Giles M. Marion

Carbonate mineral solubility at low temperatures in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system

Abstract Carbonate minerals have played an important role in the geochemical evolution of Earth, and may have also played an important role in the geochemical evolution of Mars and Europa. Several models have been published in recent years that describe chloride and sulfate mineral solubilities in concentrated brines using the Pitzer equations. Few of these models are parameterized for subzero temperatures, and those that are do not include carbonate chemistry. The objectives of this work are to estimate Pitzer–equation bicarbonate–carbonate parameters and carbonate mineral solubility products and to incorporate them into the FREZCHEM model to predict carbonate mineral solubilities in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system at low temperatures (≤25°C) with a special focus on subzero temperatures. Most of the Pitzer-equation parameters and equilibrium constants are taken from the literature and extrapolated into the subzero temperature range. Solubility products for 14 sodium, potassium, magnesium, and calcium bicarbonate and carbonate minerals are included in the model. Most of the experimental data are at temperatures ≥ −8°C; only for the NaHCO3-NaCl-H2O and Na2CO3-NaCl-H2O systems are there bicarbonate and carbonate data to temperatures as low as −21.6°C. In general, the fit of the model to the experimental data is good. For example, calculated eutectic temperatures and compositions for NaHCO3, Na2CO3, and their mixtures with NaCl and Na2SO4 salts are in good agreement with experimental data to temperatures as low as −21.6°C. Application of the model to eight saline, alkaline carbonate waters give predicted pHs ranging from 9.2 to 10.2, in comparison with measured pHs that range from 8.7 to 10.2. The model suggests that the CaCO3 mineral that precipitates during seawater freezing is probably calcite and not ikaite. The model demonstrates that a proposed salt assemblage for the icy surface of Europa consisting of highly hydrated MgSO4 salts and natron (Na2CO3 · 10H2O) is an incompatible salt assemblage.

The Search for Life on Europa: Limiting Environmental Factors, Potential Habitats, and Earth Analogues

The putative ocean of Europa has focused considerable attention on the potential habitats for life on Europa. By generally clement Earth standards, these Europan habitats are likely to be extreme environments. The objectives of this paper were to examine: (1) the limits for biological activity on Earth with respect to temperature, salinity, acidity, desiccation, radiation, pressure, and time; (2) potential habitats for life on Europa; and (3) Earth analogues and their limitations for Europa. Based on empirical evidence, the limits for biological activity on Earth are: (1) the temperature range is from 253 to 394 K; (2) the salinity range is a(H2O) = 0.6-1.0; (3) the desiccation range is from 60% to 100% relative humidity; (4) the acidity range is from pH 0 to 13; (5) microbes such as Deinococcus are roughly 4,000 times more resistant to ionizing radiation than humans; (6) the range for hydrostatic pressure is from 0 to 1,100 bars; and (7) the maximum time for organisms to survive in the dormant state may be as long as 250 million years. The potential habitats for life on Europa are the ice layer, the brine ocean, and the seafloor environment. The dual stresses of lethal radiation and low temperatures on or near the icy surface of Europa preclude the possibility of biological activity anywhere near the surface. Only at the base of the ice layer could one expect to find the suitable temperatures and liquid water that are necessary for life. An ice layer turnover time of 10 million years is probably rapid enough for preserving in the surface ice layers dormant life forms originating from the ocean. Model simulations demonstrate that hypothetical oceans could exist on Europa that are too cold for biological activity (T < 253 K). These simulations also demonstrate that salinities are high, which would restrict life to extreme halophiles. An acidic ocean (if present) could also potentially limit life. Pressure, per se, is unlikely to directly limit life on Europa. But indirectly, pressure plays an important role in controlling the chemical environments for life. Deep ocean basins such as the Mariana Trench are good analogues for the cold, high-pressure ocean of Europa. Many of the best terrestrial analogues for potential Europan habitats are in the Arctic and Antarctica. The six factors likely to be most important in defining the environments for life on Europa and the focus for future work are liquid water, energy, nutrients, low temperatures, salinity, and high pressures.

Modeling aqueous ferrous iron chemistry at low temperatures with application to Mars

Abstract Major uncertainties exist with respect to the aqueous geochemical evolution of the Martian surface. Considering the prevailing cryogenic climates and the abundance of salts and iron minerals on Mars, any attempt at comprehensive modeling of Martian aqueous chemistry should include iron chemistry and be valid at low temperatures and high solution concentrations. The objectives of this paper were to (1) estimate ferrous iron Pitzer-equation parameters and iron mineral solubility products at low temperatures (from Ferrous iron Pitzer-equation parameters were derived in this work or taken from the literature. Six new iron minerals [FeCl2·4H2O, FeCl2·6H2O, FeSO4·H2O, FeSO4·7H2O, FeCO3, and Fe(OH)3] were added to the FREZCHEM model bringing the total solid phases to 56. Agreement between model predictions and experimental data are fair to excellent for the ferrous systems: Fe-Cl, Fe-SO4, Fe-HCO3, H-Fe-Cl, and H-Fe-SO4. We quantified a conceptual model for the aqueous geochemical evolution of the Martian surface. The five stages of the conceptual model are: (1) carbonic acid weathering of primary ferromagnesian minerals to form an initial magnesium-iron-bicarbonate-rich solution; (2) evaporation and precipitation of carbonates, including siderite (FeCO3), with evolution of the brine to a concentrated NaCl solution; (3) ferrous/ferric iron oxidation; (4) either evaporation or freezing of the brine to dryness; and (5) surface acidification. What began as a dilute Mg-Fe-HCO3 dominated leachate representing ferromagnesian weathering evolved into an Earth-like seawater composition dominated by NaCl, and finally into a hypersaline Mg-Na-SO4-Cl brine. Weathering appears to have taken place initially under conditions that allowed solution of ferrous iron [low O2(g)], but later caused oxidation of iron [high O2(g)]. Surface acidification and/or sediment burial can account for the minor amounts of Martian surface carbonates. This model rests on a large number of assumptions and is therefore speculative. Nevertheless, the model is consistent with current understanding concerning surficial salts and minerals based on Martian meteorites, Mars lander data, and remotely-sensed spectral analyses.

A molal-based model for strong acid chemistry at low temperatures (<200 to 298 K)

Abstract Geochemical processes occurring in cold environments on Earth, Mars, and Europa have elicited considerable interest in the application of geochemical models to subzero temperatures. Few existing geochemical models explicitly include acid chemistry and those that do are largely restricted to temperatures ≥0°C or rely on the mole-fraction scale rather than the more common molal scale. This paper describes (1) use of the Clegg mole-fraction acid models to develop a molal-based model for hydrochloric, nitric, and sulfuric acids at low temperatures; (2) incorporation of acid chemistry and nitrate minerals into the FREZCHEM model; (3) validation and limitations of the derived acid model; and (4) simulation of hypothetical acidic brines for Europa. The Clegg mole-fraction acid models were used to estimate activities of water and mean ionic activity coefficients that serve as the database for estimating molal Pitzer-equation parameters for HCl (188 to 298 K), HNO 3 (228 to 298 K), and H 2 SO 4 (208 to 298 K). Model eutectics for HNO 3 and H 2 SO 4 agree with experimental measurements to within ± 0.2°C. In agreement with previous work, the experimental freezing point depression (fpd) data for pure HCl at subzero temperatures were judged to be flawed and unreliable. Three alternatives are discussed for handling HCl chemistry at subzero temperatures. In addition to defining the solubility of solid-phase acids, this work also adds three new nitrate minerals and six new acid salts to the FREZCHEM model and refines equilibria among water ice, liquid water, and water vapor over the temperature range from 180 to 298 K. The final system is parameterized for Na-K-Mg-Ca-H-Cl-SO 4 -NO 3 -OH-HCO 3 -CO 3 -CO 2 -H 2 O. Simulations of hypothetical MgSO 4 -H 2 SO 4 -H 2 O and Na 2 SO 4 -MgSO 4 -H 2 SO 4 -H 2 O brines for Europa demonstrate how freezing can convert a predominantly salt solution into a predominantly acid solution at subzero temperatures. This result has consequences for the effects of salinity, acidity, and temperature as limiting factors for potential life on Europa. Strong acidity would limit life-forms to highly acidophilic organisms.

Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars

Abstract The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has returned observations of the Nili Fossae region indicating the presence of Mg-carbonate in small (

Martian hydrogeology sustained by thermally insulating gas and salt hydrates

Numerical simulations and geologic studies suggest that high thermal anomalies beneath, within, and above thermally insulating layers of buried hydrated salts and gas hydrates could have triggered and sustained hydrologic processes on Mars, producing or modifying chaotic terrains, debris flows, gullies, and ice-creep features. These simulations and geologic examples suggest that thick hydrate deposits may sustain such geothermal anomalies, shallow ground-water tables, and hydrogeologic activity for eons. The proposed mechanism may operate and be self-reinforcing even in today9s cold Martian climate without elevated heat flux.

Seasonal formation of ikaite (caco3 · 6h2o) in saline spring discharge at Expedition Fiord, Canadian High Arctic: Assessing conditional constraints for natural crystal growth

Abstract —Spring discharge at Expedition Fiord (Pollard et al., 1999) on Axel Heiberg Island in the Canadian High Arctic produces a variety of travertine forms in addition to a diverse collection of mineral precipitates. This paper focuses on clusters of thermally unstable crystals believed to be the mineral ikaite (CaCO3 · 6H2O) growing seasonally along two spring outflows at Colour Peak. This form of calcium carbonate mineral occurs along small sections of discharge outflow as white euhedral crystals up to 0.5 cm in length. Difficulty in sampling, storage and transport of the samples for analysis has hampered attempts to confirm the presence of ikaite by X-ray diffraction. However, various field observations and the remarkable instability of these crystals at normal ambient temperatures strengthens our argument. This paper provides a description of these particular CaCO3 · 6H2O crystals and their environmental surroundings, and attempts to determine the validity of ikaite precipitation at this site by theoretical geochemical modeling: these results are compared with other reported observations of ikaite to both understand their occurrence and help delineate their geochemical characteristics. It is believed that the restrictive combination of spring water chemistry and long periods of low temperatures characteristic of arctic climates are necessary for ikaite growth at this site. The fact that ikaite is not forming at a second group of saline springs 11 km away allows us to more specifically outline conditions controlling its presence.

Stable Isotope Ratios and the Dynamics of Caliche in Desert Soils

Calcium carbonate is deposited in the soils of arid and semi-arid regions, where it forms calcic or petrocalcic soil horizons that are often known as caliche. When the parent materials are calcareous, massive deposits of secondary carbonate may form through the dissolution and reprecipitation of the parent minerals. However, caliche also forms on noncalcareous parent materials, as calcium derived from the weathering of aluminosilicate minerals or from atmospheric deposition is carried into the soil profile by the downward percolation of rainwater. Caliche is ubiquitous in the arid regions of all continents and forms a major pool in the global carbon cycle (Schlesinger 1982). The rate of deposition is typically 1.0 to 3.5 g CaCO3 m-2 yr-1 (Schlesinger 1985).

Calculation of equilibria in CO2–water–salt systems using the Frezchem model

M. V. Mironenkoa, *, V. B. Polyakovb, and G. M. Marionc aVernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia bKorzhinsky Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Akademika Osip’yana 4, Chernogolovka, 142432 Russia cDesert Research Institute 2215 Roggio Parkway, Reno, NV, 89512, USA *e-mail: mironenko@geokhi.ru Received August 24, 2015; accepted September 24, 2015