Bernard Marty

Helium isotopes and CO2 in volcanic gases of Japan

The isotope composition of He and its relationship to CO2 have been investigated in gases sampled on 12 active volcanoes of Japan. The highest 3He4He ratios reach the lower limit of the mid-ocean ridge basalt (MORB) range and have been recorded at Kusatsu-Shirane Volcano, which presented a phreatic activity during the sampling period, and at Satsuma-Iojima Volcano, where the highest temperature (786°C) was recorded. 3He4He variations (3.1·10−6–9.8·10−6) suggest that, although 3He comes from the upper mantle, part of 4He is released from the crust in the volcano area. Time elapsed after volcanic events and distance from volcanics centers appear the major factors controlling these variations. n nCO23He molar ratios (4.5·109–29·109) are systematically higher than the MORB average of 2·109. The apparent excess of CO2 (relative to 3He) cannot be entirely due to physical or chemical fractionation during the transfer of gases from the magmatic source to the surface. It is attributed to the effect of crustal outgassing or the contribution of a few percent of subducted sediments to the source of arc magmas. The last figure is in agreement with other geochemical tracers (Nd, Sr and pb isotopes, rare earth elements, 10Be, etc.). The estimated CO2 flux at arcs is in the order of (1–5)·1011 mol yr.−1 and represents <20% of the carbon flux at ridges. A mass balance of carbon at arcs shows that most sedimentary carbonates must be accreted rather than subducted. The flux of 3He at arc volcanoes represents <13% of the total volcanic 3He flux.

Helium isotope fluxes and groundwater ages in the Dogger Aquifer, Paris Basin

The Dogger aquifer of the Paris Basin has been shown to be first-order homogeneous in its helium concentration and its helium isotopic ratio, although slight differences are noted in relation to specific chemical water types. The helium isotopic composition of the Dogger aquifer contains a small component of mantle helium that is associated with a magmatic input, although the heat accompanying this magma is small. Constraints on the age of the overlying Albien aquifer are used to calculate the flux of helium out of the Dogger. This constraint allows a calculation of the residence time of waters in the Dogger aquifer of 4 Myr (+, - 50%), which is significantly longer than estimates derived from hydrologic models and hydrologic testing. It is possible that the hydrologic ages and the helium ages can be reconciled by a vertically layered Dogger aquifer with gradients in He, salinity, and fluid flow. Vertical profiles of fluid composition, helium concentration, and permeability are required in the Paris Basin to resolve this question.

Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt

Abstract Knowledge of the rare gas distribution between crystal and silicate melt is a key requisite when modelling the evolution of the Earth-atmosphere system. The concentrations and isotope compositions of He and Ar in glass and coexisting olivine in chemical equilibrium of a picritic MORB basalt from the Mid-Atlantic Ridge at 38°N have been measured. In an attempt to resolve components trapped in fluid inclusions from components dissolved in the glass or the olivine phases, gases were extracted from both phases by vacuum crushing and fusion. Since (1) olivine also contains glass inclusions and (2) the glass may be partially degassed, only upper limits of the partition coefficients between olivine and melt can be proposed. These upper limits (0.008 for He and 0.003 for Ar) indicate that rare gases behave as incompatible elements during magmatic processes. This validates assumptions about the efficiency of rare gas degassing from the solid Earth.

Continuous monitoring of distal gas emanations at Vulcano, southern Italy

The increasing activity of Vulcano Island (Italy) since 1985 led to the initiation of continuous geochemical monitoring of the lateral soil gas emissions. On the basis both of their relative geochemical characteristics and of local considerations, three gaseous components were selected for monitoring, namely CO2, He and 222Rn. Monitoring has been performed by means of specific analysers. Gases extracted from a water well located at the foot of the active cone were selected for monitoring, on the basis of their geochemical and isotopic characters that indicate their genetic link with central high temperature fumarolic gases emitted at the crater. Very strong variations of gas composition can be observed within one day (from 1 to about 94% for CO2). Some variations display a daily character and can be correlated with that of atmospheric pressure. The three monitored gases are highly correlated, suggesting very high kinetics of gas transfer in the system. Because of these considerable variations of chemical composition, bulk concentrations obviously are not suitable for monitoring at Vulcano. However, the evolution with time of ratios such as 222Rn/CO2 and He/CO2 (the latter being corrected for atmospheric contamination) supplies numerical parameters that the expected to characterize the intensity of the degassing process. A new input of magmatic gases, that would lead to an increase in the 222Rn/CO2 and He/CO2 ratios, should therefore be detected by such a monitoring station.

Nitrogen, helium and argon in basalt: a static mass spectrometry study☆

An analytical system for the simultaneous determination of nitrogen, helium and argon contents and 3He4He, 40Ar36Ar ratios in rocks by static mass spectrometry has been designed. Gases are extracted following either stepwise heating or vacuum crushing. The purification of nitrogen can be controlled by monitoring signals at masses 12, 14, 28 and 29. Helium and argon isotopic ratios can be analysed in the same fraction, allowing control on the distribution of mantle-derived and atmosphere-derived components. Concentrations of nitrogen down to few ppb N are routinely measured. The concentration and distribution of volatiles in a N-MORB sample from the East Pacific Rise having a low vesicularity of 0.18% have been investigated. The mean N2 content is (1.66 ± 0.49) · 10−8 mol g−1 (0.46 ± 0.14 ppm), associated with a mean, non-atmospheric 40Ar36Ar ratio of 8435 ± 2279. The N240Ar ratio of the vesicle fraction is 79, significantly lower than the bulk-rock value (N240Ar = 345), suggesting either contrasted N2ue5f8Ar solubilities, kinetic disequilibrium between N2 and Ar, or, more probably, selective addition of surface-derived nitrogen after vesiculation. This work illustrates the potential of a combined rare gas-nitrogen approach when observing mantle-derived samples with very low amounts of nitrogen.

Helium-3 and CO2 fluxes from subaerial volcanoes estimated from polonium-210 emissions

The volcanic flux of 210Po, a radiogenic and radioactive isotope belonging to the decay chain of 238U, has been quantified from its concentration in volcanic plumes and its mean residence time in the troposphere (Lambert et al., 1982). Measurements of 3He, CO2 and 210Po in the high-temperature gases of a few volcanoes allow estimates of 3He and CO2 subaerial fluxes to be made. Several factors affecting the 210Po/3He ratio in volcanic gases are discussed. The 3He flux from subaerial volcanoes, (1.0±0.5) × 102 mol/yr, is one order of magnitude lower than the MOR flux. The volcanic flux of CO2 into the atmosphere, (2±0.5) × 102 mol/yr, is comparable to the carbon flux from the mantle at mid-ocean ridges. We propose that most of carbon involved in subaerial volcanism is derived from the mobilisation of carbon present at the surface of the Earth.

Separation of noble gas mixtures from petroleum and their isotopic analysis by mass spectrometry

Abstract Considerable information on oil exploration can be gained from the isotopic measurement of noble gases dissolved in petroleum. However, this analysis is complicated by (i) incomplete degassing, due to the high solubility of noble gas in petroleum, and (ii) pollution of the analytical devices by organic compounds. Here, a specific facility for the separation of noble gases from petroleum is described. The separation is based on the release of the gases dissolved in petroleum into the headspace of an evacuated container (flashing). Purification of noble gases from reactive gases is performed by using CuO furnaces and Ti–Zr alloy getters.