Rapport de sondages et d'analyses, Le Kilian et les carrières anciennes de trachyte dans la Chaîne des Puys (Puy-de-Dôme)

En 2008, l'existence de carrières souterraines médiévales avait été mise en évidence dans la pente ouest du Bois de Manson qui domine la dépression du Cratère Kilian, au pied sud du puy de Dôme. En 2009-2010, des sondages et prospections ont été étendus à tout l'ensemble du Kilian de façon à préciser l'étendue et, si possible, la chronologie de son exploitation dans le passé. Ces travaux ont permis d'observer, dans le fond du cratère et sur son flanc interne ouest, des amoncellements de déblais d'un volume considérable, témoignant d'une extraction de roche à grande échelle durant le haut Moyen Âge et très probablement aussi à l'époque gallo-romaine. Une nouvelle carrière souterraine a été découverte dans la pente interne ouest du cratère. La base du remplissage de cette carrière a livré des charbons datés entre la fin du IV e siècle et le début du VI e siècle par le radiocarbone, tandis que le sommet du remplissage contenait des tessons de céramique datables, par leur typologie, de la fin du V e siècle au début du VIII e siècle. L'état actuel des investigations conduit à faire l'hypothèse que les gallo-romains ont exploité, au fond du cratère, un trachyte compact dont on ne trouve aujourd'hui que les déchets de taille, et dont les affleurements sont masqués par les déblais, tandis que les artisans du Moyen Âge ont recherché un trachyte plus tendre dans les pentes hautes du cratère. Le Kilian doit donc s'ajouter aux trois sources actuellement connues de trachyte d'oeuvre dans le passé, à savoir les volcans Sarcoui, Aumône (ou petit Suchet) et Cliersou. Dès cette découverte, en 2008, s'est posée la question de savoir quelle part éventuelle le trachyte du Kilian avait pu prendre dans la construction du temple de Mercure au sommet du puy de Dôme et dans l'agglomération gallo-romaine située au col de Ceyssat. Pour y répondre, une campagne d'analyses géochimiques et pétrographiques été engagée pour caractériser, aux fins de comparaison, non seulement les trachytes du Kilian et ceux des ruines gallo-romaines, mais, de plus, les trachytes du Cliersou, de l'Aumône et du Sarcoui. Ces analyses ont finalement montré qu'il est possible de faire une discrimination statistiquement significative entre les différents trachytes étudiés, à l'exception de ceux du couple Cliersou-Aumône dont les laves sont très peu différentes les unes des autres. Le résultat le plus remarquable est que tous les trachytes gallo-romains échantillonnés (dont 10 échantillons distincts au temple de Mercure et 10 au col de Ceyssat) se rattachent sans ambiguïté au Kilian. En toute rigueur, ces nouvelles données ne permettent pas d'exclure sans appel la possibilité d'utilisation à l'époque gallo-romaine, au temple de Mercure et au col de Ceyssat, de trachytes provenant d'autres sources que le Kilian (cas des chaperons de mur du col de Ceyssat, provenant du puy de Dôme). Cependant, il faut ajouter qu'un examen visuel des trachytes d'oeuvre dans ces deux sites, portant sur un nombre de moellons et d'éléments architecturaux bien supérieur au nombre de ceux qui ont été analysés, conduit à conclure que leur source est probablement commune. Cette conclusion est basée sur un faciès minéralogique particulier, observable à l'oeil nu ou à la loupe

The effect of different functional groups on the ligand-promoted dissolution of NiO and other oxide minerals

Sets of homologous ligands were used to probe the dissolution of oxide minerals through experiments on bunsenite (NiO). The ligand sets have primary amine, hydroxyl, and carboxyl functional groups and form five-membered, bidentate, ring complexes at the mineral surface. A set of ligands that has only two metal-coordinating functional groups (ox, en, gly) was compared with a set of larger, but similar, ligands (nta, tren) that link three sets of functional groups with a tertiary amine. Experiments were also conducted with hydroxyl ligands (tea), ammonia (NH3), and ligands containing ring nitrogen (pic). The dissolution rates of NiO(s) in the presence of these ligands established close consistency between metal detachment from a dissolving surface and the mechanisms of ligand exchange around dissolved Ni(II)-ligand complexes. The solution pH, however, is an important complicating factor. Metals compete with protons for ligand sites and this protonation changes the ligand structure and reactivity. Several types of protonation lead to different species at the mineral surface and this greatly complicates the rate laws for dissolution. The speciation will be particularly complicated for large-molecular-weight ligands with functional groups that protonate over a wide pH range. In terms of a rate law, protonation of ligand functional groups at the surface is distinct from protonation of structural oxygens at the mineral surface. These are different surface complexes (species) for the purpose of the rate law

Quantification of intraplate volcanism magmatic processes: New partition coefficients between minerals & alkaline melts

Partition coefficients (D) are key tools to identify and quantify processes such as partial melting, fractional crystallization, and melt/rock interactions and depend strongly on thermodynamic parameters and on the melt composition. Intraplate volcanism, in particular in continental rift area, is characterized by the abundance of silica-undersaturated and alkali-rich magmas. Partition coefficients are choice tools to understand the evolution of intraplate volcanism, however, in the case of alkali-rich magmas, few D data are available. In order to provide the community with new quantitative tools, we have quantified D between alkali-rich silicate melt and clinopyroxene (Cpx), nepheline (Neph), garnet (Grt), apatite (Ap) and wollastonite (Woll) based on natural samples. All those phases are widespread in alkaline systems and their crystallization may control the differentiation of alkaline magma series. In alkaline magmas and carbonatites Cpx display sinusoidal REE pattern with unusual enrichments in heavy REE (HREE) and in Zr-Hf. We show that the specific DREE partitioning of Cpx in some alkaline systems is not consistent with the standard models assuming incorporation of all REE in the M2 site; rather HREE substitutes both in M1 and M2 sites; depending on the Cpx chemistry (Fe3+, Mn, Mg, AlIV). A new parametrized model based on Cpx major element composition is provided in Baudouin et al. (2020). New D values are also provided for the partitionning of trace elements between alkaline melts and Grt, Ap, and Neph. More specifically, we show that DREE varies from 0.2 (DLa), to 15 (DLu) for Grt (Ti-andradite), from 5 (DLa) to 1 (DLu) for Ap, and from 0.006 (DLa) to 0.001 (DLu) for Neph. Rb (0.7), Sr (0.12) and Nb (0.001) are also incompatible elements in Neph. The first D between Woll and silicate melts are presented in Baudouin & France (2019). Zr and Nb are strongly incompatible in Woll (D < 0.01), and DREE increase from DLa = 0.2 to DLu = 3. The crystallization of Woll eventually strongly influence REE fractionation during magmatic differentiation of alkali-rich melts and should therefore be considered if we are to fully understand trace element evolution and partitioning in alkaline magma series. Baudouin et al. (2020) Contrib Min Pet 175, 1-15. Baudouin & France (2019) Chem Geol 523, 88-94

Quantification of intraplate volcanism magmatic processes: New partition coefficients between minerals & alkaline melts

Partition coefficients (D) are key tools to identify and quantify processes such as partial melting, fractional crystallization, and melt/rock interactions and depend strongly on thermodynamic parameters and on the melt composition. Intraplate volcanism, in particular in continental rift area, is characterized by the abundance of silica-undersaturated and alkali-rich magmas. Partition coefficients are choice tools to understand the evolution of intraplate volcanism, however, in the case of alkali-rich magmas, few D data are available. In order to provide the community with new quantitative tools, we have quantified D between alkali-rich silicate melt and clinopyroxene (Cpx), nepheline (Neph), garnet (Grt), apatite (Ap) and wollastonite (Woll) based on natural samples. All those phases are widespread in alkaline systems and their crystallization may control the differentiation of alkaline magma series. In alkaline magmas and carbonatites Cpx display sinusoidal REE pattern with unusual enrichments in heavy REE (HREE) and in Zr-Hf. We show that the specific DREE partitioning of Cpx in some alkaline systems is not consistent with the standard models assuming incorporation of all REE in the M2 site; rather HREE substitutes both in M1 and M2 sites; depending on the Cpx chemistry (Fe3+, Mn, Mg, AlIV). A new parametrized model based on Cpx major element composition is provided in Baudouin et al. (2020). New D values are also provided for the partitionning of trace elements between alkaline melts and Grt, Ap, and Neph. More specifically, we show that DREE varies from 0.2 (DLa), to 15 (DLu) for Grt (Ti-andradite), from 5 (DLa) to 1 (DLu) for Ap, and from 0.006 (DLa) to 0.001 (DLu) for Neph. Rb (0.7), Sr (0.12) and Nb (0.001) are also incompatible elements in Neph. The first D between Woll and silicate melts are presented in Baudouin & France (2019). Zr and Nb are strongly incompatible in Woll (D < 0.01), and DREE increase from DLa = 0.2 to DLu = 3. The crystallization of Woll eventually strongly influence REE fractionation during magmatic differentiation of alkali-rich melts and should therefore be considered if we are to fully understand trace element evolution and partitioning in alkaline magma series. Baudouin et al. (2020) Contrib Min Pet 175, 1-15. Baudouin & France (2019) Chem Geol 523, 88-94

Analyse texturale et géochimique d'un polissoir à rainures du gisement magdalénien de Duruthy (Sorde, Landes, France)

International audienc

Airborne magnetic data compared to petrology of crustal scale shear zones from southern Madagascar: A tool for deciphering magma and fluid transfer in orogenic crust,

International audienceThe southern part of Madagascar consists of a granulitic metamorphic belt with a complex Proterozoic shear zone network. Aeromagnetic maps reveal sharp magnetic spatial gradients, especially across shear zones. All shear zones are associated with high magnetic values, except one, the Beraketa shear zone. Based upon relationships between rock magnetic properties, petrographic and aeromagnetic data, we show that the magnetic signal is controlled by variations in proportions of iron-rich oxides. Their nature and texture are variable and complex. Magnetite and ilmenite are often observed together showing intergrowths texture, suggesting possible lamellar magnetism. Detailed petrographic observations of the Zazafotsy shear zone show that a strong magnetic signal is correlated with metamorphic reactions and especially with biotite breakdown to peritectic phases such as orthopyroxene and iron-rich oxides (metamorphic charnockitization). Magmatic material can migrate easily inside the Zazafotsy shear zone and inside the fold hinges close to the shear zone, increasing the kinetics of charnockitic reaction. In opposition, inside the Beraketa shear zone, lower magnetic values are correlated with the absence of iron-rich oxides. This is interpreted as back reaction melting. Thus, peritectic phases, such as iron-rich oxides, react with the water released when magmas crystallise to produce biotite. Inside the Zazafotsy shear zone, iron-rich oxides are stable because part of the migmatite was segregated and escaped with dissolve H2O. In this case, back reaction was no longer possible

Standard thermodynamic properties of aqueous lanthanides and solubility of synthetic pur Ndmonazite

COMMUNICATION ORAL

Large-scale ultrasonic cleaning system: Design of a multi-transducers device for boat cleaning (20 kHz)

International audienc

Uranium and thorium partitioning in the bulk silicate Earth and the oxygen content of Earth’s core

International audienceThis study investigates the partitioning of U and Th between molten metal and silicate liquid (DUandDTh) during Earth’score-mantle differentiation. We report new Th partition coefficients between chondritic silicate melt and various Fe-rich alloysin the system Fe-Ni-C-S-Si as determined by experiments in a multi-anvil apparatus at 3–8 GPa, 2073–2373 K, and oxygenfugacities from 1.5 to 5 log units below the iron-wu ̈stite (IW) buffer. By compiling all existing data on molten metal-silicateliquid partitioning of U and Th, we develop global models of U and Th partitioning between the mantle and core throughoutEarth’s accretion. The calculated concentrations in the Bulk Silicate Earth (BSE) are in agreement with previous studies(UBSE= 11.42 ± 0.45 ppb and ThBSE= 43.20 ± 1.73 ppb), whereas the contents of these radioactive elements in the Earth’score remain negligible. Compared to recent geochemical estimations, the calculated (Th/U)BSEsupports EL rather than EHenstatite chondrites as the reduced building blocks of the Earth. Furthermore, we demonstrate that Th is much more sensitivethan U to the oxygen content of the metallic phase. To reproduce the Th/U ratio of the BSE within its uncertainties, the oxy-gen content of the Earth’s core must be lower than 4.0 wt%. By combining other existing constraints, this suggests that thecore contains 2.0–4.0 wt% O. The calculations of U and Th concentrations and Th/U in the BSE developed herein can be usedas new constraints for determining the concentrations of other refractory lithophile elements in the BSE as soon as theirmetal-silicate partition coefficients are well constrained over the conditions of core segregation