Temporal evolution and geochemical variability of the South Pacific superplume activity

International audienceWe are presenting a new set of K/Ar ages and geochemical analyses obtained on deep-sea samples dredged in 1999 on several seamounts of the Cook–Austral volcanic chains in the Pacific Ocean. The new geochemical results, together with published data on island samples, allow us to reveal a time evolution of the mantle source composition as well as an increase in geochemical variability of the superplume responsible for the regional South Pacific Superswell. Three identified volcanic stages of 58–40, 33–20 and 20–0 Ma are identified with signatures of mantle reservoir composition varying from close to C to N-MORB-types and C/HIMU-type, respectively. Using a geodynamic reconstruction for the most recent volcanic period, from 20 Ma to present, three hotspot tracks are needed to explain the several volcanic episodes observed within the limited geographical area of the central part of the Cook–Austral chains. At the scale of a single volcano, different magmatic phases can also be identified with different ages and geochemical signatures, emphasizing the importance of structural control, either crustal or lithospheric, in the location of the magmatic outputs. These observations, taken together, are in good agreement with a model where each hotspot could sample a small volume of the large very heterogeneous plume responsible for the regional South Pacific Superswell

Helium isotopes on the Pacific-Antarctic ridge (52.5°–41.5°S)

International audienceThe first isotopic data and concentrations of helium are reported for the Pacific-Antarctic ridge between 52.5°S and 41.5°S. The 4He/3He ratio is extremely homogeneous over more than 1200 km, with a mean ratio of 99,275 (R/Ra = 7.29) and a standard deviation of 2719 (0.19), which is the lowest dispersion observed for the global mid oceanic ridge system. Moreover, the Menard T.F. is a frontier between two mantles with slightly different helium isotopic ratios (96,595 ± 1520 and 100,347 ± 2330). No difference in the helium concentration between the two ridge segments defined by the Menard T.F. can be observed, as well as no significant difference in the U and Th contents suggesting that the difference in helium isotopic ratio is old (>500 My) and may represent a slight difference in degassing or/and trace element depletion histor

The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

In this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific–Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from −76 to −48‰−48‰, with an average value of −61‰−61‰. The lack of correlation between Cl content and either H_2O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific–Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of −60±5‰−60±5‰, as supported by the most recent data from North Atlantic N-MORB

Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S

International audienceMajor, trace element and Sr-Nd-Hf isotope data in basalts collected along the Pacific-Antarctic Ridge (PAR) axis between 53 and 41°S, far from any hotspot influence, reveal tight coherent geochemical variations within the depleted MORB mantle. All samples are located below the Pacific reference line defining two sub-oceanic mantle domains on each side of the Easter microplate. The data extend the PAR 66-53°S field towards more radiogenic Sr (0.70264), less radiogenic Nd (ɛ = 7.7) and Hf (ɛ = 11.4) values. The along ridge geochemical variability is closely related to the morphological segmentation of the ridge. Anomalous geochemical features are attributed to the atypical morphology of two segments due to the presence of off-axis magmatism. The first order ridge discontinuity defined by the Menard transform fault separates two slightly different mantle domains, each with its own history

A low δ7Li lower crustal component: Evidence from an alkalic intraplate volcanic series (Chaîne des Puys, French Massif Central)

International audienceThe intraplate volcanic suite of the Chaîne des Puys (French Massif Central) shows a complete petrologic range, from alkali basalts to trachytes. The significant variations of trace elements and radiogenic isotopes along the series strongly support the occurrence of crustal assimilation associated with fractional crystallization (AFC). The least contaminated basalts are clearly related to a HIMU-type reservoir (206Pb/204Pb > 19.6; 87Sr/86Sr + 4). The behavior of radiogenic isotopes suggests that the most likely crustal contaminants are meta-sediments located in the lower crust. The Li isotopic compositions of the lavas range from high δ7Li (> + 7‰) in basalts to lighter values in more evolved lavas (down to δ7Li ≈ 0‰). The mantle component, expressed in the least evolved lavas, has a heavy Li isotopic signature, in good agreement with previous δ7Li measurements of OIB lavas with HIMU affinities. The evolution of Li isotopic compositions throughout the volcanic series is in agreement with the AFC model suggested by the Sr–Nd–Pb isotopic systems. Although the behavior of Li isotopes during assimilation processes is currently poorly constrained, our calculations suggest that at least a portion of the lower crust beneath the Chaîne des Puys is characterized by a light Li isotopic composition (δ7Li < − 5‰)

Mycolactone Diffuses into the Peripheral Blood of Buruli Ulcer Patients - Implications for Diagnosis and Disease Monitoring.

BACKGROUND: Mycobacterium ulcerans, the causative agent of Buruli ulcer (BU), is unique among human pathogens in its capacity to produce a polyketide-derived macrolide called mycolactone, making this molecule an attractive candidate target for diagnosis and disease monitoring. Whether mycolactone diffuses from ulcerated lesions in clinically accessible samples and is modulated by antibiotic therapy remained to be established. METHODOLOGY/PRINCIPAL FINDING: Peripheral blood and ulcer exudates were sampled from patients at various stages of antibiotic therapy in Ghana and Ivory Coast. Total lipids were extracted from serum, white cell pellets and ulcer exudates with organic solvents. The presence of mycolactone in these extracts was then analyzed by a recently published, field-friendly method using thin layer chromatography and fluorescence detection. This approach did not allow us to detect mycolactone accurately, because of a high background due to co-extracted human lipids. We thus used a previously established approach based on high performance liquid chromatography coupled to mass spectrometry. By this means, we could identify structurally intact mycolactone in ulcer exudates and serum of patients, and evaluate the impact of antibiotic treatment on the concentration of mycolactone. CONCLUSIONS/SIGNIFICANCE: Our study provides the proof of concept that assays based on mycolactone detection in serum and ulcer exudates can form the basis of BU diagnostic tests. However, the identification of mycolactone required a technology that is not compatible with field conditions and point-of-care assays for mycolactone detection remain to be worked out. Notably, we found mycolactone in ulcer exudates harvested at the end of antibiotic therapy, suggesting that the toxin is eliminated by BU patients at a slow rate. Our results also indicated that mycolactone titres in the serum may reflect a positive response to antibiotics, a possibility that it will be interesting to examine further through longitudinal studies

Genomic history of the seventh pandemic of cholera in Africa.

The seventh cholera pandemic has heavily affected Africa, although the origin and continental spread of the disease remain undefined. We used genomic data from 1070 Vibrio cholerae O1 isolates, across 45 African countries and over a 49-year period, to show that past epidemics were attributable to a single expanded lineage. This lineage was introduced at least 11 times since 1970, into two main regions, West Africa and East/Southern Africa, causing epidemics that lasted up to 28 years. The last five introductions into Africa, all from Asia, involved multidrug-resistant sublineages that replaced antibiotic-susceptible sublineages after 2000. This phylogenetic framework describes the periodicity of lineage introduction and the stable routes of cholera spread, which should inform the rational design of control measures for cholera in Africa

Initiation of a plume-ridge interaction in the South Pacific recorded by high-precision Pb isotopes along Hollister Ridge

The southern Pacific Ocean offers the rare possibility to study a situation where a spreading ridge (the Pacific-Antarctic Ridge (PAR)) migrates toward a fixed hot spot (the Louisville hot spot) (Small, 1995). Hollister Ridge is a 450 km long linear structure whose position, between the PAR axis and the most recent edifices of the Louisville hot spot trail, led some authors to suggest that the ridge is genetically related to the hot spot (Small, 1995; Wessel and Kroenke, 1997). Mapping and sampling of the ridge in 1996 revealed, however, that the contribution of the Louisville plume material to its mantle source is minor and suggested that it might be the result of intraplate deformation (Géli et al., 1998; Vlastélic et al., 1998). We report new, highly precise Pb isotopic data from Hollister Ridge, which (1) confirm that the maximal contribution of the Louisville plume, in the centrally, volcanic active part of the ridge, probably does not exceed 20% (15 and 35% for lower and upper limits) and (2) reveal through time an increasing plume influence. The initiation of the Louisville plume involvement in the source of Hollister Ridge is estimated to have occurred between 1.04 and 0.77 Myr ago. It thus followed closely the most recent volcanic activity reported along the Louisville trail (1.11 Ma (Koppers et al., 2004)). This suggests that Hollister Ridge has recorded the dispersion of the Louisville plume as the spreading ridge approached the hot spot. Assuming that the Louisville hot spot is located near the youngest seamount dredged along the Louisville seamount chain, Hollister Ridge lies along the shortest path of pressure release connecting the hot spot to the spreading axis. This path involves, first, an abrupt upwelling across the Eltanin fault system and, subsequently, a more progressive migration toward the spreading axis. Because Hollister Ridge is older than 2.5 Ma, the structure might not be the consequence of the plume-ridge flow. Instead, Hollister Ridge most likely emplaced through a lithospheric crack (Géli et al., 1998), which, subsequently, may have captured the plume-ridge flow