Meaningful Ages Research Articles

Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites

Zircon, with a chemical formula of ZrSiO4, is a widely used mineral for determining the crystallization age of igneous rocks. It is also used to constrain the timing of metamorphic events from its overgrowth or recrystallized domains. Furthermore, detrital zircon grains can provide information on the sedimentary provenance. Due to the trace amounts of uranium (parent) which decays into its daughter element (Pb), it is a prime geochronometer for the majority of magmatic and metamorphic rocks. With high-precision analytical instruments, such as TIMS, SIMS, and LA-ICP-MS, huge amounts of geochronological and trace element data from zircon have been generated around the globe to date. Target domains within zircon grains are analyzed to extract geochemical and geochronological records using spatially resolved techniques such as ion probes or laser ablation coupled with mass spectrometry. Before any such analysis, the zircon grains are examined for internal structures, growth zoning, and the presence of tiny inclusions. However, many researchers analyze multiple domains within single zircon grains for U-Pb isotope analysis with little regard for their internal structures, particularly crystallographic orientations. Hence, they may obtain mixed ages with variable discordance, leading to imprecise interpretation especially when the growth domains are not well-identified. Particularly, zircon grains that contain multi-growth domains or have local internal deformations within a single grain may not produce geologically meaningful age results if the analyses are conducted on mixed domains. This study presents a brief review on zircon geochronology, how to identify and visualize micro-deformations in metamorphic zircons through the EBSD analysis, and the effects of micro-deformation on age results. Examples from a case study conducted on zircons hosted in the Himalayan high-pressure eclogites are presented that show intra-grain plastically deformed domains and their effects on the corresponding age results.

Dating metamorphic processes and identifying 87Sr/86Sr inheritance using volume-coupled Rb/Sr geochronology and geochemistry of in situ white mica: A demonstration with HP/LT rocks from Syros, Greece

High-pressure/low-temperature rocks from two well-studied outcrops of the Cycladic Blueschist Unit on Syros are used to demonstrate the advantages of volume-coupled 87Rb/87Sr geochronology and geochemistry of in situ white mica. Single-spot Rb/Sr dates are interpreted based on the microstructural position of white mica combined with its chemical information, and the results are discussed within the tectonic framework of the island. The first outcrop on Katerghaki cape exposes rigid blocks of foliated eclogite within well-foliated blueschist, whereas the second outcrop on Delfini peninsula comprises massive eclogitic blocks, likely of metasomatic origin, hosted within blueschist that are interlayered with white mica schists. The rheological contrasts exhibited in both outcrops preserve (near-)peak HP/LT metamorphism/metasomatism (eclogites) followed by retrogression and deformation in lower P-T conditions (blueschist, micaschist). The white mica in eclogite from both outcrops are in equilibrium with the HP/LT mineral assemblages, do not show chemical zoning nor systematic chemical variations, and yield uniform single-spot Rb/Sr dates. Weighted averages of the Rb/Sr dates are 48.2 ± 3.7 Ma (MSWD: 0.9, n: 34; Katerghaki) and 44.5 ± 3.1 Ma (MSWD: 0.7, n: 28; Delfini), interpreted as (near-)peak HP/LT metamorphism and metasomatism, respectively, overlapping with published garnet LuHf and SmNd geochronology of the corresponding outcrops. White mica in the Katerghaki blueschist and the Delfini micaschist exhibit significant chemical zoning and correlations of Ti with major and trace elements (Al, Fe, Mg, Rb, Sr, Zn, Ba, Nb). Weighted age averages were calculated from Rb/Sr dates corresponding to high-Ti and low-Ti white mica compositions. The blueschist yields ages of 56.0 ± 10.6 Ma (MSWD: 2.2, n: 20/80) and 36.3 ± 2.1 Ma (MSWD: 0.9, n: 20/80), whereas the micaschist produced ages of 43.7 ± 3.5 Ma (MSWD: 0.5, n: 14/53) and 31.9 ± 2.7 Ma (MSWD: 1.0, n: 13/53), respectively. These results are interpreted to resolve the timing of fluid-mediated isotopic re-equilibration of white mica during deformation and retrogression (low-Ti), and reflect 87Sr/86Sr inheritance from the earlier HP/LT history (high-Ti). Overall, the results demonstrate that coupling 87Rb/87Sr geochronology with geochemistry of white mica in situ is advantageous for evaluating Rb/Sr datasets that may possess a single mica population (eclogites) or are affected by 87Sr/86Sr inheritance (blueschist, micaschist) in order to interpret geologically meaningful ages.

Paleogene Sedimentary Basin Development in Southern Nevada, USA

Abstract The cause of the transition from Mesozoic and early Cenozoic crustal shortening to later extension in the western United States is debated. In many parts of the extant Sevier hinterland, now the Basin and Range Province, the sedimentary sections that provide the most direct record of that transition remain poorly studied and lack meaningful age control. In this paper, we present field characterization supported by U-Pb detrital zircon and 40Ar/39Ar feldspar ages for ten sections across southern Nevada. We describe a newly identified basin, here named the Fallout Hills basin, which preserves >1.0 km of sedimentary deposits as old as middle Eocene, ca. 48 Ma. Deposition occurred during the 20 m.y. (million years) before the 27.60 ± 0.03 Ma Monotony Tuff blanketed much of south-central Nevada, based on 47.6 Ma and younger detrital zircon maximum depositional ages (MDAs) from near the Pintwater and Spotted Ranges. Elsewhere in southern Nevada, prevolcanic Cenozoic strata commonly form thinner (~100 m), isolated exposures that yield detrital zircon MDAs ≤10 m.y. older than (and in some cases essentially the same age as) the ca. 27–28 Ma ignimbrites that cap the sections. A variable but overall upward-fining facies pattern is observed in both the Fallout Hills basin and the thinner sections. These localized patterns imply topographic changes that are unlikely to reflect plate-scale processes and are not consistent with large-magnitude extension. Instead, variable uplift due to magmatism combined with antecedent topographic relief from thrust faulting and subsequent erosion likely provided accommodation for these deposits.

Minor minerals, major implications: using key mineral phases to unravel the formation and evolution of Earth's crust

Abstract The investigation of key minerals including zircon, apatite, titanite, rutile, monazite, xenotime, allanite, baddeleyite and garnet can retain critical information about petrogenetic and geodynamic processes and may be utilized to understand complex geological histories and the dynamic evolution of the continental crust. They act as small but often robust petrochronological capsules and provide information about crustal evolution, from local processes to plate tectonics and supercontinent cycles. They offer us insights into processes of magmatism, sedimentation, metamorphism and alteration, even when the original protolith is not preserved. In situ techniques have enabled a more in-depth understanding of trace element behaviour in these minerals within their textural context. This has led to more meaningful ages for many stages of geological events. New developments of analytical procedures have further allowed us to expand our petrochronological toolbox while improving precision and accuracy. Combining multiple proxies with multiple minerals has contributed to new interpretations of the crustal history of our planet.

A - 186 Performance Characteristics of the Structured Inventory of Malingered Symptomatology in a Diverse Community Sample of Social Security Disability Applicants.

Analyze performance characteristics of the Structured Inventory of Malingered Symptomatology (SIMS) in a diverse community sample of SSDI applicants. 532 adult SSDI applicants providing research consent were administered the SIMS in conjunction with standard psychodiagnostic, intellectual, and neuropsychological assessments performed in a private practice setting. A broad range of ages and referring diagnoses are represented. Mean age was 42years (SD = 13), with 56% female and 44% male. Based on the nature of the examination, WAIS (n = 47), WMS (n = 60), and Trail Making Test (n = 50) data were obtained, along with performance validity tests (PVTs) including Reliable Digit Span (n = 182), Dot Counting Test (n = 18), and Test of Memory Malingering (n = 70). No meaningful age or gender differences emerged across SIMS scores. The mean SIMS Total Score within this population is 22.9 (SD = 10.1). This compares to a suggested manual cutting score of 14-16 for detection of likely noncredible symptom reporting. 69% of this population produced SIMS Total Scores at or above 16, with 45% of the sample scoring 21 or above. Established studies indicate a roughly 46% base rate of symptom invalidity in SSDI applicants. Standard SIMS cutoff scores when used with this population are likely to overestimate invalidity. Upward adjustment to 21 may be indicated for the SIMS Total Score when attempting to capture a portion of the SSDI population more likely to demonstrate symptom invalidity. Individual subscale performances and correlations with concurrently administered Wechsler scales and PVTs are reported, along with data limitations and additional areas of suggested research.

Supergene phases from ferruginous duricrusts: non-destructive microsampling and mineralogy prior to (U–Th) ∕ He geochronological analysis

Abstract. Interpreting the ages of supergene mineralogical phases in laterite is complex because they consist of polycrystalline mixtures of different phases at the microscopic scale that could be crystalized at different epochs. Among the geochronometers, the (U-Th)/He method on hematite and goethite is more often used, but ages can be difficult to interpret due to phases mixing. To resolve this issue, this study proposes a methodology for performing detailed mineralogical analysis of hematite and goethite single grains prior to their dating using the (U-Th)/He method. Strictly non-destructive mineralogy of single grains is not achievable by classical tools, such as conventional powder XRD (X-ray diffraction; requiring at least some milligrams of powder) or SEM (scanning electron microscopy; that can contaminate the grain by coating or fixing). Therefore, we performed X-ray diffraction patterns of single grains using high-flux X-ray beams from both a rotating anode (XRD_rotat) laboratory diffractometer and a synchrotron beamline (XRD_synch) and compared the results in order to design a method based on XRD_rotat only. For this purpose, two samples from the pisolitic facies of a Brazilian ferruginous duricrust (Alto Paranaíba region, Minas Gerais State, Brazil) were chosen because they presented a usual heterogeneity. Rietveld refinements of the XRD patterns obtained from both XRD_rotat and XRD_synch yielded similar results for the weight percentage ratio of the main phases and mean coherent domain sizes and less similar results for Al substitution rates, thus validating the XRD_rotat approach. No beam damage was observed when increasing X-ray exposure time, neither on XRD patterns nor on (U-Th)/He ages. Hence, sub-millimeter, undisturbed grains can be used to analyze the mineralogy of ferruginous duricrusts by XRD_rotat with a short exposure, and the same grains can subsequently be dated by (U-Th)/He geochronology analysis. The (U-Th)/He dating of pisolitic core and cortex grains also provided meaningful ages: they revealed two evolution phases of the ferruginous duricrust, which occurred at or before the Oligocene for the pisolitic core and middle Miocene for the pisolitic cortex, agreeing with the previous model for the development of pisolites. The mineralogy of single grains selected for dating is helpful for discussing the crystallization ages, and the high-flux XRD approach may be applied to other supergene mineral parageneses used for absolute dating.

New U-Pb baddeleyite ID-TIMS ages from the intrusive high-Ti-Sr rocks of the Southern Paraná LIP, Brazil: Implications for correlations with environmental disturbances during the Early Cretaceous

Large volumes of mafic igneous rocks are commonly emplaced during Large Igneous Province (LIP) eruptions, and these mafic rocks are often contemporaneous with periods of environmental disturbances, such as global ocean anoxia, and as a result, mass extinctions. The Paraná-Etendeka LIP is no exception, and has been previously correlated with, and interpreted to be the cause of the Valanginian oceanic anoxic event (OAE), a small global environmental disturbance. Here, we present new U-Pb ID-TIMS baddeleyite ages from high-Ti-Sr mafic intrusive rocks from the Paraná LIP, in Brazil. While these data are potentially complicated by the presence of Pb-loss and inheritance, it is possible to interpret geologically meaningful ages from them. The first high-precision age is reported for the type-locality of the Florianópolis Dyke Swarm, in North Santa Catarina Island, which yields an age of 132.53 ± 0.40/0.40/0.42 Ma. We also report an age of 132.07 ± 0.27/0.30/0.33 Ma for a dolerite sill, which intrudes organic-rich sedimentary rocks of the Paraná Basin. The emplacement of high-Ti-Sr magmas at ca. 132 Ma suggests that there was up to 2 Myr of intrusive magmatism in the southern part of the Paraná LIP. Further investigation on the mafic intrusive magmatism from the Paraná LIP through robust high-precision geochronology is required to elucidate the proposed linkage more clearly with environmental changes during the Early Cretaceous.

Differentiating the incidence and burden of HIV by age among women who sell sex: a systematic review and meta-analysis.

IntroductionYoung women who sell sex (YWSS) are at heightened risk of HIV acquisition and transmission and are among the least engaged in HIV services. There is insufficient epidemiologic evidence characterizing the burden of HIV among YWSS, particularly as compared to older WSS. These data are needed to design and tailor effective HIV prevention and treatment programmes for this population.MethodsWe conducted two parallel systematic reviews and meta‐analyses to define both the immediate and long‐term HIV risks for YWSS, including among women engaged in sex work, survival sex and transactional sex. In the first review, we identified and synthesized published studies of HIV incidence comparing estimates for cisgender women ≤24 years of age versus >24. In the second review, we identified and synthesized studies of HIV prevalence, comparing estimates for cisgender women who initiated selling sex <18 versus ≥18 years. In both reviews, we completed a search of four databases for articles in any language and any geographic area published from 1 January 1980 until 12 February 2021. Included articles were assessed for quality and a random effects model was used to calculate pooled effect estimates for each review.Results and DiscussionWe identified 12 studies for the HIV incidence review and 18 studies for the HIV prevalence review. In a meta‐analysis, HIV incidence was elevated in younger (5.3 per 100 person‐years [PY]; 95% confidence interval [CI]: 3.5, 7.1) compared to older women (2.8 per 100 PY; 95% CI: 1.7, 3.9), although CIs overlapped. HIV prevalence among those who initiated selling sex <18 years of age (28.8; 95% CI: 18.9, 38.7) was higher than those who initiated later (20.5; 95% CI: 12.4, 28.6).ConclusionsThese companion reviews offer an important perspective on the relative HIV risk of engaging in selling sex at a younger age. Our findings highlight the unique and intersectional challenges YWSS face, and the importance of ensuring that health services are tailored to meet their specific needs. Research and programming should routinely stratify data into meaningful age bands to differentiate and intervene within this population.

Measuring Loneliness in Different Age Groups: The Measurement Invariance of the UCLA Loneliness Scale.

Age differences in the prevalence of loneliness have been a key focus among researchers, practitioners, and policy makers. However, the degree to which those reflect genuine differences in the experience of loneliness or the way individuals understand and respond to loneliness measures is yet to be examined. The current study explored the age measurement invariance of the 20-item Revised University of California Los Angeles, Loneliness Scale (UCLA-LSR) and its shorter forms in a U.K. sample of adults aged 18 to 99 years (M = 50.6, SD = 19.7). The fit of different structures/versions was explored through multigroup confirmatory factor analysis (CFA; N = 4,375) and local structural equation modeling (N = 19,521). Results indicated a poor and/or inconsistent structure for the 20-item UCLA-LSR and many of its shorter forms. Of the structures considered, 12 showed acceptable model fit and received age measurement invariance testing through multigroup CFA and alignment; 10 of these achieved full, partial, or approximate measurement invariance. Our findings suggest that the age measurement invariance of loneliness measures should not be assumed, and crucially, this must be explored before accurate and meaningful age comparisons can be made. Implications for measurement research, and clinical and community practice, are discussed.

In situ Lu–Hf geochronology of calcite

Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and 0.6 % obtained from calcite with Lu concentrations as low as ca. 0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary precipitation ages over a significant amount of geological time, although further research is required to constrain the closure temperature. The in situ approach allows calcite to be rapidly dated while maintaining its petrogenetic context with mineralisation and other associated mineral processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to resolve the timing of complex mineral paragenetic sequences that are a feature of many ancient rock systems.

CD4+/CD8+ Ratio and Growth Differentiation Factor 8 Levels in Peripheral Blood of Large Canine Males Are Useful Parameters to Build an Age Prediction Model.

PurposeTo build an age prediction model, we measured CD4+ and CD8+ cells, and humoral components in canine peripheral blood.Materials and MethodsLarge Belgian Malinois (BGM) and German Shepherd Dog (GSD) breeds (n=27), aged from 1 to 12 years, were used for this study. Peripheral bloods were obtained by venepuncture, then plasma and peripheral blood mononuclear cells (PBMCs) were separated immediately. Six myokines, including interleukin (IL)-6, IL-8, IL-15, leukemia inhibitory factor (LIF), growth differentiation factor 8 (GDF8), and GDF11 were measured from plasma and CD4+/CD8+ T-lymphocytes ratio were measured from PBMC. These parameters were then tested with age prediction models to find the best fit model.ResultsWe found that the T-lymphocyte ratio (CD4+/CD8+) was significantly correlated with age (r=0.46, p=0.016). Among the six myokines, only GDF8 showed a significant correlation with age (r=0.52, p=0.005). Interestingly, these two markers showed better correlations in male dogs than females, and BGM breed than GSD. Using these two age biomarkers, we could obtain the best fit in a quadratic linear mixed model (r=0.77, p=3×10-6).ConclusionsAge prediction is a challenging task because of complication with biological age. Our quadratic linear mixed model using CD4+/CD8+ ratio and GDF8 level showed a meaningful age prediction.

Trends in pulmonary embolism mortality rates by age group in the United States, 1999–2019

IntroductionAcute pulmonary embolism (PE) is a major cause of mortality in the United States. Recent reports indicate that PE-related mortality rates have increased among individuals younger than 65 years old. It remains unclear whether this increase in PE-related mortality is evenly distributed. A narrowly focused and clinically meaningful age group analysis is necessary. MethodsDeath certificate data from the Centers for Disease Control and Prevention Wide-Ranging Online Data for Epidemiologic Research database were examined to determine all-cause PE mortality trends from 1999 to 2019 among adults 25–39, 40–54, 55–69, 70–84, and ≥85 years old. The crude death rates for individual years and annual percentage change (APC) were calculated to determine trends. ResultsPE-related mortality rates increased among those 25–39, 40–54, and 55–69. Among individuals 25–39 years old, death rate increased from 1.8 to 2.0 (APC 0.7 [95% confidence interval (CI) 0.2 to 1.1]) between 1999 and 2014 and continued to increase from 2.0 to 2.4 (APC 4.1 [95% CI 1.8 to 6.5]) between 2014 and 2019. Among those 40–54 years old, the crude death rate increased from 5.7 to 7.5 (APC 2.0 [95% CI, 1.6 to 2.5]) between 2007 and 2019. Among those 55–69 years old the crude death rate increased from 15.6 to 18.5 (APC 2.2 [95% CI, 1.9 to 2.5]) between 2010 and 2019. Recent death rates decreased or plateaued among individuals older than 70. ConclusionsIndividuals younger than 70 years had increase in PE-related mortality between 1999 and 2019 with marked increase among those 25–39 years old.

Abstract 11287: Trends in Pulmonary Embolism Mortality Rates by Age Group in the United States, 1999-2019

Introduction: Acute pulmonary embolism (PE) is a major cause of mortality in the United States. Recent reports indicate that PE-related mortality rates have increased among individuals 25-64 and 65-79 years old and plateaued among individuals ≥80 years old. A narrowly focused and clinically meaningful age group analysis is necessary. Methods: Death certificate data from the Centers for Disease Control and Prevention Wide-Ranging Online Data for Epidemiologic Research database were examined to determine all-cause PE mortality trends from 1999 to 2019 among adults 25-39, 40-54, 55-69, 70-84, and ≥85 years old. The crude death rates for individual years were calculated by dividing the number of PE-related deaths by the corresponding population. Annual percentage change (APC) was calculated to determine trends. Results: Among individuals 25-39 years old, the death rate increased from 1.76 to 2.01 (APC 0.7 [95% confidence interval (CI) 0.2 to 1.1]) between 1999 and 2014 and continued to increase from 2.01 to 2.42 (APC 4.1 [95% CI 1.8 to 6.5]) between 2014 and 2019. Among those 40-54 years old, the death rate increased from 5.7 to 7.46 (APC 2.0 [95% CI 1.6 to 2.5]) between 2007 and 2019. Among those 55-69 years old the death rate increased from 15.64 to 18.53 (APC 2.2 [95% CI 1.9 to 2.5]) between 2010 and 2019. Among those 70-84 years old the death rate decreased from 49.46 to 43.96 (APC -0.7 [95% CI -0.8 to -0.5]) between 1999 and 2019. While death rate among those older than 85 decreased from 112.31 to 100.53 (APC -1.3 [95% CI -1.9 to -0.7]) between 2007 and 2016, the death rate among this cohort increased non-significantly between 2016 and 2019. Conclusions: Despite significant improvements in PE risk stratification and clinical management, recent PE related mortality rates increased significantly among those younger than 69 years old. The APC in mortality was highest among those 25-39 years old. Among individuals older than 70 years old, recent death rates decreased or plateaued.

Pediatric Opioid Prescribing: A Call for Calm.

Pediatric Opioid Prescribing: A Call for Calm.

Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

Abstract Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures.

Rapid erosion of the central Transantarctic Mountains at the Eocene-Oligocene transition: Evidence from skewed (U-Th)/He date distributions near Beardmore Glacier

Apatite (U-Th)/He thermochronology has the potential to reconstruct records of erosional exhumation that are critical to understanding interactions between climate, tectonics, and the cryosphere at high latitudes on million-year timescales. However this approach is often hindered by the problem of intrasample single-grain date dispersion. Here we present an extensive new apatite (U-Th)/He dataset (n = 361) from the central Transantarctic Mountains of East Antarctica between 160°E to 170°W and 84 to 86°S, and show that apparently uninterpretable data in most samples are a reflection of inadequate sampling of skewed date distributions. We outline a workflow for interpreting such dispersed data and demonstrate that geologically meaningful age interpretations are possible in the case of rapidly cooled samples, despite the wide array of potential causes for date dispersion. We show that for samples and compilations with a large number of single-grain analyses (n > ∼25), the youngest probability distribution peak represents the most likely time of fast cooling through the apatite (U-Th)/He closure temperature. When fewer grains are analyzed, the youngest peak is represented best by the minimum date or first quartile date, depending on sample size. Using this workflow, we show that since the latest Eocene, up to 8.8 km of exhumation occurred to incise the deepest point of the Beardmore Glacier trough. Rapid incision began at c. 37-34 Ma (at the latest by 34±3 Ma), coinciding with or slightly preceding the initiation of Antarctic glaciation at the Eocene-Oligocene transition, and contributed to at least 2.6 km of exhumation within the first 3-6 million years, at an apparent exhumation rate of no less than 0.4 mm/a.

Timing and origin of multiple petroleum charges in the Solveig oil field, Norwegian North Sea: A rhenium-osmium isotopic study

The Re-Os geochronometer constrains the timing of petroleum formation and provides an oil-source isotopic correlation tool for improved geologic knowledge and exploration success. However, the effects of secondary processes that may sometimes complicate interpretations are not well understood. This paper discusses Re-Os systematics in a petroleum system that experienced extensive asphaltene precipitation upon mixing of different oil phases. The Solveig oil field (formerly Luno II), Utsira high, Norwegian North Sea, comprises three structural compartments that capture different stages of the mixing process. In the central compartment, an Re-Os age of circa 40 Ma for whole-rock extracts sampling the residual (asphaltene-rich and biodegraded) oil below the oil–water contact is consistent with burial models for Paleocene–Miocene generation. A 10-m-thick, asphaltene-rich (65 wt. %) tar mat zone precipitated upon mixing of at least two components does not yield meaningful ages. The Re-Os isochrons for the free-flowing crude oil and whole-rock extracts from the oil leg above the tar mat zone indicate circa 10 to 0 Ma ages for the younger oil phase, supporting independent estimates for recent oil influx. Decreasing 187Os/188Os from maltenes to asphaltenes in oil from the southeastern compartment are highly unusual and provide geochemical evidence for recent mixing, consistent with the strong asphaltene-in-oil gradient. Very low and remarkably uniform Re/Os ratios for all petroleum phases, combined with relatively low and uniform 187Os/188Os ratios, strongly support isotopic overprint of the Re-Os system, likely during water–oil interaction. Despite this overprint, Re-Os ages retain information on primary events, whereas initial 187Os/188Os ratios are strongly affected.

How Can Technical Aspects Help Improving K-Ar Isotopic Data of Illite-Rich Clay Materials into Meaningful Ages? The Case of the Dominique Peter Uranium Deposit (Saskatchewan, Canada)

Previously published K-Ar dating results of &lt;2 μm illite separates from uranium-hosting and associated barren rocks from Dominique Peter district of the Carswell circular structure in the Athabasca Basin (Canada) were considered to trace four distinct tectonic-thermal events at 1447 ± 45, 1282 ± 40 and 1184 ± 15 (all errors at ± 2 Ma, with a younger, less constrained episode at ~900 Ma. Recent analyses of K-Ar ages of additional &lt;0.2 μm illite fractions from a few initial samples demonstrate that the intermediate ages at ~1280 and ~1185 Ma result, in fact, from mixing of two generations of illite that precipitated at ~1450 and ~900 Ma. They have, therefore, no stratigraphic value, while the two tectonic-thermal episodes at 1448 ± 48 and 937 ± 39 Ma appear to be historically sound. In fact, the analytical procedure of isotopic dating clay materials is of more importance than is often stated. For instance, a safe way to evaluate and constrain best numerical isotopic data of clay separates into ages is combining data of two size splits from several samples. If such age data, especially from size fractions of indurated host rocks, are scattered and point towards the higher data of the coarser fractions, they are potentially enriched in earlier crystallized K-rich components, and should consequently be discarded. The occurrence of detrital or early-crystallized components in clay-rich separates becomes a serious concern when comparing ages generated by various isotopic methods on mineral separates of various whole rocks. It is especially verified in very old, metal-rich deposits such as the uranium-rich deposits of the Saskatchewan Basin. These deposits and their host rocks were studied extensively by a large spectrum of isotopic methods on many types of rocks in a widely dispersed area, and for which the numerical statistics became, sometimes, more central in the interpretation than the specificity of the successive events in the host rocks.

Height growth–rate at a given height: A mathematical perspective for forest productivity

Understanding the height growth of trees is a fundamental component for scientific knowledge and management of forest ecosystems. The height of dominant trees at a reference-age, commonly known as the site-index, is the most widely used forest productivity indicator globally. Yet, it has been criticized for its restricted applicability to monospecific and even-aged forests, making it unreliable for natural forests or mixed-species where there is not a single meaningful age. Here, I develop a mathematical perspective for using height growth-rate at a reference-height as a new type of site index. I provide the mathematical basis for the proposed index and illustrate its application by fitting a nonlinear mixed-effects differential equation model to tree height growth data of three Nothofagus species in southern Chile. The proposed index allows us to foresee and analyze growth patterns, not only by representing growth-rates as a function of time but also of size. In doing so, the proposed index makes tree growth and productivity analyses accessible to a broader community of researchers.

Bypassing the Suess-effect: Age determination of charcoal kiln remains using OSL dating

Ancient forests all over Europe often preserve remains of (pre)historic charcoal production (kilns), informing on past forest composition and anthropogenic woodland exploitation. So far, the chronology of these features has been entirely derived from 14C-dating of associated charcoal. Though generally successful, 14C-dating cannot provide meaningful age information for post-1650 CE features, while most of the archaeological remains of charcoal kilns date to this period. Here, we investigate the potential of quartz-based optically stimulated luminescence (OSL) dating of sediments that were heated during charcoal production as a more precise alternative tool for age determination of these kilns.Using a set of 14 samples collected from both relict charcoal kilns and the underlying parent material, we first document the quartz OSL characteristics in terms of the procedural tests commonly used to assess the appropriateness of the single-aliquot regenerative-dose (SAR) protocol. The results suggest that the measurement procedure allows determining equivalent doses both accurately and precisely. We then demonstrate how the linear part of the dose-response curve can be exploited to examine completeness of resetting and/or post-depositional disturbance without compromising instrument time, precision and laboratory accuracy. Optical ages derived from these distributions are consistent with the available independent (14C) age information for the charcoal kilns. We conclude that OSL can provide an accurate and viable alternative to 14C and will be especially useful to constrain post-1650 CE features in time. Our results also suggest that the approach may allow establishing relative chronologies with a time resolution of 2–4%.