that zircon age peaks from ancient sedimentary successions are often out of phase with the detrital zircon record obtained from modern sediments. The growth and diminishment of the zircon age peaks through time implies the presence of continental crust whose age is not proportionately represented in the modern record, and therefore that the current crustal archive is biased. However, when the detrital zircon record is viewed in terms of its evolution through time, that is taken as a time-lapse view of continental growth, it appears there never was a time in Earth history without an associated zircon age peak. The analysis of detrital zircon age peaks presented herein also reveals an evolution that can be broadly divided into three temporal groupings that broadly correspond with phases of Earth's tectonic evolution, namely pre-supercontinent continental growth (pre-2.1 Ga), Earth's middle age (2.1–0.8 Ga), and post-onset of modern-day plate tectonics (post-0.8 Ga). These three groupings each display increasing degrees of zircon age diversification with time, and are a likely result of a net increase of preserved continental crust through time. The presence of these three tectonic states in multiple geologic proxies (detrital zircon ages, changing styles of metamorphism, paleogeography/supercontinents) suggests that while the growth of the continental crust is continuous, the tectonic processes that shape the long-term preservation of the crust have evolved over geologic time.
Abstract
Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the ...petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.
Evidence of glaciation is recognized in Earth's history from 2.9 Ga to the present, providing important information regarding the climatic, sedimentary, and tectonic evolution of our planet. Glaciers ...are highly effective agents of erosion that generate and transport large amounts of sediments for 100ʹs of kilometres. Therefore, detrital zircon geochronology of glaciogenic sedimentary rocks can elucidate the provenance and evolution of the sedimentary record through time. In this study, we investigate the Paleoproterozoic Glaciation Event which is hypothesized to be the oldest global glaciation in Earth's history (2.4 to 2.2 Ga). We identify distinct sedimentary provenance patterns between the Archaean supercratons Sclavia and Superia, which, comprised most of the extant Archean crust. The detrital zircon of Sclavia exhibit increasing dissimilarity through time whereas the detrital zircon ages of Superia show consistent age peaks through time. Coupled with the absence of glacial deposits and predominance of marine sedimentary successions within Sclavia, the detrital zircon provenance likely indicates limited subaerial exposure of continental crust in Sclavia with glaciers narrow mountain belts producing no diamictites. This model resolves previous issues related to hypotheses proposing high-obliquity low latitude glaciation of Superia as the only explanation available.
Abstract
Earth’s earliest continental crust is dominated by tonalite–trondhjemite–granodiorite (TTG) suites, making these rocks key to unlocking the global geodynamic regime operating during the ...Archaean (4.0–2.5 billion years ago Ga). The tectonic setting of TTG magmatism is controversial, with hypotheses arguing both for and against subduction. Here we conduct petrological modeling over a range of pressure–temperature conditions relevant to the Archaean geothermal gradient. Using an average enriched Archaean basaltic source composition, we predict Ba concentrations in TTG suites, which is difficult to increase after magma generated in the source. The results indicate only low geothermal gradients corresponding to hot subduction zones produce Ba-rich TTG, thus Ba represents a proxy for the onset of subduction. We then identify statistically significant increases in the Ba contents of TTG suites worldwide as recording the diachronous onset of subduction from regional at 4 Ga to globally complete sometime after 2.7 Ga.
Geological observations and numerical models imply that Archean continents were mostly submarine. In contrast, approximately one third of modern earth's surface area consists of subaerial continental ...crust. To temporally constrain changes in the subaerial exposure of continents, we evaluate the eruptive environment (submarine vs subaerial) of 3.4–2.0 Ga continental large igneous provinces (LIPs). Our results indicate that up until 2.4 Ga LIPs predominantly erupted onto submerged continents. This period of low freeboard was punctuated by local subaerial eruptions at 2.8–2.7 Ga and 2.5 Ga. From 2.4 Ga–2.2 Ga, extensive subaerial continental volcanism is recorded in six different cratons, supporting widespread subaerial continents at this time. An increase in exposed continental crust significantly impacts atmospheric and oceanic geochemical cycles and the supply of nutrients for marine bioproductivity. Thus, the 2.4–2.2 Ga high‐freeboard conditions may have triggered the earliest global glaciation event and the first significant rise of atmospheric oxygen.
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