De l'emploi des anesthésiques durant l'accouchement

Doctorat en sciences médicalesinfo:eu-repo/semantics/nonPublishe

Extreme U–Th Disequilibrium in Rift-Related Basalts, Rhyolites and Granophyric Granite and the Timescale of Rhyolite Generation, Intrusion and Crystallization at Alid Volcanic Center, Eritrea

Rhyolite pumices and co-erupted granophyric (granite) xenoliths yield evidence for rapid magma generation and crystallization prior to their eruption at 15·2 ± 2·9 ka at the Alid volcanic center in the Danikil Depression, Eritrea. Whole-rock U and Th isotopic analyses show 230Th excesses up to 50% in basalts <10 000 years old from the surrounding Oss lava fields. The 15 ka rhyolites also have 30–40% 230Th excesses. Similarity in U–Th disequilibrium, and in Sr, Nd, and Pb isotopic values, implies that the rhyolites are mostly differentiated from the local basaltic magma. Given the (230Th/232Th) ratio of the young basalts, and presumably the underlying mantle, the (230Th/232Th) ratio of the rhyolites upon eruption could be generated by in situ decay in about 50 000 years. Limited (5%) assimilation of old crust would hasten the lowering of (230Th/232Th) and allow the process to take place in as little as 30 000 years. Final crystallization of the Alid granophyre occurred rapidly and at shallow depths at 20–25 ka, as confirmed by analyses of mineral separates and ion microprobe data on individual zircons. Evidently, 30 000–50 000 years were required for extraction of basalt from its mantle source region, subsequent crystallization and melt extraction to form silicic magmas, and final crystallization of the shallow intrusion. The granophyre was then ejected during eruption of the comagmatic rhyolites

Crystallisation ages in coeval silicic magma bodies: 238U-230Th disequilibrium evidence from the Rotoiti and Earthquake Flat eruption deposits, Taupo Volcanic Zone, New Zealand

The timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent, successive eruption deposits in the Taupo Volcanic Zone, New Zealand. The earlier, caldera-forming Rotoiti eruption (>100 km3 magma) at Okataina volcano was followed, within months at most, by the Earthquake Flat eruption (10 km3 magma) from nearby Kapenga volcano; both generated non-welded ignimbrite and coeval widespread fall deposits. The Rotoiti and Earthquake Flat deposits are both crystal-rich high-silica rhyolites, with sparse glass-bearing granitoid fragments also occurring in Rotoiti lag breccias generated during caldera collapse. Here we report 238U–230Th disequilibrium data on whole rocks and mineral separates from representative Rotoiti and Earthquake Flat pumices and the co-eruptive Rotoiti granitoid fragments using TIMS and in situ zircon analyses by SIMS. Multiple-grain zircon-controlled crystallisation ages measured by TIMS from the Rotoiti pumice range from 69±3 ka (350 ka, with a pronounced peak at 70–90 ka. The weighted mean of isochrons is 83±14 ka, in accord with the TIMS data. One glass-bearing Rotoiti granitoid clast yielded an age of 57±8 ka by TIMS (controlled by Th-rich phases that, however, are not apparently present in the juvenile pumices). Another glass-bearing Rotoiti granitoid yielded SIMS zircon model ages peaking at 60–90 ka, having a similar age distribution to the pumice. Age data from pumices are consistent with a published 64±4 ka eruptive age (now modified to 62±2 ka), but chemical and/or mineralogical data imply that the granitoid lithics are not largely crystalline Rotoiti rhyolite, but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies. Similarly, although the Earthquake Flat eruption immediately followed (and probably was triggered by) the Rotoiti event, age data from juvenile material are significantly different. A multiple-grain zircon-controlled crystallisation age measured by TIMS from a representative pumice is 173±5 ka, while SIMS model ages range from 70−26+34 ka to >350 ka, with a peak at 105 ka. These age data coupled with previously published geochemical and isotopic data show that the Rotoiti and Earthquake Flat deposits were erupted from independent, unconnected magma bodies

Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U–Th and U–Pb Systematics in Zircons

Young (<65 ka) explosive silicic volcanism at Taupo volcano, New Zealand, has involved the development and evacuation of several crustal magmatic systems. Up to and including the 26·5 ka 530 km3 Oruanui eruption, magmatic systems were contemporaneous but geographically separated. Subsequently they have been separated in time and have vented from geographically overlapping areas. Single-crystal (secondary ionization mass spectrometry) and multiple-crystal (thermal ionization mass spectrometry) zircon model-age data are presented from nine representative eruption deposits from 45 to 3·5 ka. Zircon yields vary by three orders of magnitude, correlating with the degrees of zircon saturation in the magmas, and influencing the spectra of model ages. Two adjacent magma systems active up to 26·5 ka show wholly contrasting model-age spectra. The smaller system shows a simple unimodal distribution. The larger system, using data from three eruptions, shows bimodal model-age spectra. An older 100 ka peak is interpreted to represent zircons (antecrysts) derived from older silicic mush or plutonic rocks, and a younger peak to represent zircons (phenocrysts) that grew in the magma body immediately prior to eruption. Post-26·5 ka magma batches show contrasting age spectra, consistent with a mixture of antecrysts, phenocrysts and, in two examples, xenocrysts from Quaternary plutonic and Mesozoic–Palaeozoic metasedimentary rocks. The model-age spectra, coupled with zircon-dissolution modelling, highlight contrasts between short-term silicic magma generation at Taupo, by bulk remobilization of crystal mush and assimilation of metasediment and/or silicic plutonic basement rocks, and the longer-term processes of fractionation from crustally contaminated mafic melts. Contrasts between adjacent or successive magma systems are attributed to differences in positions of the source and root zones within contrasting domains in the quartzo-feldspathic (<15 km deep) crust below the volcano

Preliminary conceptual design of an electronuclear system for space applications

International audienceThe purpose of this paper is to present a preliminary conceptual design of a nuclear power system for space applications. This design was produced within the framework of a collaboration between CNES and CEA. The targeted electrical power of the system is 10 kWe for a period of at least 10 years. The scalability towards larger (30 kWe) and lower (about 1 kWe) power has been investigated. The system must be technologically “affordable” and robust over a 10-year horizon, which de facto eliminates the choice of too innovative and unqualified technologies in conditions representative of the missions; the system must be able to be launched in Ariane 6 while respecting the launcher mass and size capacities. Given the intermediate power level targeted and the potential use of the system on the Moon or Mars, the CEA favored design options associated with a compact and monolithic radiator (allowing it to be landed) rather than minimizing the system mass. Taking into account the desired characteristics for the nuclear power system, the CEA proposes main conceptual design features which includes: a thermal spectrum core whose fuel is UO$_2$ enriched to 19.75% in U$^{235}$, zirconium fuel cladding, a zirconium hydride (ZrH$_2$) moderator, and a beryllium reflector. The proposed shield is made of tungsten and lithium hydride (LiH). A core cooling system made up of 150 steel heat pipes operating with a sodium–potassium alloy (NaK) has been pre-designed and a thermoelectric energy converter made up of SiGe thermo-elements (hot temperature of 950 K and cold temperature of 700 K) generates electrical power. A “hot” carbon composite radiator lined with titanium NaK heat pipes rejects the heat of the system. This system has intrinsic redundancies conferred by the heat pipes and the large number of thermo-elements of the converter as well as high passivity, in particular in its ability to adapt its power to electricity demand. In its version limited to 10 kWe, the system whose height is about 5 m, enables to keep a height of 13 m for a payload to be launched in the Ariane 6 fairing and its mass is estimated at about 2 tons. To access a version of the system capable of producing 30 kWe, an additional mass estimated at around 1 ton is to be expected as well as a lower useful height in the fairing under the system, which would then be 10 m