A. Dapoigny

Université de Bretagne Occidentale, Brest, Brittany, France

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Publications (21)32.33 Total impact

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    ABSTRACT: The (3)He ingrowth technique is based on the detection of the tritium radioactive daughter, (3)He, by mass spectrometry. Over the last three decades it has been used extensively in oceanography and groundwater studies for measurements of very low to ultra-low levels of tritium. To compare it with the best available methods of radioactive counting, we applied this method to measure the massic activity of a tritiated-water primary standard prepared by the French Laboratoire National Henri Becquerel (LNHB). One liter of a low-level tritium solution was prepared from a small aliquot of the LNHB standard by two-step gravimetric dilution with tritium-free groundwater. Sixteen samples of this solution were analyzed at the LSCE-Saclay noble gas facility, using the (3)He ingrowth method. The massic activity of the prepared solution was also measured by Liquid Scintillation Counting using the Quantulus LNHB counter and an internal calibration method with the LNHB tritiated water standard. All results agree within their standard uncertainty.
    Applied radiation and isotopes: including data, instrumentation and methods for use in agriculture, industry and medicine 12/2013; · 1.09 Impact Factor
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    ABSTRACT: a b s t r a c t Following the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contam-inated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer 3 H/ 3 He method (t 1/2 = 12.3 a) and anthropogenic tracers including CFC and SF 6 are investigated along with the water mass natural tracers Na, Cl, 18 O and 2 H. The groundwater is stratified, as evidenced by Na and Cl concentrations and stable isotopes (18 O, 2 H). In the upper aeolian layer, the Na–Cl relationship corresponds to evapotranspiration of precipitation, while in the underlying alluvial layer, an increase in Na and Cl with depth suggests both water–rock interac-tions and mixing processes. The 3 H/ 3 He and CFC apparent groundwater ages increase with depth, ranging from 'recent' (1–3 a) at a 2 m depth below the groundwater table to much higher apparent ages of 50–60 a at 27 m depth below the groundwater table. Discrepancies in 3 H/ 3 He and CFC apparent ages (20–25 a and 3–10 a, respectively) were observed during the 2008 campaign at an intermediate depth immediately below the aeolian/alluvial sand limit, which were attributed to the complex water transfer processes. Extremely high SF 6 concentrations, well above equilibrium with the atmosphere and up to 1112 pptv, are attributed to significant contamination of the soils following the nuclear reactor explosion in 1986. The SF 6 concentration vs. the apparent groundwater ages agrees with this interpretation, as the high SF 6 concentrations are all more recent than 1985. The persistence of the SF 6 concentration suggests that SF 6 was introduced in the soil atmosphere and slowly integrated in the groundwater moving along the hydraulic gradient. The apparent age distribution in the lumped parameter models suggests an exponential or piston flow model in the upper geological section, followed by more pronounced mixing processes in the lower section. Ó 2012 Published by Elsevier Ltd.
    Applied Geochemistry 07/2012; 27(7):1304-1319. · 1.71 Impact Factor
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    ABSTRACT: The 140-m-thick, clay-rich Callovo-Oxfordian (COX) layer of the eastern Paris Basin, France, is being considered by the French Nuclear Waste Agency (Andra) as a long-term underground nuclear waste repository. Andra has selected a 250 km2 area (transposition zone) to be further characterised, especially in view of the confinement properties exhibited by the COX. This study reports the helium concentrations and isotopic ratios of water samples from the aquifers above and below the COX, which are the Oxfordian and Dogger aquifers, respectively. The samples were collected from five drilling sites (2007–2008). Both the Oxfordian and Dogger groundwaters are of meteoric origin and have accumulated radiogenic He with a 3He/4He ratio of approximately 0.02 Ra. The He concentrations in the groundwaters are two orders of magnitude higher than in the air-saturated water in the Oxfordian and approximately 10 times higher in the Dogger than in the Oxfordian. One borehole was drilled down into the Triassic sediments, allowing a sample to be collected from the Bundsandstein aquifer. Here, the He concentration is of the same order of magnitude as those measured in the Dogger, but the isotopic ratio is slightly higher at 0.04 Ra. However, this ratio is approximately 10 times lower than those measured by Marty et al. (2003) in waters collected closer to the Trias recharge and tagged with mantle-derived He. This lower ratio is most likely due to a significant and rapid slowdown of the circulation in this aquifer, allowing substantial radiogenic He accumulation. A key conclusion of this study concerns the lateral non-homogeneity of the studied area: He concentrations are higher in the northern part of the transposition zone. This observation can be explained by a longer residence time of the waters and/or a higher input of He from the basement. A simplified 2D model of He transport shows that the second hypothesis alone does not allow fitting of the data, thus implying more stagnant water in this area. The indicative residence times derived from this coarse model are 0.3–0.5 Myr (Oxfordian) and 0.5–0.6 Myr (Dogger) for the borehole located in the centre of the investigated zone; for the northernmost borehole, they reach 0.8–1.4 Myr for the Oxfordian and 1.5 Myr for the Dogger.Highlights► He geochemistry in aquifers below and above the COX clay-rich layer, eastern France. ► This COX layer is considered for a long-term underground nuclear waste repository. ► Lateral non-homogeneity of He concentrations across the 250 km2 study area is observed. ► Indicative residence times are derived from a simplified 2D He transport model.
    Physics and Chemistry of the Earth Parts A/B/C 10/2011; 36:1511-1520. · 1.04 Impact Factor
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    ABSTRACT: The article presents studies on estimation of age of groundwater from the unconfined aquifer in sandy Quaternary deposits at the international radioecological experimental site at Chernobyl nuclear power plant site using “tritium – helium-3” (3H-3He) method, and interpretation of the obtained data using groundwater flow modeling. A good agreement in isotope dating estimates and groundwater modeling data was obtained. Results of these studies suggest that the long-term (during last 60 years) infiltration recharge rate at the first terrace of the Pripyat River constituted 200 mm/y. Successful application of 3H-3He method creates preconditions for its more wide application for studies of water exchange and radionuclide migration in groundwater at Chernobyl site.
    Geologichnyi Zhurnal (Geological Journal, Kiev; ISSN 0367-4290). 12/2010; 2010(4):119-124.
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    ABSTRACT: The design, setup and performance of a mass spectrometric system for the analysis of low to very low-level tritium in environmental samples are described. The tritium concentration is measured indirectly by the (3)He ingrowth from radioactive decay after complete initial degassing of the sample. The analytical system is fully computer-controlled and consists in a commercial helium isotope mass spectrometer coupled with a high vacuum inlet system. A detection limit of 0.15 Bq/kg is routinely obtainable for sample sizes of 20g of water equivalent and an accumulation time of three months. Larger samples (and/or longer accumulation time) can be used to obtain lower detection limits. In addition to the benefit of a lower detection limit, another advantage of this non-destructive method lies in the simplicity of the analytical procedure which strongly limits the risk of contamination. An inter-comparison was successfully performed with the conventional beta counting technique on lyophilized grass samples, in a range of tritium concentrations of environmental interest. It shows that the (3)He mass spectrometry method yields results that are fully consistent with the conventional liquid scintillation technique over a wide range of tritium concentrations.
    Journal of Environmental Radioactivity 11/2009; 101(2):185-90. · 3.67 Impact Factor
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    ABSTRACT: As part of a multidisciplinary project aimed at studying mid-ocean ridge processes near the Azores, fifty water column profiles were analyzed for 3He/4He ratios in dissolved helium (a well-known hydrothermal tracer) from 36°N to 40°N along the Mid-Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large delta 3He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3He plume north of the Açor Fracture Zone (north AFZ site), with a CH4/3He ratio indicative of a basaltic-hosted hydrothermal system. Clear 3He and CH4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 +/- 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26°N).
    Geochemistry Geophysics Geosystems 01/2008; 90(3). · 2.94 Impact Factor
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    ABSTRACT: As part of a multidisciplinary project aimed at studying mid-ocean ridge processes near the Azores, fifty water column profiles were analyzed for He-3/He-4 ratios in dissolved helium (a well-known hydrothermal tracer) from 36 degrees N to 40 degrees N along the Mid-Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large delta(3) He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3 He plume north of the Acor Fracture Zone (north AFZ site), with a CH4/He-3 ratio indicative of a basaltic-hosted hydrothermal system. Clear He-3 and CH4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3 He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 +/- 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26 degrees N).
    Geochemistry Geophysics Geosystems - G3 (1525-2027) (American Geophysical Union), 2008-03 , Vol. 9 , N. 3 , P. NIL_33-NIL_45. 01/2008;
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    ABSTRACT: The Creys-Malville nuclear plant, located on the left bank of the Rhône, was shut down in 1998. The facilities are currently in their initial stage of dismantling. In order to establish a baseline for tritium in the vicinity of the site prior to the main dismantling phase, we carried out a monitoring program between 2002 and 2005 in the main terrestrial and aquatic compartments of the local environment. Tritium levels in the groundwaters and in the Rhône waters correspond to the regional tritium concentration in precipitation. The data obtained for the terrestrial environment are also in good agreement with the regional background and do not show any specific signature linked to the nuclear plant. The various aquatic compartments of the Rhône (fish, plant, sediment) are significantly enriched in tritium both upstream and downstream of the power plant: although Tissue-Free Water Tritium concentrations are in equilibrium with the river water, the non-exchangeable fraction of organic bound tritium in plants and fishes shows values which outpace the river water background by one to two orders of magnitude, and up to four to five orders of magnitude in the sediments. This tritium anomaly is not related to the nuclear plant, as it is already present at the Swiss border 100km upstream of the site. Although fine particles of tritiated polystyrene entering the composition of the luminous paints used by the clock industry have been suspected on several occasions, the exact nature and the origin of this tritium source remain unknown and require further investigations.
    Journal of Environmental Radioactivity 02/2007; 94(2):107-18. · 2.12 Impact Factor
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    ABSTRACT: Noble gases have been used successfully for several decades in studies of groundwater dynamics and paleoclimate (Kipfer et al. 2002). However, in spite of the likelihood that sediment pore waters may provide high resolution noble gas archives of pale-oenvironmental and paleoceanographic changes, they have not been analyzed widely for this information because of difficulties in collecting samples without gas loss and/or contamination problems. For example, severe gas loss was indicated in an early attempt to measure noble gas concentrations in pore fluids of some Deep-Sea Drilling Project (DSDP) sediment cores that were squeezed after recovery (Broecker 1973), yet the presence of excess helium could be qualitatively demonstrated (Clarke et al. 1973). Sano et al. (1992) developed an alternative sampling technique to withdraw and collect gases through core liners using a glass syringe with a stainless steel needle, but they obtained only reliable isotopic and elemental ratios (3 He/ 4 He and 4 He/ 20 Ne) without quantitative concentration data. Alternatively, in-situ sampling also was explored. During the DSDP, Barnes developed tools designed to sample gases in sedimentary pore fluids through a probe extending ahead of the drilling bit (Barnes 1973, 1988). Torres et al. (1995) used an improved version of this tool, known as the Water Sampler-Temperature-Pressure (WSTP) and concluded that gas bubbles significantly affected measured noble gas concentrations. Moreover, this equipment was rather complex to operate, could be deployed only on drilling programs, and was able to recover only one sample per cast. To study dissolved helium in lake sediment pore-waters, Stephenson et al. (1994) improved the in-situ probe by using ping-pong balls (Dyck and Da Silva 1981) as semi-permeable gas samplers that equilibrate with the surrounding environ-ment. However, the system needed to be left in the sediment for at least 4 d to get a complete equilibration between helium in the ball and in the pore-water. Moreover, the probe was not suitable for water depths exceeding 55-m because the balls could not withstand greater hydrostatic pressures. It is only recently that ship-based sampling has been revis-ited. Brennwald et al. (2003) designed a new method for the quantitative sampling and extraction of dissolved noble gases from sediment cores. Bulk sediment was transferred by squeez- Abstract Owing to their inertness and contrasted composition in the various earth reservoirs, helium isotopes are powerful tracers of a number of processes pertaining to geophysics and geochemistry. Because sediments cover a large portion of the earth's surface, helium isotope geochemistry of sediment pore-waters is of particular interest. In spite of this potential, its development has been hampered by the difficulty of collecting samples without gas loss and/or contamination problems. We developed a new method for the sampling and the quantitative extraction of dissolved helium from sediment pore-waters, leading to the determination of 3 He and 4 He con-centration profiles. Core sampling is non-destructive (no squeezing). The principle of the method is to use stan-dard copper tubes (1.2 cm OD/25 cm in length), subsequently sealed with clamps, to take mini-cores along the sediment core immediately following its retrieval. In the lab, the sediment is transferred from the copper tube to a noble gas extraction line by applying pressurized helium-free water at one end of the copper tube. This tech-nique allows dissolved helium to be recovered and analyzed using standard procedures for water samples. Tests were carried out successfully on an artificial core equilibrated with air to check the extraction efficiency in the same conditions as for real cores. The validity of the method was further confirmed by acquiring a vertical heli-um profile from a real marine core from the Zaire deep-sea fan, illustrating some possible applications. Elise Fourré, LSCE/Orme des merisiers, 91191 Gif-sur-Yvette cedex, France (elise.fourre@cea.fr).
    01/2007;
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    ABSTRACT: Tritium concentration was measured in snow deposited at the GRIP site (central Greenland) and at the Vostok station (east Antarctica) from snow pits covering the period 1980–1990. The objective of the study was to investigate tritium concentrations in polar regions several decades after the bomb peak of the sixties and to put them in the context of available data for environmental tritium in the Arctic and the Antarctic over the last five decades. The tritium content of the samples was measured by mass spectrometry using the helium-3 regrowth method. In Antarctica, the tritium concentrations are in the range 70–110 TU. The comparison of the bomb tritium history at different locations show that tritium levels increase moving inland, where vapour pressure becomes extremely low and therefore more sensitive to the intrusion of stratospheric air masses highly enriched in tritium. Although most tritium fallout occurred in the Northern hemisphere, the tritium levels in central Greenland in the 80's, in the range 10–40 TU, are significantly lower than at Vostok. Unlike Antarctica, no such continental effect is observed in Greenland, due to the higher water vapour content of the air masses, as evidenced by the much higher snow accumulation rate. Whereas tritium fallout in Antarctica appears to occur as a result of direct injections of stratospheric tritium during winter, Arctic fallout are the result of the dominant spring injection of stratospheric air at mid-latitude, in line with the deposition of other stratospheric tracers.
    Earth and Planetary Science Letters. 01/2006;
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    ABSTRACT: The results of an environmental survey of tritium in the vicinity of Creys-Malville nuclear plant are presented. The plant, which was shut down in 1998, is currently in its initial stage of dismantling. Measurements in the terrestrial environment do not provide any indication that tritium levels are significantly above the regional background. Tritium concentrations in groundwaters and in the river Rhône waters are at their background level too, with values in the range 0.94-1.64 Bq/l. In contrast, the various aquatic compartments of the river (fishes, plants, sediments) show enriched non-exchangeable bound tritium concentrations both upstream and downstream of the power plant, with values up to 14.7, 49 and 1495 Bq/kg (dry weight) respectively. Although fine particles of tritiated compounds entering in the composition of luminous paints have been suspected on several occasions, the nature and the origin of this tritium source, which is not related to the nuclear plant, remain unknown and will require further investigations.
    http://dx.doi.org/10.1051/radiopro:2005s1-112. 01/2005;
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    ABSTRACT: During the DIVA 1 cruise in May 1994, a series of 19 dives was conducted using the French submersible Nautile at the topographic highs of three volcanic segments centered at 37°17′N, 37°50′N and 38°20′N, respectively, south of the Azores Triple Junction (ATJ) on the Mid-Atlantic Ridge (MAR). Hydrothermal fluids were collected at the Lucky Strike (LS) hydrothermal field, discovered at 37°17′N in 1993, and at a new hydrothermal site called Menez Gwen (MGw) discovered at 37°50′N during this cruise. Both systems are relatively shallow compared to other MAR systems with seafloor depths of 1700 and 850 m, respectively, characteristics which make them unique among the already known sites on the MAR. The characteristics of the LS fluids are the same as in 1993 and include temperatures ranging from 170°C to 324°C, variable chlorinities lower than seawater, low hydrogen sulfide (<3.0 mmol/kg), high Ba concentrations (up to 80 μmol/kg), low metal concentrations and high gas contents, and distinct chemical end-members indicative of significant geographic control of the venting system. In contrast, the very clear MGw fluids at 37°50′N show a rather uniform exit temperature (285°C) and chemical compositions with chlorinities (360–380 mmol/kg) lower than at LS and corresponding lower concentrations of cations, H2S (1.8 mmol/kg), metals and silica (8–11.5 mmol/kg), due to the lower temperature (T) and pressure (P) of the system. In the two systems, fluid chemistry is strongly affected by phase separation. At the low-pressure conditions of these sites, phase relations in the NaCl–H2O system dictate production of an extremely low salinity. The higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single-phase hydrothermal seawater is the dominant process controlling Cl concentrations. Depletions and enrichments of elements in solutions are also explained by the involvement of fluid-rock reactions. The enrichment of Ba, K, Cs and Rb in the fluids is linked to the enriched character of these elements in basalts. Relatively high pH and Ca, low Li, Sr and metals are related to the highly altered basaltic substrate in the reaction zone. Low Sr/Ca and high Ca/Na ratios are explained by albitization within the crust. The increase of degassing when approaching the Azores may be related to the carbon-enriched basaltic crust near the Azores hot spot. Isotopic ratios show that CO2, CH4 and helium gases have a magmatic signature. Compared to other deeper sites on the MAR, all fluids collected at LS and MGw are gas-enriched, except for H2S and helium, whose both concentration and isotopic ratio compare well with other hydrothermal fluids. The high CH4 concentration associated with unsaturated hydrocarbons and the high CH4/3He ratios also suggest a contribution of CH4 generated by serpentinization of ultramafic rocks by Fischer–Tropsch catalysis of CO2 reduction.
    Chemical Geology 01/2000; 171(1). · 3.15 Impact Factor
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    ABSTRACT: Given a multigraph G = (V, E), the Edge Coloring Problem (ECP) calls for the minimum number  of colors needed to color the edges in E so that all edges incident with a common node are assigned different colors. The best known polynomial time approximation algorithms for ECP belong to a same family, which is likely to contain, for each positive integer k, an algorithm which uses at most ?((2k + 1) + (2k - 2))/2k? colors. For k = Keywords: Approximation algorithm; Edge coloring; Matching Document Type: Short Communication DOI: http://dx.doi.org/10.1016/S0020-0190(98)00138-0 Affiliations: DEIS, Universita di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Publication date: October 15, 1998 $(document).ready(function() { var shortdescription = $(".originaldescription").text().replace(/\\&/g, '&').replace(/\\, '<').replace(/\\>/g, '>').replace(/\\t/g, ' ').replace(/\\n/g, ''); if (shortdescription.length > 350){ shortdescription = "" + shortdescription.substring(0,250) + "... more"; } $(".descriptionitem").prepend(shortdescription); $(".shortdescription a").click(function() { $(".shortdescription").hide(); $(".originaldescription").slideDown(); return false; }); }); Related content In this: publication By this: publisher In this Subject: Computer Science , Library Science By this author: Charlou, J.L. ; Fouquet, Y. ; Bougault, H. ; Donval, J.P. ; Etoubleau, J. ; Jean-Baptiste, P. ; Dapoigny, A. ; Appriou, P. ; Rona, P.A. ; Costa, V.A.F. ; Caprara, A. ; Rizzi, R. GA_googleFillSlot("Horizontal_banner_bottom");
    Information Processing Letters 01/1998; 68(1). · 0.49 Impact Factor
  • International Journal of Heat and Mass Transfer 01/1998; 42(1). · 2.52 Impact Factor
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    ABSTRACT: As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH4 concentration is among the highest found along the Mid-Atlantic Ridge and . CH4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH4 concentrations correspond to only weak 3He anomalies. This CH4-rich plume is also associated with active fault zones that expose peridotite rocks. CH4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.
    Geochimica et Cosmochimica Acta 01/1998; · 3.88 Impact Factor
  • Geochimica et Cosmochimica Acta 01/1998; 62(13). · 3.88 Impact Factor
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    ABSTRACT:  Hydrothermal vent fields south of the Garret Fracture zone were sampled for the isotope composition of helium and oxygen ([18O]H2O/[16OH2O). The helium isotopes end-member (3He / 4He=8.3×R a and [4He]≈1.2–2.4×10-5 cm3 STP g-1) is quite similar to other known hydrothermal sites pointing to the homogeneous helium composition of the upper mantle. The δ18O end-member value (δ18O≈0.5–0.6‰) confirms previous suggestions from other sites and from isotope modeling, that hydrothermal fluids are slightly enriched in 18O relative to the ocean as a result of water–rock interactions at high temperature.
    Geo-Marine Letters 07/1997; 17(3):213-219. · 1.85 Impact Factor
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    ABSTRACT: Simultaneous acoustic imaging of the seafloor and detection of particle-rich plumes in the overlying water column have been used to identify and determine the tectonic setting of high-temperature ‘black smoker’ hydrothermal activity along 200 km of the Mid-Atlantic Ridge between 36° and 38°N. Using this approach, we have identified hydrothermal signals at 7 different locations. These results indicate a higher incidence of hydrothermal activity along this section of ridge axis than has been reported elsewhere along slow spreading ridges.Our data show that the majority of hydrothermal sites here are located near to non-transform offsets rather than at the centres of individual ridge segments. We suggest that this intersection of fabrics, associated with ridge discontinuities and the spreading process, is instrumental in focussing hydrothermal flow at these localities. Future strategies of exploration for hydrothermal activity on slow-spreading ridges may need to be revised accordingly.
    Earth and Planetary Science Letters. 01/1996;
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    ABSTRACT: Hydrothermal fluids were sampled for dissolved gases at TAG (26°N-MAR) during two Alvin dive series, in April-May 1993 and March 1995, respectively 17 months before and 4 months after Ocean Drilling Program Leg 158. Total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude, even if some increase in H2S, CO2 and CH4 was noticeable in the 1995 samples. No significant difference was observed in helium concentration and helium isotopic ratio (3He/4He=8.2 +/-0.1 Ra). The CH4/3He ratio found in black smokers sampled in 1993 is around 9×106, close to ratio found in MAR basalts. In 1995, this ratio is around 4 times higher, due to the CH4 increase. The delta13C in CO2, measured on two black smokers sampled in 1993, is uniform at -8.4 to -8.8% (versus PDB), while a delta13C value of -13% is measured at a new site sampled in 1995, located 40 meters east of the main Black Smokers Complex (BSC). CH4/3He and 13C values both point to a mainly magmatic (abiogenic) origin of the carbon species in the system. Overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium, CO2 and CH4 of magmatic origin, even if some thermogenic CH4 contribution seems likely in the 1995 samples collected in the new active area. We can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the TAG mound and temporarily enhanced the hydrothermal circulation, by opening new pathways favouring transient input of CH4 rich fluids, before coming back to a new steady state.
    Geophysical Research Letters 01/1996; 23:3491-3494. · 3.98 Impact Factor
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    01/1996;