Precise Age Determination Research Articles

Precise Pb baddeleyite geochronology by the thermal extraction-thermal ionization mass spectrometry method

The ability to easily obtain precise (~0.1%) age determinations on mafic rocks can facilitate identification of contemporaneous magmatic events across continents and hence open new avenues of research investigations related to large scale mantle processes. We present high precision Pb age determinations on baddeleyite (ZrO2), a common accessory mineral in mafic rocks using the recently developed thermal extraction-thermal ionization mass spectrometer (TE-TIMS) procedure for dating of zircon. In order to constrain the conditions for thermal extraction of Pb from baddeleyite, a sequence of SEM back-scattered electron images were acquired along with energy-dispersive X-ray spectroscopy spot analysis of baddeleyite grains embedded in silica beads at progressively increasing temperature between 1200 and 1590°C. Replicate analyses of twelve preheated (at 1250°C) and silica embedded baddeleyite grains from the Paleoproterozoic Phalaborwa carbonatite using this procedure have yielded a 207Pb–206Pb weighted mean age of 2060.3±0.4Ma, consistent with previously reported 207Pb–206Pb ages for this carbonatite occurrence determined by the conventional U–Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) technique. Fourteen baddeleyite fractions from a large Paleoproterozoic N–S striking mafic dyke swarm in the Dharwar craton yielded a 207Pb–206Pb weighted mean age of 2215.9±0.3Ma, also consistent with previously determined conventional ages. All data were corrected for mass fractionation of 0.18%/amu as determined previously on zircon. Both data sets scatter outside measurement errors, implying an external 2 sigma error of 0.055% in the case of the carbonatite and 0.043% for the mafic dyke, which we suggest represents the reproducibility of the fractionation correction. Our results demonstrate that thermal extraction 207Pb–206Pb ages are comparable with the conventional U–Pb ages in both accuracy and precision. This procedure does not need ultra-clean laboratory conditions (<1pg procedure blanks), a prerequisite for conventional high precision U–Pb geochronology, and hence can be easily practised in most TIMS isotope geochemistry laboratories.

Precise age determination of a single plutonium particle using inductively coupled plasma mass spectrometer

Abstract Age-dating of single plutonium oxide particles ∼ 1 µm in diameter was demonstrated using eight particles. For this demonstration, the particles were prepared from a standard reference material SRM-947 that had been stored for 3.9 years since the last Pu purification. The Pu particles were separately dissolved, and Am and Pu were chemically separated from the solution via anion-exchange chromatography. The isotope ratios of Am and Pu in the eluted fractions were measured using a high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS). The precision and accuracy of the Pu age were improved through the addition of an 243Am spike to the sample solutions and by the trace chemical separation of Pu and Am. The determined age in this work was in good agreement with the expected age with high accuracy (deviation: 7.1–105 days) and precision (uncertainty: 0.16–0.5 years (k = 1)). These results indicate that a combination of ICP-MS and chemical separation after spiking with 243Am provides an effective tool for the analysis of environmental samples collected during nuclear safeguard inspections.

Characterisation of the Rota Wewa tank cascade system in the vicinity of Anuradhapura, Sri Lanka

A complex and sustainable watershed management strategy was implemented in Sri Lanka during the ancient Anuradhapura period, from the 5th century BC to the 11th century AD. Like modern watershed management strategies, it focused on flood prevention, soil erosion control, water quality control and water storage for irrigation. Tank cascade systems were the key element of these ancient watershed management installations. The wewas investigated were constructed in valleys characterised by fluvial accumulation. Sedimentological analyses of these tank cascade systems show that a precise age determination and the reconstruction of sediment and water f luxes as triggered by human-environment interactions are difficult. This is caused by the shallow character of the wewas leading to the steady redeposition of the tank sediments by wave motions during the wet season and agricultural use of the desiccated wewas during the dry season. Beyond, the sediments analysed allow to distinguish between the weathered parent bedrock and the overlying sediments. A differentiation between wewa deposits and the underlying fluvial deposits remains challenging.

Precision Analysis of Three Aging Structures for Amphidromous Dolly Varden from Alaskan Arctic Rivers

AbstractThe accuracy of population statistics and the validity of management actions they motivate are in part dependent on the acquisition of quality age determinations. Such data for northern‐form Dolly Varden Salvelinus malma have been traditionally garnered using otoliths, despite little research investigating the consistency of this or alternative nonlethal techniques. To address these gaps, the precision of age determinations generated from scales, otoliths, and fin rays was examined for 126 amphidromous Dolly Varden collected from two Arctic rivers. Three independent readers, age‐bias plots, coefficients of variation (CVs), and percent agreement (PA) were used to estimate bias and precision for among‐reader, within‐structure comparisons and within‐reader, among‐structure comparisons. Among‐reader, within‐structure tests of CVs suggested that otoliths produced more precise age determinations than fin rays. Furthermore, the CV for scales was intermediate to and not significantly different from those for otoliths and fin rays. Age‐bias plots suggested that, scales consistently underestimated age relative to otolithsbeginning at age 6. Underestimation was also apparent, but less distinct, within fin ray–otolith and scale–fin ray comparisons. Potential sources of error and management implications are discussed. Because scale and otolith ages exhibited little bias within cohorts younger than age 6, age may be determined nonlethally in these cohorts using scales; otoliths should be used otherwise.Received April 27, 2012; accepted May 13, 2013

Dinoflagellate cysts from the Szlachtowa Formation (Jurassic) and adjacent deposits (Jurassic-Cretaceous) of the Grajcarek Unit at Szczawnica-Zabaniszcze (Pieniny Klippen Belt, Carpathians, Poland)

The Jurassic–Cretaceous fine-clastic dark deposits of the Grajcarek Unit (Pieniny Klippen Belt, Carpathians, Poland) show lithological similarities, which may lead to erroneous age correlations. Their well-exposed outcrop at Szczawnica-Zabaniszcze allows detailed sampling for their precise age determination. For this purpose, dinoflagellate cysts have been studied from a set of samples collected from the Szlachtowa, Opaleniec, Pieniny Limestone, Kapuśnica and Wronine formations exposed at that site (additional sample from the Pieniny Limestone Formation was studied). The Szlachtowa Formation, a very characteristic lithostratigraphic unit, which consists of black shale and sandstones extremely rich in mica flakes, occasionally referred to literature as the so-called “black flysch”, yielded rich and well-preserved assemblages. Their age is Middle Jurassic, spanning the Aalenian to Lower Bajocian. The exposures of the Szlachtowa Formation at Szczawnica-Zabaniszcze are located close to the outcrops of the Opaleniec Formation. The calcareous spotted shale from this lithostratigraphic unit yielded rich dinoflagellate cyst assemblage characteristic for Late Bathonian (Early Callovian?). The Opaleniec Formation tectonically contacts with very similar non-calcareous spotted shale of the Wronine Formation. Its age, based on infrequent dinoflagellate cysts is Aptian–Albian. Similar, Lower Aptian age was concluded for the Kapuśnica Formation. Late Barremian–earliest Aptian dinoflagellate cysts have been found in the underlying topmost part of the Pieniny Limestone Formation.

ANALYSIS OF TRIASSIC ARCHOSAURIFORM TRACKWAYS: DIFFERENCE IN STRIDE/FOOT RATIO BETWEEN DINOSAUROMORPHS AND OTHER ARCHOSAURIFORMS

Fossilized trackways have rarely been analyzed quantitatively to examine major trends and patterns in evolution despite their potential utility, especially in understanding locomotory evolution. In the present study, trackways of Triassic archosauriforms were analyzed. The analyses showed foot and stride lengths of archosauriforms increased from the Early to Middle Triassic, especially those of dinosauromorphs, which tripled. Dinosauromorphs were much smaller in foot length and stride length compared to other archosauriforms during the Early Triassic. They reached similar stride length compared with other archosauriforms during the Middle Triassic and similar foot length in the Late Triassic. Stride/foot ratio is significantly higher in dinosauromorphs compared to other archosauriforms throughout the Triassic. This relatively long stride length of dinosauromorphs is attributed to either faster speed or higher relative hip height that was probably caused by their digitigrade foot posture. Analyses of trackway data sets, especially in combination with precise trackmaker assignment and age determination, would bring us more thorough knowledge about locomotory evolution of tetrapods that complements body fossil evidence.

Zircon U–Pb dating and Hf isotope analysis on the Taihua Complex: Constraints on the formation and evolution of the Trans-North China Orogen

Precise age determination and source tracing of the granitic and pegmatitic gneisses of the Taihua Complex in the Xiaoqinling area are important to address the debates on the timing and processes of the amalgamation of the Western and Eastern Blocks along the Trans-North China Orogen. This paper conducted zircon dating of metamorphic rocks from the Xiaoqinling area. The results demonstrate that the collision between the Western and Eastern Blocks occurred in the Paleoproterozoic. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon uranium–lead (U–Pb) analyses yielded consistent results. The Early Paleoproterozoic (the first period) age of 2346±28Ma was determined for granitic gneiss in the Taihua Complex and 2328±14Ma for the Xiaohe granite pluton that intruded into the granitic gneiss. The gneissose pegmatitic granite and granitic veins that intruded into the granitic gneiss yielded LA-ICP-MS zircon ages of 1881±24 and 1866±19Ma, respectively. These Paleoproterozoic ages strongly indicate that the ocean between the Eastern and Western Blocks were completely subducted at ∼1.85Ga. The ɛHf(t) values of the zircons from the granitic gneiss can be divided into a positive group (0.44 to 3.77) and a negative group (−3.11 to −1.27). The two-stage hafnium (Hf) model ages (TDM2(Hf)) corresponding to the positive values are between 2672 and 2887Ma. The ɛHf(t) values of the zircons from the Xiaohe granite range from −2.49 to 2.58, with the Hf isotope model ages ranging from 2742 to 3022Ma. Thus, the Xiaohe granite could have resulted from the partial melting of the Late Archean lower crust caused by subduction and the presence of juvenile mantle material. The primary magma of the granitic gneiss of the Paleoproterozoic Taihua Complex was derived from the differentiation of the depleted Late Archean mantle source with a small portion derived from the remelting of the subducted lower crust.

Archaeological and palaeopathological study on the third/second century BC grave from Turfan, China: Individual health history and regional implications

This paper presents an archaeological–palaeopathological case study from Turfan (western China). Although this area is located outside of the two focus regions of the special issue (Baikal and Hokkaido), it once belonged to the same cultural sphere and experienced intensive contacts and exchanges with the neighbouring regions. The case study presented here combines detailed palaeopathological analysis of human skeletal remains with precise age determination and archaeological and regional contexts, demonstrating the high potential of such studies in arid and semi-arid China. The current paper also presents an unusual early case of prosthetic leg use from western China. The skeleton of a 50–65-year-old man and his wooden leg prosthesis discovered in a tomb in Shengjindian graveyard (Turfan, China) have been examined. Macroscopic lesions observed on ribs two to eleven, between the fifth and sixth cervical vertebra, and on the left knee joint are probably due to tuberculosis infection. The inflammatory process led to osseous ankylosis of the knee, fixing it at 135° flexion and 11° internal rotation, making walking impossible. The lost mobility of the disabled shank was regained by using an externally fitted wooden prosthesis which consists of thigh stabilizer, peg, leather straps, sheep/goat horn reinforcement of the peg tip, and horse/Asiatic ass hoof as sink resistance. Heavy traces of wear and absence of muscle atrophy indicate long-term use of the prosthesis. Ten radiocarbon dates on the prosthesis, human bones and wood pieces from the same grave suggest the most probable age of the burial is about 300–200 BC (68% confidence interval), thus introducing the oldest functional leg prosthesis known to date. The results provide some support for a DNA-based hypothesis that the spread of tuberculosis in Asia was related to intensified inter-regional contacts and higher residential mobility during the first millennium BC.

Geochronological constraints on the age of a Permo–Triassic impact event: U–Pb and 40Ar/39Ar results for the 40 km Araguainha structure of central Brazil

Impact cratering has been a fundamental geological process in Earth history with major ramifications for the biosphere. The complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination, hampering the assessment of the effects of an individual event in the geological record. We demonstrate the utility of a multi-chronometer approach in our study of samples from the 40km diameter Araguainha impact structure of central Brazil. Samples of uplifted basement granite display abundant evidence of shock deformation, but U/Pb ages of shocked zircons and the 40Ar/39Ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history. Mixed results are obtained from in situ40Ar/39Ar spot analyses of shocked igneous biotites in the granite, with deformation along kink-bands resulting in highly localized, partial resetting in these grains. Likewise, spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material, the age of the glass itself, and post-impact devitrification. The timing of crater formation is better assessed using samples of impact-generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization. Granular aggregates of neocrystallized zircon form a cluster of ten U–Pb ages that yield a “Concordia” age of 247.8±3.8Ma. The possibility of Pb loss from this population suggests that this is a minimum age for the impact event. The best evidence for the age of the impact comes from the U–Th–Pb dating of neocrystallized monazite and 40Ar/39Ar step heating of three separate populations of post-impact, inclusion-rich quartz grains that are derived from the infill of miarolitic cavities. The 206Pb/238U age of 254.5±3.2Ma (2σ error) and 208Pb/232Th age of 255.2±4.8Ma (2σ error) of monazite, together with the inverse, 18 point isochron age of 254±10Ma (MSWD=0.52) for the inclusion-rich quartz grains yield a weighted mean age of 254.7±2.5Ma (0.99%, 2σ error) for the impact event. The age of the Araguainha crater overlaps with the timing of the Permo–Triassic boundary, within error, but the calculated energy released by the Araguainha impact is insufficient to be a direct cause of the global mass extinction. However, the regional effects of the Araguainha impact event in the Paraná–Karoo Basin may have been substantial.

Evaluating Re–Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments

Successful application of the 187Re–187Os geochronometer has enabled the determination of accurate and precise depositional ages for organic-rich sedimentary rocks (ORS) as well as establishing timing constraints of petroleum generation. However, we do not fully understand the systematics and transfer behaviour of Re and Os between ORS and petroleum products (e.g., bitumen and oil). To more fully understand the behaviour of Re–Os systematics in both source rocks and petroleum products we apply hydrous pyrolysis to two immature hydrocarbon source rocks: the Permian Phosphoria Formation (TOC=17.4%; Type II-S kerogen) and the Jurassic Staffin Formation (TOC=2.5%; Type III kerogen). The laboratory-based hydrous pyrolysis experiments were carried out for 72h at 250, 300, 325 and 350°C. These experiments provided us with whole rock, extracted rock and bitumen and in some cases expelled oil and asphaltene for evaluation of Re–Os isotopic and elemental abundance.The data from these experiments demonstrate that the majority (>95%) of Re and Os are housed within extracted rock and that thermal maturation does not result in significant transfer of Re or Os from the extracted rock into organic phases. Based on existing thermodynamic data our findings suggest that organic chelating sites have a greater affinity for the quadravalent states of Re and Os than sulphides.Across the temperature range of the hydrous pyrolysis experiments both whole rock and extracted rock 187Re/188Os ratios show small variations (3.3% and 4.7%, for Staffin, respectively and 6.3% and 4.9% for Phosphoria, respectively). Similarly, the 187Os/188Os ratios show only minor variations for the Staffin and Phosphoria whole rock and extracted rock samples (0.6% and 1.4% and 1.3% and 2.2%). These isotopic data strongly suggest that crude oil generation through hydrous pyrolysis experiments does not disturb the Re–Os systematics in ORS as supported by various studies on natural systems.The elemental abundance data reveal limited transfer of Re and Os into the bitumen from a Type III kerogen in comparison to Type II-S kerogen (0.02% vs. 3.7%), suggesting that these metals are very tightly bound in Type III kerogen structure. The 187Os/188Os data from the pyrolysis generated Phosphoria bitumens display minor variation (4%) across the experimental temperatures, with values similar to that of the source rock. This indicates that the isotopic composition of the bitumen reflects the isotopic composition of the source rock at the time of petroleum generation. These data further support the premise that the Os isotopic composition of oils and bitumens can be used to fingerprint petroleum deposits to specific source rocks.Oil generated through the hydrous pyrolysis experiments does not contain appreciable quantities of Re or Os (∼120 and ∼3ppt, respectively), in contrast to natural oils (2–50ppb and 34–288ppt for Re and Os, respectively), which may suggest that kinetic parameters are fundamental to the transfer of Re and Os from source rocks to oils. From this we hypothesise that, at the temperatures employed in hydrous pyrolysis, Re and Os are assimilated into the extracted rock as a result of cross-linking reactions.

Precise age determination of mafic and felsic intrusive rocks from the Permian Emeishan large igneous province (SW China)

Radiogenic isotopic dating using different (mainly micro-beam analyses) techniques of rocks from the Emeishan large igneous province (ELIP) has yielded a range of ages from the Capitanian to the Early Triassic. In contrast, estimates based on paleontological and paleomagnetic data suggest the ELIP was emplaced rapidly (≤2Ma) and that volcanism ended by ~257Ma. New zircon CA–TIMS U–Pb ages, that have superior precision compared to ages from micro-beam analyses, were obtained from strategic intrusive rocks of the Panxi region (Inner Zone) of the ELIP. The Huangcao (258.9±0.7Ma), Daheishan (259.1±0.5Ma), Woshui (259.6±0.5Ma) and Cida (258.4±0.6Ma) syenitic and granitic plutons and three mafic dykes (259.2±0.4Ma; 259.5±0.8Ma, 257.6±0.5Ma) yielded a narrow range of ages between >257Ma and ~260Ma. The new results are consistent with the estimates from magneto-biostratigraphic data and a rapid emplacement model. The new Wuchiapinqian ages of the Huangcao and Daheishan plutons suggests that magmatism may have lasted significantly less than 10Ma as previously suggested, and the precise ages of the mafic dykes refute the hypothesis that Emeishan ‘high-Ti’ basalts represent the waning stages of the ELIP magmatism. The short duration of magmatism also implies that degassing of the host sediments (mostly carbonates), and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought, which significantly changes the loading of atmosphere per time unit. The latter corollary potentially has severe environmental effects that may have contributed to causing the end-Guadalupian mass extinction.

On the applicability of post-IR IRSL dating to Japanese loess

Recent work on infrared stimulated luminescence (IRSL) dating has focussed on finding and testing signals which show less or negligible fading. IRSL signals measured at elevated temperature following IR stimulation at 50°C (post-IR IRSL) have been shown to be much more stable than the low temperature IRSL signal and seem to have considerable potential for dating. For Early Pleistocene samples of both European and Chinese loess natural post-IR IRSL signals lying in the saturation region of the laboratory dose response curve have been observed; this suggests that there is no significant fading in nature. As a contribution to the further testing of post-IR IRSL dating, we have used 18 samples from two Japanese loess profiles for which quartz OSL and tephra ages up to 600 ka provide age control. After a preheat of 320°C (60 s), the polymineral fine grains (4–11 μm) were bleached with IR at 50°C (200 s) and the IRSL was subsequently measured at 290°C for 200 s. In general, the fading uncorrected post-IR IRSL ages agree with both the quartz OSL and the tephra ages. We conclude that the post-IR IRSL signal from these samples does not fade significantly and allows precise and accurate age determinations on these sediments.

New discoveries of glirids and eomyids (Mammalia, Rodentia) in the Early Miocene of the Junggar basin (Northern Xinjiang province, China)

We report herein new discoveries of Gliridae and Eomyidae (Mammalia, Rodentia) from the new Early Miocene locality XJ 200604, located northwest to the Burqin city, northern Xinjiang province, China. These specimens represent three taxa of Gliridae, including a new species (Miodyromys asiamediae nov. sp., Microdyromys aff. orientalis and Eliomys? sp.), and four taxa of Eomyidae (Asianeomys aff. engesseri, Asianeomys sp., Keramidomys sp. and Eomyidae indet). The identification of two forms that are closely related to Microdyromys orientalis and Asianeomys engesseri suggests an Early Miocene age of the locality but does not allow a more precise age determination. The new discoveries of Gliridae and Eomyidae again reveal the relative scarcity of the two families in the Early Miocene of Central Asia, in contrast to the coeval rich European record. Such a discrepancy in taxonomic diversity and abundance is hypothesized as resulting from different environmental evolutions during the Early Miocene, most notably the onset of a mid-latitude dry climate in Central Asia linked to combine effects of the Tibetan Plateau uplift and the retreat of the Paratethys Sea.

Late Quaternary climatic history of NW Romania: results from Th/U dating of speleothems

Speleothems are secondary minerals that precipitate in caves, most commonly as stalagmites, stalactites and flowstones. They are primarily composed of calcium carbonate, precipitated by slow degassing of carbon dioxide from supersaturated waters entering the cave gallery. Trace elements may be transported in solution and co-precipitated in calcite. One of these, uranium, when in concentration that exceeds 0.01 ppm allows precise age determination by U-series dating methods. Speleothem growth is a sensitive indicator of both temperature and humidity, and changes in either one or both of these can cause periods of enhanced or reduced growth. Furthermore, the worldwide karst landscape distribution and the study of speleothems growth interval enable regional chronologies to be drawn up. Uranium concentration in speleothems depends on several factors: uranium content of the host rock, residence time in the underground water, HCO 3 - availability etc., and thus is highly variable from one speleothem to another or even in the same speleothem

Time evolution of high-energy emissions of low-mass stars

Stellar ages are extremely difficult to determine and often subject to large uncertainties, especially for field low-mass stars. We plan to carry out a calibration of the decrease in high-energy emissions of low-mass GKM stars with time, and therefore precise age determination is a key ingredient. The overall goal of our research is to study the time evolution of these high-energy emissions as an essential input to studing exoplanetary atmospheres. We propose to determine stellar ages with a methodology based on wide binaries. We are interested in systems composed of a low-mass star and a white dwarf (WD), where the latter serves as a stellar chronometer for the system. We aim at obtaining reliable ages for a sample of late-type stars older than 1 Gyr. We selected a sample of wide binaries composed by a DA type WD and a GKM companion. High signal-to-noise, low-resolution spectroscopic observations were obtained for most of the WD members of the sample. Atmospheric parameters were determined by fitting the spectroscopic data to appropiate WD models. The total ages of the systems were derived by using cooling sequences, an initial-final mass relationship and evolutionary tracks, to account for the progenitor life. The spectroscopic observations have allowed us to determine ages for the binary systems using WDs as cosmochronometers. We obtained reliable ages for 27 stars between 1 and 5 Gyr, which is a range where age determination becomes difficult for field objects.

Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.

Some aspects of age assessment of Holocene loess colluvium: OSL and 137Cs dating of sediment from Biała agricultural area, South Poland

At the end of the Pleistocene in Central-Eastern Europe, the accumulation of periglacial loess had already finished. From the end of loess accumulation to the establishment of grass cover and forest succession in this area, a phase of active modeling of the land surface by slope processes occurred. Before the beginning of the Holocene, this activity stopped and mollisol and alfisol formation started. These soils are preserved beneath Holocene colluvia. Until the Neolithic age, the area of loess formations in Poland was uninhabited. The extensive Neolithic agriculture locally induced soil erosion, causing slopewash on farmedslopes. The scale of this process and establishing the age of the successive stages of the prehistoric soil erosion in loess deposits in Poland is a key problem in the investigations of the human–environment relationship in prehistory. Chronostratigraphy of the preserved Holocene colluvia is very helpful in the solution of these problems. Optical (OSL) dating was applied in chronostratigraphical analyses. Due to expected partial bleaching during sediment transport, a single-aliquot regenerative dose protocol (SAR) was applied. Insufficient bleaching during transport on the slope is expected to be a serious problem for luminescence dating of loess colluvia, and causes overestimation of the OSL age. Quartz from the study site was suitable for age determination by SAR. The youngest 50-year old colluvia were also studied using the 137Cs method. The reliability of the obtained results is discussed in this paper, as well as the influence of environmental processes and type of the applied methods on chronological assignments. The precise age determination of loess colluvia is important for study of the human impact on the environment. The results of OSL dating show that soil erosion started in the Neolithic, with three phases of soil erosion: Neolithic, Middle Ages, and modern.

Testing substellar models with dynamical mass measurements

We have been using Keck laser guide star adaptive optics to monitor the orbits of ultracool binaries, providing dynamical masses at lower luminosities and temperatures than previously available and enabling strong tests of theoretical models. We have identified three specific problems with theory: (1) We find that model color–magnitude diagrams cannot be reliably used to infer masses as they do not accurately reproduce the colors of ultracool dwarfs of known mass. (2) Effective temperatures inferred from evolutionary model radii are typically inconsistent with temperatures derived from fitting atmospheric models to observed spectra by 100–300 K. (3) For the only known pair of field brown dwarfs with a precise mass (3%) and age determination (≈25%), the measured luminosities are ~2–3× higher than predicted by model cooling rates (i.e., masses inferred from L bol and age are 20–30% larger than measured). To make progress in understanding the observed discrepancies, more mass measurements spanning a wide range of luminosity, temperature, and age are needed, along with more accurate age determinations (e.g., via asteroseismology) for primary stars with brown dwarf binary companions. Also, resolved optical and infrared spectroscopy are needed to measure lithium depletion and to characterize the atmospheres of binary components in order to better assess model deficiencies.

The construction and development of SHRIMP I: An historical outline

In 1973 Bill Compston advocated the building of an ion microprobe at the Research School of Earth Sciences (RSES) at the Australian National University (ANU). The commercial ion probes available at this time were too small to have sufficient sensitivity for trace element analysis and too low in mass resolution to avoid molecular interferences. The project commenced in 1974 with the appointment of a former ANU PhD student Steve Clement who had expertise in beam transport theory. To achieve high sensitivity and high mass resolution, beam transport theory indicated that a much larger magnet than in any commercially available mass spectrometer would be required. Clement chose an ion optical design, by Professor Matsuda of Osaka University in 1974, which had the required combination of high mass resolution and high transmission. Clement's job was to produce the detailed scientific designs and machine drawings for the new instrument as well as testing the completed instrument. Clement coined the term SHRIMP- Sensitive High Resolution Ion Micro Probe. By the end of 1977 nearly all the components had been manufactured and the big electromagnet had been successfully tested. In the following year the secondary mass analyzer was assembled and tested using a thermal ionization source and showed great promise with flat-topped peaks at 5000 resolution and 50% transmission with 50 V energy spread. At this stage the machine had far exceeded the specifications for the available commercial ion probes. Continued development during 1981 to the point where the original design specifications were fully realized was time consuming since learning how to use the entirely novel instrument was no simple task; no one else had an instrument like SHRIMP. The application of the instrument to zircon U–Pb geochronology established the necessary operating conditions for measuring Pb isotopic compositions and the elemental ratios Pb/U and U/Zr from 20 μm diameter spots on single zircon grains. Application of this in the early 1980s started a revolution in Precambrian geology by the ability to produce rapidly accurate and precise age determinations on structurally complex zircon samples.

Late Jurassic-Early Cretaceous Foraminifera from Štramberk-type Limestones, Polish Outer Carpathians

Foraminifera studies were carried out on the Stramberk-type limestones which occur as pebbles to small boulders (exotics) within the uppermost Jurassic-Palaeogene flysch deposits of the Polish Outer Carpathians. The studies were focused on foraminifera from (1) reef and peri-reef facies, and (2) lagoonal algal-foraminiferal facies. The paper deals with 34 species (including 8 in open nomenclature) representing 30 genera, 22 families, 14 superfamilies, 11 suborders, and 7 orders. A more precise age determination has been obtained for 30 exotics, representing a Tithonian (mostly) to Valanginian age. Foraminiferal assemblages from at least two exotics suggest a Valanginian age as indicated by the co-occurrence of the species: Meandrospira favrei (CHAROLLAIS, BRONNIMANN & ZANINETTI), Montsalevia salevensis (CHAROLLAIS, BRONNIMANN & ZANINETTI), Patellina turriculata DIENI & MASSARI, Neotrocholina valdensis REICHEL, Hechtina praeantiqua BARTENSTEIN & BRAND, Istriloculina eliptica (IOVCHEVA) and I. emiliae NEAGU. Previous studies based on ammonites and calpionellids indicated the Tithonian to the Early Berriasian age of the Stramberk Limestone in Moravia (the Czech Republic), and their equivalents in Poland. However, Valanginian shallow-water limestones occur as pebbles in some deposits of the Outer Carpathians in the Czech Republic. Further evidence for Valanginian shallow-water marine carbonate sedimentation (algal facies) in the Polish part of the Tethyan margin comes from recent studies of the substrate drilled in the Carpathian Foredeep.