Éditeur(s) : HAL CCSD
Résumé : East African Rift (EAR) is the divergent plate boundary. EAR exposes different stages of extension, from early stage rifting in Tanzania to oceanic accretion in Afar (Ethiopia). Manyara basin is the southernmost rift system of the east branch of EAR with recent volcanism (< 1.5 Ma) and a seismic swarm in the lower crust (20 – 40 km). Due to its location and tectonic setting, the Manyara basin offers the opportunity to study the earliest stage of rift initiation. Manyara volcanism is composed of several types of hyper-alkaline lavas as Mg-nephelinites (Mg# > 55) (Labait, Kwaraha), calciocarbonatite (Kwaraha) and evolved nephelinites (Mg# < 35) (Hanang).Mg-nephelinites (Labait and Kwaraha) are primary lavas mainly composed of olivine and clinopyroxene (cpx). Geochemical modelling from trace elements suggests that these primary magmas result from a degree of partial melting < 1 % from a CO2-garnet-phlogopite-bearing peridotite. These magmas have an asthenospheric source at depth > 120 km (lava carries xenoliths with equilibrium conditions > 4 GPa). The minerals were crystallized from a magma with a low H2O content (0.1 and 0.5 wt% H2O). The calciocarbonatite and evolved nephelinites are derived from Mg-nephelinites by fractional crystallization and immiscibility processes. Hanang nephelinites are silica- and alkaline-rich lavas (44.2 – 46.7 wt % SiO2, 9.5 –12.1 wt % Na2O+K2O, respectively) composed by cpx, Ti-garnet, nepheline, apatite and titanite. Complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites record magmatic differentiation involving open system with carbonate-silicate immiscibility and primary melt replenishment. The low H2O content of cpx (3 – 25 ppm wt. H2O) indicates that at least 0.3 wt % H2O was present at depth during carbonate-rich nephelinite crystallization at 340 – 640 MPa and 1050 – 1100 °C. The study of hosted-nepheline melt inclusions from Hanang allows constraining the late magmatic evolution of nephelinites during storage and magma ascent. Melt inclusions are composed by a silicate trachytic glass, a carbonate phase and a shrinkage bubble. Trachytic glass contains high content in CO2 (0.43 wt %, SIMS analyses), sulfur (0.21 – 0.92 wt % S), chlorine (0.28 –0.84 wt % Cl) and H2O low content (< 0.1 wt %, Raman analyses). Immiscibility process leading to the formation of carbonate occurs in a closed system during rapid magma ascent between 200 – 500 MPa. The carbonate phase is a Ca-Na-K-S-rich and anhydrous carbonate (33 wt % CaO, 20 wt % Na2O, 3 wt % K2O, and 3 wt % S). The pre-immiscible liquid has a phonolitic composition with 6 ± 1.5 wt % CO2 at 700 MPa. A preliminary study of melt inclusions by XANES spectroscopy and whole rocks by Mössbauer spectroscopy was used to determine these Manyara lavas were formed at oxidizing conditions (~ ΔFMQ +1.5).The early stage rifting volcanism (Manyara Basin) is characterized by CO2-rich and H2O-poor magmas from at least 120 km below the rift escarpment. The presence of CO2-rich magmas and the small amount of volcanic rocks erupted at the surface may indicate that the storage and percolation of these magmas at depth is a potential trigger for deep seismic swarms.
Le rift Est africain (REA) est une frontière de plaque en extension. Ce rift présente plusieurs stades d’extension, de l’initiation du rift en Tanzanie jusqu’à l’accrétion océanique en Afar. Le bassin de Manyara se situe le plus au sud de branche Est du REA. Il est caractérisé par la présence de volcanisme récent (< 1,5 Ma) et d’un essaim sismique dans la croûte inférieure (20 – 40 km). De par sa localisation et son contexte tectonique, le bassin de Manyara offre l’opportunité d’étudier le stade le plus précoce de l’initiation du rift. Le bassin de Manyara est composé de plusieurs types de laves hyperalcalines, les néphélinites magnésiennes (Mg# > 55) (Labait, Kwaraha), de calciocarbonatite (Kwaraha) et des néphélinites différenciées (Mg# < 35) (Hanang).Les néphélinites magnésiennes (Labait et Kwaraha) sont des laves primaires composées d’olivines et de clinopyroxènes (cpx). La modélisation géochimique des éléments en trace suggère que ces magmas primaires résultent d'un degré de fusion partielle ≤ 1 % à partir d'une péridotite à grenat et phlogopite. Ces magmas proviennent d’une profondeur > 120 km (présence de xénolites avec des conditions d’équilibre > 4 GPa). Les minéraux ont cristallisés à partir d’un magma pauvre en eau (0,1 et 0,5 pds % H2O). La calciocarbonatite et les néphélinites différenciés sont issues des néphélinites magnésiennes par cristallisation fractionnée et processus d’immiscibilité. Les néphélinites du Hanang sont riches en éléments alcalins (9,5 – 12,1 pds % Na2O+K2O) et en silice (44,2 – 46,7 pds% SiO2) et sont composés de cpx, grenat, néphéline, titanite et apatite. La zonation complexe dans les cpx (par exemple, changement brusque de Mg#, Nb/Ta, et H2O) implique une différenciation magmatique en système ouvert avec immiscibilité de liquide carbonaté et silicaté ainsi qu’un remplissage de la chambre magmatique avec des liquides primaires. La faible teneur en eau des cpx (3 – 25 ppm H2O) indique la présence d’un magma pauvre en eau (0,3 pds % H2O) lors de la cristallisation des cpx à des conditions crustales (340 – 640 MPa et 1050 – 1100 °C). L’étude des inclusions vitreuses dans les néphélines de Hanang permet de contraindre l'évolution magmatique tardive des néphélinites et le comportement des éléments volatils (CO2, H2O, S, F, Cl) lors du stockage et de la remontée du magma. Les inclusions vitreuses sont composées d’un verre trachytique, d’une phase carbonatée et d’une bulle de rétraction. Le verre trachytique contient du CO2 (0,43 pds % CO2, analyses SIMS), du soufre (0,21 à 0,92 pds% S), du chlore (0,28 – 0,84 pds % Cl) et très peu d’H2O (< 0,1 pds % H2O, analyses Raman). Le processus d’immiscibilité conduisant à la formation du carbonate se produit dans un système fermé pendant l'ascension rapide du magma, entre 200 – 500 MPa. La phase carbonatée est un carbonate anhydre et riche en Ca-Na-K-S (33 pds % CaO, 20 pds % Na2O, 3 pds % K2O, et 3 pds % S). Le liquide pré-immiscible a une composition phonolitique avec 6 ± 1,5 pds % CO2 à une pression de 700 MPa. Une étude préliminaire des inclusions par spectroscopie XANES et des roches par spectroscopie Mössbauer a permis de déterminer que les laves de Manyara se sont formées à conditions oxydantes (~ ∆FMQ +1,5).À l’initiation du rift, le volcanisme dans le bassin de Manyara est caractérisé par des magmas riches en CO2 et pauvres en H2O issus d’au moins 120 km de profondeur sous l'escarpement du rift. La présence de ces magmas riches en CO2 et la faible quantité de roches volcaniques émises à la surface peuvent indiquer que le piégeage et la percolation de ces magmas en profondeur est un déclencheur potentiel des essaims sismiques profonds.
https://tel.archives-ouvertes.fr/tel-01563231

NNT : 2016MONTT114
tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231v2/document
https://tel.archives-ouvertes.fr/tel-01563231/file/2016_BAUDOUIN_archivage.pdf

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
We report oil the discovery of magnetite nanoparticles ranging in size from 2 to 14 full in the mineralized zones of the Pena Colorada iron-ore deposit, southern Mexico. Micrometric scale magnetite was magnetically reduced and divided into distinct size ranges: 85-56 mu m, 56-30 mu m, 30-22 mu m, 22-15 mu m, 15-10 mu m, 10-7 mu m and 7-2 mu m. Nanometric-scale magnetite in the size range 2-14 nm was identified. The magnetite was characterized by X-ray diffraction, transmitted and reflected light microscope, high-resolution transmission electron microscopy (TEM), high angle annular dark field, Mossbauer spectroscopy and its magnetic properties. Crystallographic identification of nanostructures Was performed using high-resolution TEM. Characteristic changes were observed when the particles make the size transition from micro- to nanometric sizes, as follows: (1) frequency-dependent magnetic Susceptibility percentage (chi FD%) measurements Show high Values (13%) for the 2-14 nm fractions attributed to dominant fractions of superparamagnetic particles; (2) variations of chi FD% < 4.5% in fractions of 56-0.2 mu m occur in association with the presence of microparticles formed by magnetite aggregates of nanoparticles (< 15 nm) embedded in berthierine; (3) Mossbauer spectroscopy results identified a superparamagnetic fraction; (4) nanometric and 0.2-3 mu m grain size magnetite particles require a magnetic field up to 152 mT to reach saturation during the isothermal remanent magnetization experiment; (5) coercivity and remanent magnetization of the magnetite increase when the particle size decreases, probably due to parallel Coupling effects; (6) two-magnetic Susceptibility versus temperature experiments of the same 2-14 nm sample show that the reversibility during the second heating is due to the formation of new magnetite nanoparticles and growth of those already present. during the first heating process.
ISSN: 1343-8832

hal-00413053
https://hal.archives-ouvertes.fr/hal-00413053

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
Wadsleyites with various iron contents were synthesized at ~12-14 GPa and 1400 °C under oxidizing and hydrous conditions in coexistence with enstatite. The samples were studied using micro-X-ray absorption near edge structure (XANES) and micro-Mössbauer spectroscopy to determine the ferric iron contents in polyphasic samples and secondary ion mass spectrometry (SIMS) to determine the water concentrations. XANES and Mössbauer analyses show that ferric iron content increases with increasing total iron content, and reaches a maximum of ~30% Fe3+/Fetotal. Two XANES results were cross-checked by Mössbauer analysis and both methods are in reasonable agreement. The use of Fourier transform infrared spectroscopy reveals a new protonation scheme in wadsleyite, with a significant proportion of protons associated with the high-frequency band at 3611 cm−1 and a new band located at 3500 cm−1. The intensity of these two bands is higher for Fe3+-rich wadsleyite. SIMS analyses show that water contents in wadsleyite vary from 4500 to 9400 ppm H2O by weight. Pyroxene water contents range from 790 to 1600 ppm wt H2O. The concentration of water in both phases decreases with increasing iron content. The partition coefficient of water between wadsleyite and pyroxene varies between 5 and 9 and increases with increasing Fe-number of wadsleyite [i.e., XFe/(XFe + XMg) × 100 ratio]. The divalent cation concentrations (i.e., Mg2+ + Fe2+), the Si as well as the H content in wadsleyite decrease with increasing Fe3+ content, indicating an incorporation mechanism via substitution into the metal (Me = Mg2+ and Fe2+) and Si sites with a ratio of 5/3 for (Fe3++H+):Me and of 5/1 for (Fe3++H+):Si, similarly as in the dry system. Thus, coupled substitution of Fe3+ and H+ does not affect the incorporation mechanism of Fe3+, but does affect the location of H+, which is partly incorporated at tetrahedral edges forming [FeSi′-(OH)o*]X neutral defects substituting for Si. In this model 25% of the ferric iron occupies the tetrahedral sites.
ISSN: 0003-004X

hal-00758071
https://hal.archives-ouvertes.fr/hal-00758071
DOI : 10.2138/am.2012.3869

Éditeur(s) : HAL CCSD
Résumé : International audience
In orogens, shortening is mainly accommodated by thrusts, which constitute preferential zones for fluid–rock interactions. Fluid flow, mass transfer, and mineralogical reactions taking place along thrusts have been intensely investigated, especially in sedimentary basins for petroleum and uranium research. This study combines petrological investigations, mineralogical quantifications, and geochemical characterizations with a wide range of analytical tools with the aim of defining the fluid properties (nature, origin, temperature, and redox) and fluid–host rock interactions (mass transfers, recrystallization mechanisms, and newly formed synkinematic mineralization) in the Pic-de-Port-Vieux thrust fault zone (Pyrenees, Spain). We demonstrate that two geochemically contrasted rocks have been transformed by fluid flow under low-grade metamorphism conditions during thrusting. The hanging-wall Triassic red pelite was locally bleached, while the footwall Cretaceous dolomitic limestone was mylonitized. The results suggest that thrusting was accompanied by a dynamic calcite recrystallization in the dolomitic limestone as well as by leaching of iron via destabilization of iron oxides and phyllosilicate crystallization in the pelite. Geochemical and physical changes highlighted in this study have strong implications on the understanding of the thrust behavior (tectonic and hydraulic), and improve our knowledge of fluid–rock interactions in open fluid systems in the crust.
Contributions to Mineralogy and Petrology

hal-01575548
https://hal-univ-fcomte.archives-ouvertes.fr/hal-01575548
DOI : 10.1007/s00410-017-1394-5

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
We have investigated the partitioning of Fe3+ between orthopyroxene (Opx) and garnet (Grt) in well-equilibrated mantle xenoliths using Mössbauer spectroscopy. The samples cover a wide range of P–T conditions (2.1–6.6 GPa, 690–1,412 °C) and geothermal gradients, and are thus representative for Earth’s upper mantle in both on-craton and off-craton continental settings. Garnet has Fe3+/Fetot ratios of 0.03–0.13 and Fe2O3 contents of 0.24–1.00 wt%. Orthopyroxene has, on average, lower Fe3+/Fetot ratios (0.01–0.09) and Fe2O3 contents (0.05–0.63 wt%). In low-pressure, high-temperature samples, however, Opx is systematically richer in Fe2O3 than the coexisting Grt. The Fe3+ Opx/Grt partition coefficient (DOpx/GrtFe3+) shows no obvious relationship with temperature, but increases with decreasing pressure and with increasing NaOpx. The observed Opx/Grt Fe3+ systematics imply that the Opx–Grt Fe–Mg exchange thermometer is not robust against redox changes if total Fe is treated as Fe2+. An approximate evaluation of errors on T estimates due to redox effects predicts negligible deviations for strongly reduced conditions (<65 °C), but potentially large deviations (> to ≫100 °C) for strongly oxidized conditions, especially at very high pressure and when both P and T are calculated by iteration.
ISSN: 0010-7999

hal-01121606
https://hal.archives-ouvertes.fr/hal-01121606
DOI : 10.1007/s00410-014-1101-8

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present archeointensity data carried out on pieces of incense burners from the ancient Maya city of Palenque, Chiapas, Mexico, covering much of the Mesoamerican Classic period, from A.D. 400 to A.D. 850. We worked on pieces from 24 incense burners encompassing the five Classic ceramic phases of Palenque: Motiepa (A.D. 400-500), Cascadas (A.D. 500-600), Otulum (A.D. 600-700), Murcielagos (A.D. 700-770), and Balunté (A.D. 770-850). All the samples come from highly elaborate, flanged pedestal of incense burners that are undoubtedly assigned to a ceramic phase by means of their iconographic, morphological and stylistic analyses. Archeointensity measurements were performed with the Thellier-Thellier's method on pre-selected samples by means of their magnetic properties. We obtained archeointensities of very good technical quality from 19 of 24 pieces, allowing the determination of a precise mean value for each ceramic phase, between View the MathML source29.1±0.9μT and View the MathML source32.5±1.2μT. The firing temperatures of ceramics were estimated with Mössbauer spectroscopy between 700 °C and 1000 °C. These values ensure that a full thermo-remanent magnetization was acquired during the original heating. Our results suggest a relative stability of the field intensity during more than 400 years in this area. The abundance of archeological material in Mesoamerica contrasts with the small amount of archeomagnetic data available that are, in addition, of uneven quality. Thus, it is not possible to establish a trend of intensity variations in Mesoamerica, even using the global databases and secular variation predictions from global models. In this context, our high technical quality data represent a strong constraint for the Mesoamerican secular variation curve during the first millennium AD. The corresponding Virtual Axial Dipole Moments (VADM) are substantially smaller than the ones predicted by the last global geomagnetic models CALS3k.4, suggesting the need for additional data to develop a regional model and a reference curve for Mesoamerica.
ISSN: 0012-821X

hal-00828696
https://hal.archives-ouvertes.fr/hal-00828696
DOI : 10.1016/j.epsl.2012.12.035