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1.
Helium isotope analyses are central to modern earth science and measured by many noble gas laboratories around the globe (Burnard, 2013; Wieler et al., 2002), spanning a wide spectrum of fundamental research – from identifying primordial reservoirs in the Earth mantle to paleoclimate reconstructions. The CRONUS-Earth initiative included the manufacturing, distribution and analysis of a pyroxene reference material (CRONUS-P) that was designed to be useful for internal reliability control of 3He measurements within a few percent and potentially for 4He on a higher level of uncertainty.This short paper describes the CRONUS-P material and its performance as 3He and 4He reference sample for noble gas laboratories. The companion paper by Blard et al. 2015 describes in depth the inter-laboratory helium isotope experiment within CRONUS-Earth.We show normalized helium isotope data of CRONUS-P measured at three different noble gas laboratories. Data from all three laboratories show no relation between helium isotope concentrations and sample mass, implying that the material is homogeneous. The data show that CRONUS-P is useful as an internal standard for 3He within better 2% (1σ) and for 4He within better 10%.  相似文献   

2.
3He is among the most commonly measured terrestrial cosmogenic nuclides, but an incomplete understanding of the 3He production rate has limited robust interpretation of cosmogenic 3He concentrations. We use new measurements of cosmogenic 3He in olivine from a well-dated lava flow at Tabernacle Hill, Utah, USA, to calibrate the local 3He production rate. The new 3He measurements (n = 8) show excellent internal consistency and yield a sea level high latitude (SLHL) production rate of 123 ± 4 at g?1 yr?1 following the Lal (1991)/Stone (2000) scaling model [Lal, D., 1991. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, 104, 424–439.; Stone, J.O., 2000. Air pressure and cosmogenic isotope production. Journal of Geophysical Research, 105, 23753–23759.]. We incorporate the new measurements from Tabernacle Hill in a compilation of all published production rate determinations, characterizing the mean global SLHL production rates (e.g. 120 ± 9.4 at g?1 yr?1 with Lal (1991)/Stone (2000)). The internal consistency of the global 3He production rate dataset is as good as the other commonly used cosmogenic nuclides. Additionally, 3He production rates in olivine and pyroxene agree within experimental error. The 3He production rates are implemented in an age and erosion rate calculator, forming a new module of the CRONUS-Earth web-based calculator, a simple platform for cosmogenic nuclide data interpretation [Balco, G., Stone, J., Lifton, N.A., and Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology, 3, 174–195.]. The 3He calculator is available online at http://www.cronuscalculators.nmt.edu/.  相似文献   

3.
We performed an interlaboratory comparison study with the aim to determine the accuracy of cosmogenic 21Ne measurements in quartz. CREU-1 is a natural quartz standard prepared from amalgamated vein clasts which were crushed, thoroughly mixed, and sieved into 125–250 μm and 250–500 μm size fractions. 50 aliquots of CREU-1 were analyzed by five laboratories employing six different noble gas mass spectrometers. The released gas contained a mixture of 16–30% atmospheric and 70–84% non-atmospheric (predominantly cosmogenic) 21Ne, defining a linear array on the 22Ne/20Ne-21Ne/20Ne three isotope diagram with a slope of 1.108 ± 0.014. The internal reproducibility of the measurements is in good agreement with the formal analytical precision for all participating labs. The external reproducibility of the 21Ne concentrations between labs, however, is significantly overdispersed with respect to the reported analytical precision. We report an average reference concentration for CREU-1 of 348 ± 10 × 106 at [21Ne]/g[SiO2], and suggest that the 7.1% (2σ) overdispersion of our measurements may be representative of the current accuracy of cosmogenic 21Ne in quartz. CREU-1 was tied to CRONUS-A, which is a second reference material prepared from a sample of Antarctic sandstone. We propose a reference value of 320 ± 11 × 106 at/g for CRONUS-A. The CREU-1 and CRONUS-A intercalibration materials may be used to improve the consistency of cosmogenic 21Ne to the level of the analytical precision.  相似文献   

4.
Precise 40Ar/39Ar age determinations made on basalt groundmass collected from the SP and upper and lower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields of Arizona, USA, yield ages of 72 ± 4, 97 ± 10, and 123 ± 12 ka (2σ; relative to Renne et al., 2010, 2011, full external precision), respectively. Previous ages of the SP lava flow include a K–Ar age of 70 ± 8 ka and OSL ages of 5.5–6 ka. 40Ar/39Ar age constraints, relative to the optimization model of Renne et al. (2010, 2011), of 81 ± 50 and 118 ± 64 ka (2σ; full external precision) were previously reported for the upper and lower Bar Ten lava flows, respectively. The new 40Ar/39Ar ages are within uncertainty of previous age constraints, and are more robust, accurate, and precise. Preliminary cosmogenic 3He and 21Ne production rates from the Bar Ten flows reported by Fenton et al. (2009) are updated here, to account for the improved quality of the 40Ar/39Ar data. The new 40Ar/39Ar age for the SP flow yields cosmogenic 3He and 21Ne production rates for pyroxene (119 ± 8 and 26.8 ± 1.9 at/g/yr; error-weighted mean, 2σ uncertainty; Dunai (2000) scaling method) that are consistent with production rate values reported throughout the literature. The 40Ar/39Ar and cosmogenic 3He and 21Ne data support field observations indicating the SP flow has undergone negligible erosion. The SP flow contains co-existing phenocrysts of olivine and pyroxene, as well as xenocrysts of quartz in a fine-grained groundmass facilitating cross-calibration of cosmogenic production rates and production-rate (3He, 10Be, 14C, 21Ne, 26Al, and 36Cl). Thus, we propose the SP flow is an excellent location for a cosmogenic nuclide production-rate calibration site (SPICE: the SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations).  相似文献   

5.
In-situ cosmogenic 3He exposure ages of pyroxene phenocrysts from basalts from the Upper Neostromboli formation in southwest Stromboli date its eruption at 7.0 ± 0.3 ka (1σ, n = 3, Ginostra site) and 6.8 ± 0.2 ka (1σ, n = 10, Timpone del Fuoco site) respectively. Correlation of our new data to previous K/Ar and palaeomagnetic ages from the northwestern Neostromboli phase suggests that it erupted within a confined period between roughly 6 and 14 ka. The low uncertainty on the 3Hecos ages as well as on individual exposure ages (4.4–8.7%) demonstrates that 3Hecos exposure dating is a viable tool for dating Holocene basalt lavas. The ages compare favourably to uncertainties obtained for radiocarbon dating of similar rocks.  相似文献   

6.
Reference materials are key for assessing inter-laboratory variability and measurement quality, and for placing analytical uncertainty bounds on sample analyses. Here, we investigate four years of data resulting from repeated processing of the CRONUS-N reference material for cosmogenic 10Be and 26Al analyses. At University of Vermont, we prepared a CRONUS-N aliquot with most of our sample batches from 2013 to 2017; these reference material samples were then distributed to four different accelerator mass spectrometry facilities, yielding 73 10Be analyses and 58 26Al analyses. We determine CRONUS-N 10Be concentrations of (2.26 ± 0.14) x 105 atoms g−1 (n = 73, mean, 1 SD) and 26Al concentrations of (1.00 ± 0.08) x 106 atoms g−1 (n = 58, mean, 1 SD). We find a reproducibility of 6.3% for 10Be and 7.7% for 26Al (relative standard deviations). We also document highly variable 27Al and Mg concentrations and a 10Be dispersion twice as large as the mean AMS analytic uncertainty. Analyses of the CRONUS-N material with and without density separation demonstrate that non-quartz minerals are present in the material and have a large impact on measured concentrations of 27Al, 10Be, and impurities; these non-quartz minerals represent only a very small portion of the total mass (0.6–0.8%) but have a disproportionally large effect on the resulting data. Our results highlight the importance of completely removing all non-quartz mineral phases from samples prior to Be/Al extraction for the determination of in situ cosmogenic 10Be and 26Al concentrations.  相似文献   

7.
Terrestrial cosmogenic nuclide concentrations in sediment are used to quantify mean denudation rates in catchments. This article explores the differences between the 10Be concentration in fine (sand) and in coarse (1–3 or 5–10 cm pebbles) river sediment. Sand and pebbles were sampled at four locations in the Huasco Valley, in the arid Chilean Andes. Sand has 10Be concentrations between 4.8 and 8.3·105 at g−1, while pebbles have smaller concentrations between 2.2 and 3.3·105 at g−1. It appears that the different concentrations, systematically measured between sand and pebbles, are the result of different denudation rates, linked with the geomorphologic processes that originated them. We propose that the 10Be concentrations in sand are determined by the mean denudation rate of all of the geomorphologic processes taking place in the catchment, including debris flow processes as well as slower processes such as hill slope diffusion. In contrast, the concentrations in pebbles are probably related to debris flows occurring in steep slopes. The mean denudation rates calculated in the catchment are between 30 and 50 m/Myr, while the denudation rates associated with debris flow are between 59 and 81 m/Myr. These denudation rates are consistent with those calculated using different methods, such as measuring eroded volumes.  相似文献   

8.
Well-dated records of tropical glacier fluctuations are essential for developing hypotheses and testing proposed mechanisms for past climate changes. Since organic material for radiocarbon dating is typically scarce in low-latitude, high-altitude environments, surface exposure-age dating, based on the measurement of in situ produced cosmogenic nuclides, provides much of the chronologic information on tropical glacier moraines. Here, we present a locally calibrated 10Be production rate for a low-latitude, high-altitude site near Quelccaya Ice Cap (∼13.95°S, 70.89°W, 4857 m asl) in the southeastern Peruvian Andes. Using an independent age (12.35 +0.2, −0.02 ka) of the late glacial Huancané IIa moraines based on thirty-four bracketing radiocarbon ages and twelve 10Be concentrations of boulders on the moraines, we determine a local production rate of 43.28 ± 2.69 atoms gram−1 year−1 (at g−1 yr−1). Reference 10Be production rates (i.e., production rates by neutron spallation appropriate for sea-level, high-latitude sites) range from 3.97 ± 0.09 to 3.78 ± 0.09 at g−1 yr−1, determined using scaling after Lal (1991) and Stone (2000) and depending on our assumed boulder surface erosion rate. Since our boulder surface erosion rate estimate is a minimum value, these reference production rates are also minimum values. A secondary control site on the Huancané IIIb moraines suggests that the 10Be production rates are at least as low as, or possibly lower than, those derived from the Huancané IIa moraines. These sea-level, high-latitude production rates are at least 11–15% lower than values derived using the traditional global calibration dataset, and they are also lower than those derived from the late glacial Breque moraine in the Cordillera Blanca of Peru. However, our sea-level, high-latitude production rates agree well with recently published, locally calibrated production rates from the Arctic, New Zealand, and Patagonia. The production rates presented here should be used to calculate 10Be exposure ages in low-latitude, high-altitude locations, particularly in the tropical Andes, and should improve the ability to compare the results of studies using 10Be exposure-age dating with other chronological data.  相似文献   

9.
Stable cosmogenic isotopes such as 3He and 21Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for 3He and 21Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic 3He and 21Ne can be readily measured in biotite and hornblende. 21Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure 21Nehbl/21Neqtz = 1.35 ± 0.03 and 21Nebio/21Neqtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g? 1 yr? 1 relative to a 21Neqtz production rate of 19.0 ± 1.8 at g? 1 yr? 1. We show that nucleogenic 21Ne concentrations produced via the reaction 18O(α,n)21Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic 21Ne by comparison to nucleogenic 22Ne produced from the reaction 19F(α,n)22Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic 3He dating, but that 3He is lost from biotite at Earth surface temperatures. Comparison of 3He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for 3He production via the reaction 6Li(n,α)3H → 3He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced 3He represents ~ 40% of total 3He production in our samples, and must be considered on a sample-specific basis if 3He dating in hornblende is to be widely implemented.  相似文献   

10.
The chronology and origin of volcanism of Ascension Island, South Atlantic Ocean, is poorly resolved. Here we use in situ produced cosmogenic 3He in olivine and pyroxene phenocrysts from well-preserved lava flows to date the main sub-aerial basalt volcanism on the island. Etching olivine separates in HF/HNO3 appears to remove a significant proportion of the implanted radiogenic 4He contribution. Average exposure ages of each flow corrected for radiogenic He range from 328 ka to 186 ka and are used to refine the chronology and stratigraphy of the island. Magmatic 3He/4He ratios derived from in vacuo crushing are in the range of 6.3–7.3 RA. This range is lower than the neighbouring Mid-Atlantic Ridge segment (6–8°S) but slightly higher than measured in regional ocean islands of St. Helena, Tristan da Cunha and Gough. Combining these data with new trace element data and published radiogenic isotope ratios it appears that the Ascension Island magmatism is a mix of HIMU mantle material, typified by basalts from St. Helena, and depleted MORB-source mantle.  相似文献   

11.
The production rate of cosmogenic 3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against 10Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have 10Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals 3He correlates with 10Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, 3He/10Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for 3He of 226 atoms g? 1 yr? 1 in zircon, 254 atoms g? 1 yr? 1 in apatite, 177 atoms g? 1 yr? 1 in kyanite, and 153 atoms g? 1 yr? 1 in garnet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the 3He/10Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of 3He via thermal neutron capture on 6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic 3He and 10Be are distinct from each other at least at this location over the last ～ 10 kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates.  相似文献   

12.
In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic 3He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the 40Ar/39Ar incremental heating method on groundmass samples and in situ cosmogenic 3He on olivine and clinopyroxene phenocrysts. 40Ar/39Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.  相似文献   

13.
Based on cosmogenic 10Be and 26Al analyses in 15 individual detrital quartz pebbles (16–21 mm) and cosmogenic 10Be in amalgamated medium sand (0.25–0.50 mm), all collected from the outlet of the upper Gaub River catchment in Namibia, quartz pebbles yield a substantially lower average denudation rate than those yielded by the amalgamated sand sample. 10Be and 26Al concentrations in the 15 individual pebbles span nearly two orders of magnitude (0.22 ± 0.01 to 20.74 ± 0.52 × 106 10Be atoms g−1 and 1.35 ± 0.09 to 72.76 ± 2.04 × 106 26Al atoms g−1, respectively) and yield average denudation rates of ∼0.7 m Myr−1 (10Be) and ∼0.9 m Myr−1 (26Al). In contrast, the amalgamated sand yields an average 10Be concentration of 0.77 ± 0.03 × 106 atoms g−1, and an associated mean denudation rate of 9.6 ± 1.1 m Myr−1, an order of magnitude greater than the rates obtained for the amalgamated pebbles. The inconsistency between the 10Be and 26Al in the pebbles and the 10Be in the amalgamated sand is likely due to the combined effect of differential sediment sourcing and longer sediment transport times for the pebbles compared to the sand-sized grains. The amalgamated sands leaving the catchment are an aggregate of grains originating from all quartz-bearing rocks in all parts of the catchment. Thus, the cosmogenic nuclide inventories of these sands record the overall average lowering rate of the landscape. The pebbles originate from quartz vein outcrops throughout the catchment, and the episodic erosion of the latter means that the pebbles will have higher nuclide inventories than the surrounding bedrock and soil, and therefore also higher than the amalgamated sand grains. The order-of-magnitude grain size bias observed in the Gaub has important implications for using cosmogenic nuclide abundances in depositional surfaces because in arid environments, akin to our study catchment, pebble-sized clasts yield substantially underestimated palaeo-denudation rates. Our results highlight the importance of carefully considering geomorphology and grain size when interpreting cosmogenic nuclide data in depositional surfaces.  相似文献   

14.
The possibility of quantifying surface processes in mafic or volcanic environment using the potentialities offered by the in situ-produced cosmogenic nuclides, and more specifically by the in situ-produced 10Be, is often hampered by the rarity of quartz minerals in the available lithologies. As an alternative to overcome this difficulty, we explore in this work the possibility of relying on feldspar minerals rather that on quartz to perform in situ-produced 10Be measurements in such environments. Our strategy was to cross-calibrate the total production rate of 10Be in feldspar (P10fsp) against the total production rate of 3He in pyroxene (P3px) by measuring 3He and 10Be in cogenetic pyroxene (3Hepx) and feldspar (10Befsp). The samples were collected from eight ignimbritic boulders, exposed from ca 120 to 600 ka at elevations ranging from 800 to 2500 m, along the preserved rock-avalanche deposits of the giant Caquilluco landslide (18°S, 70°W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22°S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of 3Hepx - 10Befsp total production ratios whose weighted-mean is 35.6 ± 0.5 (1σ). The obtained weighted-mean 3Hepx - 10Befsp total production ratio combined with the local 3Hepy total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL 10Be in situ-production rate in feldspar mineral (P10fsp) of 3.57 ± 0.21 at.g−1.yr−1 (scaled for the LSD scaling scheme, the ERA40 atm model and the VDM of Lifton, 2016).Despite the large elevation range covered by the whole dataset (800–4300 m), no significant variation of the 3Hepx - 10Befsp total production ratios in pyroxene and feldspar was evidenced. As an attempt to investigate the effect of the chemical composition of feldspar on the total 10Be production rate, major and trace element concentrations of the studied feldspar samples were analyzed. Unfortunately, giving the low compositional variability of our dataset, this issue is still pending.  相似文献   

15.
Cosmogenic 3He can be used to date a wide range of mineral phases because it is produced from all target elements and can be readily measured above atmospheric contamination. Calcite is a particularly attractive target mineral due to its natural abundance, large crystal size (>1 mm), and low He closure temperature (<70 °C), which limit non-cosmogenic 3He components (Copeland et al., 2007). However, several recent studies have shown that some calcite may not be retentive to helium, even under surface temperatures (Cros et al., 2014; Copeland et al., 2007). This study thus explores 3He retention and production in natural calcite samples at four different sites. Samples from two high elevation sites appear retentive to 3He over 10 kyr timescales, whereas two additional sites clearly suffer from diffusive loss of 3He. Step-degassing experiments suggest that diffusion in calcite is controlled by multiple diffusion domains, with an apparent activation energy of 25–27 kcal mol−1. Although minor 3He loss is expected from the smallest diffusion domains, the observed kinetics cannot explain the poor retention at all sites. We thus propose that opaque (non-transparent) calcite may be more retentive due to the presence of imperfections in the crystal lattice. We conclude that 3He dating of calcite shows promise in some settings. However, because retention depends on crystallographic variability it must be evaluated on a case-by-case basis until robust criteria for retention can be identified.  相似文献   

16.
Single-crystal (U-Th)/He dating of 32 apatite and zircon crystals from an impact breccia yielded a weighted mean age of 663 ± 28 ka (n = 3; 4.2 % 2σ uncertainties) for the Monturaqui impact structure, Chile. This ~350 m diameter simple crater preserves a small volume of impactite consisting of polymict breccias that are dominated by reworked target rock clasts. The small size, young age and limited availability of melt material for traditional geochronological techniques made Monturaqui a good test to define the lower limits of the (U-Th)/He system to successfully date impact events. Numerical modeling of 4He loss in apatite and zircon crystals shows that, for even small craters such as Monturaqui, the short-lived compressional stage and shock metamorphic stage can account for the observed partial to full resetting of (U-Th)/He ages in accessory minerals. Despite the distinctly different 4He diffusion parameters of apatite and zircon, the 2σ-overlapping youngest ages are recorded in both populations of minerals, which supports the inference that the weighted mean of the youngest (U-Th)/He population is the age of formation of this impact structure.  相似文献   

17.
We have used cosmogenic 3He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano. The post-collapse flows have exposure ages ranging from 62 to 11 ka. The analysis of multiple flow tops on each lava flows, often at different elevations, provides an internal check for age consistency and the exposures ages conform with stratigraphic level. The exposure ages suggest that volcanic activity along the western branch of the triple-armed rift zone was more or less continuous from before 62 ka to approximately 11 ka. The absence of magmatic activity for the last 11 kyr reflects a structural reconfiguration of the volcano and may be related to renewed flank instability. This volcanic hiatus is similar in duration to that observed in the Canary Islands. Replicate 3He exposure ages of a pre-collapse flow (123.0 ± 5.2 ka) brackets the time of the Monte Amarelo collapse between 62 ka and 123 ka. Reproducible cosmogenic 3He exposure ages of less than 123 ka from flows away from major erosion features demonstrates that the technique is a viable alternative to the radiocarbon, K/Ar and 40Ar/39Ar chronometers for dating recent volcanism in arid climate zones.  相似文献   

18.
The Huancané II moraines deposited by the Quelccaya Ice Cap in southern Peru were selected by the CRONUS-Earth Project as a primary site for evaluating cosmogenic-nuclide scaling methods and for calibrating production rates. The CRONUS-Earth Project is an effort to improve the state of the art for applications of cosmogenic nuclides to earth-surface chronology and processes. The Huancané II moraines are situated in the southern Peruvian Andes at about 4850 m and ∼13.9°S, 70.9°W. They are favorable for cosmogenic-nuclide calibration because of their low-latitude and high-elevation setting, because their age is very well constrained to 12.3 ± 0.1 ka by 34 radiocarbon ages on peat bracketing the moraines, and because boulder coverage by snow or soil is thought to be very unlikely. However, boulder-surface erosion by granular disintegration is observed and a ∼4% correction was applied to measured concentrations to compensate. Samples from 10 boulders were analyzed for 10Be, 26Al, and 36Cl. Interlaboratory bias at the ∼5% level was the largest contributor to variability of the 10Be samples, which were prepared by three laboratories (the other two nuclides were only prepared by one laboratory). Other than this issue, variability for all three nuclides was very low, with standard deviations of the analyses only slightly larger than the analytical uncertainties. The site production rates (corrected for topographic shielding, erosion, and radionuclide decay) at the mean site elevation of 4857 m were 45.5 ± 1.6 atoms 10Be (g quartz)−1 yr−1, 303 ± 15 atoms 26Al (g quartz)−1 yr−1, and 1690 ± 100 atoms 36Cl (g K)−1 yr−1. The nuclide data from this site, along with data from other primary sites, were used to calibrate the production rates of these three nuclides using seven global scaling methods. The traditional Lal formulation and the new Lifton-Sato-Dunai calibrations yield average ages for the Huancané samples that are in excellent-to-good agreement with the radiocarbon age control (within 0.7% for 10Be and 36Cl and 6% for 26Al). However, all of the neutron-monitor-based methods yielded ages that were too young by about 20%. The nuclide production ratios at this site are 6.74 ± 0.34 for 26Al/10Be in quartz and 37.8 ± 2.3 (atoms 36Cl (g K)−1) (atom 10Be (g SiO2)−1)−1 for 36Cl/10Be, in sanidine and quartz, respectively.  相似文献   

19.
As part of the NSF-funded program CRONUS-Earth, a series of natural reference materials for in situ produced 26Al, 10Be, 14C, and 36Cl were prepared and circulated to United States, Australian, and European laboratories for analysis to explore the comparability of results from the different laboratories and generate preliminary consensus values for a range of reference material. Such reference materials, which did not exist for these isotopes, assist laboratories in independently assessing quality and are useful to quantify precision and accuracy. Currently, most researchers report only internal analytical uncertainties for all results. While researchers have acknowledged the need for realistic inter-laboratory uncertainties for in situ produced cosmogenic isotopes, few previous studies have addressed this issue. Two samples (denoted A and N) were provided for 26Al, 10Be and in situ 14C analysis, one from the Antarctic, high in 26Al and 10Be and the other from Australia, lower in both 26Al and 10Be. Both samples were prepared to quartz at the University of Vermont. For each sample, results have been summarised in terms of the mean reported concentration, standard deviation both between (inter) and within (intra) laboratories to describe inter- and intra-laboratory variability. Coefficients of variation (CoV) expressed as a percentage of the mean are also reported. For in-situ 14C, a small number of laboratories reported results, so they are summarised separately. Initial uncorrected results for 10Be for samples A and N showed significant variation (greater than 8% CoV) in results. When corrected to a common standardisation basis, the CoV was 2.9% for 10Be measurements of sample A (high concentration) and to 4.1% for sample N (lower concentration), which is closer to typical cosmogenic samples. 26Al measurements had greater variation; a CoV of 4.9% was achieved for sample A (high concentration) but for the lower concentration sample N, the CoV was 10.1%.  相似文献   

20.
Combining cosmogenic 3He and 21Ne (3Hec and 21Nec) measurements on both pyroxene and olivine from the Pleistocene Bar Ten flows (85–107 ka) greatly increases our ability to evaluate the accuracy of 3Hec and 21Nec production rates and, therefore, 3Hec and 21Nec surface exposure ages. Comparison of 3Hec and 21Nec age-pairs yielded by experimentally determined production rates and composition-based model calculations indicates that the former give more accurate surface exposure ages than the latter in this study. However, experimental production rates should be adjusted to the composition of the minerals being analyzed to obtain the best agreement between 3Hec and 21Nec ages for any given sample. 21Nec/3Hec values are 0.400 ± 0.029 and 0.204 ± 0.014 for olivine and pyroxene, respectively, in Bar Ten lava flows, in agreement with previously published values, and indicate that 21Nec/3Hec in olivine and pyroxene is not affected by erosion and remains constant with latitude, elevation, and time (up to 10 Myr). Samples with 21Nec/3Hec that do not agree with these values may indicate the presence of non-cosmogenic helium and/or neon. The neon three-isotope diagram can also indicate whether or not all excess neon in mineral separates comes from cosmogenic sources. An error-weighted regression for olivine defines a spallation line [y = (1.033 ± 0.031)x + (0.09876 ± 0.00033)], which is indistinguishable from that for pyroxene (Schäfer et al., 1999). We have derived a production rate of 25 ± 8 at/g/yr for 21Nec in clinopyroxene (En43–44) based on the 40Ar/39Ar age of the upper Bar Ten flow. Our study indicates that the production rate of 21Nec in olivine may be slightly higher than previously determined. Cosmogenic 3He and 21Ne remain extremely useful, particularly when paired, in determining accurate eruption ages of young olivine- and pyroxene-rich basaltic lava flows.  相似文献   

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