Role of fluorine in the transfer of Be and the formation of beryl deposits: a thermodynamic model.

After a thorough study on the tracing of the emerald origin by oxygen data by Zwaan et al. [12], the paper by Moine et al. [4] entitled ‘The role of fluorine in the formation of the Mananjary emerald deposits (eastern Madagascar)' demonstrates through a thermodynamic model that the metasomatic transformation of amphibolites into fluorine-enriched phlogopites was the cause for beryl precipitation. – This paper is important as it shows that fluorine can be a dominant ligand for some metals, like beryllium. As underlined by the authors, the reason for the BeF affinity in solution is that beryllium, despite the fact it is divalent, still displays a high ionic potential due to its small ionic radius. – Moine et al. [4] calculate that a fluid equilibrated with F-phlogopite (XF=0.7) at 500 °C and 2 kbar contains 275 ppm total fluorine, compared to 55 ppm dissolved in fluids buffered by wollastonite–quartz–fluorite (WQF) in the same conditions [11]. This paper therefore contributes to enlighten that fluorine-rich fluids can circulate through rocks in some natural geological settings. More precisely, it shows that rocks dominantly formed of hydrous silicates constitute a remarkable environment where are contradicted previous conclusions in the literature that “fluoride concentrations in natural hydrothermal solutions are restricted by equilibrium constraints involving fluorite, topaz and other fluoride-bearing minerals with low solubilities” (e.g., [2] and references therein). – The numerous local crystal-chemical factors that control (OH–F) exchanges in hydrous silicates (smectites, micas, amphiboles, tourmalines) are far from being understood and modelled for any given composition. However, this paper is a good illustration of the fact that, at least for a limited number of amphibole and biotite end-members with restricted Fe-free Mg-rich compositions, a thermodynamic database is now available that satisfactorily accounts for fluorination reactions. Numerous systematic structural studies of synthetic end-members have shown that some magnesian minerals like talc, phlogopite or tremolite, host the hydroxyl group in a symmetrical trioctahedral 36[Mg3] environment. Fluorination of such minerals is easy in all proportions and controlled by simple local crystal-chemical parameters. Firstly, there is no steric limitation to the complete substitution of OH− by F− in these minerals because the ionic radii of these two anions are very similar. Also, the proton exchanges one charge with the hydroxyl oxygen and is independent of any lattice atom. Such a hydroxyl group, which behaves as a separate entity, can easily be replaced by fluorine [8]

Accuracy of PIXE analyses using a funny filter

Light and heavy trace elements can be analysed simultaneously by particle-induced X-ray emission (PIXE) using an Al-funny filter (a filter with a hole drilled at its centre, placed in front of the detector window). The treatment of spectra, performed using the GUPIX code, requires the determination of an instrumental constant H. In theory and when using ordinary filters, H is a constant corresponding to the detection solid angle. In contrast, we have observed that H varies with X-ray energy using a funny filter. This is due to an inaccurate description of the geometry of the detection: detector-to-target distance, filter thicknesses and hole diameter of the funny filter. We develop a methodology to determine the effective values of the four geometrical parameters from a calibration of H using standards. In turn, H recalculated using these effective values is a constant equivalent to the effective detection solid angle

Lifetime of superheated water in a micrometric synthetic fluid inclusion

A synthetic pure water fluid inclusion presenting a wide temperature range of metastability (Th - Tn ≈ 50°C; temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, measured metastability lifetimes for the 7 data sets follow the exponential Reliability distribution, i.e., the probability of non nucleation within time t equals . This enabled us to calculate the half-life periods of metastability τ for each of the selected temperature, and then to predict τ at any temperature T > Tn for the considered inclusion, according to the equation τ(s) = 22.1 × e1.046×ΔT , (ΔT = T - Tn). Hence we conclude that liquid water in water-filled reservoirs with an average pore size ≈ 104 µm3 can remain superheated over geological timelengths (1013s), when placed in the metastable field at 24°C above the average nucleation temperature, which often corresponds to high liquid tensions (≈ -50 MPa)

Lifetime of superheated water in a micrometric synthetic fluid inclusion

International audienceA synthetic pure water fluid inclusion presenting a wide temperature range of metastability (Th - Tn ≈ 50°C; temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, measured metastability lifetimes for the 7 data sets follow the exponential Reliability distribution, i.e., the probability of non nucleation within time t equals . This enabled us to calculate the half-life periods of metastability Τ for each of the selected temperature, and then to predict Τ at any temperature T > Tn for the considered inclusion, according to the equation Τs , (∃T = T - Tn). Hence we conclude that liquid water in water-filled reservoirs with an average pore size ≈ 104 µm3 can remain superheated over geological timelengths (107s), when placed in the metastable field at 10°C above the average nucleation temperature, which often corresponds to high liquid tensions (≈ -120 -70 MPa)

Experimental superheating of water and aqueous solutions

International audienceThe metastable superheated solutions are liquids in transitory thermodynamic equilibrium inside the stability domain of their vapor (whatever the temperature is). Some natural contexts should allow the superheating of natural aqueous solutions, like the soil capillarity (low T superheating), certain continental and submarine geysers (high T superheating), or even the water state in very arid environments like the Mars subsurface (low T) or the deep crustal rocks (high T). The present paper reports experimental measurements on the superheating range of aqueous solutions contained in quartz as fluid inclusions (Synthetic Fluid Inclusion Technique, SFIT) and brought to superheating state by isochoric cooling. About 40 samples were synthetized at 0.75 GPa and 530-700 °C with internally-heated autoclaves. Nine hundred and sixty-seven inclusions were studied by micro-thermometry, including measuring the temperatures of homogenization (Th: L + V → L) and vapor bubbles nucleation (Tn: L → L + V). The Th-Tn difference corresponds to the intensity of superheating that the trapped liquid can undergo and can be translated into liquid pressure (existing just before nucleation occurs at Tn) by an equation of state. Pure water (840-935 kg m−3), dilute NaOH solutions (0.1 and 0.5 mol kg−1), NaCl, CaCl2 and CsCl solutions (1 and 5 mol kg−1) demonstrated a surprising ability to undergo tensile stress. The highest tension ever recorded to the best of our knowledge (−146 MPa, 100 °C) is attained in a 5 m CaCl2 inclusion trapped in quartz matrix, while CsCl solutions qualitatively show still better superheating efficiency. These observations are discussed with regards to the quality of the inner surface of inclusion surfaces (high P-T synthesis conditions) and to the intrinsic cohesion of liquids (thermodynamic and kinetic spinodal). This study demonstrates that natural solutions can reach high levels of superheating, that are accompanied by strong changes of their physico-chemical properties

Advances in multi-elementary analysis of fluid or solid micro-crystalline inclusions (12

International audienceX-Ray-based analytical methods can be applied in an absolute fashion, provided that matrix effects are calculated and that parameters related to instrumental factors are controlled (Newbury, 1986). When EPMA (Electron Probe Micro Analysis) was conceived at the end of the 60's, this potentiality was abandoned, as the instrument works in a relative fashion, by comparison with standards. At the end of the 70's and in the 80's, PIXE (Proton Induced X-Ray Emission) by contrast was applied in a way that preserved the possibility of an absolute application: the computer programs developed to interpret PIXE spectra calculate matrix effects and also integrate instrumental factors (e.g., Maxwell et al., 1989). In spite of this advantage, and also despite the fact that PIXE application extends to trace element analysis, the development of PIXE in the scientific community was sluggish, in deep contrast with the widespread applications of EPMA. In the field of Earth Sciences particularly, EPMA was recognized by the Mineralogical Society of America to have had 'a revolutionary, profound impact on mineralogy and petrology'. In the same time, PIXE applications remained mainly restricted to trace element analysis, and the potential accuracy of the method was never clearly realized. A first aim of this presentation is to show that, using a simple standardization procedure, the multi-elementary absolute capability of PIXE can be revealed. This in turn changes PIXE into a tool of quantitative mineralogy and trace element geochemistry. We then show that, by coupling PIXE to PIGE (Proton Induced Gamma Ray Emission) and RBS (Rutherford Back Scattering) spectrometries, the Nuclear Microprobe becomes a tool for quantitative mineralogy s.l. and geochemistry, i.e., an instrument to analyze all major to trace elements from Li to U in minerals and their inclusions. In the second part of the presentation, we illustrate the capability of µ-PIXE to analyze in situ individual fluid inclusions that have been carefully localized in space and time. The Hercynian French Massif Central and its sedimentary eastern margin are part of a large European Carbonic Province, which hosts numerous deep CO2 reservoirs and carbonic springs (Blavoux, Dazy, 1990). Carbonic fluids are present at all stages of the long-lived evolution of this crustal segment, from deep metamorphic fluids involved in a thrusting event at 340 M.a to mantle-derived volcanic CO2 related to Neogene volcanism. In order to characterize the main aquo-carbonic fluid reservoirs through time in this crustal segment, we present preliminary data on the trace element content of aquo-carbonic inclusions trapped in the schists at peak and retrograde metamorphic conditions, and compare them to contemporaneous granite-related fluid inclusion

The European Space Analogue Rock Collection (ESAR) at the OSUC-Orleans for in situ planetary missions

International audienceThe ESAR is a collection of well-characterised planetary analogue rocks and minerals that can be used for testing in situ instrumentation for planetary exploration. An online database of all relevant structural, compositional and geotechnics information is also available to the instrument teams and to aid data interpretation during missions

Cathodoluminescence Instrumentation for Analysis of Martian Sediments

International audienceThe morphologic study of the surface of Mars reveals that liquid water existed during the first few hundred millions of years of the planet's 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

Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan

International audienceAccretionary prisms constitute ideal targets to study fluid circulation and fluid-rock interactions at depths beyond the reach of active margin deep drilling. The highest-grade rocks from the Shimanto Belt on Kyushu were buried under 3-5 kbars at ~ 300°C (Toriumi and Teruya, 1988). They contain abundant quartz veins, formed throughout burial and exhumation and variably affected by brittle and ductile deformation.Cathodoluminescence (CL) reveals the existence of two distinct types of quartz, characterized by a blue and brown color, respectively. CL-blue quartz fills macro-veins (width ≥ 10μm), while CL-brown quartz is present in micro-veins (width ~ 1 − 10μm) and ductilely recrystallized domains. On the basis of microstructures, the fluids associated with the CL-blue and CL-brown quartz are interpreted as “external” and “local”, respectively. Quartz growth rims of alternating CL colors as well as mutually cross-cutting veins show that the two fluids cyclically wetted the host rock.From fluid inclusions analysis, the fluid associated with CL-blue quartz has a salinity similar to seawater, while the fluid associated with CL-brown quartz is less saline. In addition, CL-blue quartz is richer in aluminum than the CL-brown one. In contrast to the salinity/aluminum signature, the δ18O isotopic signature of both quartz types is similar and buffered by host rock. The difference between the preservation of the salinity signature of the fluid and the loss of its δ18O signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration.The “local” fluid, associated with CL-brown quartz, reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals. The “external” fluid associated with CL-blue quartz is interpreted as seawater or pore water from shallow (depth<1-2 km below seafloor) sediments. We propose that downward percolation of shallow water to depths ~ 10km is a transient process associated with mega-earthquakes

A new method of reconstructing the P-T conditions of fluid circulation in an accretionary prism (Shimanto, Japan) from microthermometry of methane-bearing aqueous inclusions

International audienceIn paleo-accretionary prisms and the shallow metamorphic domains of orogens, circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane, producing two types of fluid inclusions: methane-bearing aqueous and methane-rich gaseous fluid inclusions. In such geological settings, where multiple stages of deformation, veining and fluid influx are prevalent, textural relationships between aqueous and gaseous inclusions are often ambiguous, preventing the microthermometric determination of fluid trapping pressure and temperature conditions. To assess the P-T conditions of deep circulating fluids from the Hyuga unit of the Shimanto paleo-accretionary prism on Kyushu, Japan, we have developed a new computational code, applicable to the H2O-CH4-NaCl system, which allows the characterization of CH4-bearing aqueous inclusions using only the temperatures of their phase transitions estimated by microthermometry: Tmi, the melting temperature of ice; Thyd, the melting temperature of gas hydrate and Th,aq, homogenization temperature. This thermodynamic modeling calculates the bulk density and composition of aqueous inclusions, as well as their P-T isochoric paths in a P-T diagram with an estimated precision of approximatively 10 %. We use this computational tool to reconstruct the entrapment P-T conditions of aqueous inclusions in the Hyuga unit, and we show that these aqueous inclusions cannot be cogenetic with methane gaseous inclusions present in the same rocks. As a result, we propose that pulses of a high-pressure, methane-rich fluid transiently percolated through a rock wetted by a lower-pressure aqueous fluid. By coupling microthermometric results with petrological data, we infer that the exhumation of the Hyuga unit from the peak metamorphic conditions was nearly isothermal and ended up under a very hot geothermal gradient. In subduction or collision zones, modeling aqueous fluid inclusions in the ternary H2O-CH4-NaCl system and not simply in the binary H2O-NaCl is necessary, as the addition of even a small amount of methane to the water raises significantly the isochores to higher pressures. Our new code provides therefore the possibility to estimate precisely the pressure conditions of fluids circulating at depth