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The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ~0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.

Laser depth profiling of laboratory-induced helium diffusion profiles in natural zircon confirms that helium diffusivity is crystallographically controlled and significantly anisotropic. Experiments on Mud Tank zircon with low degrees of alpha radiation damage (5.6 × 1016 to 1.3 × 1017 α/g) indicate that c ‖ diffusion is ∼400 to 700 times faster than a ‖ diffusion over the experimental temperature range investigated (400–600 °C). This magnitude of diffusive anisotropy implies that zircon crystals with different crystal morphologies record different helium closure temperatures. Zircon diffusion models commonly used in thermal-kinematic modeling programs do not properly account for diffusive anisotropy, and consequently, are likely to over- or underestimate helium closure temperatures in low-damage zircon. Additional experiments on pieces of a large Sri Lankan zircon crystal with strong radiation damage zoning demonstrate that both c ‖ and a ‖ diffusivity – as well as the magnitude of diffusive anisotropy – decrease with increasing radiation damage over an alpha dose range of ∼4.2 × 1017 to 8.5 × 1017 α/g. Decreases in diffusivity appear to reflect changes in the diffusion coefficient D0 and not the activation energy for diffusion. While we did not design our experiments to explore the effect of trace element geochemistry on helium diffusion in zircon in detail, our results suggest that such an effect may be significant.

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has ...

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to...

Most thermochronological studies aimed at constraining exhumation rates rely on bedrock datasets. Often, they involve the analysis of samples collected along an elevation profile in terrains with h...

Since initially developing laser ablation (U-Th)/He procedures for high-spatial-resolution dating of monazite more than a decade ago, our research group has refined the technique to the point that ...

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan.

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40Ar/39Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor (RBHs/GA1550) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; RTCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; RFCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; RACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes. Zircons from the rhyolite of Bodie Hills are strongly zoned in U and Th, and U/Pb geochronologic analyses suggest multiple generations of zircon growth. The youngest ages indicate a crystallization age of 9.97 ± 0.08 Ma (2σ); arguably similar to the 40Ar/39Ar results when residence time is considered, but previous episodes of zircon growth began at least 400 ka prior to eruption. Precise (U - Th)/He thermochronology of these zircons is difficult because crystal-to-crystal variations in the magnitude and complexity of their U + Th zoning complicates accurate alpha ejection calculations. Replicate conventional (single-crystal) (U - Th)/He ages are more widely dispersed than predicted by analytical uncertainties, but laser ablation (U - Th)/He ages are more reproducible, with an inverse variance-weighted mean of 9.71 ± 0.58 Ma.

Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

Paired U/Pb and (U-Th-Sm)/He dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high- and low-temperature thermal evolution of sediment source terrains. Here we present the results of exploratory applications of the laser ablation double-dating (LADD) method – originally developed for detrital zircon research – to the widely available Durango fluorapatite standard in order to evaluate the practicality of the method. Sixty-two laser-ablation analyses of a single large crystal of Durango fluorapatite yielded an inverse-variance weighted mean 206Pb/238U date of 31.46 ± 0.48 Ma and an inverse-variance weighted mean (U-Th-Sm)/He date of 31.75 ± 0.60 Ma, both of which are in good agreement with previously published conventional dates. While these results are encouraging, several factors suggest that LADD may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation. These principally reflect the propensity for apatites to have comparatively lower U + Th concentrations, and thus lower radiogenic He and Pb concentrations, as well as high concentrations of common Pb. These factors contribute to substantially higher analytical imprecision for most LADD U/Pb dates for apatite, occasionally too high for the dates to be geologically useful. Reasonably precise laser ablation (U-Th-Sm)/He dating of detrital apatites requires relatively large crystal sizes (≥ 100 μm in the shortest dimension), with the minimum useful size increasing with decreasing (U-Th-Sm)/He apparent age. In contrast to the geological interpretation of LADD datasets for detrital zircons, the interpretation of datasets for detrital apatites is less straightforward. In particular, researchers should consider carefully the possibility that (U-Th-Sm)/He apparent age distributions are biased by the need to analyze only larger apatites.

High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the high-pressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

Our knowledge of the Solar System has been greatly advanced by exploration with robotic spacecraft, but there are many limitations on the instruments those can carry. More detailed information can be extracted by analyzing samples of Solar System bodies in Earth laboratories. Meteorites provide some natural samples, but the body they came from is often unknown, and they may not represent typical material. Recent years have seen increased interest in directly retrieving samples from other worlds and bringing them back to Earth, a process known as sample return. The most primitive carbonaceous asteroids preserve information about the formation and early evolution of the Solar System and thus have been a high priority for sample return. The Hayabusa mission collected 1500 dust particles from asteroid (25143) Itokawa and returned them to Earth in 2010. A successor mission, Hayabusa2, visited asteroid (162173) Ryugu with the goal of obtaining a larger mass of sample, which is due to arrive on Earth in December 2020. Meanwhile, on 31 December 2018, the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft arrived at another primitive asteroid, (101955) Bennu, with plans to return at least 60 grams of material. Writing in Science and Science Advances , members of the OSIRIS-REx team present results from the survey and reconnaissance phases of the mission. Detailed maps of surface properties were used to identify promising sites for sample collection and provide the necessary scientific context. These data provide information on Bennu's composition, constrain its formation process, and show how its surface evolved. They show that Bennu's surface material has been modified by exposure to space weathering, contains abundant organic material, and has evidence of past alteration by liquid water. OSIRIS-REx successfully collected its sample of Bennu on 20 October 2020; it is due to arrive on Earth in 2023.

Jeremy Jackson Kip Hodges The COVID-19 pandemic is causing devastating mortality, with the highest rates of intensive care unit hospitalization and morbidity among older adults, men, and those with certain preexisting conditions, most notably cardiopulmonary diseases, obesity, and diabetes. In addition, a host of interrelated socioeconomic factors—including race, ethnicity, occupation, and poverty—increase the risks of COVID-19 infection for people of color, health care professionals, and other essential workers. These factors are, in turn, influenced by conditions of the human environment including chronic levels of air pollution, most notably fine particulate matter (PM2.5) that is a well-established risk factor for death from cardiovascular and pulmonary obstructive diseases. This raises the question of whether long-term exposure to higher levels of PM2.5 increases the severity of COVID-19 and, if so, what measures might be taken …

Rainfall rates set the scaling of relief and river incision in tectonically active landscapes., The ongoing debate about the nature of coupling between climate and tectonics in mountain ranges derives, in part, from an imperfect understanding of how topography, climate, erosion, and rock uplift are interrelated. Here, we demonstrate that erosion rate is nonlinearly related to fluvial relief with a proportionality set by mean annual rainfall. These relationships can be quantified for tectonically active landscapes, and calculations based on them enable estimation of erosion where observations are lacking. Tests of the predictive power of this relationship in the Himalaya, where erosion is well constrained, affirm the value of our approach. Our model allows estimation of erosion rates in fluvial landscapes using readily available datasets, and the underlying relationship between erosion and rainfall offers the promise of a deeper understanding of how climate and tectonic evolution affect erosion and topography in space and time and of the potential influence of climate on tectonics.

New 40Ar/39Ar thermochronological and deformation temperature analyses in the Scandian (c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr−1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr−1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c. 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a−1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge. Supplementary material:40Ar/39Ar analytical data for muscovite (Supplementary Data Table 1), 40Ar/39Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

Dramatic, short-term improvements in the global environment have accompanied the COVID-19 pandemic. Intrigued by these observations, we commissioned a series of papers for Science Advances dedicated to studies of pandemics from an environmental perspective. The first in this series is a paper by Liu et al . on the decline of nitrogen dioxide over China since the outbreak of COVID-19. Collectively, the series will explore how the Earth system as a whole – not just the human societies developed on it – is affected by pandemics like COVID-19, and how environmental conditions can influence the spread and severity of infection. The responses of governments around the world to the COVID-19 catastrophe is an ongoing, essentially uncontrolled experiment. We should not forget that this experiment is playing out in the context of another uncontrolled experiment of …

Pablo A. Marquet Shahid Naeem Jeremy B. C. Jackson Kip Hodges “ Biology must become central to climate change science and policy formulation. The planet does not work just as a physical system; that reality needs to become fundamental to the way we pursue the science and derive policy recommendations. ”—Thomas E. Lovejoy ( 1 ) This planet is the home of life, born into existence and transformed over 3.8 billion years into a continuous tapestry, covering all possible places from the deep ocean floors to mountain summits. Ours is a bioclimatic world in which every organism, from bacterium to blue whale, inseparably contributes to the climate and surface conditions of Earth. This tapestry, of which we are a part, is unraveling, with its delicate patterns and motifs denigrated to near invisibility, disappearing at a rate and magnitude that rivals that of the great mass extinction events of the past ( 2 , 3 ). This fading to nonexistence is making us unfortunate witnesses to the accumulated consequences of human actions over the past 10,000 years. Happily, though, we are now increasingly empowered by science and can act to abate ongoing trends and protect planetary resources before the essential threads of life’s coherence become completely eroded. Each year since 1995, participants in the United Nations Framework Convention on Climate Change (UNFCCC) meet as a Conference of the Parties (COP) to determine how best to address the increasingly harmful changes now taking place. This Special Collection of articles has been timed to coincide with the COP25 meeting scheduled to be held in Spain this year. The Collection provides comprehensive review articles and original research by leading authorities on recent advances in the study of interactions between biodiversity and climate that deepen our understanding of bioclimatic changes and can provide guidance on how best …

The Chinese Tianshan is located in the south of the Central Asian Orogenic Belt and formed during final consumption of the Paleo-Asian Ocean in the late Palaeozoic. In order to further elucidate the tectonic evolution of the Chinese Tianshan, we have established the temperature-time history of granitic rocks from the Chinese Tianshan through a multi-chronological approach that includes U/Pb (zircon), 40Ar/39Ar (biotite and K-feldspar), and (U-Th)/He (zircon and apatite) dating. Our data show that the central Tianshan experienced accelerated cooling during the late Carboniferous- to early Permian. Multiple sequences of complex multiple accretionary, subduction and collisional events could have induced the cooling in the Tianshan Orogenic Belt. The new 40Ar/39Ar and (U-Th)/He data, in combination with thermal history modeling results, reveal that several tectonic reactivation and exhumation episodes affected the Chinese central Tianshan during middle Triassic (245–210 Ma), early Cretaceous (140–100 Ma), late Oligocene-early Miocene (35–20 Ma) and late Miocene (12–9 Ma). The middle Triassic cooling dates was only found in the central Tianshan. Strong uplift and deformation in the Chinese Tianshan has been limited and localized. It have been concentrated in around major fault zone and the foreland thrust belt since the early Cretaceous. The middle Triassic and early Cretaceous exhumation is interpreted as distal effects of the Cimmerian collisions (i.e. the Qiangtang and Kunlun-Qaidam collision and Lhasa-Qiangtang collision) at the southern Eurasian margin. The Cenozoic reactivation and exhumation is interpreted as a far field response to the India-Eurasia collision and represents the beginning of modern mountain building and denudation in the Chinese Tianshan.

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523 Ma and 88.0 to 138.9 Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474 Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520 Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15 × 1019 α /g, spanning the complete damage spectrum. We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample’s thermal history.

Many planetary surfaces in the Solar System have experienced prolonged bombardment. With each impact, new rocks can be assembled that incorporate freshly generated impact melts with fragments of older rocks. Some breccias can become polygenetic, containing multiple generations of impact melt products, and can potentially provide important insights into the extensive bombardment history of a region. However, the amount of chronological information that can be extracted from such samples depends on how well the mineral-isotopic systems of geochronometers can preserve the ages of individual melt generations without being disturbed by younger events. We model the thermal evolution of impact melt veins and the resulting loss of Ar from K-bearing phases common in impact melt breccias to assess the potential for preserving the 40Ar/39Ar ages of individual melt generations. Our model results demonstrate that millimeter-scale, clast-free melt veins cause significant heating of adjacent host rock minerals, and can cause detectable Ar loss in contact zones that are generally thinner than, and at most about the same thickness as, the vein width. The incorporation of cold clasts in melt veins reduces the magnitudes of heating and Ar loss in the host rocks, and Ar loss can be virtually undetectable for sufficiently clast-rich veins. Quantitative evidence of the timing of impacts, as measured with the 40Ar/39Ar method, can be preserved in polygenetic impact melt breccias, particularly for those containing mm-scale bodies of clast-bearing melt products.

Highlights • The South Tibetan detachment system played a major role in Himalayan evolution. • Near Mt Everest, the detachment system accommodated large displacements under both brittle and ductile conditions. • Rapid cooling of footwall rocks reflected tectonic denudation by brittle slip from ca. 15.6 to at least 13.0 Ma. • Thermal–kinematic modeling suggests displacement on the detachment to be at least 61 km. Abstract North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) are tectonically important features of the Himalayan–Tibetan orogenic system. The STDS is best exposed in the N–S-trending Rongbuk Valley in southern Tibet, where the primary strand of the system – the Qomolangma detachment – can be traced down dip from the summit of Everest for a distance of over 30 km. The metamorphic discontinuity across this detachment implies a large net displacement, with previous studies suggesting >200 km of slip. Here we refine those estimates through thermal–kinematic modeling of new (U–Th)/He and 40Ar/39Ar data from deformed footwall leucogranites. While previous studies focused on the early ductile history of deformation along the detachment, our data provide new insights regarding the brittle–ductile to brittle slip history. Thermal modeling results generated with the program QTQt indicate rapid, monotonic cooling from muscovite 40Ar/39Ar closure (ca. 15.4–14.4 Ma at ca. 490 °C) to zircon (U–Th)/He closure (ca. 14.3–11.0 Ma at ca. 200 °C), followed by slower cooling to apatite (U–Th)/He closure at ca. 9–8 Ma (at ca. 70 °C). Although previous work has suggested that ductile slip on the detachment lasted only until ca. 15.6 Ma, thermal–kinematic modeling of our new data suggests that rapid (ca. 3–4 km/Ma) tectonic exhumation by brittle–ductile to brittle fault slip continued to at least ca. 13.0 Ma. Much lower modeled exhumation rates (≤0.5 km/Ma) after ca. 13 Ma are interpreted to reflect erosional denudation rather than tectonic exhumation. Projection of fault-related exhumation rates backward through time suggests total slip of ca. 61 to 289 km on the Qomolangma detachment, with slightly more than a third of that slip occurring under brittle–ductile to brittle conditions.

The (U–Th)/He method has been applied to constrain the formation ages of the Clearwater West and East impact structures of Quebec, Canada. Zircons were separated from impact melt samples derived from a surface exposure at Clearwater West (32 km diameter), and from a drill core at Clearwater East (26 km diameter). The (U–Th)/He results indicate ages of 280±27 Ma (2σ, n=7) for Clearwater West, and 450±56 Ma (2σ, n=8) for Clearwater East. Our (U–Th)/He date for Clearwater West supports the findings of previous Rb–Sr (266±15 Ma; 2σ) and 40Ar/39Ar (280±4 Ma and 283.8±2.2 Ma, 2σ) impact melt studies. Our (U–Th)/He date for Clearwater East also overlaps with previously published 40Ar/39Ar dating results, which yielded U-shaped spectra, with ‘maximum’ and ‘best-estimate’ dates of ∼ 460–470 Ma. Our results support the contention, previously based solely on 40Ar/39Ar data, that the Clearwater West and East impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair.

The comparison of K–Ar and 40Ar/39Ar geochronologic data published by different laboratories is markedly hindered by the inconsistent use of the parameters necessary to convert isotopic analyses to dates. This problem is particularly acute when we try to evaluate the significance of datasets obtained prior to the development of community consensus values for basic decay constants, isotopic abundances, and the ages of common monitor minerals. Unfortunately, the effect of using different parameters for the same dataset can sometimes exceed the quoted analytical precision of derived dates. We created the Argon Age Recalculator, or ArAR, to help researchers account for such discrepancies in a simple, efficient manner, allowing for more robust comparisons among datasets and more effective compilation of existing datasets using self-consistent parameter sets. ArAR is freely available as a platform independent executable application at: http://group18software.asu.edu.

Kip V. Hodges Harrison H. Schmitt July marked the beginning of a 3-year-long celebration of one of the most remarkable periods of human exploration in history: the Apollo missions to the Moon. Reflections on Apollo have inspired a renewed international interest in sustained lunar exploration. While there are many motivations for traveling to the surface of the Moon, experience shows that a sustained program of science activities on the lunar surface would yield unique and invaluable scientific data. Although the Apollo missions themselves ended nearly five decades ago, Apollo science continues today. Building on field scientific observations and sampling by the Apollo astronauts, we now know that the Moon is very old, forming perhaps only a few tens of million years after the 4.56-billion-year origin of Earth and the Solar System. Unfortunately, our planet preserves only sparse evidence of its own ancient history as a consequence of the continuous recycling of Earth’s crust through plate tectonics. There are no plate tectonics on the Moon and none of the erosional effects of wind, water, and flowing ice that drive most of the surface evolution of Earth. As a consequence, the Moon represents an …

40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

Climatology Regional studies of paleoclimate provide important insights into how different parts of the Earth system respond to global climate change. Geochemical data gathered for cave deposits from northeast Greenland have now provided the first paleoclimate record for the High Arctic during an interglacial warming event extending from 588 to 549 thousand years ago. Moseley et al. show that the High Arctic at that time was at least 3.5°C warmer than today during that interval, with extensive permafrost thaw and markedly increased precipitation. Comparisons with datasets for that interval from elsewhere in the world suggest that the Arctic regions were affected more substantially during this warming event, and the same can be anticipated as anthropogenic global warming continues into the future. Sci. Adv. 10.1126/sciadv.abe1260 (2021).

Water Resources Freshwater resources are limited on oceanic islands, and their scarcity complicates sustainability efforts. Attias et al. identified an important new freshwater resource in the submarine environment offshore of the island of Hawai'i. Previous studies suggested that as much as 40% of water input to the onshore aquifer is missing and unavailable for use. The salinity of seawater makes it more electrically conductive than freshwater, and electromagnetic imaging surveys off the coast of Hawai'i revealed large quantities of freshwater in deep submarine sediments just offshore. These alternative water resources improve the prospects of sustainable water extraction for Hawai'i, and the authors speculate that the same mechanism may operate on other volcanic islands. Sci. Adv. 10.1126/sciadv.abd4866 (2020).

X-ray fluorescence (XRF) spectroscopy is a well-established and commonly used technique in obtaining diagnostic compositional data on geological samples. Recently, developments in X-ray tube and detector technologies have resulted in miniaturized, field-portable instruments that enable new applications both in and out of standard laboratory settings. These applications, however, have not been extensively applied to geologic field campaigns. This study investigates the feasibility of using developing handheld XRF (hXRF) technology to enhance terrestrial field geology, with potential applications in planetary surface exploration missions. We demonstrate that the hXRF is quite stable, providing reliable and accurate data continuously over a several year period. Additionally, sample preparation is proved to have a marked effect on the strategy for collecting and assimilating hXRF data. While the hXRF is capable of obtaining data that are comparable to laboratory XRF analysis for several geologically-important elements (such as Si, Ca, Ti, and K), the instrument is unable to detect other elements (such as Mg and Na) reliably. While this limits the use of the hXRF, especially when compared to laboratory XRF techniques, the hXRF is still capable of providing the field user with significantly improved contextual awareness of a field site, and more work is needed to fully evaluate the potential of this instrument in more complex geologic environments.

The Himalaya grow as the Indian Plate is thrust beneath Tibet. Analysis of surface deformation caused by the 2015 Gorkha earthquake suggests slip on smaller-scale faults at the foot of the high Himalaya help build Earth’s highest peaks.

Mature orogenic systems built by continent-continent collision feature orogenic plateaus flanked by accretionary wedges. Thermal-mechanical models of these systems predict the development of a thermally weakened orogenic infrastructure that is capable of lateral flow toward the orogenic foreland. Such flow, if it occurs, strongly influences the evolutionary pathway of a wedge. Although the architecture of a wedge features numerous large-displacement faults, three are preeminent in mature orogens: one that marks the base of the wedge and two others that mark the base and top, respectively, of the weakened infrastructure. These structures represent major decoupling horizons separating domains with distinctive deformational and thermal histories. Reviews of the geology of orogenic wedges in two mature orogenic systems—the Cenozoic Himalaya and the Paleozoic East Greenland Caledonides—show how this simple conceptual model provides a valuable context for studies of how collisional orogenic systems develop and how they interact with the surrounding lithosphere.

Excimer laser technologies enable a rapid and effective approach to simultaneous U/Pb geochronology and (U–Th)/He thermochronology of a wide range of detrital accessory minerals. Here we describe the ‘laser ablation double dating’ (LADD) method and demonstrate its viability by applying it to zircon and titanite crystals from the well-characterized Fish Canyon tuff. We found that LADD dates for Fish Canyon zircon (206Pb/238U – 28.63 ± 0.11 Ma; (U–Th)/He – 28.38 ± 0.73 Ma) are statistically indistinguishable from those obtained through established, traditional methods of single-crystal dating. The same is true for Fish Canyon titanite LADD dates: 206Pb/238U – 28.08 ± 0.90 Ma; (U–Th)/He – 27.98 ± 0.86 Ma. As anticipated, given that LADD involves the analysis of smaller amounts of material than traditional methods, it yields dates with higher analytical uncertainty. However, this does not substantially reduce the utility of the results for most applications to detrital datasets. An important characteristic of LADD is that it encourages the chemical characterization of crystals by backscattered electron, cathodoluminescence, and/or Raman mapping prior to dating. In addition, by permitting the rapid and robust dating of crystals regardless of the degree of their abrasion during sedimentary transport, the method theoretically should yield dates that are more broadly representative of those of the entire population of detrital crystals in a natural sample.

Prior studies have proposed tectonic and climatic mechanisms to explain surface uplift throughout the Bhutan Himalaya. While the resulting enigmatic, low-relief landscapes, elevated above deeply incised canyons, are a popular setting to test ideas of interacting tectonic and climatic forces, when and why these landscapes formed is still debated. We test the idea that these landscapes were created by a spatially variable and recent increase in rock uplift rate associated with the formation of structural duplexes at depth. We utilize a new suite of erosion rates derived from detrital cosmogenic nuclide techniques, geomorphic observations, and a landscape evolution model to demonstrate the viability of this hypothesis. Low-relief landscapes in Bhutan are eroding at a rate of ~70 m/Ma, while basins from surrounding steep landscapes yield erosion rates of ~950 m/Ma, demonstrating that this portion of the range is in a transient period of increasing relief. Applying insights from our erosion rates, we explore the influence of an active duplex on overlying topography using a landscape evolution model by imposing a high rock uplift rate in the middle of a mountain range. Our simulations show that low-relief landscapes with thick alluvial fills form upstream of convex knickpoints as rivers adjust to higher uplift rates downstream, a pattern consistent with geologic, geomorphic, and thermochronometric data from Bhutan. With our new erosion rates, reconstructed paleo-river profiles, and landscape evolution simulations, we show that the low-relief landscapes were formed in situ as they were uplifted ~800 m in the past ~0.8–1 Ma.