Alteration assemblages in Martian meteorites: implications for near-surface processes

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals – which comprise up to 1 vol.% of the meteorites – provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 – 150 °C) brines. This is consistent with the simple mineralogy of these assemblages – Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays – and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event post-dating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last 1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50–250 mbar of CO2 being trapped in the uppermost 200–1000 m of the martian crust. Large fractionations in 18O between igneous silicate in the meteorites and the secondary minerals (30) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (400°C) and have been taken to suggest low temperatures (e.g. 150°C) of precipitation from a hydrous fluid

Experimental Determination of Phase Equilibria in the System H{sub 2}O-CO{sub 2}-NaCl at 0.5 Kb from 500 to 800C

An understanding of activity-composition (a/X) relations and phase equilibria for halite-bearing, mixed-species supercritical fluids is critically important to many geological and industrial applications. The authors have performed experiments on the phase equilibria of H{sub 2}O-CO{sub 2}-NaCl fluids from 500 C to 800 C at 500 bars, conditions of significant importance in studies of magma-hydrothermal systems, geothermal reservoirs and some ore deposits, to obtain highly accurate and precise data for this ternary system. These experiments are conducted using a double capsule technique. An excess of NaCl is placed in an inner Pt capsule, which is crimped shut and placed in an outer capsule containing H{sub 2}O and CO{sub 2}. During the experiment NaCl dissolves out of the inner capsule, and is deposited in the outer capsule during the quench. After the experiment the capsule is opened, and the amount of NaCl remaining in the inner capsule determined by dissolution. The difference between the initial and final amounts of NaCl in the inner capsule yields the solubility of NaCl at the P-T conditions of the experiment. At 500 C data from these experiments suggest that the vapor comer of the three-phase field lies near X(H{sub 2}O) = 0.760, X(NaCl) = 0.065, which is a significantly more water-rich composition than suggested by previous models. As expected, increasing temperature increases the solubility of NaCl in the NaCl-vapor field. For example, at intermediate H{sub 2}O/CO{sub 2} ratios the vapor field extends from approximately near X(H{sub 2}O) = 0.66, X(NaCl) = 0.06 at 500 C to near X(H{sub 2}O) = 0.65, X(NaCl) = 0.08 at 600 C

Overview of fundamental geochemistry basic research at the Oak Ridge National Laboratory

Researchers in ORNL`s Geochemistry and High Temperature Aqueous Chemistry groups are conducting detailed experimental studies of physicochemical properties of the granite-melt-brine system; sorption of water on rocks from steam-dominated reservoirs; partitioning of salts and acid volatiles between brines and steam; effects of salinity on H and O isotope partitioning between brines, minerals, and steam; and aqueous geochemistry of Al. These studies contribute in many ways to cost reductions and improved efficiency in the discovery, characterization, and production of energy from geothermal resources

Excess Free Energies and Activity-Composition Relations for H{sub 2}O-CO{sub 2} Fluids at 400{degree}C and 1-4000 Bars

Experimentally determined excess molar volumes (V{sup ex}) for H{sub 2}O--CO{sub 2} fluids at 400{degree}C and 100-4000 bars have been used to calculate excess free energies (G{sup ex}) and activity-composition (a-X) relations for these mixtures. Excess free energies are continuously positive and asymmetric toward H{sub 2}O at all pressures up to 4000 bars, rising to peak values of approximately 1300, 1800, 2000 and 2100 J/mol at 500, 1000, 2000 and 4000 bars, respectively. Calculated activities for H{sub 2}O and CO{sub 2} vary correspondingly, increasing: substantially from 0 to 1000 bars, moderately from 1000 to 2000 bars, and slightly from 2000 to 4000 bars. In addition, because G{sup ex} is asymmetric toward H{sub 2}O at 400{degree}C and pressures up to at least 4000 bars, a-X relations for H{sub 2}O are distinctly different from a-X relations for CO{sub 2}. These results imply that H{sub 2}O--CO{sub 2} fluids are strongly nonideal at 400{degree}C and all pressures above approx. 300 bars, despite the fact that peak values for V{sup ex} decrease from approx. 50 cm{sup 3}/mol at 300 bars to approx. 1 cm{sup 3}/mol at 2000 bars, and remain small to pressures as high as 5000 bars. Excess free energies and a-X relations for H{sub 2}O--CO{sub 2} fluids at 400{degree}C and pressures up to 4000 bars calculated from semi-empirical equations of state generally suggest significantly smaller positive deviations from ideality

Experimental Determinations of the Activity-Composition Relations and Phase Equilibria of H{sub 2}O-CO{sub 2}-NaCl Fluids

An understanding of activity-composition (a/X) relations and phase equilibria for halite-bearing, mixed-species supercritical fluids is critically important in many geological and industrial applications. The authors have performed experiments on the a/X relations and phase equilibria of H{sub 2}O-CO{sub 2}-NaCl fluids at 5OO C, 500 bars, to obtain highly accurate and precise data for this ternary system. H{sub 2}O-CO{sub 2}-NaCl samples were reacted at a (H{sub 2}O) = 0.350, 0.425, 0.437, 0.448, 0.560, 0.606, 0.678, 0.798, and 0.841. Results indicate that fluids with these activities lie in the vapor-NaCl two-phase region, and that a fluid with the last value has a composition close to the three-phase (vapor + brine + halite) field. Data from these experiments and NaCl solubility runs also suggest that the vapor comer of the three-phase field lies near X(H{sub 2}O) = 0.760, X(NaCl) = 0.065, which is a significantly more water-rich composition than suggested by the model of [1]

Microstuctural Characterization of Water-Rich Boehmite (AlO(OH)): TEM Correlation of Apparently Divergent XRD and TGA Results

An understanding of the solid-phase thermodynamics and aqueous speciation of aluminum is critical to our ability to understand and predict processes in a wide variety of geologic and industrial settings. Boehmite (AlO(OH)) is an important phase in the system Al2O3-H2O that has been the subject of a number of structural and thermodynamic studies since its initial synthesis [1] and discovery in nature [2]. Unfortunately, it has long been recognized that thermogravimetric analysis (TGA) of both synthetic and natural boehmite samples (that appear well crystallized by powder XRD methods) yields significant excess water - typically losing 16-16.5 wt. % on heating as compared with a nominal expected weight loss of 15.0 wt. % [3,4]. The boehmite used in our experiments was synthesized hydrothermally from acid-washed gibbsite (Al(OH)3) at 200°C. Powder XRD and SEM examination showed no evidence of the presence a contaminant phase. The TGA patterns do not suggest that this is due to adsorbed water, so a structural source is likely. We therefore undertook to examine this material by TEM to clarify this phenomenon.Boehmite is orthorhombic (a = 0.0285nm, b = 0.1224nm and c = 0.0365nm, Amam). The crystals were tabular with major surfaces normal to &lt;010&gt;. Simple powder dispersals onto holey carbon films typically resulted in b-axis orientations parallel to the electron beam. To view other orientations, specimens were mulled in M-Bond epoxy, pressed between plates of single-crystal Si (aligning the boehmite tablets parallel to the Si plates) while the epoxy cured. Electron transparent thin foils normal to the silicon plates were produced by argon ion milling techniques. Sample stability in the electron beam was dramatically improved by cooling to −140°C using an 2 cold stage.</jats:p