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

Large Mantle Heterogeneities in H2O and deltaD Below the Southwest Indian Ridge (35-70° E)

No abstract available

Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures : Implications for volatiles from igneous rocks and minerals

The extraction of water from igneous rocks and minerals is classically achieved by induction heating of a platinum alloy crucible where the sample has been deposited. Here, we show that chemical interaction between water and Pt–10%Rh crucibles occurs at high temperature. Known amounts of water were reacted with a Pt crucible held at high temperatures (900–1300 °C) for 5–10 min and then recovered. The experiments show that on average 20% of the water was lost to the crucible during the reaction, and that the isotopic composition of the remaining water was shifted by up to 25‰. Only 20–50% of the lost water was recoverable by re-heating the crucible at 1300 °C. Repeated experiments using the same standard water on the crucible showed a decrease of the isotopic shift to only 2‰. This is compatible with a memory effect of the Pt–10%Rh crucible. We propose that a large amount (at least several tens of μmol) of water remains trapped in the crucible and that partial isotopic exchange between trapped and introduced water affects subsequent isotopic composition of injected water. We conclude that the use of Pt alloys, as crucibles or foils, to extract water from rocks or minerals should be avoided. The interaction highlighted in this study shed light on previously inconsistent observations made on several mantle-derived samples. Fourteen basaltic samples were also re-analyzed without using a metal crucible and show isotopic compositions of hydrogen enriched by 10–20‰ in deuterium relative to previous studies. The isotopic composition of the upper mantle is closer to −60‰, rather than −80‰ as postulated earlier

Physical characteristics and triggering mechanisms of the 2009–2010 landslide crisis at Montagne Pelée volcano, Martinique: implication for erosional processes and debris-flow hazards

International audienceAbstract Flank destabilizations are common processes in the life of a volcano. Apart from giant landslides with recurrence times of tens of thousands of years and whose deposits are identified in the bathymetry around the islands, less voluminous but more frequent erosional landslides contribute significantly to the morphological development of the topography. In this paper, we present a detailed description of a landslide sequence that occurred in 2009 and 2010 on the western flank of Montagne Pelée volcano, originating at the Samperre cliff. This sequence is characterized by two main events, in August 2009 and May 2010, and hundreds of smaller collapses. From seismic data and high resolution topographic data from airborne Lidar, collapses are counted and volumes of the main events are estimated. The May 2010 landslide has removed 2.1 millions of m3 of debris, which were subsequently remobilized during several hazardous lahars. The mean rates of erosion deduced from these volumes indicate that this kind of erosional landslide could represent a long term contribution of the same order of magnitude as giant flank collapses. The characterization of the runout of the landslides and of the Samperre cliff slopes provide important information for risk assessment, in particular for the risk of lahars that threaten the population living on the lower slopes

Carbon and hydrogen isotope constraints on degassing of CO<SUB>2</SUB> and H<SUB>2</SUB>O in submarine lavas from the Pitcairn hotspot (South Pacific)

International audienceThe analysis of CO2-δ13C-H2O-δD in vesicles of 35 submarine volcanic glasses reveals that open-system degassing, following closed-system degassing, is responsible for the observed variations in volatile concentration and isotopic ratios. The least degassed samples allow the assessment of minimum estimates for the flux of CO2 and H2O from the Pitcairn hotspot: 2.24 +/- 0.17 104 tons C yr-1 and 1.32 +/- 0.82 105 tons H2O yr-1. The results also suggest that the carbon in the source of the Pitcairn hotspot may have a δ13C value around -6‰, 2‰ lower than the source of mid-ocean ridge basalts. This indicates either a possible contribution from the lower mantle or the possible involvement of decarbonated recycled materials with an unusual carbon isotopic signature

DIC concentration and

International audienc

ERDA of hydrogen content in hydrous and nominally anhydrous mantle phases

International audienc

Hydrogen partitioning between melt, clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H2O

International audienceAbstract To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a midocean ridge basalt (MORB) + 6 wt.% H2O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H2O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H2O is between 1,300 and 1,325C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with [7 wt.% H2O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H2O. DH cpx-gt is 11 ± 3, DH cpx-melt is 0.023 ± 0.005 and DH gt-melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of DH gt-melt, but DH gt-melt increases with TiO2 in the garnet. As also found by previous studies, values of DH cpx-melt increase with Al2O3 of the crystal. For garnet pyroxenite, estimated values of DH pyroxenite-melt decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H2O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H2O in their pyroxenes