Daniel Grimaud

Helium and carbon geochemistry of hydrothermal fluids from the North Fiji Basin spreading ridge (southwest Pacific)

The North Fiji Basin is one of the active marginal basins in the southwestern Pacific. Hydrothermal fluid emanations were located at two sites on the Central ridge of the basin. High-temperature fluids (230–290°C) venting from anhydrite chimneys at a 17°S site have end-member compositions of: 11.0–14.5 mmol/kg of CO2; 30.4–43.5 μmol/kg of CH4; and 2.3–4.5 × 10−5 cm3STP/g of He. Taking phase separation processes into account, the gas abundances are not enriched compared with the mid-oceanic ridge hydrothermal systems. Isotopic compositions of CO2 (δ13C = −6.2 to −5.7‰PDB), CH4 (δ13C = −20 to −18‰PDB), and helium (RRA = 9.0–10.0) are comparable to the mid-oceanic ridge signature. Together with basalt helium data, the helium isotopic signature may be attributed to the incorporation of a hotspot-like primitive component. Low-temperature shimmering fluids (the highest measured temperature was 5.2°C) associated with biological communities at a site at 18°50′S show slight chemical anomalies, in some species, in SiO2, Mn, Li, pH and CH4, and helium isotope ratios distinct from ambient seawater. Evaluated helium isotopic compositions prior to dilution fall between RRA = 8.4 and 8.9, supporting the suggestion of high-temperature hydrothermal activity at this site, although this was not observed by dive expeditions. The gas geochemistry of these two different types of fluids show several similar characteristics to the mid-oceanic ridge hydrothermal systems. This result is in accordance with previous petrological studies which demonstrated a dominant N-MORB source signature and a co-existing OIB source influence of the North Fiji Basin magmatism.

Modelling of the evolution of ground waters in a granite system at low temperature: the Stripa ground waters, Sweden

Abstract From chemical data collected by Nordstrom and his colleagues on Stripa ground waterswe have tried to model the evolution of the chemical composition of a ground water in a granitic system at low temperature. The existence of two end-member ground water compositions made it possible first, to test the conventional model of a geothermal system according to which an overall equilibrium between the waters and a given mineral assemblage can be defined, and then to show that such a model could be extended to low temperatures (10°C). Conversely, if we know the mineral assemblage, the equilibration temperature and the charge of the mobile ions (in this case, Cl), the composition of the solution is entirely fixed. In our model of the Stripa ground waters, the existence of two end-member ground water compositions can be explained by an evolution from a “kaolinite-albite-laumontite” equilibrium to a “prehnite-albite-laumontite” equilibium, the latter requiring less Al than the former. We have also emphasized the importance of the Cl ion concentrions ofof the ground waters, because they cab be considered as indicators of the degree of reaction progress between rock and water, thus determining the degree of equilibration of the system.

Chemistry of hydrothermal fluids from the 17°S active site on the North Fiji Basin Ridge (SW Pacific)

Abstract In June 1989 a diving cruise of the French submersible Nautile was made along the North Fiji Basin spreading ridge. The objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the Kaiyo cruise in 1988. Active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge. The transparent venting fluid showed significant salinity depletion. The extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the Cl-depleted hydrothermal fluid from the ashes vent site in the Juan de Fuca Ridge. These features suggest that the present hydrothermal end member resulted from subcritical phase separation, which generated a low-salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents.

Fluctuation of chemical compositions of the phase-separated hydrothermal fluid from the North Fiji Basin Ridge

Ishibashi, J., Grimaud, D., Nojiri, Y., Auzende, J.-M. and Urabe, T., 1994. Fluctuation of chemical compositions of the phase-separated hydrothermal fluid from the North Fiji Basin Ridge. In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STARMER French-Japanese Program. Mar. Geol., 116: 215-226. On the central ridge of the North Fiji Basin (southwest Pacific), hydrothermal activities were located in 1989. It was the second discovery of the hydrothermal fluid undergo phase separation. During a dive mission of Slrinkai 6500 in 1991, we revisited the hydrothermal area and collected fluid samples from the same anhydrite chimney. Fluid composition of the end member shows no significant changes during this two years. Other fluid ventings 150 m apart show similar chemistry, suggesting that the phase separation and segregation processes occur throughout the hydrothermal area. Small fluctuation in chemical composition of the fluids from two sites is attributed to subseafloor fluid-rock interaction after the phase segregation.

Chemical study of geothermal waters of Central Tibet (China)

Abstract Thirteen of about 300 areas of thermal springs in Tibet have been studied in detail. The springs fall into four groups, the most important and interesting of which produces silica, boron, lithium and cesium-rich waters. Chemical geothermometers agree within a few degrees in the range 200–220°C. A second group includes waters of the same type, but secondary reactions decrease the silica concentrations and increase the Ca and Mg contents. The third and fourth groups are similar to Pyrenean alkaline waters and to CO2-rich waters from Massif Central respectively; deep temperatures are low to moderate.

Hydrothermal 3He and manganese plumes at 19°29′S on the Central Indian Ridge

We report the discovery of a hydrothermal helium-3 plume north of the Indian Triple Junction, on the Central Indian Ridge at 19°29′S. Water column δ3He anomalies show a spike of 33.8% at 2460 meter depth, i.e. the highest δ3He anomaly recorded so far in the Indian ocean. The helium-3 data are correlated with manganese, giving a Mn/3He ratio of 700 mol/cm3 (3.8×104 g/cm3STP). The CH4/3He ratio deduced from available CH4/Mn data in the same area is around 7×106. These values are close to what is observed on water column plumes on the East Pacific Rise. On a regional scale, the 3He distribution over the Indian Triple Junction exhibits a strong δ3He increase at the Ridge crest, which appears to spread eastwards through the deep Indian ocean.

In situ geological and geochemical study of an active hydrothermal site on the North Fiji Basin ridge

Abstract In June 1989 a diving cruise by the submersibleNautile was carried out on the North Fiji Basin spreading ridge. The objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes. This operation was the third cruise of the French-Japanese Starmer project following the two cruises of the R.V.Kaiyo in 1987 and 1988. Six dives along the spreading axis between 16°58′S and 17°00S show that the axial graben consists of alternating N15—trending horsts and grabens. Extinct hydrothermal sites have been observed all along the graben. These consist of fossil chimneys, oxide staining and dead shells. At 16°59′S an active chimney has been discovered and called the “White Lady” because of its almost exclusively anhydrite composition. This chimney expels a peculiar water characterized by low chlorinity and a 285°C maximum temperature. In the northern extremity of the N15 axis a wide fossil hydrothermal site has been explored and sampled. It is located in an area cut by N15, N140 and N60-trending faults and fissures.

Composition isotopique du strontium d'eaux interstitielles extraites de sédiments récents: un argument en faveur de l'homogénéisation isotopique des minéraux argileux

The 87Sr86Sr ratio of interstitial waters squeezed from recent sediments of the Pacific Ocean increases from 0.70920 ± 0.00017 (2σ) to 0.70960 ± 0.0023 and 0.70984 ± 0.00040 at 1.50 and 1.60 m depth—reference value for sea water: 0.70910 ± 0.00035. This variation underlines the likely existence of exchanges between the clays and the interstitial environment. The isotopic homogenization of strontium between the clays and their environment, result of these changes, becomes thus a credible phenomenon. This is a new argument for the dating of clays by the RbSr method.

Influence of mobile ion concentrations on the chemical composition of geothermal waters in granitic areas. Example of hot springs from piemonte (Italy)

The six hot springs from Vinadio and the springs from Valdieri (Piemonte, Italy) have similar emergence temperatures (∼50°C), similar deep temperatures (∼115°C) and their chloride content varies from 0.9 to 30 mmol/kg. Major elements and some trace elements concentrations (Li, Rb, Cs, Sr, Ba, Mn) correlate closely with Na concentrations. The correlations in a log-log diagram are linear with a slope close to the electric charge of the ion. This is explained, for major elements, by an equilibrium between “a complete assemblage” of minerals (Michard, 1987) and a water containing varying amounts of chloride. It is suggested, from the Cl/Br ratio, that chloride originates by halite dissolution during the descent of the water.