Olivine/melt partitioning of the transition metal cations, Fe2+, Mn2+, Co2+, and Ni2+, together with Mg2+ and Ca2+, has been examined experimentally as a function of melt composition at ambient pressure. Melt structure was inferred from bulk-chemical composition, existing structural data, and Fe-57 resonant absorption Mossbauer spectroscopy. Under isothermal conditions, K-D(i-Mg)(olivine/melt) = (C-i/C-Mg)(olivine)(C-i/ C-Mg)(melt) , is an exponential function of melt NBO/T for i = Ca2+, Mn2+, Co2+, and Ni2+. For i = Fe2+, the relationship is parabolic with maximum K-D(Fe2+-Mg)(olivine/melt)-values at NBO/T near 1. At constant melt NBO/T, K-D(i-Mg)(olivine/melt) increases systematically with decreasing cation radius, an effect that is more pronounced the more polymerized the melt. The K-D(i-Mg)(olivine/melt) is also a positive and linear function of Na/(Na + Ca) of Al-free melts. This latter effect results from changes in Q(n)-species abundance governed by Na/(Na + Ca) of the melts. The enthalpy of the exchange equilibrium, i(olivine) + Mg-melt = i(melt) + Mg-olivine, derived from the temperature-dependence of K-D(i-Mg)(olivine/melt), is also a positive function of the ionic radius of the, i-mg) cation. The relationship of enthalpy to melt polymerization also depends on cation radius. The K-D(Fe2+-Mg)(olivine/melt) does not, however, follow this trend possibly because the bond distance, d(Fe2+-O), in the melts depends on melt composition.

We report a comprehensive imaging study including confocal microRaman spectroscopy, scanning electron microscopy (SEM), and 3-D extended focal imaging light microscopy of carbonate globules throughout a depth profile of the Martian meteorite Allan Hills (ALH) 84001 and similar objects in mantle peridotite xenoliths from the Bockfjorden volcanic complex (BVC), Svalbard. Carbonate and iron oxide zoning in ALH 84001 is similar to that seen in BVC globules. Hematite appears to be present in all ALH 84001 carbonate-bearing assemblages except within a magnesite outer rim found in some globules. Macromolecular carbon (MMC) was found in intimate association with magnetite in both ALH 84001 and BVC carbonates. The MMC synthesis mechanism appears similar to established reactions within the Fe-C-O system. By inference to a terrestrial analogue of mantle origin (BVC), these results appear to represent the first measurements of the products of an abiotic MMC synthesis mechanism in Martian samples. Furthermore, the ubiquitous but heterogeneous distribution of hematite throughout carbonate globules in ALH 84001 may be partly responsible for some of the wide range in measured oxygen isotopes reported in previous studies. Using BVC carbonates as a suitable analogue, we postulate that a low temperature hydrothermal model of ALH 84001 globule formation is most likely, although alteration (decarbonation) of a subset of globules possibly occurred during a later impact event.

The influence on olivine/melt transition metal (Mn, Co, Ni) partitioning of Al3+ double left right arrow Si4+ substitution in the tetrahedral network of silicate melt structure has been examined at ambient pressure in the 1450-1550 degrees C temperature range. Experiments were conducted in the systems NaAlSiO4-Mg2SiO4-SiO2 and CaAl2Si2O8-Mg2SiO4-SiO2 with about 1 wt% each of MnO, CoO, and NiO added. These compositions were used to evaluate how, in silicate melts, Al3+ double left right arrow Si4+ substitution and ionization potential of charge-balancing cations affect activity-composition relations in silicate melts and mineral/melt partitioning.

The structure of silicate glasses and the corresponding liquids at high pressure and their structure-property relations remain difficult questions in modern physical chemistry, geochemistry, and condensed matter physics. Here we report high-resolution solid-state 0-17 3QMAS NMR spectra for mixed cation Ca-Na silicate glasses quenched from melts at high pressure up to 8 GPa. The spectra provide the experimental evidence for the varying pressure-dependence in two different types of nonbridging oxygen (NBO) environments (i.e., Na-O-Si and mixed {Ca,Na}-O-Si) in the single glass composition. The percentage of NBO drops significantly with increasing pressure and is a complex function of melt composition, including cation field strength of network modifying cations. A decrease in NBO fraction with pressure is negatively correlated with the element partitioning coefficient between crystals and liquids at high pressure.

The two most abundant network-modifying cations in magmatic liquids are Ca2+ and Mg2+. To evaluate the influence of melt structure on exchange of Ca2+ and Mg2+ with other geochemically important divalent cations (in-cations) between coexisting minerals and melts, high-temperature (1470-1650 degrees C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg2SiO4-CaMgSi2O6-SiO2 with <= 1 wt% m-cations (Mn2+, CO2+, and Ni2+) substituting for Ca2+ and Mg2+. The bulk melt NBO/Si-range (NBOISi: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca2+, Ca2+-NBO) is linearly related to NBOISi, whereas fraction of Mg2+-NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of KD(m-mg) with NBO/Si(Ca) for the exchange equilibrium, librium, m(melt) + mg(otivine) reversible arrow m(olivine) + Mg-melt, is linear. KD(m-mg) decreases as an exponential function of increasing ionic potential, Z/r(2) (Z: formal electrical charge, r: ionic radius-here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, Delta H, decreases linearly with increasing Z/r(2) [Delta H = 261(9)-81(3)-Z/r(2) (angstrom(-2))]. From existing information on (Ca,Mg)O-SiO2 melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Q(n)-species in the melts. The negative partial derivative KD(m-mg)/partial derivative(Z/r(2)) and partial derivative(Delta H)/partial derivative(Z/r(2)) is because increasing Z/r(2) is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Q(n)-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts. (C) 2008 Elsevier Ltd. All rights reserved.

A diopside composition silicate glass containing 8 wt% Fe2O3 was prepared from melt equilibrated at 1500 degrees C and different redox conditions in the range logf(O2) = -0.7 (air) to logf(O2) = -6. The Fe2+/Fe-T was measured using Mossbauer spectroscopy. The Mossbauer data were used to calibrate Raman-scattering intensity variations of the same samples as a function of oxidation state, providing a simple empirical method to determine the redox ratio of this glass. This new Micro-Raman-based method has been used to quantify redox profiles across partially oxidized samples. No significant Fe2+/Fe-T, gradients were found (values were constant from the surface to the center), although the average oxidation state was observed to increase as a function of time. The former result contrasts with O self-diffusion profiles measured with the ion microprobe on diopside glasses prepared at similar experimental conditions, for which strong isotopic gradients were found at the sample scale (corresponding to a self-diffusion coefficient for O at 1450 degrees C of 1 x 10(-11) m(2)/s). Local oxidation of Fe in the melt therefore appears to occur independently of long-range diffusion of O from the sample surface. A mechanism capable of explaining this observation is proposed based upon the fact that redox gradients result in the generation of electromotive forces. This results in a powerful driving force to wipe out redox gradients through fast electron transfer. However, migration of electrons alone would result in unfavorable charge gradients, in particular at the surface of the sample. At the temperature of our experiments, the local mobility of O is apparently sufficient to compensate the migration of electrons. Despite rapid charge transfer, the bulk oxidation state of our sample is nevertheless limited by the addition of external O. The time dependence of the bulk oxidation state of our samples can be modeled by a constant rate of O diffusion across the interface of 2.1 10(-7) m/s. However, the bulk oxidation state of the liquid is also found to be concordant with variations calculated assuming that diffusion of O is the rate-limiting mechanism. This apparent paradox may be explained if the characteristic time-scales of O self-diffusion in the sample volume and of O incorporation at the sample surface are similar. We suggest that this is indeed the case, given that both of these processes are likely to be limited by the frequency of bond-breaking and bond-forming events in the liquid.

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (f(H2)), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1-2.5 GPa pressure and 1300-1500 degrees C temperature ranges. The f(H2) was controlled at values defined by the magnetite-hematite (MH), Mn3O4-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H2O.

The solubility and solution mechanisms of reduced C-O-H volatiles in Na(2)O-SiO(2) melts in equilibrium with a (H(2) + CH(4)) fluid at the hydrogen fugacity defined by the iron-wustite + H(2)O buffer [f(H2)(IW)] have been determined as a function of pressure (1-2.5 GPa) and silicate melt polymerization (NBO/Si: nonbridging oxygen per silicon) at 1400 degrees C. The solubility, calculated as CH(4), increases from similar to 0.2 wt% to similar to.5 wt% in the melt NBO/Si-range similar to 0.4 to similar to 1.0. The solubility is not significantly pressure-dependent, probably because f(H2)(IW) in the 1-2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H(2) + CH(4)) fluid varied by similar to 14%. in the NBO/Si-range of these melts.

The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S-298 - S-0 values of 56.7 and 116 J/mol for FeO and Fe2O3, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO-"FeO"-Al2O3-SiO2 system and that association of Ca2+ and Na+ with Fe3+ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements. (C) 2009 Elsevier Ltd. All rights reserved.