Biography:

In the past C. Chauvel has collaborated on articles with F. Nauret and C. Balu-Maestro. One of their most recent publications is Correlated trace element-Pb isotope enrichments in Indian MORB along 18–20°S, Central Indian Ridge. Which was published in journal Earth and Planetary Science Letters.

More information about C. Chauvel research including statistics on their citations can be found on their Copernicus Academic profile page.

C. Chauvel's Articles: (3)

Correlated trace element-Pb isotope enrichments in Indian MORB along 18–20°S, Central Indian Ridge

AbstractThe Central Indian Ridge (CIR), between 18° and 20°S, shows topographic and chemical characteristics, which suggest interaction of the ridge with a mantle plume. In order to investigate the previously postulated input from the Réunion plume (presently located ∼ 1000 km off-axis to the west) on the CIR, we present chemical and isotopic compositions of basalts, collected on and off the CIR axis between 18° and 20°S.We distinguish two geographical groups of samples, called On-Axis and Gasitao, respectively. The On-Axis group is characterized by unradiogenic Sr and Pb isotope ratios and high εNd values. Gasitao group basalts have lower SiO2, are more depleted in incompatible elements and have more radiogenic Sr–Pb isotope ratios and lower εNd values than the On-Axis group. The two groups form two distinct, subparallel linear arrays in 207Pb/204Pb–206Pb/204Pb space. While the Gasitao array trends towards Réunion plume compositions, and therefore appears to contain some Réunion-type plume material, this is not the case for the On-Axis array. Along the ridge axis, Pb isotopes become more radiogenic from south to north, and incompatible trace elements become more enriched, but the compositional field of Réunion lavas is not a suitable end member for the Pb isotope and highly incompatible element trends (e.g. Ba/Nb). This indicates that the geochemical enrichment seen in the On-Axis region is not related to Réunion-type plume material.Basalts from both groups show both positive and negative Eu anomalies, which are strongly correlated with Sr/Nd ratios, thus indicating both gains and losses of feldspar phenocrysts. However, this has little effect on ratios of other trace elements. The trace element enrichment patterns are strongly correlated with Pb isotope ratios, with the most E-MORB-like samples having the most radiogenic Pb isotopic compositions. Using the trace element (TE)/Pb ratios versus 206Pb/204Pb correlations, and by extrapolating these linear correlations to TE/Pb = 0, we constrain possible 206Pb/204Pb ratios of the enriched and depleted endmembers. These lie at 18.3 ≤ 206Pb/204Pb ≤ 18.8 for the depleted and enriched components, respectively, and not very far outside the range of the actual data. We infer that the CIR MORB, between 18° and 20°S are generated by partial melting of a heterogeneous source consisting of an enriched component and a normal, depleted upper-mantle peridotite. The nature of the enriched component is a matter of speculation. As noted, its composition is different from known Réunion plume compositions. Instead, it may represent recycled (oceanic) crustal material, perhaps derived from a subducted oceanic island. It could also be formed by a “metasomatic” enrichment process similar to that modeled by Donnelly et al. [K.E. Donnelly, S.L. Goldstein, C.H. Langmuir, M. Spiegelman. Origin of enriched ocean ridge basalts and implications for mantle dynamics. Earth Planet. Sci. Lett. 226 (2004) 347–366] to explain “E-type” MORB compositions. In either case, the location of the enriched anomaly on the CIR near the intersection with the Gasitao Ridge appears to be coincidental, because the Gasitao enrichment can be traced to the Réunion plume, whereas the On-Axis group enrichment cannot. We speculate that the Réunion plume flow might be deflected towards the South by the hot upwelling E-MORB mantle, because the southernmost On-Ridge sample does fall on the Gasitao-Réunion trend.

OncologieDepistage des cancers du sein : quelle imagerie pour quelles femmes

ObjectifsConnaître la place des différentes techniques d’imagerie pour le dépistage organisé et individuel du cancer du sein.Connaître le cadre de la valeur ajoutée de la mammographie numérique.Savoir les indications et les résultats de l’échographie associée à la mammographie connaître les protocoles et résultats du dépistage des femmes génétiquement prédisposées.Points clésLa mammographie est la seule technique actuellement retenue pour la dépistage organisé.La mammographie numérique est plus sensible que la mammographie analogique chez les femmes non ménopausées et/ou de moins de 50 ans et/ou en cas de densité mammaire élevée.L’échographie présente un taux de détection équivalent à celui de la mammographie en cas de seins denses.Chez les femmes génétiquement prédisposées et à haut risque l’IRM annuelle est associée à l’imagerie conventionnelle et nécessite des systèmes de prélèvements dédiés.RésuméLe taux de détection par la mammographie analogique dans les campagnes est d’environ 3 cancers pour 1 000 femmes dépistées. La mammographie numérique est plus sensible lorsque la densité est élevée; la valeur ajoutée de la mammographie numérique concerne les femmes pré ou non ménopausées et/ou de moins de 50 ans. L’échographie associée à la mammographie augmente la détection des cancers au prix d’une VPP de la biopsie néanmoins faible ( < 10 %).L’IRM pour les femmes à haut risque détecte deux fois plus de cancers que la mammographie numérique et l’échographie nécessitant le recours à des méthodes de prélèvements adaptées.

Do decades of tropical rainfall affect the chemical compositions of basaltic lava flows in Mount Cameroon?

AbstractTo evaluate the effects of tropical rainfall on the compositions of basaltic lavas, we studied 20th century lava flows from Mount Cameroon in Africa. Weathering conditions are extreme because the climate is particularly warm and humid, and vegetation grows extremely quickly on the flows. The high rainfalls and dense vegetation contribute to rapid and intense degradation of the volcanic rocks and should cause significant changes in chemical composition. Such effects need to be quantified to constrain how young a lava flow must be so that its trace element and isotopic composition remains representative of the original magma.Fresh inner parts and altered flowtops of four different lava flows were sampled and analysed for major and trace elements as well as O, U, Sr, Nd and Pb isotopic compositions. Four samples of the 1999 eruption were also analysed to constrain the composition of fresh basalts.Almost all major and trace elements display similar concentrations in inner and outer parts of the same flow. This is notably the case for elements such as K, Rb and Sr, which are highly mobile during weathering. The lack of variation suggests that the overall composition of the lava flows has not been significantly affected. However, some systematic chemical changes are observed: Loss-on-ignition (LOI) and δ18O increase slightly from inner parts of flows to near surface samples; Na and, to a lesser extent, U display significant losses in the outer samples. We interpret the Na loss in terms of hydration leading to exchange between Na+ and H+ ions. This process, associated with oxidation of Fe2+ to Fe3+, accounts for the larger loss-on-ignition in the outer parts of flows. A change in U contents is only observed in the 1922 flow, which is covered by dense vegetation. This emphasizes the role that complexation by organic ligands plays in U mobility. While U is not completely immobile, all volcanic rocks are in secular equilibrium ((234U/238U)≈1), indicating limited interaction between meteoric waters and basalts. Sr and Nd isotopic compositions remain constant and although 206Pb/204Pb ratios vary from inner to outer parts of the lavas, the changes are not systematic and cannot be attributed to weathering.The absence of significant chemical mobility and substantial isotopic exchange suggests very limited interaction between water and lavas at the centimeter scale, due, most probably, to their relatively young age. This work shows that over a period of about 100 years, no detectable geochemical changes are observed. These results are very promising for petrological and geochemical studies of ocean island basalts located in tropical areas since they provide direct evidence of very limited trace element mobility at the century time scale.

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