Damien Guillaume

Calibration of methane analysis by Raman spectroscopy in H2O-NaCl-CH4 fluid inclusions

Abstract Calibration of the determination of CH 4 /H 2 O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions at different NaCl concentration (0, 0.05, 0.66, 0.98, 1.00, 1.60, 2.25 and 3.5 m NaCl). Spectra of the stretching bands of methane and water in the aqueous phase were collected at variable temperatures up to a few degrees above the homogenisation temperature. The composition of the aqueous phase for temperatures below the homogenisation temperature was calculated with a computer program, using the model of Duan et al. [Geochim. Cosmochim. Acta 56 (1992) 1451]. Results show the dependency of the estimate of the CH 4 concentration on salinity: at constant CH 4 concentration, the CH 4 /H 2 O area ratio of Raman bands decreases with increasing salinity from 0 to 1.6 m and remains constant up to 3.5 m NaCl. The P – T projection of the isopleth of a natural fluid inclusion is deduced from the homogenisation temperature, its composition obtained from cryometry (mNaCl eq.) and Raman analysis (mCH 4 ) ratio. This methodology was applied to a sample from the Cave-in-Rock MVT deposit (fluorite–Pb–Zn district, southern Illinois, USA) presenting petroleum fluid inclusions associated with fluid inclusions of the system H 2 O–NaCl–CH 4 . Hydrocarbon isochore intersects the isopleth of the H 2 O–NaCl–CH 4 inclusions at the homogenization temperature, which validates this procedure.

EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON EXPERIMENTAL CONVERSION OF DIOCTAHEDRAL SMECTITE TO BERTHIERINE, CHLORITE, VERMICULITE, OR SAPONITE

AbstractIn deep geological repositories for high-level nuclear wastes, interactions between steel canisters and clay-rich materials may lead to mineralogical transformations with a loss of the confining properties of the clays. Experiments simulating the conversion of smectite to Fe-rich clay phases in contact with Fe metal have been carried out to evaluate such a possibility by taking into account the effects of a series of critical parameters, including temperature, pH, and Fe/clay (Fe/C) and liquid/clay (L/C) ratios. The mineralogical and chemical transformations observed in these experiments have been compared with data from the literature, and subsequently used to propose a conceptual model for the main mineralogical transformations which can be expected in clay formations surrounding high-level nuclear waste repositories. In the presence of Fe metal and under low oxygen fugacity (<-40) the main mineralogical sequences are as follows:n (1)up to 150°C, under neutral pH, and L/C > 5: dioctahedral smectite (di-sm) → 7 Å Fe-rich phase (berthierine, odinite-cronstedtite) for large Fe/C ratios (>0.5), or di-sm → Fe-rich di-sm + Fe-rich trioctahedral smectite (tri-sm) for small Fe/C ratios (0.1)(2)up to 150°C, under alkaline pH (10–12), and L/C > 5: di-sm → Fe di-sm (±palygorskite) for a small Fe/C ratio (0.1)(3)at 300°C, Fe/C = 0.1, and L/C > 5: di-sm → Fe-rich saponite → trioctahedral chlorite + feldspar + zeolite (near-neutral pH); di-sm → Fe-rich vermiculite + mordenite (pH 10–12).n Low temperatures (<150°C) and large L/C and Fe/C ratios seem to favor the crystallization of the serpentine group minerals instead of Fe-rich trioctahedral smectites or chlorites, the latter being favored by higher temperatures. The role of L/C and Fe/C ratios and the competition between them at different temperatures is a crucial point in understanding the transformation of smectite in contact with Fe metal.

Sulfur radical species form gold deposits on Earth

Significance Gold resources on Earth result from an exceptional concentration phenomenon yielding metal contents in ore a thousand to a million times higher than those in common rocks. We show that this process is controlled by sulfur radical ions (S3−), which strongly bind Au in aqueous solution at elevated temperatures and pressures and allow very efficient extraction, transport, and deposition of gold by geological fluids. Thus, the most inert metal of the periodic table may be very mobile, which explains key features of known gold deposits and offers new possibilities for resource prospecting. Furthermore, the high capacity of the radical ions to solubilize gold may be used for its selective extraction from ores and hydrothermal synthesis of Au-based nanomaterials. Current models of the formation and distribution of gold deposits on Earth are based on the long-standing paradigm that hydrogen sulfide and chloride are the ligands responsible for gold mobilization and precipitation by fluids across the lithosphere. Here we challenge this view by demonstrating, using in situ X-ray absorption spectroscopy and solubility measurements, coupled with molecular dynamics and thermodynamic simulations, that sulfur radical species, such as the trisulfur ion S3−, form very stable and soluble complexes with Au+ in aqueous solution at elevated temperatures (>250 °C) and pressures (>100 bar). These species enable extraction, transport, and focused precipitation of gold by sulfur-rich fluids 10–100 times more efficiently than sulfide and chloride only. As a result, S3− exerts an important control on the source, concentration, and distribution of gold in its major economic deposits from magmatic, hydrothermal, and metamorphic settings. The growth and decay of S3− during the fluid generation and evolution is one of the key factors that determine the fate of gold in the lithosphere.

Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement

Alteration experiments on natural monazite crystals (Manangotry standard, Madagascar) under alkali conditions at 300, 400, 500 and 600 °C and 200 MPa were conducted to clarify mechanisms behind incomplete resetting of U-Th-Pb geochronological systems in monazite replaced by dissolution and precipitation processes. Above 400 °C, experimental products show typical replacement textures: a compositionally distinct monazite rim, referred as altered rim, surrounds the primary monazite (Mnz1). Isotopic and electron microprobe U-Th-Pb in situ dating of the altered rim yields intermediate ages between pristine monazite (555 Ma) and complete experimental resetting (0 Ma). Lead is systematically detected in altered rims, with concentration decreasing from 400 °C to 600 °C. The origin of incomplete resetting is elucidated with transmission electron microscope images that reveal an incomplete replacement of Mnz1 by a secondary monazite (Mnz2) within the altered rim. With increasing temperature, the size and volume of the Mnz2 within the altered rim become more important. Because no structural Pb or Pb nanoinclusions were observed, Pb in the altered rim is attributed to the Mnz1 component. Partial resetting of U-Th-Pb systems depends on the nanomixture of different Mnz1 proportions in the analyzed volume, and explains the higher rejuvenation at 600 °C than at lower temperatures. Although microanalytical techniques have the spatial resolution to date micrometer-sized rims, they are unable to resolve a nanoscale mixture of pristine and secondary monazite that could occur in altered rims formed by fluid-driven replacement, especially at low temperatures. Porosity and/or inclusions and complex age scattering in zoned monazite are significant markers that can indicate a possible nano-sized partial replacement.

Mineralogical characterization and genesis of hydrothermal Mn oxides from the flank of the Juan the Fuca Ridge

Abstract Several sites of active hydrothermal flow have been found on the eastern flank of the Juan de Fuca Ridge. These sites are typically located along the edge of basaltic outcrops where sediment is thin. We present data on Mn-oxides formed on such outcrops (Zona Bare and Grinin Bare). These oxides are either black-layered crust or soft micro-concretions found in partially altered sediments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of Mn crusts indicate the presence of well-crystallized todorokite and birnessite encrusting detrital minerals and replacing siliceous fossil. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) analyses were used to identify amorphous and poorly crystallized Mn-rich phases in partially altered sediments and crusts. TEM of impregnated samples showed textural evidence suggesting that amorphous Mn oxides are incrusting cellular structures that could be bacteria. The valence state of Mn in these oxides was determined by parallel electron energy loss spectroscopy (PEELS). Results indicate that todorokite and birnessite have an average valence state of about 3.7 whereas the poorly crystallized Mn-rich phases have a lower valence state. These data suggest that the formation of hydrothermal Mn concretions occurs in several steps. The initial step is the adsorption or precipitation of Mn, Fe, and Si around cell-wall bacteria, extracellular polymers, and siliceous fossil remains. These mineralizations are poorly crystallized phyllomanganates, which progressively increase in size and crystallinity to give the final birnessite and todorokite products. All of these Mn-rich phases are the result of interactions between hydrothermal fluid and sediments and formed in areas where hydrothermal fluids discharge through the sediment.

Dissolved methane in water: temperature effect on Ramanquantification in fluid inclusions

Abstract This study confirms the ability of Raman spectroscopy for the quantification of methane in aqueous fluids trapped ininclusions. The procedure is based on the use of reference synthetic inclusions for calibration at various salinities. The temperature effect, which was suspected to be weak, has been confirmed using a natural metastable liquid inclusion. No temperature correction is needed between 80 and 175 °C. The Raman decomposition of the water stretching band using methane as an internal reference gives us the opportunity to follow the state of liquid water between 24 and 278 °C. The decrease of the hydrogen bonding contribution with temperature is responsible for the increase of the Raman CH 4 /H 2 O peak area ratio.

In situ Raman spectroscopy identification of the S3− ion in S-rich hydrothermal fluids from synthetic fluid inclusions

Abstract The chemical forms of sulfur in geological fluids control the behavior of this element and associated base and precious metals in magmatic, hydrothermal, and metamorphic environments. However, these forms are insufficiently known at elevated temperature (T) and pressure (P). In this study, sulfur speciation in model aqueous solutions of thiosulfate and sulfur (~3 wt% of total S) was examined by in situ Raman spectroscopy on synthetic fluid inclusions at T-P-pH-redox conditions typical of porphyry Cu-Au-Mo deposits. Fluid inclusions were entrapped at 2 kbar and 600 or 700 °C in quartz that served as a container for the high T-P fluid. Then, the inclusion-bearing quartz samples were re-heated and examined by Raman spectroscopy as a function of T and P (up to 500 °C and ~1 kbar). At T < 200 °C, all fluid inclusions show sulfate (SO4 2- ± HSO4 -) and sulfide (H2S ± HS-) in the aqueous liquid phase and elemental sulfur (S8) in the solid/molten phase; these results agree both with thermodynamic predictions of sulfur speciation and the common observation of these three S forms in natural fluid inclusions. At T > 200-300 °C, in addition to these S species, the S3- ion was found to appear and grow with increasing temperature to at least 500 °C. The formation of S3- is rapid and fully reversible; its Raman signal disappears on cooling below 200 °C, and re-appears on heating. These new data confirm the recent findings of S3 - in similar aqueous solutions at P of 5-50 kbar and T > 250 °C; they suggest that S3- may account for some part of dissolved sulfur and serve as a ligand for chalcophile metals in fluids from subduction zones and related Cu-Au-Mo deposits. This work demonstrates that in situ approaches are required for determining the true sulfur speciation in crustal fluids; it should encourage future spectroscopic investigations of natural fluid and melt inclusions at high temperatures and pressures close to their formation conditions.

Soil Calcium Availability Influences Shell Ecophenotype Formation in the Sub-Antarctic Land Snail, Notodiscus hookeri

Ecophenotypes reflect local matches between organisms and their environment, and show plasticity across generations in response to current living conditions. Plastic responses in shell morphology and shell growth have been widely studied in gastropods and are often related to environmental calcium availability, which influences shell biomineralisation. To date, all of these studies have overlooked micro-scale structure of the shell, in addition to how it is related to species responses in the context of environmental pressure. This study is the first to demonstrate that environmental factors induce a bi-modal variation in the shell micro-scale structure of a land gastropod. Notodiscus hookeri is the only native land snail present in the Crozet Archipelago (sub-Antarctic region). The adults have evolved into two ecophenotypes, which are referred to here as MS (mineral shell) and OS (organic shell). The MS-ecophenotype is characterised by a thick mineralised shell. It is primarily distributed along the coastline, and could be associated to the presence of exchangeable calcium in the clay minerals of the soils. The Os-ecophenotype is characterised by a thin organic shell. It is primarily distributed at high altitudes in the mesic and xeric fell-fields in soils with large particles that lack clay and exchangeable calcium. Snails of the Os-ecophenotype are characterised by thinner and larger shell sizes compared to snails of the MS- ecophenotype, indicating a trade-off between mineral thickness and shell size. This pattern increased along a temporal scale; whereby, older adult snails were more clearly separated into two clusters compared to the younger adult snails. The prevalence of glycine-rich proteins in the organic shell layer of N. hookeri, along with the absence of chitin, differs to the organic scaffolds of molluscan biominerals. The present study provides new insights for testing the adaptive value of phenotypic plasticity in response to spatial and temporal environmental variations.

Production of synthetic fluid inclusions in the H2O-CH4-NaCl system using laser-ablation in fluorite and quartz

Synthesis of fluid inclusions is made by the pre-formation of cavities with a diameter greater than 10 μm by laser ablation using a Nd-YAG pulsed laser focused through a Cassegrain objective. Ablation cavities inside the crystals are connected to the surface by microcracks, which serve as the fluid channels. In fluorite crystals, the microcracks heal at 200°C during four-week experiments and produce large inclusions corresponding to the ablation cavities and smaller ones in the microcracks. In quartz crystals, cavity sealing occurs by quartz overgrowths on faces perpendicular to the c axis at 300°C during four weeks. No cavity sealing occurs at 200°C in quartz. Micro-thermometric data demonstrate that fluid inclusions have trapped representative samples of the experimental fluids. Raman and infrared spectra indicate that fluid inclusions synthesised by our laser technique may be suitable for spectroscopic analysis.

High-temperature and high-pressure water solubility in ethylbenzene to 200°C and 1 kbar and the acetic acid effect

Abstract Water solubility in hydrocarbon systems is of great interest for deep oil fields. A new autoclave has been designed to measure phase equilibria in water-hydrocarbon systems up to 400°C and 1.5 kbar. It has been applied for the measurement of water solubility in ethylbenzene with or without acetic acid to 200°C and 1 kbar in the two-phase field. Water solubility was measured by the Karl Fisher method. The acetic acid concentration was measured by FT-IR microspectroscopy. Both the experimental procedure and analytical techniques were validated by showing the consistency of our data with those of Heidman et al. (“High-temperature mutual solubilities of hydrocarbons and water,” AIChE J. 31, 376–384, 1995) along the liquid-liquid-vapor curve. At constant pressure, the solubility of water in ethylbenzene increases significantly with temperature. On the other hand, at constant temperature, the solubility of water is constant to 1 kbar at 100°C, and decreases slightly with pressure at 150 and 200°C. Data were regressed by the Krichevsky-Kasarnovsky equation to obtain estimates of the Henry’s law constant and estimates of the molar volume of water at infinite dilution. Acetic acid increases the solubility of water in ethylbenzene and fractionates preferentially into the aqueous phase.