Isochron Approach Research Articles

LEAD-LEAD DATING REVEALS PERMIAN REMOBILIZATION OF NIOBIUM MINERALIZATION AT BAYAN OBO

Abstract The Bayan Obo deposit hosted by the H8 unit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe). Permian granites are widely exposed in the mining area and have a close spatial association with the Nb mineralization. Whether the granites contributed Nb or only remobilized existing mineralization is important for understanding the controls of ore formation. Previous studies have mostly focused on the REEs, whereas research on Nb has been limited. This is due mainly to the difficulty of accurately determining the age of the Nb mineralization because of the fine-grained and texturally complex nature of the Nb-bearing minerals and their exceptionally low U content. Although microbeam techniques show promise in tackling the aforementioned challenges, their application is hampered by matrix effects caused by the diverse composition of Nb-bearing minerals. Here we report the application of a high-precision secondary ion mass spectrometry (SIMS) Pb-Pb isochron approach that enables young samples (i.e., <500 Ma) to be dated without matrix-matched reference materials. A variety of Nb-bearing minerals from eastern Bayan Obo were analyzed, yielding Pb-Pb isochron ages of 276 ± 10 Ma (pyrochlore, 394–6,864 ppm U in the rim and 6,563–19,858 ppm in the core), 277 ± 36 Ma (fersmite, 18–61 ppm U; fergusonite-Ce, 45–95 ppm U), and 257 ± 46 Ma (aeschynite, 342–1,006 ppm U). In combination with the deposit geology and petrographic observations, these ages link the Nb mineralization to ~270 Ma granites. As these granites are not particularly rich in Nb, skarn formation during granite emplacement is interpreted to have remobilized the existing Nb mineralization, which increased the grain size of the Nb-bearing minerals—a key factor facilitating their extraction. Our study shows that high-precision SIMS Pb-Pb analysis holds promise for directly dating mineralization without matrix-matched reference materials. It also emphasizes the need to consider the role of the Nb remobilization at Bayan Obo and elsewhere.

53Mn‐53Cr chronology and ε54Cr‐Δ17O genealogy of Erg Chech 002: The oldest andesite in the solar system

AbstractThe meteorite Erg Chech (EC) 002 is the oldest felsic igneous rock from the solar system analyzed to date and provides a unique opportunity to study the formation of felsic crusts on differentiated protoplanets immediately after metal–silicate equilibration or core formation. The extinct 53Mn‐53Cr chronometer provides chronological constraints on the formation of EC 002 by applying the isochron approach using chromite, metal–silicate–sulfide, and whole‐rock fractions as well as “leachates” obtained by sequential digestion of a bulk sample. Assuming a chondritic evolution of its parent body, a 53Cr/52Cr model age is also obtained from the chromite fraction. The 53Mn‐53Cr isochron age of 1.73 ± 0.96 Ma (anchored to D'Orbigny angrite) and the chromite model age constrained between and Ma after the formation of calcium‐aluminium‐rich inclusions (CAIs) agree with the 26Al‐26Mg ages (anchored to CAIs) reported in previous studies. This indicates rapid cooling of EC 002 that allowed near‐contemporaneous closure of multiple isotope systems. Additionally, excess in the neutron‐rich 54Cr (nucleosynthetic anomalies) combined with mass‐independent isotope variations of 17O provides genealogical constraints on the accretion region of the EC 002 parent body. The 54Cr and 17O isotope compositions of EC 002 confirm its origin in the “noncarbonaceous” reservoir and overlap with the vestoid material Northwest Africa 12217 and anomalous eucrite Elephant Moraine 92023. This indicates a common feeding zone during accretion in the protoplanetary disk between the source of EC 002 and vestoids. The enigmatic origin of iron meteorites remains still unresolved as EC 002, which is more like a differentiated crust, has an isotope composition that does not match known iron meteorite groups that were once planetesimal cores.

Lithological fabric as a proxy for Rb–Sr isotopic complexity

In situ Rb–Sr dating, made possible by recent analytical advancement, is a powerful chronometer that enables the timing of crystal growth or cooling to be determined for fabric forming minerals (e.g. alkali feldspar, muscovite, biotite). In contrast with accessory phase geochronology, Rb–Sr geochronology can be deployed on minerals that constitute a significant component of a wide range of rock types. Here we report laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) biotite Rb–Sr ratios in metaigneous and metasedimentary samples from across Western Australia. These biotite samples span a diversity of structural, metamorphic, and temporal states, yet highlight common salient features of this chronometer. Biotite in these samples received its isotopic compositions from completely to partially homogenized Sr reservoirs, as evident from isochronous to more complex distributions of isotopic ratios. The utility of the classic isochron approach is contrasted with biotite model ages for the geologic interpretation of Rb–Sr results for samples with and without constrained initial 87Sr/86Sr ratio. Model ages calculated from individual analyses aid in identifying multiple discrete events that may have only partially equilibrated isotopic systems. Thus, single-analysis model ages can enable the extraction of thermochronological information from complex Rb–Sr data sets, if interpreted under consideration of their geologic context. Textural analysis of thin sections via Fourier component image analysis quantifies the preferred orientation of biotite fabrics. This analysis reveals a correlation between diversity of grain orientation and biotite age complexity. Such a simple texture-to-age relationship highlights a potential pre-analysis screening tool to facilitate sample selection where Rb–Sr isochrons are sought.Results reveal a thermal event at ca. 550–500 Ma in the Lamboo Province, West Kimberley, and multiple contact metamorphic events at ca. 633 Ma and 619–615 Ma that occurred in the Yeneena Basin, Paterson Orogen. Biotite from the Yilgarn Craton yields a strong E-W trend in cooling ages, from 2477 Ma in the central Yilgarn Craton to 580 Ma at the western margin of the craton. These ages support slow cooling and thermo-tectonic quiescence after cratonisation in the central Yilgarn Craton, while the western margin was subject to Neoproterozoic reworking. Placed into a regional geologic context, these results highlight the sensitivity of the in situ Rb–Sr dating method to constrain the timing of geologic events that are not commonly captured by other isotopic systems.

Transport stability profiling- a proposed generic protocol.

Diagnostic samples are exposed to a spectrum of variables during transport to laboratories; therefore, the evaluation of a rather comprehensive stability profile of measurands is warranted. While appropriate testing standards have been established for pharmaceuticals and reagents, this is not the case for diagnostic samples. The aim of our work was to develop and evaluate a protocol applicable to diagnostic samples. An isochronous approach with representation of temperature and exposure duration in a two-dimensional matrix was established. The deviations of the measurement results from the baseline associated with the exposure are evaluated with respect to the measurement uncertainty of the analytical measurement procedure applied. Variables ofthe experiment are documented in a standardized matrix. As a proof-of-concept, we profiled the stability patterns of a number of measurands at four temperature levels over up to 72h in primary serum sample tubes. The protocol proved to be workable and allowed the description of a comprehensive stability profile of a considerable number of compounds based on 21 small-volume primary samples collected from each volunteer and exposed according to this protocol. A straightforward and feasible isochronous protocol can be used to investigate in detail the effects of different pre-processing conditions on the stability of measurands in primary samples during transport to diagnostic laboratories. This is of significance as pre-analytical logistics become increasingly important with the centralization of analytical services.

Uranium-thorium isotope systematics of cold-seep carbonate and their constraints on geological methane leakage activities

Marine methane hydrates are a huge and dynamic carbon reservoir found mainly at the continental margins, and their stability might be affected by climate-associated pressure and temperature changes on the seafloor. Reconstructing the growth history of cold-seep carbonates, which formed during seafloor methane leakage, could help to constrain methane hydrate stability in the geological past. However, U-Th isotope systematics of these complex carbonate cements have not yet been systematically investigated on the micro-scale, leaving uncertainties in the U-series geochronology which is commonly applied to date the seep carbonates. In this study, we have developed multiple in-situ analytical methods, including U-Th isotope analysis by laser ablation MC-ICPMS, elemental concentration mapping by laser ablation ICPMS, as well as organic distribution mapping by Raman Spectroscopy, to provide insights into the U-series geochemistry and geochronology for the different types of cold-seep carbonates. Our result demonstrates that 238U and 232Th of these carbonates are dominantly derived from seawater and detrital particles, respectively. As the [230Th/232Th] (activity ratio) of micro-domains with high [232Th/238U] is negligibly affected by U decay, we have directly determined the initial [230Th/232Th] of the cold-seep carbonates which is 0.7 ± 0.1 (2 SD, n = 12). In general, U-series isotopes show closed-system behavior within our studied analytical precision in the interior of the seep cements, and the initial [230Th/232Th] derived from the isochron approach is consistent with the in-situ direct determination. A notable exception is a calcitic pipe sample with highly enriched U in its rim which has also experienced post-depositional mobilization. Our method is then applied to a large set of seep carbonate samples from the upper continental slope of the northern South China Sea (SCS). The obtained U-series ages provide evidence that cold seep was likely continuously active since at least ∼72 ka in the northern SCS, but the timing and duration of methane leakage vary between different sites within this region. While the upper continental slope methane hydrates are highly susceptible to destabilization under changing bottom water conditions, our extensive dating on seep carbonates from the SCS further indicates that bottom water P-T conditions could exert contrasting impacts on methane hydrate stability at different depths within the same geological setting. Overall, the in-situ U-series geochronological method developed in our study has the potential to date cold-seep carbonates efficiently and reliably, offering new opportunities to probe into the history of methane leakage in the late Quaternary.

Can Serum be a Match for Urine in the Regulatory Analysis of Boldenone in Cattle? A Systematic Comparison Between Detection Window, Stability, and Enzymatic Hydrolysis

This work involved a systematic comparison between serum and urine for the monitoring of anabolic androgenic steroids in livestock. Incurred samples were collected over 120 days from crossbred steers treated with intramuscular injections containing boldenone undecylenate. Independent high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) methods were used for the assessment of the respective detection windows, which were larger for serum samples. Both matrices presented adequate performance in terms of long-term stability, assessed using an isochronous approach during 196 days at -20 °C and for five freeze-thaw cycles. The effectiveness of the enzymatic hydrolysis reaction using Helix pomatia juice was also compared. The calculated concentrations in serum samples were not statistically influenced by the deconjugation reaction. On the other hand, urine hydrolysis conditions were studied using a 33 Box-Behnken Design, in which a central point condition led to a satisfactory deconjugation performance. It could be observed that serum exhibited equivalent or better performance than urine for most of the evaluated criteria; thus, its inclusion in the regulatory analysis of boldenone in cattle is supported.

ESR/U-series and ESR dating of several Middle Pleistocene Italian sites: Comparison with 40Ar/39Ar chronology

The stratigraphic sequences of numerous Palaeolithic sites of Central and Southern Italy, very rich in both archaeological and palaeontological remains, have also recorded Pleistocene volcanic events through volcanic ash deposits (tephra). They allow the establishment of an accurate chronological framework by comparing results obtained by 40 Ar/ 39 Ar dating method on single volcanic K-feldspar crystals, with those derived from ESR and ESR/U-series analyses on fluvial bleached quartz and tooth enamel respectively. Since 2009, these three methods were hence applied on samples of several Middle Pleistocene sites of Central and Southern Italy including both volcanic and archaeological levels (from the west to the east): La Polledrara di Cecanibbio, Isoletta, (Latium), Guado San Nicola, Isernia La Pineta (Molise), Valle Giumentina (Abruzzo) and Venosa Notarchirico (Basilicata). The obtained results, covering a time range from 660 to 350 ka, prove that such a multi-method approach when possible is essential to constrain the chronology of each site and allow the recognition of the specific limitations due to the lack of quartz for ESR or to complex geochemical histories in teeth rendering difficult the ESR/U-series method. Despite these limitations, the ESR framework is globally in agreement with the 40 Ar/ 39 Ar chronology, while ESR/U-series dates can be underestimated for the oldest sites. In such cases, an isochron approach attests however of the quite good reliability of the palaeodosimetric reconstruction and the observed age underestimation could be related to other factors affecting the ESR age determination.

Cosmogenic 3He exposure ages of basaltic flows from Miller Knoll, Panguitch Lake, Utah: Using the alternative isochron approach to overcome low-gas crushes

Determining the ages of young volcanic rocks is important for understanding the tectono-magmatic development of geologic terranes. Usually, if rocks are old enough the 40 Ar/ 39 Ar or K–Ar techniques can provide reliable ages. However, when rocks are younger, they often lack enough daughter product to resolve an age. Cosmogenic 3 He methods provide an alternative for determining the eruption age of relatively recent mafic/intermediate lava flows. We sampled morphologically young basaltic andesite flows south of Miller Knoll, near Panguitch Lake on the Markagunt Plateau in southern Utah. We took two samples in close proximity from two different areas on the flows and separated both olivine and pyroxene. The typical protocol is to crush mineral separates on-line to determine an inclusion-hosted magmatic 3 He/ 4 He component. Then the powders are heated in a furnace to release the total 3 He and 4 He component and the crushed component is subtracted to determine the cosmogenic 3 He component. Unfortunately, in the case of the Miller Lake flows, 3 He yield from on-line crushes was below detection. An alternative isochron approach, which obviates the need for crush data, was first described by Cerling and Craig (1994) and more fully by Blard and Pik (2008). In this approach the 3 He/ 4 He of the total gas released from furnace heating is plotted vs. 1/ 4 He. If the samples plot on a line, then the resulting y-intercept is the magmatic 3 He/ 4 He and the slope of the line is the cosmogenic 3 He component, which determines the exposure age. Our data create good isochrons (MSWD = 0.76 and 0.14) with magmatic 3 He/ 4 He of 4.7–4.9 Ra. Concentrations of cosmogenic 3 He are 2.13 and 2.61 × 10 7 atoms g −1 after correction for radiometric 4 He using the R correction factor and measured and estimated U and Th concentrations in whole rock and minerals, respectively. Using the LSDn scaling routine and an online exposure age calculator, we determine zero erosion exposure ages of 32 ± 3 ka for the upper part of the flow and 34 ± 4 ka for the lower part of the flow. • We determined 3 He exposure ages of a basaltic flow on the Markagunt Plateau, Utah. • Olivine separates had low gas crushes and unresolvable crush/magmatic 3 He/ 4 He ratios. • We utilized the alternative isochron approach for determining cosmogenic 3 He. • Zero erosion exposure age (LSDn) of the Black Rock Valley flow is 32–34 ± 4 ka.

In-situ K-Ar dating on Mars based on UV-Laser ablation coupled with a LIBS-QMS system: Development, calibration and application of the KArMars instrument

Absolute dating is needed to check and calibrate the relative Martian chronology presently available from meteoritic crater counting. For such purpose, a new analytical system (KArMars) has been developed to experiment the feasibility of in-situ K-Ar for future landing planetary missions. It consists in a laser ablation-based system built to vaporize a reproducible volume of rock. Potassium content is measured by laser-induced breakdown spectroscopy (LIBS) and argon by Quadrupole Mass Spectrometry (QMS). Instrument calibration, and checking of the reliability of the measurements for Martian analyses, requires terrestrial analogues. For such purpose, total chemistry, electron microprobe analyses, flame absorption spectrometry and mass spectrometry have been performed in order to qualify the stoichiometry, the mineralogy, the K concentration and the Ar isotopic composition from a collection of old terrestrial rocks. These new analyses coupled with published data allowed us to select mineral phases (e.g. feldspars, phlogopite, muscovite, amphibole) showing a large range of K content (0.3–8.5 wt%). All these mineral phases display a K-Ar age older than 300 Ma. Hence, the content of radiogenic Ar atoms per gram is within the range of Martian samples (on the order of 1 Ga for 0.4 wt% of K). We have shown that the ablated mass can be precisely estimated by measurement of Ar extracted from a similar mineral of known amount of radiogenic Ar content per gram. Then, it has been possible to derive a well-defined relationship between the ablation time and the sample mass extracted from UV-laser ablations for different minerals. Using an isochron approach from multiple ablations of several samples analyzed as unknowns, we have shown that KArMars can be successfully used for dating samples with different compositions, and with ages varying by an order of magnitude from 0.3 to 3 Ga. In-situ K-Ar ages obtained here for plagioclase and amphibole crystals display uncertainties as low as about 10%, and with accuracy, i.e. difference with the reference age, of only 10%. Such results further support in-situ K-Ar dating based on QMS and LIBS instruments as a reliable approach to be implemented onboard Martian rovers. In case a return mission is made possible, KArMars can also be a valuable tool to select the most relevant samples for return and ensure onsite which samples are representative of most of the surface at a given landing site.

In-house validation of a liquid chromatography–tandem mass spectrometry method for the determination of selective androgen receptor modulators (SARMS) in bovine urine

ABSTRACTA sensitive and robust LC–MS/MS method allowing the rapid screening and confirmation of selective androgen receptor modulators in bovine urine was developed and successfully validated according to Commission Decision 2002/657/EC, chapter 3.1.3 ‘alternative validation’, by applying a matrix-comprehensive in-house validation concept. The confirmation of the analytes in the validation samples was achieved both on the basis of the MRM ion ratios as laid down in Commission Decision 2002/657/EC and by comparison of their enhanced product ion (EPI) spectra with a reference mass spectral library by making use of the QTRAP technology. Here, in addition to the MRM survey scan, EPI spectra were generated in a data-dependent way according to an information-dependent acquisition criterion. Moreover, stability studies of the analytes in solution and in matrix according to an isochronous approach proved the stability of the analytes in solution and in matrix for at least the duration of the validation study. To identify factors that have a significant influence on the test method in routine analysis, a factorial effect analysis was performed. To this end, factors considered to be relevant for the method in routine analysis (e.g. operator, storage duration of the extracts before measurement, different cartridge lots and different hydrolysis conditions) were systematically varied on two levels. The examination of the extent to which these factors influence the measurement results of the individual analytes showed that none of the validation factors exerts a significant influence on the measurement results.

Reply to and comment on “The usage of 238U/207Pb vs 206Pb/207Pb linear regressions for the LA-ICP-MS U-Pb dating of cassiterite”

Isotope dilution thermal ionization mass spectrometry (ID-TIMS) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) are two popular analytical methods used to constrain the age of minerals, including cassiterite. Compared to ID-TIMS, the LA–ICP-MS analytical method is often favoured due to its simple analytical procedure and sample preparation, although the methodology constantly needs modification. Our recent paper on dating cassiterite from the Baiganhu W-Sn ore field in NW China based on LA–MC-ICP-MS and ID-TIMS methods has received a response from Dr. Leonid A. Neymark (2017). Definitely, Neymark is appreciated, and his suggestions are helpful to improve the methodology of cassiterite U–Pb geochronology. Based on a discussion on the cassiterite U–Pb dating method, we re-confirm the validity of the cassiterite U-Pb ages for the Baiganhu W-Sn ore field. The initial non-radiogenic 204Pb used in the U-Pb isochron approach can be replaced by the non-radiogenic 207Pb, because the radiogenic 207Pb decayed from 235U can be ignored in cassiterite with a high Pb content. Therefore, the cassiterite U-Pb isochron yields an age similar to that obtained from T-W concordia for the same sample, testifying the validity of the cassiterite 238U/207Pb–206Pb/207Pb isochron method.

Stability of ethyl glucuronide in hair reference materials after accelerated aging

Two different hair reference materials, one produced from authentic hair displaying an ethyl glucuronide (EtG) content of about 25pg/mg and one obtained by fortification of blank hair to an EtG level of 85pg/mg were submitted to accelerated aging between 4°C and 60°C for periods between one and 24 months. Subsequently, the EtG content was determined in the aged samples and untreated reference samples stored at −22°C under repeatability conditions following the so-called isochronous approach. The EtG content remained stable even at 40°C for 24 months and at 60°C over six months. This is in contrast to many organic analytes contained in trace concentrations in diverse matrices. A slight but significant increase of the recovered EtG in case of authentic hair samples having been exposed for 24 months between 4°C and 60°C may be due to a temperature-driven process that allows increased recoveries of the physiologically embedded EtG.

How many and from where? Assessing the sensitivity of exposure durations calculated from paired bedrock 14C/10Be measurements in glacial troughs

We present a sensitivity analysis of the isochron approach of Goehring et al. (2013) for paired measurements of in situ14C/10Be from glacially sculpted bedrock surfaces. This analysis tests how sensitive the resulting exposure durations from this technique are to both the number of samples analyzed and their locations along a glacial trough transect, using a dataset from Goehring et al. (2011) as a test case. A simple equally weighted combinatorial approach was employed to (1) generate non-repetitive combinations of n subsets of samples arranged from the ten possible samples (where n < 10), and (2) estimate the exposure duration and uncertainty for each set of simulations. Results from the Goehring et al. (2011) data indicate that five samples evenly distributed along a transect parallel to the ice margin are the minimum number of samples required for this method, while eight or more samples provide an optimal combination of accuracy and precision at the 1σ level. These findings should be applicable to paired in situ14C/10Be measurements from other polished bedrock troughs at glacial margins, but need further experimental confirmation.

An in situ14C–10Be Bayesian isochron approach for interpreting complex glacial histories

We present a Bayesian isochron approach to interpret measurements of multiple cosmogenic nuclides from glacially modified bedrock surfaces with complex exposure histories. An isochron approach explicitly incorporating glacial erosion is ideally suited for this problem; such erosion must be accounted for but has traditionally been ignored. Previous methods required treating each sample individually (to account for glacial erosion) and subsequently averaging results for the entire dataset. Geological considerations, however, suggest a more robust approach is to treat samples in the dataset here (and samples from other conceivable datasets) simultaneously. The Bayesian isochron method is applied to a previously published set of in situ14C and 10Be measurements from a set of samples spanning the forefield of the Rhone Glacier, Switzerland. Results indicate 6.4 ± 0.5 kyr of integrated exposure and 4.7 ± 0.5 kyr of cumulative burial, similar to previous estimates, but with much smaller uncertainties. The reduced uncertainties result from fitting the exposure and burial duration to the entire dataset, while explicitly accounting for glacial erosion. The method presented here should be applicable with minor modifications in a number of geologic settings, and further demonstrates the utility of paired in situ10Be and 14C measurements for unraveling complex exposure histories over during the Holocene and late Pleistocene.

Micron-scale coupled carbon isotope and nitrogen abundance variations in diamonds: Evidence for episodic diamond formation beneath the Siberian Craton

The internal structure and growth history of six macro-diamonds from kimberlite pipes in Yakutia (Russia) were investigated with cathodoluminescence imaging and coupled carbon isotope and nitrogen abundance analyses along detailed core to rim traverses. The diamonds are characterised by octahedral zonation with layer-by-layer growth. High spatial resolution SIMS profiles establish that there is no exchange of the carbon isotope composition across growth boundaries at the μm scale and that isotopic variations observed between (sub)zones within the diamonds are primary. The macro-diamonds have δ13C values that vary within 2‰ of −5.3‰ and their nitrogen contents range between 0–1334at. ppm. There are markedly different nitrogen aggregation states between major growth zones within individual diamonds that demonstrate Yakutian diamonds grew in multiple growth events. Growth intervals were punctuated by stages of dissolution now associated with <10μm wide zones of nitrogen absent type II diamond. Across these resorption interfaces carbon isotope ratios and nitrogen contents record shifts between 0.5–2.3‰ and up to 407at. ppm, respectively. Co-variation in δ13C value–nitrogen content suggests that parts of individual diamonds precipitated in a Rayleigh process from either oxidised or reduced fluids/melts, with two single diamonds showing evidence of both fluid types. Modelling the co-variation establishes that nitrogen is a compatible element in diamond relative to its growth medium and that the nitrogen partition coefficient is different between oxidised (3–4.1) and reduced (3) sources. The reduced sources have δ13C values between −7.3‰ and −4.6‰, while the oxidised sources have higher δ13C values between −5.8‰ and −1.8‰ (if grown from carbonatitic media) or between −3.8‰ and +0.2‰ (if grown from CO2-rich media). It is therefore concluded that individual Yakutian diamonds originate from distinct fluids/melts of variable compositions. The diamond-forming fluids within the cratonic mantle beneath the Siberian Craton record significant variations in composition and volume and include both oxidised and reduced sources. These observations suggest that dating diamond inclusions using an isochron approach will best provide geologically meaningful ages if inclusions can be shown to be genetically (spatially) related.

A chronological framework for the Clyde Foreland Formation, Eastern Canadian Arctic, derived from amino acid racemization and cosmogenic radionuclides

The most extensive terrestrial outcrops of glacial and glaciomarine deposits in the Eastern Canadian Arctic are exposed in sea cliffs along the Clyde Foreland and Qivitu Peninsula of Baffin Island. Collectively known as the Clyde Foreland Formation (CFF), these stacked deposits record at least seven glacial advances. Despite having been the focus of numerous investigations spanning nearly 50 years, no numerical chronological framework for the age of the deposits has been established. Previous studies relied on biostratigraphy and amino acid racemization (AAR) geochronology and postulated that the oldest units were Late Pliocene to Mid-Pleistocene in age. In this paper, we use a cosmogenic radionuclide isochron approach to determine a minimum age for the burial of a paleosol preserved within the CFF. Abundant palynomorphs in the paleosol are dominated by cool-climate taxa. Combining the paleosol burial age with a compilation of published and new CFF AAR data for marine bivalves Hiatella arctica and Mya truncata, we statistically define seven CFF aminozones and develop a piecewise isoleucine AAR calibration model for Baffin Island. From this, we estimate the minimum age of each aminozone, although the propagation of errors through all calculations produces large uncertainties for each age estimate. The youngest three CFF units, known as the Kogalu, Kuvinilk, and Cape Christian members, were most likely deposited during glaciations in the Mid- to Late-Pleistocene. The paleosol formed prior to 1.15 ± 0.20 Ma, and the underlying aminozones represent sedimentation during Early Pleistocene or latest Pliocene glaciations and record early advances of Laurentide ice across Baffin Island.

Effectiveness of combined unspiked K–Ar and 40Ar/ 39Ar dating methods in the 14C age range

In this study, the effectiveness of combined unspiked K–Ar and 40Ar/ 39Ar dating methods as currently applied now at LSCE in the 14C age range was evaluated by studying two 30,000 years old phonolites from Tenerife (Canary Islands). New 40Ar/ 39Ar ages obtained in this work are comparable with the K–Ar ages, and of similar precision. This remarkable agreement obtained between the two dating methods in this young age range, validates the new LSCE 40Ar/ 39Ar facility and provides a very promising approach to calibrate the Quaternary timescale and to date key events such as geomagnetic field instabilities and climatic changes. The approach combining the 40Ar/ 39Ar and the unspiked K–Ar methods has the advantage that all the basic assumptions, which rule the K–Ar clock, are checked via the isochron approach. In the event of positive check, the 40Ar/ 39Ar and unspiked K–Ar ages can be pooled to produce very precise ages.

An isochron approach to 21Ne cosmic ray exposure dating by activation with deuteron–deuteron fusion neutrons

This paper describes an alternative approach to cosmogenic 21Ne ( 21Ne cos) exposure dating that entails irradiating Mg-bearing samples in a deuteron–deuteron (D–D) neutron generator to produce 22Ne by the reaction 25Mg(n,α) 22Ne Mg. Following irradiation and provided that the 22Ne Mg/ 22Ne cos ratio is greater than ~ 10 3, one can identify cosmogenic 21Ne using a 21Ne/ 20Ne vs. 22Ne/ 20Ne isochron. The method is analogous to identifying radiogenic 40Ar using a conventional 40Ar/ 36Ar vs. 39Ar K/ 36Ar isochron. With knowledge of the 21Ne production rate and the relationship between 22Ne Mg and Mg content (determined by co-irradiating a standard of known Mg concentration), the slope of the isochron ( 21Ne cos/ 22Ne Mg) can be translated into an exposure age. In this way a 21Ne exposure age is calculated without making assumptions regarding the isotopic composition of the trapped or cosmogenic components. This technique, if proven to be viable, will be applicable to minerals that contain a stoichiometric abundance of Mg (e.g., orthopyroxene, clinopyroxene, and olivine).

Uranium-series dating of fen peat horizons from pit Nachtigall in northern Germany

Peat and other terrestrial sediments are important archives of climate forcing. This study discusses the numerical age determination of stratigraphically important Quaternary sequences from clay pit Nachtigall near Holzminden in northern Germany, which have been the subject of debate concerning their chronological classification since they were first described by Grupe in 1929. Uranium-series dating ( 230Th/U) of different organic horizons as well as sedimentology, including loss-on-ignition (LOI) and palynology was carried out to set up a more reliable chronological framework for the Weser river area. 230Th/U ages were calculated using an isochron approach with the leachate/leachate method for chemical preparation. Open system requirements and specification of sample selection criteria for organic samples are investigated by determination of uranium and thorium composition of the rim layers. Different organic sediments are investigated. The organic deposits from Nachtigall show Middle Pleistocene 230Th/U ages ranging from 227 +9 −8 ka to 201 +15 −13 ka and appear to represent a warm temperate phase preceding the Eemian Interglacial (MIS 5e) and the Drenthe Stadial (MIS 6), indicating a correlation with marine isotope substage MIS 7c. These results are in good agreement with the investigation of the pollen spectra indicating a correlation with a warm stage within the Lower Saalian Complex.

Uranium-series dating of travertine from Süttő: Implications for reconstruction of environmental change in Hungary

Terrestrial carbonate formations, such as travertines, speleothems and lake sediments, are important archives of terrestrial climate forcing. At the Süttő section in Hungary, a complex succession of travertine is covered by a loess-palaeosol sequence; both are high resolution terrestrial archives of climate and environment change. Uranium-series ( 230Th/U) dating was carried out to set up a more reliable chronological frame for the travertine. As the growth of travertine is very complex, pore cements may cause serious problems for precise dating. The state of alteration of primary spar and micrite was characterised by microscopic, cathodoluminescence and microprobe analyses. The travertine from Süttő showed homogeneous phases of primary calcite, minor micropores and rare pore cements. The carbon isotopic composition indicates that the source of carbon was a mixture of atmospheric and soil derived CO 2. Calculated water temperatures based on oxygen isotope data range from 22 °C to 31 °C. An attempt to provide an independent relative dating approach by palaeomagnetic techniques failed. 230Th/U ages were determined applying an isochron approach using the leachate/leachate method. As the travertine deposits have a dense structure, the bulk sampling method was successfully applied in determining uranium-series ages with much higher precision than former studies with alpha spectrometry. Travertines from Süttő yielded Mid-Pleistocene ages ranging from the antepenultimate glacial to the penultimate interglacial (310–240 ka). These results are in agreement with those from optically stimulated luminescence (OSL) dating and amino acid racemisation (AAR) dating of the overlying sediment indicating at least an MIS 7 age for the travertine.