Philippe Jean-Baptiste

Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14′N, MAR)

The Flores diving cruise was part of the MAST III-AMORES (1995–1998) program funded by the European Union. One of the major achievements of the Flores cruise was the discovery of the Rainbow hydrothermal field hosted in ultramafic rocks south of the Amar segment on the Mid-Atlantic ridge (MAR). The Rainbow hydrothermal fluids exhibit temperatures of 365 °C, pH of 2.8, high chlorinity (750 mmol/kg), and low silica (6.9 mmol/kg). The uniformity in endmember major, minor, trace element concentrations and gas contents suggests that all Rainbow fluids originate from the same deep source. Although H2S content is relatively low (1.20 mmol/kg), all vent fluids show extraordinary high H2 (16 mmol/kg), CH4 (2.5 mmol/kg) and CO (5 μmol/kg) endmember concentrations compared to fluids collected from other vent sites along the MAR. Hydrogen represents more than 40% of the total gas volume extracted from the fluids. At Rainbow, H2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization. Given that exposures of ultramafic rock may be common, particularly along slow-spreading ridges, the production of H2 may have important implications for microbial activity at and beneath the seafloor.

Compared geochemical signatures and the evolution of Menez Gwen (37°50′N) and Lucky Strike (37°17′N) hydrothermal fluids, south of the Azores Triple Junction on the Mid-Atlantic Ridge

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.

Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

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 Charlou and Donval 1993 , Charlou et al 1997 . 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.

Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy

We report the first detailed investigation of both helium and carbon isotopes in groundwaters and gases of Mt. Etna, providing new insight into the distribution, origin and budget of magmatic gas release at this very active volcano [1]. A mantle-derived magmatic component, with ultimate3He/4He ratio of 6.9 ± 0.2 Ra and δ13C of about −4‰, is identified in both types of fluids, depending on their location and the extent of their dilution by either air (gases) or a mixture of dissolved air and organic carbon (waters). Apart from the summit zone, this magmatic component is preferentially concentrated in CO2-rich groundwaters that issue from two remote sectors of the south-southwest and eastern volcano flanks, where its proportion increases with the altitude of meteoric recharge (or the length of pathflow) of the waters. Such a pattern suggests that, in addition to possible local gas input, the groundwaters collect much of their dissolved magmatic He and C while they infiltrate and flow through one of the more elevated, gas-effusing parts of the volcanic pile, among which the south-southeast fracture zone of Etna is the best candidate. These observations provide a new framework for remote geochemical monitoring of the volcano. The3He/4He ratio of the magmatic gas end-member coincides with that of helium trapped in the He-rich olivine crystals of Etna basalts (mean range: 6.7 ± 0.4 Ra, [2,3]), pointing to its negligible dilution by radiogenic He from the crustal basement and further constraining the3He/4He ratio of the present-day Etna magma. While being lower than the typical MORB value of 8 Ra, this ratio is the highest for an active volcano in continental Europe and probably tracks a relatively radiogenic upper mantle zone that is upwelling beneath this region [4]. The estimated outputs of mantle-derived CO2 and3He from Etna account for 10% and 15%, respectively, of estimates for global subaerial volcanic emissions. This huge contribution results from continuous degassing of mostly unerupted He- and C-rich alkaline basaltic magma, which occurs principally through the central open conduits and secondarily through the flanks of the volcano. Groundwaters carry only a minor fraction (≈ 3%) of total emitted CO2 and3He.

Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

-European Union, -Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica; -CNR–Gruppo Nazionale per la Vulcanologia.

Helium and methane measurements in hydrothermal fluids from the mid-Atlantic ridge: The Snake Pit site at 23°N

Methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the Snake Pit area (MAR, 23°N) as part of the Hydrosnake cruise with the submersible Nautile. The samples contain large quantities of helium and methane with endmember values of 4.6 × 10−5 cm3 STP/g for helium and 1.38 × 10−3 cm3 STP/g for methane. The helium isotopic ratio 3He/4He is 8.4 times the atmospheric ratio. These values are strikingly similar to those already obtained on the East Pacific Rise. The CH4/3He ratio is also quite similar (CH4/3He= 2.6 × 106) suggesting a non-biogenic origin of this gas in the fluids. Assuming that the range of variation of the 3He concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid-ocean ridges, the global mass flow involved in the hydrothermal circulation at the ridges axis is estimated between 1000 and 3000 m3 of water/sec, leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years. This estimate implies that, in addition to the individual vents (smokers), there is an extensive diffuse circulation of hydrothermal fluids along the ridges, in agreement with recent observations concerning heat budgets at hydrothermal sites in the Eastern Pacific.

Gases and helium isotopes in high temperature solutions sampled before and after ODP Leg 158 drilling at TAG Hydrothermal Field (26°N, MAR)

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, C02 and CH4 was noticeable in the 1995 samples. No significant difference was observed in helium concentration and helium isotopic ratio ( 3 He/ 4 He=8.2 +/-0.1 Ra). The CH4/ 3 He ratio found in black smokers sampled in 1993 is around 9 x 10 6 , close to ratio found in MAR basalts. In 1995, this ratio is around 4 times higher, due to the CH4 increase. The δ 13 C in C02, measured on two black smokers sampled in 1993, is uniform at -8.4 to - 8.8‰ (versus PDB), while a δ 13 C value of -13‰ is measured at a new site sampled in 1995, located 40 meters east of the main Black Smokers Complex (BSC). CH4/ 3 He and 13 C 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, C02 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.

Design and performance of a mass spectrometric facility for measuring helium isotopes in natural waters and for low-level tritium determination by the 3He ingrowth method

The design and performance of a mass spectrometric system for the measurement of helium isotopes and very low tritium concentrations in natural waters are described and discussed in the light of analytical precision and accuracy. The system consists of a VG 3000 mass spectrometer with a fully automated inlet system for preparation and purification of the samples. Along with this mass spectrometric system, different custom-fabricated units are described, especially designed for taking samples, extracting helium or degassing tritium samples prior to the mass spectrometric analysis. The 3He detection limit of the system is close to 10(-16) cm3 STP corresponding to a tritium level of 0.003 TU for a 500 g water sample stored six months for 3He regrowth. A vertical oceanic tritium profile from the south hemisphere is presented as an illustration of the systems capability to detect very low tritium concentrations in the environment.

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (∼100°C) aquifer of mixed marine–meteoric water. The measured 3He/4He ratios and δ13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO2 ratio, constrain a 3He/4He ratio of 7.3±0.1 Ra and a δ13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/4He ratio but higher than MORB CO2/3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.

Energy policy and climate change

The problem of massive emissions of carbon dioxide (CO2) from the burning of fossil fuels and their climatic impact have become major scientific and political issues. Future stabilization of the atmospheric CO2 content requires a drastic decrease of CO2 emissions worldwide. Energy savings and carbon sequestration, including CO2 capture/storage and enhancement of natural carbon sinks, can be highly beneficial, although it is suggested that both economic and climatic feedbacks could nullify part of the gains achieved. Fossil fuels (coupled with CO2 capture), and lower-carbon hydrogenated fuels such as natural gas are still expected to play an important role in the future. Nevertheless, stabilizing atmospheric CO2 concentration in a growing world economy, now dependent on fossil fuels for 85% of its energy, will also require a vast increase in the supply of carbon-free power. Among these energy sources, hydropower and nuclear energy (operated under western safety and environmental standards) are the most readily available sources capable of supplying vast amount of energy at a competitive price. Wind power is also to be encouraged, as it is expected to approach the competitiveness threshold soon. The French example, where fossil fuel CO2 emissions were cut by 27% in a matter of a few years (1979–1986) despite increasing energy consumption, suggests that implementing CO2 stabilization is technically feasible at a competitive price.