Rapid uranium-series age screening of carbonates by laser ablation mass spectrometry

Uranium-series dating is a critical tool in quaternary geochronology, including paleoclimate work, archaeology and geomorphology. Laser ablation (LA) methods are not as precise as most isotope dilution methods, but can be used to generate calendar ages rapidly, expanding the range of dating tools that can be applied to late Pleistocene carbonates. Here, existing LA methods are revisited for corals (cold- and warm-water) and speleothems spanning the last 343 thousand years (ka). Measurement of the required isotopes (²³⁸U, ²³⁴U, ²³⁰Th and ²³²Th) is achieved by coupling a laser system to a multi-collector inductively-coupled-plasma mass spectrometer (MC-ICPMS) using a combination of a single central ion counter and an array of Faraday cups. Each sample analysis lasts for ∼4.3 min, and fifty samples can be measured in 12 h with an automated set up, after a day of sample preparation. The use of different standard materials and laser systems had no significant effect on method accuracy. Uncertainty on the measured (²³⁰Th/²³⁸U) activity ratios ranges from 5.4% to 7.6% for (²³⁰Th/²³⁸U) ratios equal to 0.7 and 0.1 respectively. Much of this uncertainty can be attributed to the heterogeneity of the standard material (²³⁰Th/²³⁸U) at the length scale of LA. A homogeneous standard material may therefore improve measurement uncertainty but is not a requirement for age-screening studies. The initial (²³⁴U/²³⁸U) of coral samples can be determined within ∼20‰, making it useful as a first indicator of open-system behaviour. For cold-water corals, success in determination of (²³²Th/²³⁸U) – which can affect final age accuracy – by LA depended strongly on sample heterogeneity. Age uncertainties (2 sigma) ranged from <0.8 ka at 0–10 ka, ∼1.5 ka at 20 ka to ∼15 ka at 125 ka. Thus, we have demonstrated that U-series dating by LA-MC-ICPMS can be usefully applied to a range of carbonate materials as a straightforward age-screening technique.     

Testing a new combined (U,Th)–He and U/Th dating approach on Plio-Pleistocene calcite speleothems

The geochronology of cave deposits in the Cradle of Humankind UNESCO World Heritage Site in South Africa provides a timeframe essential for the interpretation of its fossils. The uranium-lead (U–Pb) and uranium-thorium disequilibrium (U/Th) dating of speleothems, mostly flowstones that underlie and blanket the fossil-bearing sediments, have been effective in this sense, but U–Pb is limited by the requirement of ∼1 ppm U concentrations and low common Pb contents, and U/Th has a c. 500 ka limit of applicability. Here we report age results for calcite-aragonite speleothems obtained using a new combined uranium-thorium-helium ((U,Th)–He) and U/Th dating routine. We reproduced within analytical uncertainty, the published U–Pb or U/Th ages for (a) flowstone in three drill core samples in the range 2000–3000 ka, (b) a flowstone hand sample taken at surface with an age of 1800 ka, and (c) five underground flowstone samples in the range 100–800 ka. Calcite retentivity for He under cave conditions is thus demonstrated. In the few cases where helium loss was observed in speleothems, only some of the subsamples were affected, and to varying degrees, suggesting loss by lattice damage not related to diagenetic processes, rather than volume diffusion. In the 100 to 800 ka range, the combined U/Th disequilibrium and (U,Th)–He method also yielded reliable values for initial (²³⁰Th/²³⁸U) and (²³⁴U/²³⁸U) activity ratios. Importantly, most subsamples had high initial (²³⁰Th/²³⁸U) values, ranging from 1.0 to 19.7, although having low Th/U ratios. This is probably due to incorporation of Fe–Mn oxides-hydroxides dust, on which ²³⁰Th was previously adsorbed. Such samples are mostly not dateable by U/Th without the additional input from the He analysis. If not detected and corrected for, such high initial (²³⁰Th/²³⁸U) values can lead to inaccurate U/Th and U–Pb ages. Our study shows that the incorporation of He analysis in U/Th dating has broad potential application, with four methods for calculating the ages, in carbonates from different environments where U-Pb or U/Th dating would not work.     

Fine chronology of volcanic processes using 238U-230Th systematics

The²³⁰Th-²³⁸U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (²³⁰Th/²³²Th)₀ ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship.     

U-series dating of bone in an open system: The diffusion-adsorption-decay model

A new theory is described for the uptake of U in an open system applied to the dating of archaeological bones. Analytical solutions are obtained for the rate of radioactive decay of ²³⁸U, ²³⁴U and ²³⁰Th as a function of position for the case where both ²³⁸U and ²³⁴U diffuse across a bone, and where external supply of ²³⁴U is not in equilibrium with ²³⁸U. The new theory constitutes a forward model for predicting ²³⁸U, ²³⁴U and ²³⁰Th activity profiles across a bone given an age and diffusion coefficient. The forward model can be used in an inversion process whereby observations of activity profiles of ²³⁸U, ²³⁴U and ²³⁰Th as a function of position are used to infer the bone age of a sample together with robust measures of uncertainty. Differences from previous studies are that no closed system assumptions are required and no apparent age calculations necessary, while diffusion of ²³⁴U across the bone is accounted for in the inversion process. The procedure also does not require U-concentration profiles for the calculation of model parameters. The measurement of U-concentration profiles are, however, useful for the assessment of the reliability of the calculated results. Because of the assumption of constant ²³⁴U/²³⁸U ratios at the boundaries of the bone, DAD age results are generally older than closed system U-series results derived from the same isotopic data. Allowance is made for both correlated and uncorrelated errors in activity measurements as well as theoretical error caused by inhomogeneities in the sample. The implementation of the new approach (which we term the DAD model for Diffusion–Adsorption–Decay) is straightforward and efficient enough to allow estimation of age and its uncertainty on a desktop computer. Software for performing age estimation with the new model is available from the corresponding author.     

Paleomagnetism and radiochemical age estimates for Late Brunhes polarity episodes

Several reversed polarity magnetozones occur within deep-sea sediment core CH57-8 from the Greater Antilles Outer Ridge, within sediment of latest Pleistocene/Late Brunhes age. The uppermost reversed interval spanning 31 data points coincides with the X faunal zone of the Last Interglacial Period. Radiochemical dating of cores CH57-8 and KN25-4 has shown that all the reversed polarity magnetozones are significantly younger than the Brunhes/Matuyama boundary at 0.7 m.y. B.P. A variation of the excess²³⁰Th method was used, in which²¹⁰Po and²³⁸U were the actual radionuclides measured. In a third core from the Mid-Atlantic Ridge, our²¹⁰Po results were similar to those which others obtained earlier by direct²³⁰Th measurements.     

Evidence for contamination of recent Hawaiian lavas from230Th-238U data

²³⁰Th-²³⁸U radioactive disequilibrium was studied in the historical lava flows of the Mauna Loa and Kilauea, Hawaii. Large variations of the (²³⁰Th/²³²Th) ratio among lavas of the same volcano that were erupted at a few years' interval are interpreted as due to contamination. The contamination probably occurs by assimilation of zeolitic minerals formed by seawater interaction while the magma resides in a superficial chamber.     

The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite

The (U-Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼1 Ma, chemical fractionation within the ²³⁸U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [²³⁰Th/²³⁸U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He ‘ages’ that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U-Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U-Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.     

New Th-U and C age determinations from Piton des Neiges volcano, Reunion — A revised chronology for the Differentiated Series

²³⁰Th-²³⁸U age measurements were carried out on 18 samples of lava from Piton des Neiges. These lavas range from basalts to trachytes. The ages extend from 270,000 to 12,000 a B.P. and cover most of the period in which Differentiated Series magmas were erupted. New ¹⁴C ages on charcoal sampled in young pyroclastic deposits are also presented. The late-stage evolution of the volcano is discussed in the light of these new ages and a chronology of the explosive activity is given. The oldest pyroclastic flow (Ste. Suzanne) is more than 220,000 a; other pyroclastic units were emitted between 220,000 and 110,000 a. There were at least two episodes of caldera formation: the first episode is inferred to have occurred about 190,000 a ago; the last occurred between 190,000 and 110,000 a. Although the most recent lava activity was mainly concentrated on the eastern flank of the volcano and around the summit area, it also occurred, about 70,000 a ago, in the southwestern part of the massif. The youngest explosive activity (about 50,000 to 20,000 a) on the eastern flank of the volcano has now been dated by three different methods (¹⁴C and Th-U ages on block- and ash-flow tuffs, K-Ar ages on associated domes). Youthful explosive activity (about 35,000 to 12,000 a) on the western flank of the volcano is also demonstrated by ¹⁴C dating on charcoal and by a Th-U age of 12,000 a on a trachytic nuée ardente deposit. This last age indicates that activity may have persisted later than previously supposed and implies a high rate of erosion for the Cirque de Mafate.     

Early diagenetic imprints and U–Th isotope systematics of fossil land snail shells from the Chinese Loess Plateau

The loess-paleosol sequence on the Chinese Loess Plateau provides a unique archive that records climate change in East Asia in the Quaternary, yet absolute dating of the loess deposits is challenging due to the lack of directly datable materials. Fossil land snail shells, which are made from aragonite, are widely preserved in the loess deposits and have long been used to reconstruct past environmental changes. U-series dating of fossil land snail shells has the potential to provide a route for absolute dating of the loess deposits but remains largely unexplored. In this study, we present the first systematic investigation on the U-series isotope geochemistry as well as the early diagenetic imprints of fossil land snail shells (Cathaica sp.) from the Mangshan loess-paleosol sequence in Henan province, central China. Several geochemical techniques, including Raman microscopy, SEM, LA-ICPMS, LA-MC-ICPMS, solution-MC-ICPMS, and AMS ¹⁴C dating, were employed to investigate the mineralogy, chemical and isotopic compositions of both modern and fossil snail shells to micrometer level. Our results show that the fossil Cathaica sp. shells are overall characterized by a higher degree of porosity and elevated contents of organic matter compared to live-collected shells of the same species. The layers with higher porosity and organic matter content in the fossil Cathaica sp. shell are also found to be enriched in Na, Mg, Mn, Ba, and U, indicating diffusion and adsorption of these elements by specific surface binding sites of either aragonitic lattice or organic compounds of the fossil shell. Combining in-situ measurements using LA-MC-ICPMS with solution U-series determination, we further demonstrate that fossil Cathaica sp. shell is relatively homogeneous regrading both [²³⁴U/²³⁸U] and [²³⁰Th/²³⁸U] values although the distribution of U in the fossil shell is sample-specific. The comparison of different dating results suggests that the calculated apparent closed system U–Th ages are all systematically younger (∼6000 to 13,000 years) than the corresponding shell ¹⁴C ages and quartz SAR (single-aliquot regenerative-dose) ages from the Mangshan section. We suggest that the underestimation of U–Th ages of fossil Cathaica sp. shells is very likely caused by diagenetic uptake of U that started immediately after the burial of the shell and effectively ceased when the fossil shell was isolated from the pore waters due to persistent deposition of eolian dust at the Mangshan section. Our work on both modern and fossil Cathaica sp. shells thus provides detailed morphological and geochemical characterization for the diagenetic alteration of fossil snail shells and suggests that U-series dating of fossil land snail shells may provide age constraints for dust deposits in the semi-arid region although the timing of early diagenetic U-uptake by the fossil shells need to be better quantified for reliable age determination.     

Multi-method approach to dating glass: The case of Basiluzzo Islet (Aeolian archipelago,Italy)

A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and ⁴⁰Ar–³⁹Ar methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser ⁴⁰Ar–³⁹Ar total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic ⁴⁰Ar the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 ± 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 ± 11.4 ka (MSWD = 0.6), with an initial ⁴⁰Ar/³⁶Ar ratio of 297.8 ± 1.8; the isochron is characterised by very little spread among points. The procedure named ‘point-counting technique’ was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 ± 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 ± 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The ²³⁸U/²³²Th and ²³⁰Th/²³²Th activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46 ± 8 ka and the ²³⁴U/²³⁸U activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 ± 7.1 ka (FT plateau age), 40.6 ± 11.4 ka (⁴⁰Ar–³⁹Ar isochron age of all grains), and 46 ± 8 ka (U/Th isochron) agree at the 1σ level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic ⁴⁰Ar in the presence of high atmospheric contamination (⁴⁰Ar–³⁹Ar dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision.     

Development of U–Pb dating of uraninite using a secondary ion mass spectrometer: Selection of reference material and establishment of calibration method

A method for U–Pb isotopic dating using secondary ion mass spectrometer (SIMS) was developed for uraninite. Correlation between ²⁵¹(UO)⁺/²³⁵U⁺ and ²⁰⁶Pb⁺/²³⁵U⁺ obtained by a sensitive high‐resolution ion microprobe (SHRIMP) was adopted for a calibration from secondary ion ratios (Pb⁺/U⁺) to the atomic abundance ratios (Pb/U). In this study, a uraninite sample (²⁰⁶Pb/²³⁸U = 0.1647) collected from Faraday mine, Bancroft, Canada, is used as a reference material for the U–Pb calibration. The established method was applied to three uraninite samples collected from the Chardon, Ecarpière, and Mistamisk mines. The calibrated ²⁰⁶Pb^*/²³⁸U ratios of the three uraninites show correlation with Pb/U elemental ratios obtained using an electron probe microanalyzer (EPMA) (correlation coefficients: 0.98, 0.99, and 0.97, respectively), which indicates the reliability of the SHRIMP calibration method used in this study. The analysis of Ecarpière uraninite provides concordant U–Pb data, and a weighted average of the ²⁰⁶Pb^*/²³⁸U age is 287 Ma ±8 Ma (95 % conf.) which is consistent with the previous chronological results by SIMS. Mistamisk uraninite provides discordant U–Pb data with the upper and lower intercept ages of 1 729 and 421 Ma, which correspond to uraninite formation in association with the Hudsonian orogeny and the remobilization of uranium as pitchblende, respectively. The U–Pb age of Chardon uraninite (315 Ma) is consistent with the igneous activity of Mortagne granite, but is older than the previously reported age (264 Ma). Marcasite in the Chardon uraninite altered to goethite under the oxidizing condition, which indicates that U–Pb system in the uraninite crystallized at 315 Ma was disturbed under the oxidizing condition. The established calibration method using Faraday uraninite is useful for U–Pb isotopic dating on the scale of a few micrometers to tens of micrometers, which make it possible to obtain the accurate age of uraninite.     

Rates of cave and landform development in the Yorkshire Dales from speleothem age data

Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the ²³⁰Th/²³⁴U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from ²³⁴U/²³⁸U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature.     

UUThTh systematics and the precise measurement of time over the past 500,000 years

We have developed techniques to measure the²³⁰Th abundance in corals by isotope dilution mass spectrometry. This, coupled with our previous development of mass spectrometric techniques for²³⁴U and²³²Th measurement, has allowed us to reduce significantly the analytical errors in²³⁸U²³⁴U²³⁰Th dating and greatly reduce the sample size. We show that 6 × 10⁸ atoms of²³⁰Th can be measured to ±30‰ (2σ) and 2 × 10¹⁰ atoms of²³⁰Th to ±2‰. The time over which useful age data on corals can be obtained ranges from a few years to 500 ky. The uncertainty in age, based on analytical errors, is ±5 y (2σ) for a 180 year old coral (3 g), ±44 y at 8294 years and ±1.1 ky at 123.1 ky (250 mg of coral). We also report²³²Th concentrations in corals (0.083–1.57 pmol/g) that are more than two orders of magnitude lower than previous values. Ages with high analytical precision were determined for several corals that grew during high sea level stands 120 ky ago. These ages lie specifically within or slightly postdate the Milankovitch insolation high at 128 ky and support the idea that the dominant cause of Pleistocene climate change is Milankovitch forcing.     

Dating of Late Pleistocene terrace deposits of the River Rhine using Uranium series and luminescence methods: Potential and limitations

Uranium-series dating of pedogenic carbonate crusts from fluvial gravels is tested using Optically Stimulated Luminescence (OSL) ages as references. OSL dating yielded ages of 30–15 ka and 13–11 ka, which correlate with the cold periods of the Last Glacial Maximum and the Younger Dryas. These ages are internally coherent and consistent with the geological background and are thus regarded as reliable. Most of the U/Th results scatter widely in the ²³⁰Th/²³²Th vs. ²³⁴U/²³²Th isochron diagram, making regression unrealistic. Semiquantitative age estimates from the data were found to be mostly older than the OSL ages and the geological context. It is suggested that a heterogeneous initial ²³⁰Th input, not relatable to a detrital component, is responsible for the observed discrepancies. This input may be due to bacterial activities and Th transport on organic colloids. It appears necessary to avoid samples where bacteria could have contributed to carbonate precipitation. Further, the relative importance of this problem decreases with sample age, so that U/Th dating of sinters is expected to be more reliable in the >100 ka age range.     

U-series dating of Lake Nyos maar basalts,Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam

From previously published ¹⁴C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO₂ that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in ²³⁸U–²³⁰Th equilibrium, all the other samples possess ²³⁸U–²³⁰Th disequilibrium with 15 to 28% enrichment of ²³⁰Th over ²³⁸U. The (²²⁶Ra/²³⁰Th) activity ratios of these samples indicate small (2 to 4%) but significant ²²⁶Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of ²³⁰Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The ²³⁰Th–²²⁶Ra age of Nyos samples calculated with the (²²⁶Ra/²³⁰Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean ²³⁰Th/²³⁸U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its ²³⁰Th–²²⁶Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested.     

The Xe-Xe spectrum technique,a new dating method

A new method to date uranium-bearing minerals exclusively by means of a mass spectrometric determination of Xe and/or Kr isotopic ratios has been developed and experimentally tested. It is based on the compositional differences between Xe produced by spontaneous fission of²³⁸U in nature and Xe from²³⁵U fission induced by thermal neutrons in a nuclear reactor. Xe is extracted in 5–10 release fractions at successively higher temperatures. This relates the radiogenic Xe contained in various structural elements of different retention characteristics to the respective U concentrations and allows to account for natural Xe losses, which are quite common. A monazite monitor mineral of known age is included in each irradiation and its Xe isotopes are measured in the same way.The samples analyzed include uranium oxides, REE phosphates, tantalum niobates, zircon and others.The results and comparisons with reported ages obtained by other means demonstrate the applicability of the new dating method for minerals with U contents up to a few percent. In age spectra the low-temperature release fractions of some samples indicate radiogenic Xe losses, while at high temperatures age plateaux can be obtained.     

Multi‐method (14C, 36Cl, 234U/230Th) age bracketing of the Tschirgant rock avalanche (Eastern Alps): implications for absolute dating of catastrophic mass‐wasting

Correct and precise age determination of prehistorical catastrophic rock‐slope failures prerequisites any hypotheses relating this type of mass wasting to past climatic regimes or palaeo‐seismic records. Despite good exposure, easy accessibility and a long tradition of absolute dating, the age of the 230 million m³ carbonate‐lithic Tschirgant rock avalanche event of the Eastern Alps (Austria) still is relatively poorly constrained. We herein review the age of mass‐wasting based on a total of 17 absolute ages produced with three different methods (¹⁴C, ³⁶Cl, ²³⁴U/²³⁰Th). Chlorine‐36 (³⁶Cl) cosmogenic surface exposure dating of five boulders of the rock avalanche deposit indicates a mean event age of 3.06 ± 0.62 ka. Uranium‐234/thorium‐230 (²³⁴U/²³⁰Th) dating of soda‐straw stalactites formed in microcaves beneath boulders indicate mean precipitation ages of three individual soda straws at 3.20 ± 0.26 ka, 3.04 ± 0.10 ka and 2.81 ± 0.15 ka; notwithstanding potential internal errors, these ages provide an ‘older‐than’ (ante quam) proxy for mass‐wasting. Based on radiocarbon ages (nine sites) only, it was previously suggested that the present rock avalanche deposit represents two successive failures (3.75 ± 0.19 ka bp , 3.15 ± 0.19 ka bp ). There is, however, no evidence for two events neither in surface outcrops nor in LiDAR derived imagery and drill logs. The temporal distribution of all absolute ages (¹⁴C, ³⁶Cl, ²³⁴U/²³⁰Th) also does not necessarily indicate two successive events but suggest that a single catastrophic mass‐wasting took place between 3.4 and 2.4 ka bp . Taking into account the maximum age boundary given by reinterpreted radiocarbon datings and the minimum U/Th‐ages of calcite precipitations within the rock avalanche deposits, a most probable event age of 3.01 ± 0.10 ka bp can be proposed. Our results underscore the difficulty to accurately date catastrophic rock slope failures, but also the potential to increase the accuracy of age determination by combining methods. Copyright © 2016 John Wiley & Sons, Ltd.     

U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 5.0/-6.0 Ma, and the other orthogneiss sample con- tains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 12/?17 Ma and 516 ± 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 10/?8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Pre- cambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.     

U–Th radioactive disequilibrium and ESR dating of a barite-containing sulfide crust from South Mariana Trough

A block of sulfide crust collected from an active hydrothermal mound in an Archaean site (12°56.4′N, 143°37.9′E; depth ca. 3000 m) of the South Mariana Trough was dated using both ²³⁰Th/²³⁴U disequilibrium and electron spin resonance (ESR) methods to establish the growth duration. Eight subsamples from the sulfide crust were separated further into magnetic and non-magnetic fractions using a Franz isodynamic separator. Thirteen sulfide samples, soluble in nitric acid, yielded ²³⁰Th/²³⁴U ages of 0.3–2.2 ka. The magnetic fractions had significantly lower Th/U ratios, which enabled age determinations as precise as ±2% (2σ). The age distribution obtained for the section of sulfide crust analyzed is consistent with deposition of sulfide minerals from the upper surface of the crust to the inner side. The ²³⁰Th/²³⁴U ages of the sulfide minerals were compared with ESR ages of barites separated from 12 subsamples of the same sulfide crust. ESR ages of 0.27–1.3 ka show a spatial pattern broadly resembling that observed in ²³⁰Th/²³⁴U dating method. While there are some significant offsets, these results illustrate the potential of the two methods for use in investigation of the evolutional history of a hydrothermal system.     

SHRIMP ziron U-Pb geochronology of early Mesozoic felsic igneous rocks from the southern Lancangjiang and its tectonic implications

The SHRIMP zircon U-Pb geochronology of three typical samples, including two monzonitic granites from the Lincang batholith and a rhyolite from the Manghuai Formation are presented in the southern Lancangjiang, western Yunnan Province. The analyses of zircons for the biotite monzonitic granites from the northern (02DX-137) and southern (20JH-10) Lincang batholith show the single and tight clusters on the concordia, and yield the weighted mean ²⁰⁶Pb/²³⁸U ages of 229.4 ± 3.0 Ma and 230.4 ± 3.6 Ma, respectively, representing the crystallized ages of these granites. The zircons for the rhyolitic sample (02DX-95) from the Manghuai Formation give a weighted mean ²⁰⁶Pb/²³⁸U age of 231.0 ± 5.0 Ma. These data suggest that the igneous rocks from the Lincang granitic batholith and Manghuai Formation have a similar crystallized age. In combination with other data, it is inferred that both were generated at a narrow age span (∼230 Ma) and were originated from the postcollisional tectonic regime. An early Proterozoic ²⁰⁶Pb/²³⁸U apparent age of 1977±44 Ma is additionally obtained from one zircon from the biotite monzonitic granite (southern Lincang batholith), indicative of development of the early Proterozoic Yangtze basement in the region. These precisely geochronological data provide important constraints on better understanding the Paleozoic tectonic evolution of the Tethys, western Yunnan Province.