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Statistical sampling of a large geochemical database reveals a pervasive discontinuity about 2.5 billion years ago, indicating marked changes in mantle and deep-crustal melting, and providing a link between deep Earth processes and the rise of atmospheric oxygen on the Earth. A geochemical discontinuity in the Archaean Brenhin Keller and Blaire Schoene apply statistical sampling techniques to a geochemical database of about 70,000 samples from continental igneous rocks to produce a record of secular geochemical evolution throughout Earth's history. They find, superimposed on the expected gradual geochemical evolution attributable to secular cooling of Earth, a pervasive geochemical discontinuity approximately 2.5 billion years ago. This discontinuity is indicative of dramatic decreases in mantle melt fraction in basalts and in deep crustal melting/fractionation indicators. The Archaean/Proterozoic geochemical transition revealed by this analysis coincides with sudden atmospheric oxygenation at the end of the Archaean aeon, providing a temporal link between deep Earth geochemistry and the rise of atmospheric oxygen. The Earth has cooled over the past 4.5 billion years (Gyr) as a result of surface heat loss and declining radiogenic heat production. Igneous geochemistry has been used to understand how changing heat flux influenced Archaean geodynamics 1 , 2 , but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias 3 , 4 , 5 . Here we apply statistical sampling techniques to a geochemical database of about 70,000 samples from the continental igneous rock record to produce a comprehensive record of secular geochemical evolution throughout Earth history. Consistent with secular mantle cooling, compatible and incompatible elements in basalts record gradually decreasing mantle melt fraction through time. Superimposed on this gradual evolution is a pervasive geochemical discontinuity occurring about 2.5 Gyr ago, involving substantial decreases in mantle melt fraction in basalts, and in indicators of deep crustal melting and fractionation, such as Na/K, Eu/Eu* (europium anomaly 4 ) and La/Yb ratios in felsic rocks. Along with an increase in preserved crustal thickness across the Archaean/Proterozoic boundary 6 , 7 , these data are consistent with a model in which high-degree Archaean mantle melting produced a thick, mafic lower crust and consequent deep crustal delamination and melting—leading to abundant tonalite–trondhjemite–granodiorite magmatism and a thin preserved Archaean crust. The coincidence of the observed changes in geochemistry and crustal thickness with stepwise atmospheric oxidation 8 at the end of the Archaean eon provides a significant temporal link between deep Earth geochemical processes and the rise of atmospheric oxygen on the Earth.

Temporal correlation between some continental flood basalt eruptions and mass extinctions has been proposed to indicate causality, with eruptive volatile release driving environmental degradation and extinction. We tested this model for the Deccan Traps flood basalt province, which, along with the Chicxulub bolide impact, is implicated in the Cretaceous-Paleogene (K-Pg) extinction approximately 66 million years ago. We estimated Deccan eruption rates with uranium-lead (U-Pb) zircon geochronology and resolved four high-volume eruptive periods. According to this model, maximum eruption rates occurred before and after the K-Pg extinction, with one such pulse initiating tens of thousands of years prior to both the bolide impact and extinction. These findings support extinction models that incorporate both catastrophic events as drivers of environmental deterioration associated with the K-Pg extinction and its aftermath.

Deep-ocean O2 concentrations over the past 3.5 billion years are estimated using the oxidation state of iron in submarine basalts and indicate that deep-ocean oxygenation occurred in the Phanerozoic. Oxygen in the deep Oxygenation of the deep ocean associated with a rise in atmospheric oxygen levels in the geological past is thought to signal the emergence of modern marine biogeochemical cycles. Estimates of the timing of deep-ocean oxygenation and the related increase in atmospheric oxygen levels range from about 800 to 400 million years ago and are generally based on geochemical signatures that indirectly reflect the geochemical state of the deep ocean. This paper presents a more direct, quantitative constraint on the deep-ocean oxygen content from the Archaean to the Cenozoic based on the oxidation state of iron in submarine basalts. The authors suggest that deep-ocean oxygenation occurred in the Phanerozoic and probably not until the late Palaeozoic, less than 420 million years ago. The oxygenation of the deep ocean in the geological past has been associated with a rise in the partial pressure of atmospheric molecular oxygen (O 2 ) to near-present levels and the emergence of modern marine biogeochemical cycles 1 , 2 , 3 , 4 , 5 . It has also been linked to the origination and diversification of early animals 3 , 5 , 6 , 7 . It is generally thought that the deep ocean was largely anoxic from about 2,500 to 800 million years ago 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , with estimates of the occurrence of deep-ocean oxygenation and the linked increase in the partial pressure of atmospheric oxygen to levels sufficient for this oxygenation ranging from about 800 to 400 million years ago 3 , 5 , 7 , 11 , 13 . Deep-ocean dissolved oxygen concentrations over this interval are typically estimated using geochemical signatures preserved in ancient continental shelf or slope sediments, which only indirectly reflect the geochemical state of the deep ocean. Here we present a record that more directly reflects deep-ocean oxygen concentrations, based on the ratio of Fe 3+ to total Fe in hydrothermally altered basalts formed in ocean basins. Our data allow for quantitative estimates of deep-ocean dissolved oxygen concentrations from 3.5 billion years ago to 14 million years ago and suggest that deep-ocean oxygenation occurred in the Phanerozoic (541 million years ago to the present) and potentially not until the late Palaeozoic (less than 420 million years ago).

The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks—volcanic and plutonic rocks—and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation. A global geochemical data set of volcanic and plutonic rocks indicates that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Continental crust formation Brenhin Keller and co-authors present a global geochemical dataset of the two fundamental building blocks of the continental crust, volcanic (externally erupted) and plutonic (internally solidified) rocks. Their results indicate that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. This work indicates that in both tectonic settings, fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas.

The Great Unconformity, a profound gap in Earth’s stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a century. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terrestrial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of 3–5 vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard.

The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation averaging 3 to 5 km during Cryogenian (717 to 635 Ma) snowball Earth glaciations or, alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1,000 to 540 Ma) due to plate tectonic reorganization from supercontinent assembly and breakup. To test these hypotheses, the temporal patterns of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium- to low-temperature thermochronology and geologic information. We present inverse time–temperature simulations within a Bayesian modeling framework that record a consistent signal of relatively rapid, high-magnitude cooling of ∼120 to 200 °C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ∼3 to 5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents.

The nature of Earth’s earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism.

Trace element compositional trends in zircons separated from single hand samples have been used to infer dynamic processes in magma reservoirs. Here, we compile published zircon trace element chemistry to quantify any systematic difference between the range of compositions observed in zircon from individual volcanic and plutonic hand samples and compare these results with geochemical modeling to derive implications for magma reservoir dynamics. We find that both rock types span a wide range of hand‐sample scale variability (i.e., wide range of coefficients of variation), but there is no systematic difference in the average variability between plutonic and volcanic samples (i.e., no difference in the mean coefficient of variation). This indicates that dynamic processes related to eruption are not necessarily required as a fundamental process to create hand sample‐scale compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. Modeling of felsic systems (>68.5 wt.% SiO2) indicates that the similar average variability in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization of compositionally distinct melts locally within a magma reservoir (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melt compositions at a small spatial scale. This study provides a framework for focused studies on individual volcanic‐plutonic systems exploring how plutonic and volcanic zircon compositional variability records the time and length scales of magma reservoir processes. Plain Language Summary Studies of volcanic rocks (erupted magmas) and plutonic rocks (unerupted magmas) provide insights into dynamic processes operating in magma reservoirs (e.g., mixing, crystal‐melt separation, etc.). However, contemporaneous volcanic and plutonic rocks of the same magmatic system are rarely exposed together, thus conceptual models of magma reservoir dynamics are seldom integrated directly between volcanic and plutonic studies. Zircon is a common mineral in crustal magmas (volcanic and plutonic) and is capable of recording melt evolution via its trace element chemistry. This study aims to gain insights into magma reservoir dynamics by systematically comparing trace element compositional variability of zircon separated from individual volcanic and plutonic hand samples. Our study shows that there is no systematic difference in the average compositional variability between plutonic and volcanic hand samples. This indicates that processes leading to eruptions do not necessarily introduce compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. We further show using geochemical modeling that the observed similar average variability of zircon in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melts (i.e., open‐system behavior). Key Points Zircon trace element chemistry from volcanic and plutonic hand samples does not show a difference in their average compositional variability Similar variability suggests that processes leading to eruption do not introduce systematically more heterogeneity than present in unerupted parts of the reservoir Crystallization modeling requires open‐system behavior at a scale of decimeters to reproduce the average variability in both volcanic and plutonic hand samples

Understanding Hadean (>4 Ga) Earth requires knowledge of its crust. The composition of the crust and volatiles migrating through it directly influence the makeup of the atmosphere, the composition of seawater, and nutrient availability. Despite its importance, there is little known and less agreed upon regarding the nature of the Hadean crust. By analyzing the 87Sr/86Sr ratio of apatite inclusions in Archean zircons from Nuvvuagittuq, Canada, we show that its protolith had formed a high (>1) Rb/Sr ratio reservoir by at least 4.2 Ga. This result implies that the early crust had a broad range of igneous rocks, extending from mafic to highly silicic compositions.

Recent attempts to establish the eruptive history of the Deccan Traps large igneous province have used both U−Pb (Schoene et al., 2019) and 40Ar/39Ar (Sprain et al., 2019) geochronology. Both of these studies report dates with high precision and unprecedented coverage for a large igneous province and agree that the main phase of eruptions began near the C30n–C29r magnetic reversal and waned shortly after the C29r–C29n reversal, totaling ∼ 700–800 kyr duration. These datasets can be analyzed in finer detail to determine eruption rates, which are critical for connecting volcanism, associated volatile emissions, and any potential effects on the Earth's climate before and after the Cretaceous–Paleogene boundary (KPB). It is our observation that the community has frequently misinterpreted how the eruption rates derived from these two datasets vary across the KPB. The U−Pb dataset of Schoene et al. (2019) was interpreted by those authors to indicate four major eruptive pulses before and after the KPB. The 40Ar/39Ar dataset did not identify such pulses and has been largely interpreted by the community to indicate an increase in eruption rates coincident with the Chicxulub impact (Renne et al., 2015; Richards et al., 2015). Although the overall agreement in eruption duration is an achievement for geochronology, it is important to clarify the limitations in comparing the two datasets and to highlight paths toward achieving higher-resolution eruption models for the Deccan Traps and for other large igneous provinces. Here, we generate chronostratigraphic models for both datasets using the same statistical techniques and show that the two datasets agree very well. More specifically, we infer that (1) age modeling of the 40Ar/39Ar dataset results in constant eruption rates with relatively large uncertainties through the duration of the Deccan Traps eruptions and provides no support for (or evidence against) the pulses identified by the U−Pb data, (2) the stratigraphic positions of the Chicxulub impact using the 40Ar/39Ar and U−Pb datasets do not agree within their uncertainties, and (3) neither dataset supports the notion of an increase in eruption rate as a result of the Chicxulub impact. We then discuss the importance of systematic uncertainties between the dating methods that challenge direct comparisons between them, and we highlight the geologic uncertainties, such as regional stratigraphic correlations, that need to be tested to ensure the accuracy of eruption models. While the production of precise and accurate geochronologic data is of course essential to studies of Earth history, our analysis underscores that the accuracy of a final result is also critically dependent on how such data are interpreted and presented to the broader community of geoscientists.