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Steele-Macinnis M.,University of Arizona | Steele-Macinnis M.,Institute for Geochemistry and Petrology | Reimer J.,Paul Scherrer Institute | Bachmann S.,ETH Zurich
RSC Advances | Year: 2015

Molecular simulations have been conducted to assess the pVT properties and static permittivity of the charge-on-spring polarizable water model COS/D2 at hydrothermal conditions from 300 to 450 °C and bulk densities of 0.001 to 1.0 g cm-3. The results indicate that the model performs well for reproducing volumetric and dielectric properties of real water. The liquid-vapor coexistence curve and critical point of COS/D2 water (ca. 334 °C, 21.7 MPa and 0.32 g cm-3) are also in reasonable agreement with those of real water (ca. 374 °C, 22.1 MPa and 0.322 g cm-3). We include comparisons to other polarizable (as well as non-polarizable) water models where available. The good performance of the COS/D2 model at high temperatures and pressures is noteworthy considering that the model was parameterized exclusively at room temperature and pressure. Moreover, these results imply that COS/D2 is a suitable water model for simulating solutions of non-electrolyte and electrolyte solutes at elevated temperatures and pressures. © The Royal Society of Chemistry 2015. Source

Martin J.E.,University of Bristol | Martin J.E.,Ecole Normale Superieure de Lyon | Vance D.,Institute for Geochemistry and Petrology | Balter V.,Ecole Normale Superieure de Lyon
Proceedings of the National Academy of Sciences of the United States of America | Year: 2015

Geochemical inferences on ancient diet using bone and enamel apatite rely mainly on carbon isotope ratios (δ13C) and to a lesser extent on strontium/calcium (Sr/Ca) and barium/calcium (Ba/Ca) elemental ratios. Recent developments in nontraditional stable isotopes provide an unprecedented opportunity to use additional paleodietary proxies to disentangle complex diets such as omnivory. Of particular relevance for paleodietary reconstruction are metals present in large quantity in bone and enamel apatite, providing that biologically mediated fractionation processes are constrained. Calcium isotope ratios (δ44Ca) meet these criteria but exhibit complex ecological patterning. Stable magnesium isotope ratios (δ26Mg) also meet these criteria but a comprehensive understanding of its variability awaits new isotopic data. Here, 11 extant mammal species of known ecology from a single locality in equatorial Africa were sampled for tooth enamel and, together with vegetation and feces, analyzed for δ26Mg, δ13C, Sr/Ca, and Ba/Ca ratios. The results demonstrate that δ26Mg incorporated in tooth enamel becomes heavier from strict herbivores to omnivores/ faunivores. Using data from experimentally raised sheep, we suggest that this 26Mg enrichment up the trophic chain is due to a 26Mg enrichment in muscle relative to bone. Notably, it is possible to distinguish omnivores from herbivores, using δ26Mg coupled to Ba/Ca ratios. The potential effects of metabolic and dietary changes on the enamel δ26Mg composition remain to be explored but, in the future, multiproxy approaches would permit a substantial refinement of dietary behaviors or enable accurate trophic reconstruction despite specimen-limited sampling, as is often the case for fossil assemblages. Source

Steele-MacInnis M.,Virginia Polytechnic Institute and State University | Steele-MacInnis M.,Institute for Geochemistry and Petrology | Lecumberri-Sanchez P.,Virginia Polytechnic Institute and State University | Lecumberri-Sanchez P.,Institute for Geochemistry and Petrology | Bodnar R.J.,Virginia Polytechnic Institute and State University
Geochimica et Cosmochimica Acta | Year: 2015

Iron chloride is a common and abundant component in hydrothermal fluids in many geologic environments, yet the thermodynamic and PTx properties of FeCl2-bearing aqueous fluids are poorly known. In this study we have used the synthetic fluid inclusion technique to characterize the PTx conditions along the critical curve of the system H2O-FeCl2. For a given temperature or salinity, the critical pressure in the H2O-FeCl2 system is lower than that in the H2O-NaCl system. In contrast, the critical curves of aqueous solutions of other divalent-cation chlorides, such as MgCl2 and CaCl2, are at higher pressure than that of NaCl solutions of equivalent temperature or salinity. The results of this study provide the first quantitative constraints on the PTx extent of liquid-vapor immiscibility for FeCl2-rich fluids. Owing to the low pressure along the critical curve compared to other common aqueous chloride systems, immiscibility of FeCl2-rich fluids appears to be limited to relatively low pressures, or relatively shallow levels in the crust, compared to other saline hydrothermal fluids. © 2014 Elsevier Ltd. Source

Moore J.M.,Rhodes University | Kuhn B.K.,Rhodes University | Kuhn B.K.,Institute for Geochemistry and Petrology | Mark D.F.,Scottish Universities Environmental Research Center | Tsikos H.,Rhodes University
European Journal of Mineralogy | Year: 2011

An occurrence of the alkali-rich, Fe-Mn silicate sugilite [KNa 2(Fe3+, Mn3+, Al)2Li 3Si12O30] is reported from the Wolhaarkop manganese-rich chert breccia at the Bruce iron-ore mine, Northern Cape Province, South Africa. The upper portion of the breccia is dominated by braunite, albite, K-feldspar and aegirine, and locally hosts irregular-shaped void-fills (generally <5 cm) consisting of both zoned and unzoned assemblages of alkali-rich Mn-silicates. The zoned void-fills are characterised by marginal needles of a mineral with norrishite-like composition [KMn3+ 2LiSi4O10(O)2] penetrating into a persistent wall zone that comprises tabular serandite [NaMn2Si 3O8(OH)], albite, K-feldspar and lesser granular sugilite. The cores of larger vugs are occupied by masses of interlocking armbrusterite [K5Na6Mn3+Mn2+14(Si 9O22)4(OH)10.4H2O] and/or fibrous sugilite. Unzoned assemblages contain a similar mineralogy, comprising albite, K-feldspar, quartz, sugilite, norrishite, serandite, armbrusterite and minor witherite, strontianite and kentrolite [PbMn2O 2(Si2O7)]. Similarities to sugilite-bearing assemblages at the Wessels mine in the Kalahari Manganese Field suggest that a single regional hydrothermal alteration event in the vicinity of the unconformity between the Olifantshoek and Transvaal Supergroups was the probable genetic cause for both occurrences. 40Ar-39Ar dating of fibrous sugilite, however, yields an age of 620.2 ± 3.3 Ma (2σ) that is distinctly younger than the ages reported for the assemblages from the Kalahari Manganese Field. © 2011 E. Schweizerbart'sche Verlagsbuchhandlung. Source

Steele-MacInnis M.,University of Arizona | Steele-MacInnis M.,Institute for Geochemistry and Petrology | Ridley J.,Colorado State University | Lecumberri-Sanchez P.,University of Arizona | And 3 more authors.
Earth-Science Reviews | Year: 2016

Fluid inclusions are commonly the best available source of information on the compositions of fluids in past geologic environments. Microanalytical data, predominantly from LA-ICPMS, allow assessment of the relative abundances of chemical elements in fluid inclusions. Such data show that geologic fluids commonly contain appreciable concentrations of multiple salts in addition to NaCl, particularly KCl, CaCl2, and FeCl2 as major components. Quantification of absolute salt concentrations generally requires an internal standard concentration, which is typically derived from microthermometric measurements interpreted according to the vapor-saturated liquidus relations of simpler systems such as H2O-NaCl or H2O-NaCl-CaCl2. Here, we review and reassess compositional information obtainable from microthermometric measurements in multicomponent chloride-dominated aqueous systems. To do so, we investigate the systematics of vapor-saturated liquidus phase equilibria in complex multicomponent electrolyte solutions through thermodynamic modeling based on Pitzer's equations. We focus on low- to intermediate-salinity chloride-dominated inclusions, in which ice is the liquidus phase, and on the temperature range from subsolidus conditions to <25 °C. On the basis of measured ice and hydrohalite melting temperatures, fluids with predominantly monovalent (Na ± K) chlorides, or mixtures of monovalent and divalent cation chlorides can be identified and a robust value of mNa as a proportion of total cations be calculated. We show that microthermometric measurements alone do not allow unequivocal determination of the identity of salts that are present in addition to NaCl. In combination with microanalytical determination of cation ratios, however, robust compositional results for multi-salt aqueous fluid inclusions can be obtained using microthermometric measurements interpreted with generic H2O-(Na,K)Cl-σXn+Cln phase stability relations. © 2016 Elsevier B.V. Source

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