111 research outputs found
Essai de gĂ©ographie protohistorique en contexte de moyenne montagne : enquĂȘte en Haute-Auvergne et sur les plateaux limousins (Massif central, France)
Le nord-ouest du Massif central est un terrain privilĂ©giĂ© pour Ă©tudier lâoccupation au cours de la Protohistoire dans un contexte de moyenne montagne. Au cours dâun doctorat, plusieurs sites de hauteur de Haute-Auvergne (Cantal, Auvergne-RhĂŽne-Alpes) et de haute CorrĂšze (Nouvelle-Aquitaine) ont fait lâobjet de prospections, de sondages et dâĂ©tudes de mobilier. Pour lâĂąge du Bronze et le premier Ăąge du Fer, des donnĂ©es inĂ©dites permettent de reconsidĂ©rer les modalitĂ©s dâoccupation de ces territoires. Ă lâaide dâanalyses spatiales, les rythmes dâoccupation de ces sites et leurs caractĂ©ristiques dâimplantation ont Ă©tĂ© examinĂ©s. Des esquisses de dynamique dâoccupation peuvent ĂȘtre proposĂ©es, en particulier entre le Bronze moyen et La TĂšne ancienne. Ainsi, la disposition des sites de hauteur nâapparaĂźt pas comme anodine : elle sâappuie sur des voies de passages naturelles, plus ou moins utilisĂ©es au cours du temps, et un maillage rĂ©gulier du territoire se dessine Ă certaines pĂ©riodes.The Northwestern area of Massif Central is a privileged ground for the study of Protohistoric human occupation in a mid-mountain context. For the purpose of a PhD, many entrenched or hilltop settlements of Haute-Auvergne (Cantal, Auvergne-RhĂŽne-Alpes) and of Haute-CorrĂšze (Nouvelle-Aquitaine) have been subjected to prospections, surveys, or studies of collections. For Bronze Age and Early Iron Age, new data are now available, and allow us to reconsider the occupation modalities of these territories. With spatial analyses, the occupation rhythms of these sites and their characteristics can be observed and discussed. The first drafts of occupation dynamics in western Massif Central during Protohistory can be proposed, especially between the Middle Bronze age and La Tene I for the Haute-Auvergne. Thus, the positioning of hilltop settlements doesnât appear as random: it is based on more-or-less used in time natural passage ways, and designs a regular territorial network in certain periods
Les habitats de hauteur dans le centre-ouest du Massif central, Ă lâĂąge de lâĂąge du Bronze jusquâau dĂ©but du second Ăąge du Fer: nouvelles donnĂ©es et perspectives de recherche en Haute-CorrĂšze (Limousin) et dans le Cantal (Auvergne)
Dans le cadre dâun doctorat, plusieurs sites de hauteur de Haute-Auvergne (Cantal, rĂ©gion Auvergne-RhĂŽne-Alpes) et de Haute CorrĂšze (rĂ©gion Nouvelle Aquitaine) ont pu faire lâobjet de prospections, de sondages, ou bien dâĂ©tudes inĂ©dites de collections anciennes. Ainsi, pour lâĂąge du Bronze et le premier Ăąge du Fer, de nouvelles donnĂ©es sont aujourdâhui disponibles, et permettent de reconsidĂ©rer les modalitĂ©s dâoccupation de ces territoires de moyenne montagne, souvent vus comme dĂ©sertiques au cours de la Protohistoire. Des sondages menĂ©s en 2015 sur le site de Chastel-Marlhac (Le Monteil, Cantal), plateau basaltique protĂ©gĂ© naturellement par de grandes falaises, ont permis de documenter notamment une phase de frĂ©quentation au Bronze final 3, insoupçonnĂ©e auparavant, ainsi que des indices dâoccupation au dĂ©but et Ă la fin du second Ăąge du Fer. LâĂ©tude des collections de cĂ©ramiques provenant de lâĂ©peron barrĂ© du Suc de Lermu (Charmensac, Cantal) a quant Ă elle mis en Ă©vidence dâimportantes occupations du Bronze final 3 et de La TĂšne ancienne, dâailleurs documentĂ©es par les sondages menĂ©s en 2016. Enfin, une nouvelle Ă©tude des collections du site de hauteur du Roc de Chastel (Chastel-sur-Murat, Cantal), occupĂ© au Bronze moyen, a permis de mettre en lumiĂšre une occupation datĂ©e de la transition entre le Hallstatt final et le dĂ©but de La TĂšne. En CorrĂšze, des sites tout Ă fait inĂ©dits, dĂ©couverts rĂ©cemment grĂące Ă des prospections, attestent aujourdâhui dâune frĂ©quentation au Bronze final 3 (site du BelvĂ©dĂšre de Gratte-BruyĂšre Ă SĂ©randon, mais aussi peut-ĂȘtre au Camp de Fontjaloux Ă Saint-Etienne-aux-Clos), mais aussi au Hallstatt moyen : le site de la Croix de Chave Ă Egletons, localisĂ© sur un col, est un indice dâhabitat aux caractĂ©ristiques intĂ©ressantes, prenant place qui plus est dans un contexte de dynamique de peuplement tout particuliĂšre, au pied de la montagne limousine. En insĂ©rant ces nouvelles informations aux donnĂ©es dĂ©jĂ disponibles sur les habitats de hauteur dans lâouest du Massif central, il apparait que les rythmes dâoccupation de ces sites retranchĂ©s, ainsi que leurs caractĂ©ristiques dâimplantation, attestent dâune dynamique certaine et dâune bonne intĂ©gration de ces territoires de moyenne montagne dans des modalitĂ©s dâoccupation et des schĂ©mas de circulation plus globaux.For the purpose of a PhD, many altitude sites of Haute-Auvergne (Cantal, Auvergne-RhĂŽne-Alpes Region) and of Haute CorrĂšze (New Aquitaine Region) have been subjected to prospections, surveys, or unprecedented studies of older collections. Therefore, for Bronze age and Early Iron age, new data are now available, and allow us to reconsider the occupation modalities of these medium sized mountain territories, often thought as deserted during Protohistory. Surveys conducted in 2015 on the Chastel-Marlhac site (Le Monteil, Cantal), basalt plateau naturally protected by large cliffs, have permitted the documentation of an occupation period in the Late Bronze age III, unsuspected until then, and clues of occupation at the beginning and at the end of the Late Iron age. The study of collection of ceramics from the barred spur of Suc de Lermu (Charmensac, Cantal) has brought to light important occupations during Late Bronze age and La Tene I, otherwise documented by 2016 surveys. Finally, a new study of the collections of the altitude site of Roc de Chastel (Chastel-sur-Murat, Cantal), occupied during the middle Bronze age, has brought to light an occupation dated from the transition between Early and Late Iron age. In Correze, unknown sites, recently discovered during prospections, state of an attendance during Late Bronze age III (Belvedere of Gratte-BruyĂšre in Serandon, but maybe also Camp of Fontjaloux in Saint-Etienne-aux-Clos), but also during the middle of Early Iron age : the site of Croix de Chave in Egletons, located on a pass, is a clue for an habitat with interesting characteristics, taking place additionally in a context of peculiar settlement dynamics, at the base of the Limousin Mountain. By inserting this new information into already available data on altitude habitat in West Massif Central, it appears that the occupation paces of these entrenched sites, and their presence characteristics, state of a certain dynamic and of a good integration of these medium sized mountain territories in more global occupation modalities and movement patterns
Unravelling the magma feeding system of a young basaltic oceanic volcano
A multidisciplinary approach combining petrological, geochemical, and fluid-inclusion studies with seismic monitoring data was used to build a model of the magma feeding system of Pico volcano (Azores islands, North Atlantic Ocean). We explore how magma has ascended to the surface in the last 10 ka and how this ascent is associated with the selective activation of the three tectonic systems intersecting the volcano. The deepest and most important ponding level for all ascending magmas is located at 17.3â17.7 km and corresponds to the Moho Transition Zone (MTZ), which marks the transition from mantle rocks to ultramafic cumulates. At shallower depth ascending magmas carry >30 vol% of clinopyroxene and olivine. Each magma ascent followed a distinct path and ponded often for a limited period. Ponding levels common to all feeding systems are present at 16.3â16.7 km, 12.1â14.5 km, 9.4â9.8 km, and 7.7â8.1 km. These depths mark important discontinuities where magmas formed stacked sills and evolved through fractional crystallisation. Dense and un-decrepitated fluid inclusions show rapid ascent from the MTZ along the Lomba do Fogo-SĂŁo JoĂŁo fault (N150° system) and along the N120° regional transtensive system, despite multiple intrusions. Magma ponding at 5.6â6.8 km occurs where the N150° and N60° tectonic systems intersect each other. Here magma evolves towards plagioclase-rich and is only erupted at the summit crater and subterminal vents. This region is the source of the frequent microseismicity recorded at 4 to 7 km beneath the southern flank of Pico volcano, which might be associated with the early stages of formation of a more complex magma reservoir. The local and regional tectonics are of paramount importance in the activation of the different magma feeding systems over time. This new information is fundamental to improve the knowledge on the future eruptive behaviour of Pico volcano and can have significant implications on the mitigation of volcanic risk. This multidisciplinary approach can be applied not only to other volcanoes of the Azores but also to poorly monitored oceanic volcanoes, where magma ascent strongly depends on the activation of tectonic systems
Molecular analysis of patients with weak D and serologic analysis of those with anti-D (excluding type 1 and type 2)
Cristallization of olivine inside barred olivine and macro-porphyric chondrule
Les chondres, constituants principaux des chondrites sont des sphĂšres silicatĂ©es ferromagnĂ©siennes d'origine ignĂ©e. Ils rĂ©sultent de la fusion plus ou moins totale de prĂ©curseurs solides au cours d'Ă©vĂ©nements brefs de haute tempĂ©rature, c'est-Ă -dire des pics thermiques suivis d'Ă©pisodes de refroidissement rapide. Les chondres sont des tĂ©moins directs de la formation du systĂšme solaire et leurs conditions thermiques (surchauffe, vitesse de refroidissementâŠ) peut apporter des informations sur les contraintes thermiques de formation. Pour cela, des Ă©tudes expĂ©rimentales et pĂ©trographiques dĂ©taillĂ©es ont Ă©tĂ© rĂ©alisĂ©es sur des chondres de diffĂ©rentes textures (macro-porphyrique, porphyrique et barrĂ©). Pour reproduire la texture macro-porphyrique (peu de grands cristaux d'olivine), le liquide chondritique doit subir un Ă©pisode de surchauffe (au-dessus de la tempĂ©rature liquidus) afin de rĂ©duire la nuclĂ©ation et, ĂȘtre suivie par un refroidissement lent (quelques °C/h) afin de former les larges olivines prĂ©sentant les lacunes de cristallisation caractĂ©ristiques. Les chondres macro-porphyriques n'ont pas la mĂȘme histoire thermique que les chondres porphyriques qui ont Ă©tĂ© initialement moins chauffĂ©s (sous leurs tempĂ©ratures liquidus). En revanche, les chondres macro-porphyriques et barrĂ©s sont issus d'un Ă©pisode de surchauffe. Une Ă©tude pĂ©trographique et expĂ©rimentale sur les textures barrĂ©es confirme cette analogie. Les observations rĂ©alisĂ©es sur les textures barrĂ©es ont permis de proposer un modĂšle 3D de ces objets complexes mais aussi un modĂšle de formation des chondres barrĂ©s oĂč les bordures irrĂ©guliĂšres -permettant des interactions gaz/liquide- se forment en mĂȘme temps que les barres-dendrites muries. Une Ă©tude sur le comportement des Terres Rares (REE) entre l'olivine et le liquide a Ă©tĂ© rĂ©alisĂ©e lors d'expĂ©riences dynamiques Ă taux de refroidissement (2-1000°C/h) et paliers variables. Un haut taux de cristallisation (composition, tempĂ©ra ture de trempe basse, long palier) ou un taux de refroidissement lent favorisent un coefficient de partage bas entre l'olivine et le liquide (Kd). Toutefois, un taux de refroidissement trĂšs lent peut entraĂźner, par retard Ă la nuclĂ©ation, des croissances initiales rapides ayant pour consĂ©quence un enrichissement des olivines en REE et un Kdolivine-liquide variable.Chondrules are the main constituents of chondrites and are ferromagnesian silicate spherules of igneous origin. Chondrules result from the more or less complete melting of solid precursors during brief high temperature events, i.e., thermal peaks followed by episodes of rapid cooling. Chondrules are direct witnesses of the formation of the Solar System and the thermal conditions of their formation (superheating, cooling rate, etc.) can provide invaluable information on the formation. In order to provide their constraints, detailed experimental and petrographic studies were carried out on chondrules exhibiting different textures (macro-porphyritic, porphyritic and barred). To reproduce the macro-porphyritic texture (few large olivine crystals), the chondritic liquid must undergo a superheating episode (above the liquidus temperature) in order to reduce the number of nucleation sites and followed by an episode of slow cooling (a few °C/h) to form the large olivines and their peculiar embayments. Thus, the macro-porphyritic chondrules do not have the same thermal history than the porphyritic chondrules which were initially less heated (below their liquidus temperatures). On the other hand, the macro-porphyritic and barred chondrules come from a superheating episode. Petrographic and experimental studies on barred textures confirm this analogy. From the observations on the barred textures, a 3D model is proposed for the formation of these various complex objects. In particular, this model accounts for the formation of barred chondrules, where the irregular borders âdue to gas/liquid interactions- are formed at the same time as the bars - ripened dendrites. A study on the behavior of Rare Earth Elements (REE) between olivine and liquid was carried out during dynamic experiments, using different cooling rates (2-1000°C/h) and dwell time (direct quench or long dwell). A high crystallization rate (composition, low quenching temperature, long dw ell) or a slow cooling rate favors a low partition coefficient (Kd) between the olivine and the liquid. In turn, a very slow cooling rate can lead to a delay in nucleation, to fast initial growths resulting in an enrichment of olivines in REE and variable Kdolivine-liquid
Cristallisation des olivines dans les chondres barrés et macro-porphyriques
Chondrules are the main constituents of chondrites and are ferromagnesian silicate spherules of igneous origin. Chondrules result from the more or less complete melting of solid precursors during brief high temperature events, i.e., thermal peaks followed by episodes of rapid cooling. Chondrules are direct witnesses of the formation of the Solar System and the thermal conditions of their formation (superheating, cooling rate, etc.) can provide invaluable information on the formation. In order to provide their constraints, detailed experimental and petrographic studies were carried out on chondrules exhibiting different textures (macro-porphyritic, porphyritic and barred). To reproduce the macro-porphyritic texture (few large olivine crystals), the chondritic liquid must undergo a superheating episode (above the liquidus temperature) in order to reduce the number of nucleation sites and followed by an episode of slow cooling (a few °C/h) to form the large olivines and their peculiar embayments. Thus, the macro-porphyritic chondrules do not have the same thermal history than the porphyritic chondrules which were initially less heated (below their liquidus temperatures). On the other hand, the macro-porphyritic and barred chondrules come from a superheating episode. Petrographic and experimental studies on barred textures confirm this analogy. From the observations on the barred textures, a 3D model is proposed for the formation of these various complex objects. In particular, this model accounts for the formation of barred chondrules, where the irregular borders âdue to gas/liquid interactions- are formed at the same time as the bars - ripened dendrites. A study on the behavior of Rare Earth Elements (REE) between olivine and liquid was carried out during dynamic experiments, using different cooling rates (2-1000°C/h) and dwell time (direct quench or long dwell). A high crystallization rate (composition, low quenching temperature, long dw ell) or a slow cooling rate favors a low partition coefficient (Kd) between the olivine and the liquid. In turn, a very slow cooling rate can lead to a delay in nucleation, to fast initial growths resulting in an enrichment of olivines in REE and variable Kdolivine-liquid.Les chondres, constituants principaux des chondrites sont des sphĂšres silicatĂ©es ferromagnĂ©siennes d'origine ignĂ©e. Ils rĂ©sultent de la fusion plus ou moins totale de prĂ©curseurs solides au cours d'Ă©vĂ©nements brefs de haute tempĂ©rature, c'est-Ă -dire des pics thermiques suivis d'Ă©pisodes de refroidissement rapide. Les chondres sont des tĂ©moins directs de la formation du systĂšme solaire et leurs conditions thermiques (surchauffe, vitesse de refroidissementâŠ) peut apporter des informations sur les contraintes thermiques de formation. Pour cela, des Ă©tudes expĂ©rimentales et pĂ©trographiques dĂ©taillĂ©es ont Ă©tĂ© rĂ©alisĂ©es sur des chondres de diffĂ©rentes textures (macro-porphyrique, porphyrique et barrĂ©). Pour reproduire la texture macro-porphyrique (peu de grands cristaux d'olivine), le liquide chondritique doit subir un Ă©pisode de surchauffe (au-dessus de la tempĂ©rature liquidus) afin de rĂ©duire la nuclĂ©ation et, ĂȘtre suivie par un refroidissement lent (quelques °C/h) afin de former les larges olivines prĂ©sentant les lacunes de cristallisation caractĂ©ristiques. Les chondres macro-porphyriques n'ont pas la mĂȘme histoire thermique que les chondres porphyriques qui ont Ă©tĂ© initialement moins chauffĂ©s (sous leurs tempĂ©ratures liquidus). En revanche, les chondres macro-porphyriques et barrĂ©s sont issus d'un Ă©pisode de surchauffe. Une Ă©tude pĂ©trographique et expĂ©rimentale sur les textures barrĂ©es confirme cette analogie. Les observations rĂ©alisĂ©es sur les textures barrĂ©es ont permis de proposer un modĂšle 3D de ces objets complexes mais aussi un modĂšle de formation des chondres barrĂ©s oĂč les bordures irrĂ©guliĂšres -permettant des interactions gaz/liquide- se forment en mĂȘme temps que les barres-dendrites muries. Une Ă©tude sur le comportement des Terres Rares (REE) entre l'olivine et le liquide a Ă©tĂ© rĂ©alisĂ©e lors d'expĂ©riences dynamiques Ă taux de refroidissement (2-1000°C/h) et paliers variables. Un haut taux de cristallisation (composition, tempĂ©ra ture de trempe basse, long palier) ou un taux de refroidissement lent favorisent un coefficient de partage bas entre l'olivine et le liquide (Kd). Toutefois, un taux de refroidissement trĂšs lent peut entraĂźner, par retard Ă la nuclĂ©ation, des croissances initiales rapides ayant pour consĂ©quence un enrichissement des olivines en REE et un Kdolivine-liquide variable
Cristallisation des olivines dans les chondres barrés et macro-porphyriques
Chondrules are the main constituents of chondrites and are ferromagnesian silicate spherules of igneous origin. Chondrules result from the more or less complete melting of solid precursors during brief high temperature events, i.e., thermal peaks followed by episodes of rapid cooling. Chondrules are direct witnesses of the formation of the Solar System and the thermal conditions of their formation (superheating, cooling rate, etc.) can provide invaluable information on the formation. In order to provide their constraints, detailed experimental and petrographic studies were carried out on chondrules exhibiting different textures (macro-porphyritic, porphyritic and barred). To reproduce the macro-porphyritic texture (few large olivine crystals), the chondritic liquid must undergo a superheating episode (above the liquidus temperature) in order to reduce the number of nucleation sites and followed by an episode of slow cooling (a few °C/h) to form the large olivines and their peculiar embayments. Thus, the macro-porphyritic chondrules do not have the same thermal history than the porphyritic chondrules which were initially less heated (below their liquidus temperatures). On the other hand, the macro-porphyritic and barred chondrules come from a superheating episode. Petrographic and experimental studies on barred textures confirm this analogy. From the observations on the barred textures, a 3D model is proposed for the formation of these various complex objects. In particular, this model accounts for the formation of barred chondrules, where the irregular borders âdue to gas/liquid interactions- are formed at the same time as the bars - ripened dendrites. A study on the behavior of Rare Earth Elements (REE) between olivine and liquid was carried out during dynamic experiments, using different cooling rates (2-1000°C/h) and dwell time (direct quench or long dwell). A high crystallization rate (composition, low quenching temperature, long dw ell) or a slow cooling rate favors a low partition coefficient (Kd) between the olivine and the liquid. In turn, a very slow cooling rate can lead to a delay in nucleation, to fast initial growths resulting in an enrichment of olivines in REE and variable Kdolivine-liquid.Les chondres, constituants principaux des chondrites sont des sphĂšres silicatĂ©es ferromagnĂ©siennes d'origine ignĂ©e. Ils rĂ©sultent de la fusion plus ou moins totale de prĂ©curseurs solides au cours d'Ă©vĂ©nements brefs de haute tempĂ©rature, c'est-Ă -dire des pics thermiques suivis d'Ă©pisodes de refroidissement rapide. Les chondres sont des tĂ©moins directs de la formation du systĂšme solaire et leurs conditions thermiques (surchauffe, vitesse de refroidissementâŠ) peut apporter des informations sur les contraintes thermiques de formation. Pour cela, des Ă©tudes expĂ©rimentales et pĂ©trographiques dĂ©taillĂ©es ont Ă©tĂ© rĂ©alisĂ©es sur des chondres de diffĂ©rentes textures (macro-porphyrique, porphyrique et barrĂ©). Pour reproduire la texture macro-porphyrique (peu de grands cristaux d'olivine), le liquide chondritique doit subir un Ă©pisode de surchauffe (au-dessus de la tempĂ©rature liquidus) afin de rĂ©duire la nuclĂ©ation et, ĂȘtre suivie par un refroidissement lent (quelques °C/h) afin de former les larges olivines prĂ©sentant les lacunes de cristallisation caractĂ©ristiques. Les chondres macro-porphyriques n'ont pas la mĂȘme histoire thermique que les chondres porphyriques qui ont Ă©tĂ© initialement moins chauffĂ©s (sous leurs tempĂ©ratures liquidus). En revanche, les chondres macro-porphyriques et barrĂ©s sont issus d'un Ă©pisode de surchauffe. Une Ă©tude pĂ©trographique et expĂ©rimentale sur les textures barrĂ©es confirme cette analogie. Les observations rĂ©alisĂ©es sur les textures barrĂ©es ont permis de proposer un modĂšle 3D de ces objets complexes mais aussi un modĂšle de formation des chondres barrĂ©s oĂč les bordures irrĂ©guliĂšres -permettant des interactions gaz/liquide- se forment en mĂȘme temps que les barres-dendrites muries. Une Ă©tude sur le comportement des Terres Rares (REE) entre l'olivine et le liquide a Ă©tĂ© rĂ©alisĂ©e lors d'expĂ©riences dynamiques Ă taux de refroidissement (2-1000°C/h) et paliers variables. Un haut taux de cristallisation (composition, tempĂ©ra ture de trempe basse, long palier) ou un taux de refroidissement lent favorisent un coefficient de partage bas entre l'olivine et le liquide (Kd). Toutefois, un taux de refroidissement trĂšs lent peut entraĂźner, par retard Ă la nuclĂ©ation, des croissances initiales rapides ayant pour consĂ©quence un enrichissement des olivines en REE et un Kdolivine-liquide variable
Cristallisation des olivines dans les chondres barrés et macro-porphyriques
Chondrules are the main constituents of chondrites and are ferromagnesian silicate spherules of igneous origin. Chondrules result from the more or less complete melting of solid precursors during brief high temperature events, i.e., thermal peaks followed by episodes of rapid cooling. Chondrules are direct witnesses of the formation of the Solar System and the thermal conditions of their formation (superheating, cooling rate, etc.) can provide invaluable information on the formation. In order to provide their constraints, detailed experimental and petrographic studies were carried out on chondrules exhibiting different textures (macro-porphyritic, porphyritic and barred). To reproduce the macro-porphyritic texture (few large olivine crystals), the chondritic liquid must undergo a superheating episode (above the liquidus temperature) in order to reduce the number of nucleation sites and followed by an episode of slow cooling (a few °C/h) to form the large olivines and their peculiar embayments. Thus, the macro-porphyritic chondrules do not have the same thermal history than the porphyritic chondrules which were initially less heated (below their liquidus temperatures). On the other hand, the macro-porphyritic and barred chondrules come from a superheating episode. Petrographic and experimental studies on barred textures confirm this analogy. From the observations on the barred textures, a 3D model is proposed for the formation of these various complex objects. In particular, this model accounts for the formation of barred chondrules, where the irregular borders âdue to gas/liquid interactions- are formed at the same time as the bars - ripened dendrites. A study on the behavior of Rare Earth Elements (REE) between olivine and liquid was carried out during dynamic experiments, using different cooling rates (2-1000°C/h) and dwell time (direct quench or long dwell). A high crystallization rate (composition, low quenching temperature, long dw ell) or a slow cooling rate favors a low partition coefficient (Kd) between the olivine and the liquid. In turn, a very slow cooling rate can lead to a delay in nucleation, to fast initial growths resulting in an enrichment of olivines in REE and variable Kdolivine-liquid.Les chondres, constituants principaux des chondrites sont des sphĂšres silicatĂ©es ferromagnĂ©siennes d'origine ignĂ©e. Ils rĂ©sultent de la fusion plus ou moins totale de prĂ©curseurs solides au cours d'Ă©vĂ©nements brefs de haute tempĂ©rature, c'est-Ă -dire des pics thermiques suivis d'Ă©pisodes de refroidissement rapide. Les chondres sont des tĂ©moins directs de la formation du systĂšme solaire et leurs conditions thermiques (surchauffe, vitesse de refroidissementâŠ) peut apporter des informations sur les contraintes thermiques de formation. Pour cela, des Ă©tudes expĂ©rimentales et pĂ©trographiques dĂ©taillĂ©es ont Ă©tĂ© rĂ©alisĂ©es sur des chondres de diffĂ©rentes textures (macro-porphyrique, porphyrique et barrĂ©). Pour reproduire la texture macro-porphyrique (peu de grands cristaux d'olivine), le liquide chondritique doit subir un Ă©pisode de surchauffe (au-dessus de la tempĂ©rature liquidus) afin de rĂ©duire la nuclĂ©ation et, ĂȘtre suivie par un refroidissement lent (quelques °C/h) afin de former les larges olivines prĂ©sentant les lacunes de cristallisation caractĂ©ristiques. Les chondres macro-porphyriques n'ont pas la mĂȘme histoire thermique que les chondres porphyriques qui ont Ă©tĂ© initialement moins chauffĂ©s (sous leurs tempĂ©ratures liquidus). En revanche, les chondres macro-porphyriques et barrĂ©s sont issus d'un Ă©pisode de surchauffe. Une Ă©tude pĂ©trographique et expĂ©rimentale sur les textures barrĂ©es confirme cette analogie. Les observations rĂ©alisĂ©es sur les textures barrĂ©es ont permis de proposer un modĂšle 3D de ces objets complexes mais aussi un modĂšle de formation des chondres barrĂ©s oĂč les bordures irrĂ©guliĂšres -permettant des interactions gaz/liquide- se forment en mĂȘme temps que les barres-dendrites muries. Une Ă©tude sur le comportement des Terres Rares (REE) entre l'olivine et le liquide a Ă©tĂ© rĂ©alisĂ©e lors d'expĂ©riences dynamiques Ă taux de refroidissement (2-1000°C/h) et paliers variables. Un haut taux de cristallisation (composition, tempĂ©ra ture de trempe basse, long palier) ou un taux de refroidissement lent favorisent un coefficient de partage bas entre l'olivine et le liquide (Kd). Toutefois, un taux de refroidissement trĂšs lent peut entraĂźner, par retard Ă la nuclĂ©ation, des croissances initiales rapides ayant pour consĂ©quence un enrichissement des olivines en REE et un Kdolivine-liquide variable
The Man and the low mountains during Protohistory in Massif central : survey in two windows of study in Haute-Auvergne and Limousin
La Haute-Auvergne, qui correspond au dĂ©partement du Cantal, et la Montagne limousine, Ă cheval sur le nord de la CorrĂšze, le sud de la Creuse et lâextrĂȘme est de la Haute-Vienne, constituent la zone dâĂ©tude de cette recherche. Il sâagit de territoires de moyenne montagne qui se prĂ©sentent comme de rĂ©els conservatoires pour les amĂ©nagements de toutes Ă©poques, et notamment de la Protohistoire. Cette zone dâĂ©tude est donc un vĂ©ritable laboratoire permettant des approches statistiques et spatiales. De plus, ces territoires offrent aussi de nombreuses zones humides et tourbiĂšres grĂące auxquelles des donnĂ©es palĂ©o environnementales sont accessibles. Elles viennent ainsi complĂ©ter les informations fournies par le mobilier mĂ©tallique issu de dĂ©pĂŽt non funĂ©raire ou de dĂ©couverte isolĂ©e, par lâhabitat, notamment les sites de hauteur, mais aussi par le domaine funĂ©raire, particuliĂšrement bien reprĂ©sentĂ© grĂące Ă la bonne conservation des nĂ©cropoles tumulaires. Cette recherche aborde de maniĂšre thĂ©matique ces diffĂ©rentes donnĂ©es, pour lâĂąge du Bronze mais aussi les Ăąges du Fer, afin dâapprocher la question de lâoccupation de ces contextes topographiques particuliers, sur le long terme, ainsi que de lâinteraction Homme/milieu et des liens entre hautes terres et zones plus basses. Nous proposons ainsi un essai de gĂ©ographie protohistorique qui amĂšne discussions et nouvelles perspectives de recherche.The Haute Auvergne, located in Cantal, and Limousin Mountains across North of CorrĂšze, South of Creuse and far east of Haute Vienne, represent the study area on which this research was conducted. These are medium sized mountain territories, which represent real conservatories for all period installations, notably for Protohistory. This study area is therefore a real laboratory allowing statistic and spatial approaches. Furthermore, these territories offer numerous wetlands and bogs by which paleo-environmental data are made available. Thus, these supplements the information provided by the metallic artefacts originated from non-funeral depositories or isolated discoveries, by the settlements, notably by the hillforts, but also by the funeral domain, well represented through the good conservation of barrows cemeteries. This research has a thematic approach on these many data, for the Bronze Age but also for the Iron Ages, to apprehend the question of the occupation of these special topographic contexts on the long term, the Man/environment Interaction, and the connections between high grounds and lower areas. We here offer an essay on protohistoric geography that will lead to discussions and new research perspectives
- âŠ