Claire Ramboz

Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: II. Interpretation of fluid inclusion data in terms of immiscibility

Abstract Phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility (in Part I). Following from that point, the major limits on the accuracy, and even the validity, of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed. The practical implications of the general constraints (temperature, pressure, topology of the fluid systems) are envisaged. Emphasis is laid on some important consequences such as: the use of isochore intercepts (and the possible resulting interpretation of fluid mixing rather than unmixing), the case of highly saline inclusions, the identification and interpretation of heterogeneous trapping. The composition and density constraints on coexisting fluids are presented, and illustrated by natural examples. Taking into account all the measurable parameters in fluid inclusions (volume, temperature and nature of phase transitions, more or less complete individual spectroscopic analyses), all the available experimental data, and all the theoretical constraints, may be long and difficult. However, it is most generally very informative and productive although part of these data is often sufficient to deny unmixing. Nevertheless, a final example on metamorphic fluids demonstrates how such an approach can “prove”; and also characterize a fluid unmixing during a geologic process.

Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: I. Phase equilibria analysis — A theoretical and geometrical approach

Abstract Many occurrences of fluid immiscibility in natural geologic systems have been reported recently, most often from fluid inclusion studies. However, the interpretation of fluid inclusion data in terms of immiscibility sometimes suffers from ambiguity of the vocabulary, insufficient knowledge of the immiscibility constraints and insufficient knowledge of the topology (in the TPX space) of natural fluid systems. For such reasons some authors have been misled to erroneous interpretations. The term “chemical immiscibility” is unambiguously redefined as a multiphase multicomponent equilibrium. The consequences of this definition are directly derived from the phase rule and concern the possible equations that relate the various parameters (temperature, pressure, volumes, compositions) to each other. These equations already put constraints on the topology of the phase equilibria in fluid systems. A particular expression of the phase rule is proposed, which takes into account the multiphase-constant bulk volume-constant bulk composition constraints in fluid inclusions. The consequences of such an expression are of major importance in fluid inclusion studies. The phase relations of some simple systems that approximate quite efficiently the natural complex fluids are then detailed: H2ONaCl, CO2CH4, H2OCO2, H2OCO2NaCl. The effects of these topologies and of the supplementary constraints (constant bulk composition and constant bulk volume) assumed for fluid inclusions (isopleth-isochoric systems) are discussed.

Phase separation and fluid mixing in subseafloor back arc hydrothermal systems: A microthermometric and oxygen isotope study of fluid inclusions in the barite‐sulfide chimneys of the Lau Basin

Fluid inclusions in barite and sulfide in chimneys (both active and inactive) from three hydrothermal sites of the back arc Lau basin were studied with microthermometric and isotopic methods to determine the chemistry and evolution of hydrothermal aqueous fluids. Strontium isotope compositions of sulfides from the Lau basin reflect the presence of anhydrite and barite inclusions. The 87Sr/86Sr ratios of these two sulfate minerals vary from 0.7045 to 0.7078 and are interpreted as the result of mixing between various proportions of the hydrothermal end-member and pure seawater. The microthermometric study of fluid inclusions reveals that mixing with seawater involved different kinds of aqueous fluid end-members. A high-temperature Mg-depleted end-member of high salinity (≥5.5 wt % eq NaCl) was found at the Vai Lili site. A uncommon low-temperature Mg-rich end-member was also identified at the Hine Hina site in association with barite deposition. At the Vai Lili site, very low salinity fluids were produced in addition to a very saline brine (≥30 wt % NaCl) that was trapped inside anhydrite precipitated from an active vent at a temperature of 342°C. Oxygen isotope ratios of water inclusions range from 2‰ to 4.4‰ for chalcopyrite, barite, and sphalerite minerals. The 18O enrichment and the high salinities of many fluids from the Lau basin are accounted for by the specificity of the back arc setting. This non mid-ocean ridge setting is characterized by the shallow depth of the hydrothermal systems that allows frequent unmixing of high-temperature liquids. The abundance of silicic magmas also provided magmatic fluids (including brines) that mixed with seawater-derived aqueous fluids.

The anhydrite saturation index of the ponded brines and sediment pore waters of the Red Sea deeps

Abstract We compiled the available chemical analyses of the ponded brines and of the sediment interstitial waters for the Atlantis II, Discovery, Suakin and Valdivia deeps in the Red Sea central valley, along with the DSDP data for interstitial waters of several cores in the rift axis (Sites 225–228 of Leg 23). Recent advances in the calculation of the thermodynamic properties of concentrated solutions at high temperatures and pressures allow the study of the equilibrium conditions between anhydrite and halite, and the brines. We first tested the influence of various parameters as temperature, pressure and solution composition on the calculated anhydrite saturation index in order to provide a clear criterion for equilibrium, which is assumed when the saturation index lies within 0.9 and 1.1. We found that at 62°C and 200 bar, the pressure effect on the saturation indices is of the same magnitude as that of temperature. Our calculations point to the overall undersaturation of the free brines with respect to halite. The calculated halite saturation state of the pore waters at the four DSDP sites is consistent with the mineralogical description of the sediments in which halite is not reported, except at Site 225 where the calculated undersaturation is in contradiction with the reported presence of halite at the base of the core. From the hypothesis of equilibrium between anhydrite and the pore waters in the DSDP cores, we have estimated values of the geothermal gradients off the rift axis which fall within the highest values obtained by extrapolation of direct temperature measurements. This suggests that, at the latitude of the Atlantis II and Suakin deeps, the high heat fluxes measured along the axial trough extend off the rift axis. Our calculations show that the Upper Convective Layer (UCL) of the Atlantis II hydrothermal system has always been undersaturated over the studied period (1965–1985), which is consistent with the absence of anhydrite in sediment underlying this brine. The calcium and sulfate contents of the Lower Convective Layer (LCL) show a parallel evolution: they decrease when temperature increases. Although this behaviour suggests control of brine chemistry by equilibrium with anhydrite, our calculations show that anhydrite has reached saturation in the lower brine only in 1966 and 1976. The general regime leading to the loss of calcium and sulfate outside these periods is tentatively attributed to diffusion. Calculations show that, throughout the whole stratified brine column (from the sediment to normal Red Sea water), there is no chemical potential gradient for calcium sulfate whereas the chemical potential of sodium chloride regularly decreases. We tentatively advocate the coupling of calcium sulfate diffusion with the large sodium chloride gradient to account for the loss in Ca and SO 4 . Finally, the pore water chemistry of the Atlantis II and Discovery Deep sediments record two parallel evolutions from anhydrite undersaturation in 1966 to saturation (or a state close to) in 1976. These data are consistent with a periodical brine overspill from Atlantis II into Discovery.

Jabali, a Zn-Pb-(Ag) carbonate-hosted deposit associated with Late Jurassic rifting in Yemen

The Jabali deposit (3.8 Mt at 16% Zn, 2% Pb and 132 g/t Ag) is hosted by dolomitized platform carbonates of Kimmeridgian age at the southwestern edge of the oil-producing Wadi al Jawf rift basin in northern Yemen. Paleogeographical reconstructions demonstrate that tensional synsedimentary tectonic activity from the Late Jurassic to Early Cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsident rift basin, the unstable margin of which was the site of rapid facies changes, local disconformities and periods of emergence, as well as of dolomitization along the WNW- and NNW-striking boundary fault system. In the Jabali area, the upper part of the Jurassic sequence underwent two stages of dolomitization before emergence and deep karstic erosion. Solution cavities and depressions in the eroded surface were filled by dolomite sand and black pyritic mudstone prior to a last marine transgression of limited extent. Subsequent ore deposition and associated late dolomitization sealed the network of solution cavities, impregnating the dolomite sands and the host dolomites. Sphalerite I and wurtzite, followed by silver-bearing zoned sphalerite II associated with galena, crystallized from a cyclic influx of low-temperature (75-100°C) saline solutions. Lead isotope geochemistry indicates that the lead, zinc and silver probably originated from an Early Proterozoic basement. The dissolved metals were likely derived from the basal aquifer (detrital material of basement origin) of the evaporite-bearing sequence filling the Wadi al Jawf trough. Migrating metalliferous brines from the basin to the uplifted Jabali area, where ore deposition was favoured by a reducing environment, were probably channelled by the boundary fault system during the last stages of synsedimentary tectonic activity.

Surface blistering and flaking of sintered uranium dioxide samples under high dose gas implantation and annealing

High helium contents will be generated within minor actinide doped uranium dioxide blankets which could be used in fourth generation reactors. In this framework, it is essential to improve our understanding of the type of damage which a pellet could incur as a result of extensive helium build-up. This paper is an attempt at tackling this issue. Sintered uranium dioxide disks have been implanted with helium ions then annealed at various temperatures. Above a concentration of 0.4 at.% and above 1000°C, optical images of the sample surface revealed swollen grains and extensive areas which have exfoliated. Nuclear reaction microanalyses and atomic force microscopy observations were performed to demonstrate that helium has substantially precipitated within the swollen grains. Massive precipitation of the gas leads under these conditions to sample surface blistering which appears to precede flaking. Deuterium ion irradiations have also been performed at ambient and a direct flaking of the sample surface was observed, but for this phenomenon to be observed required much higher doses than in the He study, indicating that temperature could be an essential ingredient for gas to migrate and cause extensive precipitation. Such phenomena could possibly lead to degradation of the fuel.

Cathodoluminescence Instrumentation for Analysis of Martian Sediments

The morphologic study of the surface of Mars reveals that liquid water existed during the first few hundred millions of years of the planets history (e.g. Smith et al. 1999). The flow of water produced extensive erosion in some place, but also large sedimentary basins. With a long enough duration of the presence of liquid water and the oxidation of basalts, the emergence of biological activity may have eventually occurred, as on Earth. The detection of biomarkers at the surface of Mars is one of the main challenges of current and planned planetary exploration missions (e.g. Westall et al. 2000). Looking for a fossil or present biological activity may be approached by the search for cells, but also by the study of the results of their activity and their interface with the sedimentary environment. Such bio-sedimentations are known among the oldest terrestrial fossils and testify to the earliest terrestrial bioactivity. A discovery of such bio-sedimentations on the Martian surface would be of prime interest for addressing some of the key goals in exobiology. Cathodoluminescence (CL) is a method relevant to the search for life, as it is in line with these analytical goals of detecting bio-sedimentations (Barbin et al. 1999), and it fits well with robotic facilities usable in modern space missions (Blanc et al. 1999, Thomas et al. 2002. 2005). An established technique, cathodoluminescence is a newcomer to Martian exploration, whereit is expected to contribute to the mineralogical characterisation of sedimentary rocks, to the search for biomarkers revealing past biological activity, and to identify past geochemical conditions (Melezhik et al. 1999; Denson et al. 2007). CL is one of the best methods when the growth dynamics, microstructure, and origin of minerals need to be determined, such as with Martian sediments. CL has become an important standard technique for studying geological materials, offering a wide spectrum of applications (Marshall 1988; Barker and Kopp 1991; Barbin and Schvoerer 1997; Pagel et al. 2000). However, it is in the field of sedimentology and petrography that CL has proved to be especially valuable.