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Sarıhıdır travertines and associated manganese mineralization are located in an area known as the Central Anatolian Volcanic Province (CAVP). The topic of the study, the Sarıhıdır manganese mineralization, was deposited in association with fault zones in this tectonically-active region. Paragenesis of the mineralization includes pyrolusite, rhodochrosite and goethite with gangue minerals of calcite and quartz observed. Diagrams drawn using major oxide and trace elements like Fe–Mn–(Ni+Co+Cu)×10, Fe-Si×2-Mn, Co/Ni vs Cu+Co+Ni, Si/Al, Na/Mg, Fe/Ti vs. Al/(Al+Fe+Mn), Co+Ni/As+Cu+Mo+Pb+V+ Zn and FeO–MnO2–10× MgO show the mineralization found in the region is sourced in the hydrothermal field. The low total REE values and LREE>HREE ratio indicate the effect of hydrothermal solutions. Additionally, Ce, Eu and Ceanom anomalies indicate the source of the mineralization was hydrothermal with oxic-anoxic formation environment. The Y/Ho ratio is mean 30.1 which is lower compared to hydrothermal fluids. Mean values are δ13CPDB 12.24‰ and δ18OPDB 10.87‰ indicating deposition from fluids with a deep CO2 component. In conclusion, the Sarıhıdır mineralization and travertine were determined to have formed about 50–53 ka (Upper Pleistocene) based on U-series age data, as fault-controlled, hydrothermal-sourced mineralization.

The Tierra Blanca (TB) eruptive suite comprises the last four major eruptions of Ilopango caldera in El Salvador (≤45 ka), including the youngest Tierra Blanca Joven eruption (TBJ; ∼106 km3): the most voluminous event during the Holocene in Central America. Despite the protracted and productive history of explosive silicic eruptions at Ilopango caldera, many aspects regarding the longevity and the prevailing physicochemical conditions of the underlying magmatic system remain unknown. Zircon 238U‐230Th geochronology of the TB suite (TBJ, TB2, TB3, and TB4) reveals a continuous and overlapping crystallization history among individual eruptions, suggesting persistent melt presence in thermally and compositionally distinct magma reservoirs over the last ca. 80 kyr. The longevity of zircon is in contrast to previously determined crystallization timescales of, Plain Language Summary: The collapse of a volcano edifice into its shallow magma chamber can produce one of the most dangerous single events in nature, known as a caldera‐forming eruption. The TBJ eruption in El Salvador is of this kind and occurred around 1,500 years ago, having a profound impact on Maya societies. Because of this, it is crucial to understand the inner workings of caldera‐forming eruptions to assess volcanic risks and their mitigation. Beneath Ilopango caldera, the micrometer‐sized radioisotopically datable mineral zircon grew within different storage levels of a silica‐rich magma reservoir suggesting continuous melt presence for up to ca. 80,000 years prior to eruption. The time information given by zircon contrasts with that extracted from other, more abundant minerals from the same rocks (, Key Points: U‐Th zircon ages for the last four explosive eruptions of Ilopango caldera reveal a long‐lived magma reservoir (>80 kyr). Contrasting residence times for major minerals and zircon suggest extraction of zircon along with evolved melt from crystal residue. Melt extraction from vertically extensive, thermally zoned magma reservoir., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Rock art dating has been one of the major challenges since its discovery and recognition. The methods have evolved through the last century, beginning with the study of superpositions and style until to the application of numeric methods since the 1990s. The aim of this paper is to evaluate and publish an up-to-date database of all of the numerical dates currently available for Iberian prehistoric rock art sites. For this purpose, the manuscript reviews all the methods applied so far to Iberian rock art discussing the limits, the sampling involved, and the problems affecting the results. After that, we present and discuss the most relevant results related to each cultural graphic tradition (Palaeolithic, Levantine, Schematic and Megalithic rock art) assessing their value and limitations. Finally, we reflect on the future of rock art dating: unfortunately most of the motifs are not dateble in numeric terms, meaning we still have to combine traditional with numerical methods; but also, we need to keep working on the problems affecting these methods to be able to create a more reliable chronological framework of use to address other issues such as group mobility, cultural networks, and reutilisation of symbolic elements, to name a few.

A precisely dated speleothem isotope record from the Sieben Hengste cave system, Switzerland, reveals a Last Interglacial (LIG) climate optimum centred at 128.1 ± 1.2 ka. The oxygen isotope signal is consistent with a temperature-dominated record and shows a strong correlation with the modelled Greenland d18O signal. The comparison with speleothems formed during the Last Glacial Maximum (LGM) in the same cave indicates a 4e5‰ difference in d18O between peak LIG and peak LGM conditions, suggesting peak LIG temperatures being up to 3 C higher than the Holocene average. Pollen extracted from the stalagmite indicates a vegetation response delayed by up to 3 ka with maximum pollen concentrations recorded at 125 ± 2.2 ka. Our results support a regional vegetation dominated by thermophile tree species including Corylus (hazel), Quercus robur-type (oak), Ulmus (elm), Tilia (lime tree) and Hedera (ivy), consistent with slightly warmer conditions than today. Despite the relatively low pollen concentration, the speleothem-based pollen spectra are consistent with regional pollen assemblage zones from paleolake sediments and provide the first direct paleovegetation evidence of thermophilization during the LIG at high altitudes in the Alps. ISSN:0277-3791

Based on previous radiocarbon and U-series (Diffusion/Adsorption) dating of bone samples, the Middle Palaeolithic has been thought to persist at Gruta da Oliveira until ∼37 thousand years (ka) ago. New U-series ages for stratigraphically constraining speleothems, coupled with new luminescence ages for sediment infill, show that the site’s ∼6 m-thick archaeological stratigraphy dates entirely within a

EASPEBIOmEAGROSUP; Pedogenic carbonates (calcretes) have often been proposed as possible markers of the pedogenetic processes in soils under semi-arid climate. However, precise chronological constraints on, their formation are required in order to investigate the climatic and paleo-environmental conditions that prevailed during and after their formation, and to improve our understanding of the, physical and chemical conditions that promoted their development and preservation. Moreover, these authigenic calcium carbonate precipitates provide us with an interesting test of the U-Th radioactive disequilibrium dating method, the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems, but which remains much more questionable in highly porous, chemically and mineralogically complex media such as soil profiles. We report here U-238-U-234-Th-230 radioactive disequilibrium analyses measured by Thermo-Ionization Mass Spectrometry in calcretes from two sites in southwest India. Since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals, all ages must be computed using the isochron technique, involving the extraction of several coeval subsamples from the same soil horizon and U-Th analyses after total dissolution (TSD) method, in order to correct for the detrital component. Th-232 is usually used as index of the detrital contamination, and the isochron age is derived from the isotopic composition (Th-230/U-238; U-234/U-238) of the pure authigenic carbonate end-member of the mixing line. Our results show that each set of samples taken from decimetric blocks is characterized by a well-defined isochron line in a 3-D Osmond diagram (Th-232/U-238; Th-230/U-238; U-234/U-238). The regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model (in most cases 0.05), and mean square weighted deviations (MSWD) values around 1. The compositions of the end-members are highly variable between the different isochrons, both for the carbonate phase, indicating various ages of cristallisation, and for the detrital phase, indicating various origins and compositions. Notably, our data demonstrate that the detrital component is in all cases out of secular equilibrium, by contrast with the standard paradigm of "dirty carbonate" dating, which is based on the assumption of average continental crust Th/U elemental ratio, and secular equilibrium values of the ((Th-230/U-238) and (U-234/U-238) ratios. At the first site (Coimbatore), we obtain U-Th ages between 304 +/- 38 ka and 44 +/- 2.9 ka (2 sigma) from the base to the top of the profile. These suggest discontinuous precipitation of the different fades in good agreement with micro morphological observations. At the second study site (Gundlupet), the carbonate nodules sampled from two different profiles, yield consistent ages of 200 ka, which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event. Surprisingly, in mere contradiction with the standard model of impure carbonate dating, two puzzling samples display "vertical isochrons", i.e., variable Th-232/U-238 without any variation of the Th-230/U-238 and U-234/U-238 ratios. Such patterns require extremely high Th-232/U-238 ratios, in an as yet unidentified detrital phase. Taken at face value, these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally-driven climate variations recorded in other archives during the last glacial/interglacial cycles.

International audience; Longgupo cave, located in Chongqing Municipality, China, was discovered in 1984. Sixteen Gigantopithecus teeth and two hominid fossils accompanied with abundant mammalian fossils and stone artifacts were unearthed from the site. Previous dating result is questioned because of the complexity of deposition history. In this study, seventeen mammalian fossil teeth collected from different layers of unit C II and C III (C III') during the 2003-2006 Sino-Franco joint excavation were analyzed by combined ESR and U-series methods, and calculated with a US-ESR model. In situ dose rates were remeasured in detail in 2012, in order to refine the external dose rate determination. Uranium-series analyses indicate that no obvious uranium leaching has occurred, and all the teeth underwent a very recent uranium uptake history, except one from layer C III 3 of the south wall. The US-ESR results show that the ages of ten teeth from unit C III' of the north wall are consistent in general, about 2.35 Ma. The ages of seven samples from south wall are not in accordance with the stratigraphic order: three teeth are much younger than the other four, probably attributed to the relative higher uranium concentration and U/Th ratio in the dental tissues and significant higher in situ gamma dose rate. Nevertheless, the three teeth from C III of the south wall give an average age of - 2.48 Ma. The fossil ages obtained in this work combined with paleontological evidence and new paleomagnetic results place Longgupo at the very beginning of the Early Pleistocene. Compared with other early hominid settlements, Longgupo is one of the earliest evidences of hominid settlement in China and East Asia to date.

Deep erosional gullies dissect landscapes around the world. Existing erosion models focus on predicting where gullies might begin to erode, but identifying where existing gullies were initiated and under what conditions is difficult, especially when historical records are unavailable. Here we outline a new approach for fingerprinting alluvium and tracing it back to its source by combining bulk sediment optically stimulated luminescence (bulk OSL) and meteoric 10Be (10Bem) measurements made on gully-derived alluvium samples. In doing so, we identify where gully erosion was initiated and infer the conditions under which such erosion occurred. As both 10Bem and bulk OSL data have distinctive depth profiles in different uneroded and depositional settings, we are able to identify the likely incision depths in potential alluvium source areas. We demonstrate our technique at Birchams Creek in the southeastern Australian Tablelands—a well-studied and recent example of gully incision that exemplifies a regional landscape transition from unchanneled swampy meadow wetlands to gully incision and subsequent wetland burial by post-European settlement alluvium. We find that such historic alluvium was derived from a shallow erosion of valley fill upstream of former swampy meadows and was deposited down the center of the valley. Incision likely followed catchment deforestation and the introduction of livestock, which overgrazed and congregated in valley bottoms in the early 20th century during a period of drought. As a result, severe gully erosion was likely initiated in localized, compacted, and oversteepened reaches of the valley bottom.

Clay minerals hold some key information on time and rate of weathering processes that could be deciphered by U-series disequilibria. However a removal of organic matter is generally required prior to clay mineral chemical analyses, especially in soils. The impact of two protocols of organic matter removal on the U-series disequilibria in three different clay minerals was therefore investigated. Natural or commercial kaolinite, illite and montmorillonite were treated with hydrogen peroxide and by calcination. Both investigated treatments impact the U-series signature of clay minerals. Yet the extent of this impact strongly depends on the clay-type: while kaolinite remains almost insensitive to the treatments, illite shows some equivocal shifts since the activity ratios measured after calcination could describe more accurately the pure clay signal than initial activity ratios, and montmorillonite displays the larger reactivity. Overall, the H2O2 treatment seems to induce more disturbance than the calcination process, mostly on U isotopes in illite and montmorillonite. Due to the oxidizing power of H2O2, the redox state of U held in the clays, i.e.: U(IV) or U(VI), strongly influences the extent of U leaching. In addition, a preferential leaching of U-234 relative to U-238 was observed in montmorillonite sample only, resulting to a U isotopic ratio fractionation. Conversely, during calcination, the most pronounced effect arises for Th isotopes. The congruent feature of Th loss, i.e.: Th loss approximate to mass loss, affecting the three clay minerals suggests a loss of Th through volatilization during this treatment. U or Ra are also affected, but to a lesser extent, by this volatilization. Overall, calcination causes a two- or three-fold reduced change of the (Th-230/U-234) and (U-234/U-238) activity ratios relative to H2O2 treatment. The (Ra-226/Th-230) ratio was clearly the less sensitive to any treatment. It was only slightly affected in kaolinite after calcination, and the observed change suggests an improvement of the measured value due to a removal of a Th-bearing external matter. Albeit not perfect, calcination should therefore be preferred to H2O2 treatment for organic matter removal before U-series analyses.

Long-term leaching experiments were done using aquifer rocks of the Aquitaine Basin, ranging in age from Santonian to upper Eocene. Leachates were sampled after one hour, one day, one week, and one month, and were analyzed for U concentrations and activity ratios. Overall, U contents in the leachate decreased with increasing time. U activity ratios were characteristic of each aquifer rock. Only slight variations, when present, were found between the one-week and the one-month leachates. In most cases, the short-lived leachates (one hour, one day) displayed U activity ratios closer to the equilibrium value.

We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ~350 ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

On continental margins, oxidation of methane-rich fluids from the sediment often leads to formation of authigenic carbonate pavements on the seafloor. The biogeochemical processes involved during this carbonate precipitation are increasingly understood, but little is known about the duration or mode of carbonate crust formation. Here, we report uranium and thorium concentrations and isotope compositions for a set of 14 samples drilled across an authigenic carbonate pavement, which provide the first stratigraphy for a cold-seep carbonate crust. The 5.5-cm thick crust (NL7-CC2) was collected by submersible on the Nile deep-sea fan in an area of active fluid venting. U-Th analyses must be corrected for initial Th and measurement of co-existing sediments indicates the presence of both scavenged and detrital initial 230Th, which must be considered during this correction. The calculated 230Th/U age-depth profile for NL7-CC2 provides evidence for continuous downward carbonate precipitation at the studied location over the last ~ 5000 years. Three distinct phases can be distinguished from top to bottom with average growth rates of ~ 0.4, 5 and 0.8 cm/kyr, respectively, corresponding to carbonate precipitation rates ranging from ~ 7 to 92 μmol m2 h- 1 (rates consistent with previous estimates). High-resolution δ13C profiles [Gontharet, S., Pierre, C., Blanc-Valleron, M.-M., Rouchy, J.M., Fouquet, Y., Bayon, G., Foucher, J.P., Woodside, J., Mascle, J., The Nautinil Scientific Party, 2007. Nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the Nile deep-sea fan (eastern Mediterranean Sea). Deep Sea Res. II 54, 1292-1311] and major elements across NL7-CC2 show that the variations in carbonate precipitation rates were also accompanied by changes in carbonate mineralogy and fluid composition. We suggest that these changes primarily reflect modification of the diagenetic environment, i.e. a progressive depletion of dissolved sulphate through anaerobic oxidation of methane, caused by the initial carbonate crust formation and the resulting reduction in bioirrigation. Overall, U-Th dating of cold seep carbonates offers a promising tool to bring new insights into biogeochemical processes at cold seeps and to assess the timing and duration of fluid venting on continental margins. © 2008 Elsevier B.V. All rights reserved.

International audience; U-series studies in oceanic basalts are critical for understanding melting and melt extraction in the Earth's mantle. The combined results of a detailed geochemical study of melting and melt extraction at Theistareykir, northern Iceland, provide a strong case for melt extraction via channeled melt flow at an active spreading ridge. It has often been argued, however, that widely used melting and melt extraction models, which simulate channeled melt extraction (i.e. fractional and/or dynamic melting), can only partially explain the global U-series systematics in oceanic basalts. As a consequence, more complicated models have been invoked, which suggest different styles of melt extraction at different depths/pressures in the mantle, so-called “two-porosity models”. Alternatively, diffusion-controlled mechanisms have been proposed. Here we show that U–Th–Pa–Ra systematics in oceanic basalts can indeed be explained by models where melt transport occurs without chemical equilibrium between melt and solid when variations in all three critical melting parameters (residual porosity, upwelling rate of the solid mantle and melt velocity) are taken into account. Melting at ridges requires systematic variation of at least two critical melting parameters, most likely upwelling and melt extraction rate. Melts generated with increasing lateral distance to the ridge axis are generated with slower upwelling rates and are extracted with lower velocities than melts created closer to the ridge axis. Melting at ocean islands, on the other hand, can successfully be explained by variations in upwelling rate only. Global U-series systematics in OIB originate from superimposed global variations in upwelling velocity due to different buoyancy fluxes and from local variation in upwelling velocity as a function of radial distance to the plume center. The model proposed here is consistent with other geochemical data for oceanic basalts and strongly supports melt extraction via high-porosity channels as a general means of melt extraction from the Earth's upper mantle.

Ice sheets and deep ice cores have yielded a wealth of paleoclimate information based on continuous dating methods while independent radiometric ages of ice have remained elusive. Here we demonstrate the application of ( 234 U/ 238 U) measurements to dating the EPICA Dome C ice core based on the accumulation of 234 U in the ice matrix from recoil during 238 U decay out of dust bound within the ice. Measured ( 234 U/ 238 U) activity ratios within the ice generally increase with depth while the surface areas of the dust grains are relatively constant. Using a newly designed device for measuring surface area for small samples, we were able to estimate reliably the recoil efficiency of nuclides from dust to ice. The resulting calculated radiometric ages range between 80 ka and 870 ka. Measured samples in the upper 3100 m fall on the previously published age-depth profile. Samples in the 3200–3255 m section show a marked change from 723–870 ka to 85 ka indicating homogenization of the deep ice prior to resetting of the ( 234 U/ 238 U) age in the basal layers. The mechanism for homogenization is likely enhanced lateral ice flow due to high basal melting and geothermal heat flux.

International audience; The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U-Th-Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9-10 degrees N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large (230)Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large (226)Ra excesses at the expense of diminished (230)Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9-10 degrees N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9-10 degrees N EPR, the asymmetric, off-axis distribution of E-MORB at 9-10 degrees N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (similar to 0-3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9-10 degrees N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

Uranium (U) presents a unique challenge for ecological risk assessments (ERA) because it induces both chemical and radiological toxicity, and the relative importance of these two toxicities differs among the various U source terms (i.e., natural, enriched, depleted). We present a method for the conversion between chemical concentrations (μg L -1) and radiological dose rates (μGy h -1) for a defined set of reference organisms, and apply this conversion method to previously derived chemical and radiological benchmarks to determine the extent to which these benchmarks ensure radiological and chemical protection, respectively, for U in freshwater ecosystems. Results show that the percentage of species radiologically protected by the chemical benchmark decreases with increasing degrees of U enrichment and with increasing periods of radioactive decay. In contrast, the freshwater ecosystem is almost never chemically protected by the radiological benchmark, regardless of the source term or decay period considered, confirming that the risks to the environment from uranium's chemical toxicity generally outweigh those of its radiological toxicity. These results are relevant to developing water quality criteria that protect freshwater ecosystems from the various risks associated with the nuclear applications of U exploitation, and highlight the need for (1) further research on the speciation, bioavailability, and toxicity of U-series radionuclides under different environmental conditions, and (2) the adoption of both chemical and radiological benchmarks for coherent ERAs to be conducted in U-contaminated freshwater ecosystems. © 2009 American Chemical Society.

International audience; Travertine and calcareous tufa deposits, are important archives for Quaternary continental climate and archaeology. Here we present a comprehensive study on Holocene travertine deposition in the Thuringia basin based on a detailed survey of U-series measurements and dating that demonstrates rapid accumulation of carbonate during the early Holocene until 8000 years (up to 8 mm yr-1) followed by significant change in tra-vertine accumulation that ended around 5800 years BP at the investigated site. We further demonstrate that systematic changes of the U-isotopic composition through time occur in agreement with major changes in travertine accumulation and texture, which are likely to reflect changes of spring water U-isotopic composition rather than U-series system opening or contamination with non-carbonate particles.; Les travertins et les tufs calcaires constituent des archives importantes pour les études sur le climat continental quaternaire et l’Archéologie. Nous présentons l’étude de la formation d’un travertin holocène du bassin de Thuringe, basée sur un exposé détaillé des données uranium-thorium et des âges U-Th obtenus, qui démontrent une accumulation rapide du carbonate pendant le début de l’Holocène jusque vers 8000 ans (jusqu’à 8 mm par an) suivie par un changement significatif dans le rythme d’accumulation du travertin qui s’achève vers 5800 ans BP sur le site étudié. Nous démontrons ensuite que les changements systématiques dans la composition isotopique de l’uranium au cours du temps sont en accord avec les variations majeures de texture et d’accumulation du travertin et reflètent les changements de composition isotopique de l’uranium des eaux de source plutôt qu’une ouverture du système des séries de l’uranium voire une contamination par des particules non carbonatées.

International audience; The archaeological site of Payre (France) yielded Middle Palaeolithic layers with Neanderthal remains, which are partly bracketed by two stalagmitic flowstones. To obtain a precise chronological framework for the human occupation and the faunal remains of this site, several dating methods were used: uranium-series (U-series by alpha spectrometry and TIMS) and electron spin resonance (ESR) on stalagmitic flowstones, combined ESR/U-series on teeth and bones, and thermoluminescence (TL) on burnt flints. Most of the ages obtained for levels H to C range from 300 to 140 ka. The age-spread obtained for the anthropic (burnt flints) and faunal remains (bones and teeth) suggests that the dated levels represent several human occupation periods, near the transition from MIS8 to MIS7 (levels G–F) and from MIS6 to MIS5 (levels D–E).

International audience; The high proportion of crystallized apatite forming the bones and teeth should theoretically allow the use of fission track analysis to date vertebrate fossils when burying of the fossiliferous series did not subject them to temperatures exceeding 60 degrees C. However several major obstacles arise such as the complexity of fossils' internal structures, diagenetic modifications and substitutions of the hydroxyapatite by other minerals, and massive U uptake or loss during diagenesis. In this work, those various problems are addressed combining optical microscope observation of the fossils, a systematic fission track analysis of the best samples, alpha and gamma spectrometry and Secondary Ion Mass Spectrometry. Even if the problems caused by internal structures and mineral substitutions can be solved, U movements in and out of the fossils are generally too complex to allow fission track analysis dating of the fossils.

International audience; Andesitic products of the last magmatic eruption at La Soufrière in Guadeloupe (1530 AD) were analyzed for 238U–230Th–226Ra in order to constrain the timescales of magmatic processes preceding the eruption. Magma composition evolved during eruption from acid andesitic scoria falls and flows to andesitic lavas during the final phase of dome extrusion, with some evidence of mixing in banded pumices at the base of the first eruptive unit. Most of the least differentiated products show 238U and 226Ra excesses relative to 230Th, like many other arc lavas. The (226Ra/230Th) activity ratios of whole rocks then decrease with the degree of differentiation from 1.507 to 0.983, whereas the (230Th/238U) ratios increase from 0.942 to 1.003. The groundmasses of the least differentiated lava show a (226Ra/230Th) ratio higher than 1, but all the other samples are in secular equilibrium. These variations can be modeled by time-dependent differentiation from a common parental melt. Assuming a closed-or open-magma chamber behaviour (closed or open system), the duration of magma differentiation ranges from 30 to 70 kyr with crystallization rates ranging from 0.012 to 0.019 kyr− 1. 238U–230Th disequilibria have also been measured in minerals from one of the least differentiated products and provide crucial information on their age of crystallization, their origin and the crystal dynamics in the magma chamber. A crystallization age of 34.9 ± 8.3 kyr (2σ) is obtained using U–Th internal isochrons on pyroxenes, which is older than the differentiation time of their host lava given by time-dependant differentiation models, but younger than the differentiation time of the most evolved magma compositions. Older cumulate crystals were thus recycled in the erupted magmas, consistent with petrologic evidence for crystal-melt disequilibria. The calculated (226Ra/230Th) ratios of mineral assemblages (> 1 for most samples) suggest the presence of plagioclases younger than 8000 yr, in agreement with the fact that plagioclases do not plot on the U–Th isochron defined by pyroxenes. This study thus exemplifies the hybrid nature of arc andesitic magmas such as the 1530 AD Soufriere magma, which is generated by mixing of crystals and melts produced by fractional crystallization of a common parental magma. Our results also confirm that the magma chamber below large volcanoes such as La Soufrière of Guadeloupe is long-lived (up to tens of thousands of years). This timescale is much longer than the cooling times predicted by a closed-system thermal model, suggesting that the magma chamber was sustained by inputs of fresh and hot magma and behaves as an open system.