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 Oxford University Press (OUP)
Résumé : International audience
Phase equilibrium data pertaining to melting of simplified carbonated peridotite in the systems CaO-MgO-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-CO2 at pressures of 10-26 GPa, corresponding to similar to 300-750 km depths in the Earth, are presented. In both the studied systems, liquid compositions, with changing crystalline phase assemblage, are carbonatitic throughout the studied pressure range. In the system CMS-CO2, melting phase relations are isobarically invariant; liquid is in equilibrium with forsterite + clinoenstatite + clinopyroxene + magnesite, forsterite + majorite + clinopyroxene + magnesite, wadsleyite + majorite + clinopyroxene + magnesite, ringwoodite + majorite + calcium-silicate perovskite + magnesite, magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 12, 14, 16, 20, and 26 GPa, respectively. In the system CMAS-CO2, a phase assemblage consisting of forsterite + orthopyroxene + clinopyroxene + magnesite + garnet + melt from 10 to 14 GPa is isobarically invariant. However, owing to the disappearance of orthopyroxene at pressures greater than 14 GPa, from 16 and up to at least 26 GPa, the solidus of simplified carbonated peridotite spans a divariant surface in pressure-temperature space. The liquid coexists with wadsleyite + clinopyroxene + garnet + magnesite, ringwoodite + calcium-silicate perovskite + garnet + magnesite, and magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 16, 20, and 26 GPa, respectively. A curious, and as yet unexplained, feature of our study is an abrupt drop in the solidus temperature between 14 and 16 GPa that causes a small amount of melting of carbonated mantle in the Transition Zone of the Earth. In the systems CMS-CO2 and CMAS-CO2 liquid compositions at 16 and 20 GPa are highly calcic bona fide carbonatites; however, these liquids revert to being magnesiocarbonatites at 10-14 and 26 GPa. In the system CMS-CO2, at 16 GPa we locate an isobaric invariant point consisting of wadsleyite + clinopyroxene + anhydrous B + magnesite + melt. The presence of anhydrous B at 16 GPa and 1475 degrees C is interesting, as it lies outside the composition space of the mantle peridotite analog we have studied. However, despite the presence of two highly magnesian silicate crystalline phases, wadsleyite and anhydrous B, at 16 GPa and 1475 degrees C, the liquid composition remains calcic with molar Ca-number [Ca/(Ca + Mg) x 100] of about 63. The melting reactions at 16 and 20 GPa (with or without anhydrous B) show that lime-bearing crystalline silicates play a fairly large part in generating and controlling the composition of the liquids. At 16 GPa, in the system CMS-CO2, we also report an experimental run at 1575 degrees C, in which liquid coexists with only wadsleyite and majorite. The liquid composition is less calcic (Ca-number 54) than that for other runs at lower temperatures, but is still more calcic than liquids at 10-14 and 26 GPa in both the studied systems. At present, the likely cause for these changes in the reported phase relations is not known. For normally assumed mantle temperatures, melting in the Transition Zone of the Earth, owing to the presence of carbonate, is probably unavoidable. The depth range of the drop in the carbonated peridotite solidus closely matches that of commonly observed low seismic velocities at similar to 400-600 km depth in the Earth.
ISSN: 0022-3530

hal-00669945
https://hal.archives-ouvertes.fr/hal-00669945
DOI : 10.1093/petrology/egr048

Éditeur(s) : HAL CCSD Elsevier
Résumé : Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.
ISSN: 0016-7037

hal-01553520
https://hal.archives-ouvertes.fr/hal-01553520
DOI : 10.1016/j.gca.2016.09.030

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : Solidus phase relations of carbon dioxide-saturated (CO2 vapor) model peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 in the 1.1-2.1 GPa pressure range are reported. The solidus has a positive slope in pressure-temperature (PT) space from 1.1 to 2 GPa. Between 2 and 2.1 GPa, the melting curve changes to a negative slope. From 1.1 to 1.9 GPa, the liquid, best described as CO2-bearing silicate liquid, is in equilibrium with forsterite, orthopyroxene, clinopyroxene, spinel, and vapor. At 2 GPa, the same crystalline phase assemblage plus vapor is in equilibrium with two liquids, which are silicate and carbonatitic in composition, making the solidus at 2 GPa PT invariant. The presence of two liquids is interpreted as being due to liquid immiscibility. Melting reactions written over 1.1-1.9 GPa are peritectic, with forsterite being produced upon melting, and the liquid is silicate in composition. Upon melting at 2.1 GPa, orthopyroxene is produced, and the liquid is carbonatitic in composition. Hence, the invariance between 1.9 and 2.1 GPa is not only the reason for the dramatic change in the liquid composition over an interval of 0.2 GPa, but the carbonated peridotite solidus ledge itself most likely appears because of this PT invariance. It is suggested that because carbonatitic liquid is produced at the highest solidus temperature at 2 GPa in PT space in the system studied, such liquids, in principle, can erupt through liquid immiscibility, as near-primary magmas from depths of approximately 60 km.
ISSN: 0148-0227

hal-00939406
https://hal.archives-ouvertes.fr/hal-00939406
DOI : 10.1002/jgrb.50249

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Jacupiranga Complex is one of several Meso-Cenozoic alkaline intrusive complexes along the margins of the intracratonic Paraná Basin in southern Brazil. The complex encompasses a wide range of rock-types, including dunites, wehrlites, clinopyroxenites, melteigites-ijolites, feldspar-bearing rocks (diorites, syenites, and monzonites), lamprophyres and apatite-rich carbonatites. While carbonatites have been extensively investigated over the last decades, little attention has been paid to the silicate rocks. This study presents new geochonological and geochemical data on the Jacupiranga Complex, with particular emphasis on the silicate lithotypes. 40Ar/39Ar ages for different lithotypes range from 133.7 ± 0.5 Ma to 131.4 ± 0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9 ± 1.3 Ma. These ages indicate a narrow time frame for the Jacupiranga Complex emplacement, contemporaneous with the Paraná Magmatic Province. Most of the Jacupiranga rocks are SiO2-undersaturated, except for a quartz-normative monzonite. Based on geochemical compositions, the Jacupiranga silicate lithotypes may be separated into two magma-evolution trends: (1) a strongly silica-undersaturated series, comprising part of the clinopyroxenites and the ijolitic rocks, probably related to nephelinite melts and (2) a mildly silica-undersaturated series, related to basanite parental magmas and comprising the feldspar-bearing rocks, phonolites, lamprophyres, and part of the clinopyroxenites. Dunites and wehrlites are characterized by olivine compositionally restricted to the Fo83-84 interval and concentrations of CaO (0.13–0.54 wt%) and NiO (0.19–0.33 wt%) consistent with derivation by fractional crystallization, although it is not clear whether these rocks belong to the nephelinite or basanite series. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Some feldspar-bearing rocks (i.e. meladiorite and monzonite) show petrographic features and geochemical and isotope compositions indicative of crustal assimilation, although this may be relegated to a local process. Relatively high CaO/Al2O3 and La/Zr and low Ti/Eu ratios from the lamprophyres and calculated melts in equilibrium with cumulus clinopyroxene point to a lithospheric mantle metasomatized by CO2-rich fluids, suggesting vein-plus-wall-rock melting mechanisms. The chemical differences among those liquids are thought to reflect both variable contributions of melting resulting from veins and variable clinopyroxene/garnet proportions of the source.
ISSN: 0895-9811

hal-01583547
https://hal.archives-ouvertes.fr/hal-01583547
DOI : 10.1016/j.jsames.2017.05.009

Éditeur(s) : HAL CCSD
Résumé : EGU2012-6377- Geophysical Research Abstracts vol. 14
The hot geothermal fluid produced by the wells of the Bouillante power station, in Guadeloupe, contains various dissolved species such as silica or some metals (Zn, Pb, etc). During the exploitation, the geothermal fluid is flashing and cooling (from 260 to 160°C for the high-pressure steam-liquid separation and down to 110°C for the low-pressure phase separation of a part of the separated liquid at 160°C). Consequently, the dissolved species can form scale deposits such as amorphous silica or poly-metallic sulphides, according to the exploitation conditions, and cause serious scaling problems on the production and the surface installations. These minerals could also precipitate during the partial underground re-injection of the production fluids which is being set up in the Bouillante power station. In this study, in order to predict and prevent the possible silica precipitation at temperature lower than 160°C, during the partial underground reinjection of the production fluids of Bouillante or their thermal valorization down to 110°C temperatures, we propose to investigate on site the kinetics of silica precipitation as a function of temperature and pH. The experiments presented here were carried out on site on samples of separated waters collected after the steam-water separator at 160°C, containing about 600 ppm of SiO2, at four cooling temperatures: 22, 50, 75 and 90°C, and for pH ranging from 4 to 9. These experiments showed that the silica deposition was always initiated by a polymerization mechanism characterized by an order 2 kinetic law and the formation of a white colloidal gel, which slowed down the silica precipitation under solid form. This latter was observed much later, after the polymerization stage, and only in particular conditions (long time of atmospheric exposure, water stagnation,. . . ). The results also confirmed that the silica precipitation kinetics is strongly dependent on pH. In acidic solution, a transition period is observed before the beginning to the silica polymerization. At neutral-basic pH, there is no transition period and the rate of polymerization is maximal for pH close to 8. The undergoing modeling of these experimental data and specific surface area measurements on the colloidal gel will allow us to determinate the kinetics parameters useful for the prediction and prevention of silica precipitation under the specific exploitation conditions of Bouillante power station. This work is financially supported by ADEME (French Agency for Energy and Environment), BRGM and UAG.
EGU General Assembly 2012
Vienne, Austria

hal-00697627
https://hal-brgm.archives-ouvertes.fr/hal-00697627

Éditeur(s) : HAL CCSD Wiley
Résumé : 13 pages, 4 figures, to be published on J. Am. Cer. Soc
International audience
Since crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem. We report the occurrence of hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real-time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in extent and shape are demonstrated for a wide range of macroscopic humidity at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will enable novel insights in the chemistry of failure of oxide materials.
ISSN: 0002-7820

hal-00424984
https://hal.archives-ouvertes.fr/hal-00424984
ARXIV : cond-mat/0509209
DOI : 10.1111/j.1551-2916.2005.00765.x

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Melting phase relations of model carbonated peridotite in the system CaO-MgO-Al2O3-SiO2-CO2 from 2 to 3 GPa are reported. Experimentally produced melts, which are model carbonatites, with approximately 36-40 wt % CaO, 12-17 wt % MgO, 0.2-1.5 wt % Al2O3, 1-4 wt % SiO2, and 40-42 wt % CO2 (carbon dioxide) are present at all pressures investigated. At 2.8 and 3 GPa, carbonatitic melts are seen experimentally at temperatures that are very close to the vapor-free (CO2) peridotite solidus and are found in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet. Solidus phase relations with isobaric and pressure-temperature invariant points, defining the so-called carbonated peridotite solidus ledge, are also reported from 2.1 to 3 GPa. A divariant region exists from 2 to 2.6 GPa wherein two, compositionally different melts are present. In this region, these two melts, carbonatitic and silicate in composition, coexist with crystalline phase assemblage and free vapor. The silicate liquid has approximately 30-48 wt % SiO2 and approximately 6 to 20 wt % of dissolved CO2. The presence of carbonatitic and silicate liquids is interpreted to be due to liquid immiscibility. On the basis of melting phase relations reported here, we conclude that (a) the ledge is a feature along which model carbonatitic liquids are produced by reaction of silicates and CO2 vapor and (b) alkali-free carbonatites and silicate melts can form through melt unmixing at depths of ~60-80 km in the Earth's mantle.
ISSN: 0148-0227

hal-01053913
https://hal.archives-ouvertes.fr/hal-01053913
DOI : 10.1002/2013JB010913

Éditeur(s) : HAL CCSD American Physical Society
Résumé : We study the equilibrium properties of a liquid phase condensed at the nanoscale between the surfaces of a sharp crack in fused silica in a moist controlled atmosphere. The extension of the condensed phase along the fracture is measured by in situ atomic force microscopy phase imaging and it is shown to be determined by a critical distance between the opposite crack surfaces, which is an increasing function of humidity. The present technique is very promising for measuring the properties of confined liquids at the nanoscale as well as for modeling the physics and chemistry of slow crack propagation in glasses.
ISSN: 0031-9007

hal-00411946
https://hal.archives-ouvertes.fr/hal-00411946
DOI : 10.1103/PhysRevLett.100.165505

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.
ISSN: 0022-3530

hal-01553676
https://hal.archives-ouvertes.fr/hal-01553676
DOI : 10.1093/petrology/egw066

Éditeur(s) : HAL CCSD Elsevier
Résumé : Carbon emission at volcanic centers requires a constant balance between output (mostly by volcanism, either at plate boundaries or intraplate) and input (mostly at trench settings) of carbon from and to the Earth's mantle. The form of carbon that resides in the mantle is controlled by depth (pressure) and oxygen fugacity, the latter in turn depending on the depth and the concentration of iron in the mantle. In the shallow, lithospheric mantle, carbon is likely to be present in the oxidized form of CO2 (except under cratons where carbon is reduced to graphite or diamond). Below approximately 90 km, in the asthenosphere, the oxidized form of carbon is carbonate, either mineral or melt, depending on the thermal regime. At depths greater than approximately 150 km, the asthenospheric mantle is too reducing for carbon to stay in its oxidized form and only diamond is present, unless there is sufficient hydrogen to form reduced C–H fluids. Hence, the region located in the depth range of 90 to 150 km deep is where carbonatitic melts can most likely be produced and impregnate the surrounding mantle through metasomatism. The upper bound of this region is called the carbonate ledge. This limit prevents carbonate (either solid or molten) from ascending because of degassing and CO2 liberation. The lower bound is a redox front where redox melting (that is, melting caused by oxidation) may take place in an ascending portion of carbon-containing mantle. Carbonatite eruptions and presence of carbonate mineral inclusions in deep-seated diamonds provide evidence that these boundaries can be trespassed in some cases.An analysis of the experimental data that has bearing on silicate melting in the presence of carbon further shows that the carbonate ledge is a melting curve with a negative or flat Clapeyron (dP/dT) slope. In the carbonated ultrabasic (peridotite) systems, the carbonate ledge is located between ~ 2–3 GPa. The ledge divides the pressure–temperature space into a region of low-pressure silicate melt production, and a high-pressure region where carbonatites can be produced. Carbonatitic melts in equilibrium with mantle peridotite have compositions close to dolomitic (approximately equal amounts of Ca and Mg) with a general trend of becoming markedly more magnesian with increasing pressure. Calcic carbonatites may be stable at pressures < 2 GPa if clinopyroxene is absent. At mantle transition zone pressure range, there seems to be a melting temperature decrease (negative fusion slope), which may be caused by the stabilization of majoritic garnet.The carbonated basic (broadly eclogitic) system is more complex than the peridotitic one, because of the strong control of bulk silicate composition on melting temperatures, and hence, on melt composition. In carbonated eclogite systems, we propose that the effect of bulk composition upon all observed features can perhaps be related to silica super-saturation, or lack thereof. In some cases, high calcium (> 80 mol% CaCO3) melts can be produced, making the melting of carbonated eclogites an appealing scenario for the genesis of calcio-carbonatites in the Earth's mantle. Comparison with modeled pressure–temperature paths of subducted oceanic lithosphere shows that fusion of carbonated eclogite at depths shallower than 200 km should be expected for hot (Cascadian-type) subduction thermal regimes. On the other hand, in the case of cooler thermal regimes (Honshu-type, for instance), subducted carbonates may be stable to greater depths in Earth at trench settings, depending on the bulk composition of the system.Furthermore, high-pressure experiments show evidence of a continuum among carbonatitic, kimberlitic, melilititic, and basaltic liquids, for increasing melting degree of carbonated peridotite. This continuum has not been documented in the case of fusion of carbonated eclogite. It may be present, however, when certain sediments are fused, although the silicate melts are granitic to rhyodacitic instead of being kimberlitic in composition. Additional high-pressure work on phase relations in the simple binary system CaCO3–MgCO3 and specific focus on oxide solubility in the vapor phase have the potential to further clarify phase relations on complex silicate–carbonate systems at mantle conditions.
ISSN: 0009-2541

hal-01277770
https://hal.archives-ouvertes.fr/hal-01277770
DOI : 10.1016/j.chemgeo.2015.05.018

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
North Tanzanian Divergence is the first stage of continental break-up of East African Rift (<6 Ma) and is one of the most concentrated areas of carbonatite magmatism on Earth, with singular Oldoinyo Lengai and Kerimasi volcanoes. Hanang volcano is the southernmost volcano in the North Tanzanian Divergence and the earliest stage of rift initiation. Hanang volcano erupted silica-undersaturated alkaline lavas with zoned clinopyroxene, nepheline, andradite-schorlomite, titanite, apatite, and pyrrhotite. Lavas are low MgO-nephelinite with low Mg# and high silica content (Mg# = 22.4–35.2, SiO2 = 44.2–46.7 wt%, respectively), high incompatible element concentrations (e.g. REE, Ba, Sr) and display Nb–Ta fractionation (Nb/Ta = 36–61). Major elements of whole rock are consistent with magmatic differentiation by fractional crystallization from a parental melt with melilititic composition. Although fractional crystallization occurred at 9–12 km and can be considered as an important process leading to nephelinite magma, the complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites recorded magmatic differentiation involving open system with carbonate–silicate immiscibility and primary melilititic melt replenishment. The low water content of clinopyroxene (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. Mg-poor nephelinites from Hanang represent an early stage of the evolution path towards carbonatitic magmatism as observed in Oldoinyo Lengai. Paragenesis and geochemistry of Hanang nephelinites require the presence of CO2-rich melilititic liquid in the southern part of North Tanzanian Divergence and carbonate-rich melt percolations after deep partial melting of CO2-rich oxidized mantle source.
ISSN: 0010-7999

hal-01467390
https://hal.archives-ouvertes.fr/hal-01467390
DOI : 10.1007/s00410-016-1273-5

Éditeur(s) : Université de Bretagne Occidentale
Résumé : Ce travail a été réalisé dans un esprit de géochimie comparée des éléments de transition hygromagmatophiles (Treuil, 1973) ou hygromagmaphiles, éléments traces ayant une affinité pour le liquide silicaté.

Droits : UBO
http://archimer.ifremer.fr/doc/00034/14549/11805.pdf
http://archimer.ifremer.fr/doc/00034/14549/

Éditeur(s) : HAL CCSD Elsevier
Résumé : Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10(-9)-10(-10) m/s). These observations are based on in situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip.
ISSN: 0022-3093

hal-00412153
https://hal.archives-ouvertes.fr/hal-00412153
DOI : 10.1016/j.jnoncrysol.2007.06.090

Éditeur(s) : HAL CCSD IWA Publishing
Résumé : International audience
Anti-cancer drugs are an important class of pharmaceutical products. Methotrexate (MTX) is a folic acid antagonist used in high doses as antimetabolite in anti-cancer treatment as well as in low doses for the treatment of rheumatoid arthritis and adults' psoriasis. In the past, several anti-cancer drugs, including methotrexate, have been found in the environment. Their presence in water, especially if used for the production of drinking water, is even in low concentrations of particular interest, due to the risk to retrieve them in the consumed water and their high activity and grave effects. But prior to usage as drinking water, raw waters are treated and chlorination is a common practice in several countries. As such a treatment can lead to the formation of organochlorine in water, the study of the fate of MTX during chlorination in a batch trial was carried out. The reaction was monitored by dissolved organic carbon (DOC) and by fluorescence and UV spectroscopy. Investigation of by-products formed was done with liquid chromatography/mass spectrometry (LC/MS). Under the given experimental conditions, Methotrexate was eliminated rapidly (t1/2 around 21 min). However, DOC elimination was incomplete. Monitoring with LC-MS showed the formation of a monochlorinated transformation product of MTX. In silico analysis of the proposed transformation products for different carcinogenic, mutagenic and genotoxic endpoints with different software platforms provided no clear evidence that the possible transformation products after chlorination might be more toxic than the parent compound. However, since a number of alerts is altered after chlorination, it cannot be excluded that the toxicity of these transformation products might be modulated compared with the parent compound.
ISSN: 0043-1354

hal-01016617
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01016617
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01016617/document
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01016617/file/Monitoringsur methotrexate_final.pdf
DOI : 10.1016/j.watres.2014.03.008

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Spinel lherzolite xenoliths from Mont Briancon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (Os-187/Os-188 = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25%, of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of Sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.;Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic Sulfide with low [Os] and radiogenic Os-187/Os-188, and (ii) primary sulfides with high [Os] and unradiogenic Os-187/Os-188, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s). (C) 2009 Elsevier Ltd. All rights reserved.
ISSN: 0016-7037

hal-00454537
https://hal.archives-ouvertes.fr/hal-00454537
DOI : 10.1016/j.gca.2009.09.031

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Solidus melting phase relations are reported for carbonated eclogite in the system CaO-MgO-Al2O3-SiO2-CO2 at 12 to 25 GPa. From 12 to 16 GPa, melts are in equilibrium with clinopyroxene, stishovite, garnet, aragonite, and magnesite. At 20 and 25 GPa, melts are in equilibrium with garnet, stishovite, calcium-alumino silicate, calcium perovskite, and magnesite. Melting reactions demonstrate that from 12 to 16 GPa, stishovite is in reaction with the melt. At 20 and 25 GPa, garnet and stishovite together are produced upon melting of model, carbonated eclogite. At 20 and 25 GPa, calcium perovskite is also the phase that contributes the most toward liquid production. Melt compositions at all pressures are carbonatitic, with roughly 37-40 wt% dissolved CO2. From 12 to 16 GPa, the liquids are calciocarbonatites with Ca#(molar) of similar to 69-71; liquid compositions become less calcic with Ca# of similar to 52-55 at 20 and 25 GPa. Given these melting phase relations, suitable subduction zone adiabats do not intersect the solidus of model carbonated eclogite at depths investigated in the present study. Hence, on this basis, it is fair to say that carbonated eclogite possibly avoids melting in subduction zone settings, thereby delivering carbonate to at least moderate depths in the Earth. However, owing to local heating events, small-degree melting of carbonated eclogite is not completely precluded, and the liquids liberated from this melting can be viewed as agents of chemical mass transfer in the deep Earth. At present, however, geochemical consequences of subduction-related melting of carbonated eclogite are difficult to evaluate.
ISSN: 0148-0227

hal-00566560
https://hal.archives-ouvertes.fr/hal-00566560
DOI : 10.1029/2009JB006457