Rare gas systematics on Lucky Strike basalts (37°N, North Atlantic): Evidence for efficient homogenization in a long-lived magma chamber system?

International audienceWe present rare gas data in fresh glasses from the Lucky Strike segment located on the Mid Atlantic Ridge (∼37.3°N), close to the Azores plateau. We analyzed the helium and neon isotopes in 28 samples by melting as well as He‐Ne‐Ar‐Kr‐Xe isotopes in 9 samples by crushing. Samples were collected during the Graviluck06, MOMAR08, and Bathyluck09 cruises over a ridge length of ∼13 km (mean sample spacing of ∼500 m), and at depths ranging from 1550 m to 2174 m. The helium isotopic ratio varies between 84,410 and 88,235 (R/Ra between 8.19 and 8.56). The samples having the “most” primitive helium isotopic ratio are the enriched samples (e.g. high K2O/TiO2) although the difference to the depleted samples is small. It appears that all of our samples derive from the same and well‐homogenized magma chamber. Neon isotopes clearly show the influence of the Azores hotspot, which is not seen with helium because of lower 3He/22Ne in the plume source compared to the MORB source

Etude des éléments lithophiles et fortement sidérophiles (Pd, Pt, Ru et Ir) dans les basaltes des dorsales océaniques

PARIS7-Bibliothèque centrale (751132105) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Ultra-slow spreading ridges: a response to the interplay between mantle convection and plate tectonics

International audienceUltra-slow spreading ridges such as the South West Indian ridge or the Arctic ridge system are oddities amongst oceanic ridges. Conversely to faster oceanic ridges, petrographic and seafloor studies have shown that they are characterized by low melt supply and present low crustal thicknesses and heat flow; these features are interpreted as an evidence for a cooler sublithospheric mantle. In cartoonish sketches of plate tectonics, ridges open above upwellings, subduction zones occur over downwellings, and plates are riding over the mantle convection cells. In this study, we designed a simple yet dynamically consistent thermal convection model to test the impact of farfield forces on spreading ridges and show that this pattern is disrupted by plate tectonics. In particular, continental collisions modulate the spreading rates because resisting forces build up at plate boundaries. As a consequence, this modifies the surface boundary conditions and therefore the underlying mantle flow. We show that the ideal convection cell pattern quickly breaks down when plate motion is impeded by continental collisions in the far field. Not only the decreasing spreading rates are diagnostic, but in the same time, (i) the heat flow is decreasing at the ridge, (ii) the thermal structure of the cooling lithosphere no longer matches the cooling half-space model, and (iii) the mantle temperature beneath the ridge drops by more than 100 degrees. We compare our model predictions to available observables and show that this simple mechanism explains the atypical thermo-mechanical evolution of the South West Indian ridge and Arctic ridge system. Last, the recent S wave seismic tomography model of Debayle and Ricard (2012) reveals that only away from those two ridges does lithospheric thickening departs from the half-space cooling model, in accord with our model predictions

Ultraslow, slow, or fast spreading ridges: Arm wrestling between mantle convection and far-field tectonics

International audienceOceanic spreading rates are highly variable, and these variations are known to correlate to a variety of surfaceobservables, like magmatic production, heat flow or bathymetry. This correlation lead to classify ridges into fastand slow spreading ridges, but also into the more peculiar ultraslow spreading regime. Here we explore the dynamicrelationships between spreading ridges, plate tectonics and mantle flow. We first focus on the thermal signatureof the mantle, that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012).We show that the thermal structure of ridges gradually departs from the half-space cooling model for slow, andabove all ultraslow spreading ridges. We also infer that the sublithospheric mantle temperature decreases by morethan 150 degrees C from fast to ultraslow spreading regimes. Both observations overall indicate that the mantleconvection pattern is increasingly chaotic underneath slow and ultraslow spreading ridges. We suggest that this isdue to far-field tectonics at the other ends of lithospheric plates: not only it modulates the spreading rates but it alsoalters the convection regime by obstructing the circulation of plates, which in turn modifies the surface kinematicconditions for the convecting mantle. We test this hypothesis using a thermo-mechanical model that represents aconvection cell carrying a continental lithosphere atop. The continent gradually drifts away from the spreadingridge, from which the oceanic lithosphere grows and cools while the continent eventually collides at the oppositeside. In turn, this event drastically modifies the upper kinematic condition for the convecting mantle that evolvesfrom a mobile lid regime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heatadvection decreases with respect to thermal conduction, which causes the oceanic lithosphere to thicken faster;the oceanic plates get compressed and destabilized by a growing number of small scale transient plumes, whichdisrupt the structure of the oceanic lithospheres, lower the heat flow and may even starve ultraslow ridges frompartial melting. It follows that the spreading rate of a modern ridge mirrors its status in the global plate tectonicsframework within a unique breakup, drift, collision scenario, within the transition from mobile to stagnant lid, andthat it is the same mechanism that build mountains at converging boundaries and control spreading rates. Oceanicridges thus can be regarded as a sensor of the resisting rather than driving forces. Both the model and the seismicstructure of the mantle underneath ridges reveal that the temperature variations are largest at shallow depths inthe upper mantle, i.e. at the critical depth where the melt supply to the above ridges can be modulated, therebyalso explaining why slow and ultraslow ridges are almost exclusively associated to cold mantle. It follows that thechemistry of oceanic ridge basalts may not strictly reveal the mantle potential temperature, but the variations in thesublithospheric temperature field

Ultra-slow spreading ridges: a response to the interplay between mantle convection and plate tectonics

International audienceUltra-slow spreading ridges such as the South West Indian ridge or the Arctic ridge system are oddities amongst oceanic ridges. Conversely to faster oceanic ridges, petrographic and seafloor studies have shown that they are characterized by low melt supply and present low crustal thicknesses and heat flow; these features are interpreted as an evidence for a cooler sublithospheric mantle. In cartoonish sketches of plate tectonics, ridges open above upwellings, subduction zones occur over downwellings, and plates are riding over the mantle convection cells. In this study, we designed a simple yet dynamically consistent thermal convection model to test the impact of farfield forces on spreading ridges and show that this pattern is disrupted by plate tectonics. In particular, continental collisions modulate the spreading rates because resisting forces build up at plate boundaries. As a consequence, this modifies the surface boundary conditions and therefore the underlying mantle flow. We show that the ideal convection cell pattern quickly breaks down when plate motion is impeded by continental collisions in the far field. Not only the decreasing spreading rates are diagnostic, but in the same time, (i) the heat flow is decreasing at the ridge, (ii) the thermal structure of the cooling lithosphere no longer matches the cooling half-space model, and (iii) the mantle temperature beneath the ridge drops by more than 100 degrees. We compare our model predictions to available observables and show that this simple mechanism explains the atypical thermo-mechanical evolution of the South West Indian ridge and Arctic ridge system. Last, the recent S wave seismic tomography model of Debayle and Ricard (2012) reveals that only away from those two ridges does lithospheric thickening departs from the half-space cooling model, in accord with our model predictions

Pétrogénèse des laves de l'ïle de la Possession (archipel de Crozet) et implications pour les hétérogénéités lithologiques des sources de points chauds

Cette étude présente le premier jeu de données complet en éléments majeurs, traces et en isotopes Sr-Nd-Pb sur les laves du point chaud de Crozet (île de la Possession). La pétrologie et les variations en éléments majeurs de ces laves révèlent une suite magmatique alcaline saturée en silice. Les compositions chimiques des magmas primaires sont estimées à partir d une sélection de laves peu affectées par les processus d accumulation et de mélanges magmatiques et corrigées des effets de la cristallisation fractionnée. Deux estimations extrêmes de magmas primaires sont proposées. La première possède une composition chimique comparable à celles de liquides de fusion partielle expérimentaux d une lherzolite à grenat. La seconde estimation est similaire aux liquides de fusion expérimentaux issus d une éclogite carbonatée. La correction de cristallisation fractionnée est donc une étape critique pour relier la composition en éléments majeurs des laves de points chauds à la présence d hétérogénéités lithologiques dans leur source. A l opposé, les éléments traces terres rares sont moins sensibles à ces incertitudes de correction, et leur inversion révèle dans les deux cas une source péridotitique enrichie en éléments incompatibles par rapport aux chondrites (LaN/SmN=3). Les compositions isotopiques Sr-Nd-Pb ne montrent aucune corrélation avec les éléments majeurs ou terres rares et révèlent une source homogène et enrichie par rapport au réservoir chondritique (_Nd=+4,3). Les isotopes du Pb indiquent la présence de 2 composants mantelliques, l un peu radiogénique de type EM-1 ou DUPAL et l autre plus radiogénique. Leur mélange confère aux laves de Crozet une composition isotopique semblable au type EM-2.We present a new and comprehensive data set of major and trace elements and Sr-Nd-Pb isotopes for Crozet hotspot lavas (Possession island). Major element variations and petrological characteristics define an alkali silica-saturated magmatic suite. I estimate two extreme compositions of primary melts on the basis of selected fractionation-corrected lava compositions unaffected by accumulation and magma mixing processes. The first estimation display a major element composition approaching those of experimental partial melts of a garnet-lherzolite, whereas the second is characterized by a composition resembling to experimental partial melts of a carbonated eclogite. These contrasted results reveal that the fractionation correction is a critical step to interpret major element contents in terms of lithological source heterogeneities in oceanic island lavas. The REE systematic appears to be less sensitive to uncertainties in fractionation corrections compared to major elements. The REE inversion of estimated primary melts reveals in both cases a peridotitic source enriched in incompatible elements relative to chondrites (LaN/SmN=3). The Sr-Nd-Pb isotopic compositions do not display any correlation with major elements nor REE contents or ratios and reveal a homogeneous source with moderate time-integrated enrichment relative to the chondritic reservoir (_Nd=+4,3). The Pb isotope systematic indicates the presence of 2 mantle components in the Crozet mantle source, one relatively unradiogenic compatible with an EM-1 or DUPAL signature, and the other one relatively radiogenic. The mixing of both components gives rise to an isotopic signature akin to EM-2 type.NANTES-BU Sciences (441092104) / SudocSudocFranceF

Fe3+/ΣFe vs. FeLα peak energy for minerals and glasses: Recent advances with the electron microprobe

International audienceThis paper describes a preliminary study that attempts to determine the oxidation state of Fe (Fe3+/ΣFe) with the electron microprobe (EMP) by measuring the self-absorption induced shift of the FeLα peak emitted from minerals and glasses. In transition metals of the first row, the L-spectra exhibit common distortions, namely peak position shifts, peak shape alterations, and changes in the Lβ/Lα ratios, caused by the large difference in the self-absorption coefficients (μ/ρ) on either sides of the L3 absorption edges that are in close proximity to the Lα peak maxima. Measurements performed on α-Fe2O3 and FexO oxides have shown that self-absorption effects are stronger for the later oxide, leading to enhanced Fe2+Lα peak shift toward longer wavelengths as the beam energy increases. First measurements performed on silicates have confirmed that enhanced self-absorption of FeLα occurs on Fe2+ sites. The measurements consisted of plotting the FeLα peak position at a fixed beam energy (15 keV) against the total Fe concentration for two series of Fe2+- and Fe3+- bearing silicates. In a first step, these data have shown that both Fe2+Lα and Fe3+Lα peaks shift continuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts for Fe2+Lα. For silicates containing only Fe2+ or Fe3+, no effects of the site geometry were detected on the variations of the FeLα peak position. A second set of plots has shown the variations of the peak position relative to the previous Fe2+-Fe3+ curves of step 1, as a function of the nominal Fe3+/ΣFe, for a series of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chain and sheet silicates (e.g., pyroxenes, amphiboles, micas) exhibited strong deviations compared to other phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeLα. Intervalence-charge transfer (IVCT) mechanisms between Fe2+ and Fe3+ sites may be the origin of these deviations. These crystal-structure effects limit the accuracy of the method for mixed Fe2+-Fe3+ valence silicates. Precisions achieved for further Fe3+/ΣFe measurements strongly depend on the total Fe concentration. For basaltic glasses containing an average of 8 wt% Fe and 10% Fe3+/ΣFe, the precision is about ±2% (absolute). For low Fe concentrations (below 3.5 wt%), the uncertainty in the peak position measured by the EMP spectrometers leads to error bars that are similar to with the separation of the curves fitted to the Fe2+ and Fe3+ plots, which is propagated as prohibitive lack of precision for Fe3+/ΣFe (>70% relative). A major limitation of microbeam methods in general deals with beam damage. This aspect has been carefully studied for basaltic glasses, and optimal beam conditions have been established (in general, electron doses higher than those corresponding to 130 nA and 30 μm beam diameter should be avoided to prevent large beam induced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phases such as hydrated glasses; and (2) minerals in which FeLα is affected by large matrix effect corrections (e.g., Cr- and Ti-rich oxides where FeLα is strongly absorbed), for which the self-absorptioninduced shift of FeLα is different from that of common silicates and glasses

4D geomorphological evolution of intertropical islands

International audienceCoral reef terraces record joint variations of sea level and surface elevation. U/Th ratings on corals along withtopographic/bathymetric profiles and eustatic reconstitutions allow to locally determine the vertical rate of groundmotion, while numerical modelling of reef sequences allows to unravel the processes controlling the architecture ofsequences, and high-resolution DEMs facilitates the detailed mapping of the sequences of reef terraces. Alltogether,these methods allow to extrapolate the local vertical rates towards an unprecedented resolution for 4D kinematics.We applied our method to uplifting islands of the tectonically active Buton Archipelago, SE Sulawesi, Indonesia.The area undergoes a general uplift revealed by the ubiquitous occurrence of uplifted and folded reef sequences.Wedated some 40 samples using U/Th, acquired sonar and dGPS profiles, and constructed high-res DEMs (Pleiades).Local vertical rates (from 0.2 to 0.28 mm/yr) were determined. Detailed geomorphological mapping of the lateralvariations of the terraces are converted into time and space variations of uplift rates. Extrapolating the higher,undated terraces permits to reconstruct the overall 4D geomorphology history over the last Ma. In turns, theseresults give a unique view on the structural kinematic

Mn/Ca ratios of Ammonia tepida as a proxy for seasonal coastal hypoxia

Climate variability has major implications for marine geochemical cycles and biogenic carbonate production. Therefore, past climate-driven changes in marine environments are often inferred from geochemical data of the marine carbonate archive. Proxy calibration studies are essential for the reconstruction of such past environmental changes. Here, we use the geochemical composition of living specimens of the benthic foraminifer Ammonia tepida at three sites in a seasonally hypoxic (oxygen concentration < 63 mu mol/L) marine coastal system (Lake Grevelingen, the Netherlands) to explore the use of Mn/Ca as a proxy for coastal hypoxia. The study is based on samples from three stations along a depth transect, that show contrasts in the seasonal cycle of Mn2+ concentrations in the pore water of the surface sediment. In general, the sediment and pore water geochemistry of the three stations in Lake Grevelingen show increased Mn2+ concentrations in late winter/spring, combined with increased Mn refluxing in summer, which are due to cable bacteria activity and bottom water hypoxia/anoxia, respectively. Laser Ablation-ICP-MS (LA-ICP-MS) allowed a comparison of Mn/Ca ratios of different parts of the benthic foraminiferal test. Our results show that higher Mn/Ca ratios are registered at the deepest station, which experiences the longest and most severe seasonal periods of hypoxia/anoxia. Additionally, the signal preserved in the central part of the benthic foraminiferal tests, which is thought to reflect the entire calcification history of the analysed specimen, appears to be driven by high pore water Mn2+ concentrations due to cable bacteria activity in late winter/spring. Conversely, high Mn/Ca ratios in the last chambers reflect increased Mn refluxing in the surface sediment due to summer hypoxia/anoxia. Thus, Mn/Ca ratios of A. tepida give insight into the complex spatial and temporal variability of pore water manganese

Bovine tuberculosis in Spain, is it really the final countdown?

2023 Acuerdos transformativos CRUEBovine tuberculosis (bTB) is a severe zoonotic disease that has major impacts on both health and the economy, and which has been subjected to specific eradication programmes in many countries for decades. This manuscript highlights the relevance of this disease in the context of the European Union (EU) and summarizes the epidemiological situation and the main tools (e.g. antemortem diagnostic tests, slaughterhouse surveillance, laboratories, comprehensive databases, etc.) used to control and eradicate bTB in the various EU countries with a focus on the situation in Spain. A comprehensive description of the specific bTB epidemiological situation in Spain is provided, together with an assessment of the evolution of different epidemiological indicators throughout the last decades. Moreover, the main features of the Spanish bTB eradication programme and its control tools are described, along with the studies carried out in Spain that have allowed the updating of and improvement to the programme over the years with the aim of eradication, which has been established for 2030.Depto. de Sanidad AnimalCentro de Vigilancia Sanitaria Veterinaria (VISAVET)Fac. de VeterinariaTRUEpubAPC financiada por la UC