OIB/seamount recycling as a possible process for E-MORB genesis

International audienceThis paper deals with the origin of enriched MORB independent from any hot spot activity. Indeed, MORB enrichment was readily attributed to a ridge/hot spot interaction and in absence of identified neighboring hot spot, to more questionable processes (e.g., incipient plume or plume activity residue). More recently, the existence of enriched MORB away from any identifiable hot spot was attributed to different origins (i.e., recycled oceanic crust and/or enriched mantle after subduction metasomatism). Within this frame, we present here a new set of geochemical analyses of major and trace elements and Sr, Nd and Pb isotopes on samples collected by submersible on both intersections of the 15°20′N fracture zone with the spreading axis of the Mid-Atlantic Ridge. This area is characterized by an increasing enrichment of the lava compositions from north to south through the fracture zone. Results show that the geochemical enrichment observed with a different intensity on both sides of the fracture zone is linked to the 14°N topographic and geochemical anomaly. Our modeling shows that both trace element and isotopic compositions are consistent with a binary mixing between the regional depleted MORB mantle source and a recycled OIB/seamount, as previously proposed to explain the observed enrichment at 14°N. This model can also account for other enriched MORB i.e., the 18°-20°S region of the Central Indian Ridge, illustrating that it does not represent an isolated and local process. On the basis of our results and on the DMM isotopic evolution, the age of the recycled OIB/seamount is estimated to be ∼250 Ma, suggesting a recycling within the upper mantle. Considering the huge number of ocean islands and seamounts upon the ocean floor, their recycling into the upper mantle is a plausible process to produce enriched MORB

Heterogeneity in the mantle source of La Reunion Island

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

The 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

Evolution of the Saint Paul-Amsterdam Plateau in the last 10 m.y.

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Complex Dyke Emplacement at the Hyper-Inflated EPR 16°N Segment

International audienceThe EPR segment located between 15°22'N and 16°15'N (hereafter referred as EPR 16°N) presents a very wide (~13 km at the widest) and quite shallow (~2300 m) axial dome and is clearly over-inflated with respect to typical magmatically robust segments of the EPR. This segment is located at the intersection of the EPR with the Mathematician seamount chain, suggesting some kind of ridge-hotspot interaction which would account for the over-inflation. As a consequence of such an interaction, it has also been suggested that the ridge axis may episodically relocate further west to maintain the ridge-hotspot connection. Cruise Parisub of R/V L'Atalante took place in March-April 2010 and used Deep Sea Submersible Nautile and Autonomous Underwater Vehicle Aster-X to study this problem. The area was surveyed using L'Atalante new Simrad EM 122 echo-sounder, and a few sea-surface magnetic profiles were added to the existing data set. A higher resolution swath was achieved at a slower speed on the axial dome. AUV Aster-X completed a 30 x 4 km wide survey of the axial zone at an altitude of 70 m, collecting high resolution multibeam bathymetry and imagery, magnetics, and nephelometry. A total of 24 successful Nautile dives was carried out with videos and photos, rock sampling, and magnetic data collection, making an axial cross section up to 300 ka off-axis to investigate the inferred ridge jumps (10 Nautile dives), and a set of along axis dives to identify the active volcanic axis and search for active and fossil hydrothermal sites (13 Nautile dives). During these dives, several active and inactive hydrothermal vents have been discovered. In this presentation, we focus on the AUV survey. Unlike most fast spreading centres, which display one clear axial graben at the summit of the axial dome, the hyper-inflated dome at 16°N presents a complex set of several grabens, among which it is difficult to decipher which one are volcanically active and may be considered as the neovolcanic zone. To this end, we have used the magnetic data, adequately corrected for the effect of the AUV and reduced to total field anomaly. The resulting magnetic map shows a set on negative anomalies associated to some - but not all - grabens and help to discriminate between recently active and inactive grabens. Indeed, such negative anomalies have already been observed by Shah et al. (JGR, 2003) on the EPR at 17°28'S and 18°37'S and interpreted as marking the presence of hot dyke swarms, warmer than the Curie temperature, and the associated demagnetization of nearby basalt. Forward modelling by these authors suggests that such an anomaly can remain during about century. On the EPR 16°N, the presence of several anomalies suggests that, in some areas, several dyke swarms are "simultaneously" active (at the scale of a century). The intensity of these anomalies may reflect the size and/or the age of the dyke swarms, the stronger anomalies corresponding to the larger and/or the hottest (and therefore most recent) dykes