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

Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot and arc magmas

International audienceCarbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2 solubility in silicate melts, however, CO2 is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as δ13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2 in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in δ13C-values. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system.In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings.Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2 reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2+H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas.This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2 contents can be reasonably undertaken

Processus de dégazage et sources mantelliques dans les magmas de type MORB et OIB (le carbone, l'eau et les gaz rares : aspects chimiques et isotopiques)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

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

No abstract available

Concentration et composition isotopique en hydrogène du manteau terrestre

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

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

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ERDA of hydrogen content in hydrous and nominally anhydrous mantle phases

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