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 Verlag
Résumé : International audience
The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from < 0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO3 (sulfate-saturated systems). The chlorine partition coefficients (D (Cl) (apatite/glass) ) range from 0.4 to 1.3 (average D (Cl) (apatite/glass) = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D (F) (apatite/glass) decreases from 35 to 3. However, most of our data display a high partition coefficient (similar to 30) close to D (F) (apatite/glass) determined experimentally in felsic rock. D (S) (apatite/glass) decreases from 9.1 to 2.9 with increasing SO3 in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (> 0.2 wt% SO3). D (S) (apatite/glass) is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and hauyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of hauyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.
ISSN: 0010-7999

hal-00644803
https://hal.archives-ouvertes.fr/hal-00644803
DOI : 10.1007/s00410-011-0606-7

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The complex electrical impedance of a garnet-biotite-sillimanite residual enclave in the Neogene dacite of El Hoyazo (SE Spain) has been determined up to 978 °C at 200-300 MPa. This well studied material represents a direct sampling of the Alborán Domain thinned lower crust undergoing partial melting. The paragenesis is garnet + biotite + sillimanite + plagioclase + graphite ± cordierite coexisting with widespread (~ 10 wt.%) rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850 ± 50 °C and 500-700 MPa. The samples were used in cyclic measurements consisting in heating-cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity. In the first cycle up to 850 °C at 2-3 kbar, re-melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity (S/m) is up to − 1.5. At T > 950 °C new melt with orthopyroxene + ilmenite is produced from biotite partial breakdown and the logarithmic conductivity (S/m) is up to − 0.7. Application of mixing models as Hashin-Shtrikman bounds or Archie's Law, shows that once interconnected, melt controls the electrical conductivity of the enclave. The electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting Matrix glass of the enclave and with the new melt. Our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the Betics can be explained as effect of partial melting of a residual crust. Comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected.
ISSN: 0040-1951

hal-00823063
https://hal.archives-ouvertes.fr/hal-00823063
DOI : 10.1016/j.tecto.2012.11.003

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Bronchoalveolar lavage (BAL) is a major diagnostic tool in lung diseases, including viral respiratory infections. We aimed to better define the situations where viral tests should be performed on BAL fluid (BALF). We retrospectively studied all cases where viral tests [immunofluorescence, immunocytochemistry, viral culture, and/or polymerase chain reaction (PCR)] were performed on BALF during a period of 1 year (2008) in our institution. We compared the characteristics of patients with virus-positive versus virus-negative BALF. Of the 636 BALF samples sent to the microbiology laboratory, 232 underwent viral tests. Of these, 70 (30 %) were positive and identified 85 viruses: herpes simplex virus (HSV)-1 (n = 27), cytomegalovirus (CMV, n = 23), Epstein-Barr virus (EBV, n = 18), human herpesvirus (HHV)-6 (n = 12), respiratory syncytial virus (RSV, n = 3), rhinovirus (n = 1), and adenovirus (n = 1). The variables associated with positive viral tests on univariate analysis were immunosuppression [human immunodeficiency virus (HIV), corticosteroids >10 mg/day for ≥3 weeks, or other immunosuppressive therapy], ground-glass attenuations on computed tomography (CT) scanning, late-onset ventilator-associated pneumonia (VAP), and durations of (i) hospital stay, (ii) intensive care unit (ICU) stay, and (iii) mechanical ventilation before BAL (p < 0.01 for each comparison). On multivariate analysis, only immunosuppression [odds ratio (OR) 6.4, 95 % confidence interval (CI) [2.8-14.3], p < 0.0001] and ground-glass attenuations (OR 3.7, 95 % CI [1.8-7.7], p = 0.0004) remained associated with virus-positive BAL. None of the viral tests performed on BALF for the initial assessment of diffuse infiltrative lung disease (n = 15) was positive. PCR improved the diagnostic yield of viral tests on BALF by 50 %. Testing for viruses on BALF should be mostly restricted to immunocompromised patients with acute respiratory diseases and/or patients with unexplained ground-glass attenuations on CT scanning.
ISSN: 0934-9723

hal-00873158
https://hal.archives-ouvertes.fr/hal-00873158
DOI : 10.1007/s10096-012-1791-7
PUBMED : 23238685

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 : International audience
The decay of 187Re to 187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to 187Re decay following igneous crystallisation). This study presents new Re Os, U Th Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in 187Os/188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133 ± 0.009 (2?). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however, ?11B covariations with 187Os/188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with 187Os/188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.
ISSN: 0012-821X

hal-00320075
https://hal.archives-ouvertes.fr/hal-00320075
DOI : 10.1016/J.EPSL.2007.05.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesite-hosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt partition coefficients (Dmineral/meltDmineral/melt) for Cl and F relevant to subduction-zone processes and unravel the history of volatile depletion and enrichment mechanisms in an arc setting.Chlorine is ∼100 times more incompatible in pyroxenes (View the MathML sourceDClmineral/melt = 0.005–0.008 [±0.002–0.003]) than F (View the MathML sourceDFmineral/melt = 0.50–0.57 [±0.21–0.24]), which indicates that partial melting of mantle sources leads to strong depletions in Cl relative to F in the residues. The data set in this study suggests a strong control of melt composition on View the MathML sourceDCl,Fpyroxene/melt, in particular H2O contents and Al/(Al + Si), which is in line with recent experiments. Fluorine is compatible in Ca-amphibole in the ‘wet’ sub-arc mantle (View the MathML sourceDFamphibole/melt = 3.5–3.7 [±1.5]) but not Cl (View the MathML sourceDClamphibole/melt = 0.03–0.05 [±0.01–0.03]), indicating that amphibole may fractionate F from Cl in the mantle wedge. The inter-mineral partition coefficients for Cl and F in this study are consistent amongst different harzburgite samples, whether they contain glass or not. In particular, disseminated amphibole hosts much of the Cl and F bulk rock budgets of spinel harzburgites (View the MathML sourceDClamphibole/pyroxene up to 14 and View the MathML sourceDFamphibole/pyroxene up to 40). Chlorine and fluorine are variably enriched (up to 1500 ppm Cl and 750 ppm F) in the parental arc picrite and boninite melts of primitive pyroxenite veins (and related melt inclusions) crosscutting spinel harzburgites.Based on the data in this study, the main inferences on the behaviour of Cl and F during melting and metasomatic processes in the sub-arc mantle are as follow:(i) Melting models show that most depleted mantle protoliths of intra-oceanic arc sources can have extremely low Cl/F (0.002–0.007) before being overprinted by subduction-derived components.(ii) Chlorine has a higher percolation distance in the mantle than F. Even for small fluid or melt volumes, Cl and F signatures of partial melting are overprinted by those of pervasive percolation, which increases Cl/F in percolating agents and bulk peridotites during chromatographic interaction and/or amphibole-forming metasomatic reactions. These processes ultimately control the bulk Cl and F compositions of the residual mantle lithosphere beneath arcs, and likely in other tectonic settings.(iii) Fluxed melting models suggest that Cl enrichment in arc picrite and boninite melts in this study, and in many arc melt inclusions reported in the literature, could be related to the infiltration of high Cl/F fluids derived from subducted serpentinite or altered crust in mantle wedge sources. However, these high Cl/F signatures should be re-evaluated with new models in light of the possible overprint of pervasive percolation effects in the mantle. The breakdown of amphibole (and/or mica) in the deep metasomatised mantle at higher pressure and temperature conditions than in the slab may explain, at least in part, the positive correlations between F abundances and Cl/F in primitive arc melt inclusions and slab depth.
ISSN: 0016-7037

hal-01553610
https://hal.archives-ouvertes.fr/hal-01553610
DOI : 10.1016/j.gca.2016.10.035

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : International audience
INTRODUCTION: Hydroxyurea is an antimetabolite drug used in the treatment of myeloproliferative disorders. Common adverse effects include haematological, gastrointestinal cutaneous manifestations, and fever. Hydroxyurea-induced pneumonitis is unusual. CASE REPORT: A female patient was treated with hydroxyurea for polycythemia vera. She was admitted 20 days after commencing treatment with a high fever, productive cough, clear sputum and nausea. A chest CT-scan showed diffuse ground-glass opacities. Microbiological investigations were negative. The symptoms disappeared a few days after discontinuation of the drug and rechallenge led to a relapse of symptoms. CONCLUSION: Our case and 15 earlier cases of hydroxyurea-induced pneumonitis are reviewed. Two patterns of this disease may exist: an acute febrile form occurring within 1 month of introduction of hydroxyurea and a subacute form without fever. Even if uncommon, one should be aware of this complication of hydroxyurea.
Revue des Maladies Respiratoires

hal-01072996
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01072996
DOI : 10.1016/j.rmr.2013.09.001
PUBMED : 24878159

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.
ISSN: 0148-0227

hal-01114953
https://hal.archives-ouvertes.fr/hal-01114953
DOI : 10.1002/2014JB011381

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