The effect of oxygen fugacity and temperature on solubilities of nickel, cobalt, and molybdenum in silicate melts (original) (raw)
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Geochimica et Cosmochimica Acta, 1994
The data presented in this paper indicate that the oxidation state of Ni in the investigated melt is Ni2* over the entire range of fo, investigated. This conclusion contrasts with recent reports in the literature of an inflection in the fo, dependence of Ni solubility, which has been interpreted as solution of neutral Ni at low fo, (MORSE et al., 199 1; COLSON, 1992; EHLERS et al., 1992). We also present data for the temperature dependence of Ni solubility in the investigated melt. The solubility decreases with increasing temperature at constant fo,. The present results are in good agreement with the metal-loop-equilibration experiments reported by HOLZHEID et al. (1994)
Geochimica Et Cosmochimica Acta, 1994
The data presented in this paper indicate that the oxidation state of Ni in the investigated melt is Ni2* over the entire range of fo, investigated. This conclusion contrasts with recent reports in the literature of an inflection in the fo, dependence of Ni solubility, which has been interpreted as solution of neutral Ni at low fo, (MORSE et al., 199 1; EHLERS et al., 1992). We also present data for the temperature dependence of Ni solubility in the investigated melt. The solubility decreases with increasing temperature at constant fo,. The present results are in good agreement with the metal-loop-equilibration experiments reported by HOLZHEID et al. ( 1994)
Metal-silicate partitioning of nickel and cobalt: The influence of temperature and oxygen fugacity
Geochimica et Cosmochimica Acta, 1994
We report new metal-silicate melt partitioning experiments for Co and Ni. One atmosphere, gas-mixing ( Hz-CO2 or CO-CO,) experiments were run at 1300,1425, and 155O'C and at oxygen fugacities between air and near or below the iron-wiistite (IW) buffer. Bulk melt composition, at the diopsideanorthite-forsterite eutectic, was constant in all runs. Metals in these experiments were platinum alloys permitting a large oxygen fUgacity range to be investigated while maintaining fixed bulk melt composition. Since a~~~~orn~sition relations of Pt-Ni and Pt-Co alloys are reasonably well known, we calculated equilibrium constants for the metal-silicate partitioning reactions. Variations of the equilibrium constants as a function of experimental parameters allowed us to monitor the chemical behavior of Ni and Co in the silicate melt.
Geochimica et Cosmochimica Acta, 1992
Partitioning of nickel and cobalt between olivine, silicate melt, and metal has been determined in the system olivine-albite-anorthite-CaO at 1350°C and 1 atm over the oxygen fugacity (Jo,) range 10-5.5 to 1O-'2.6 atm. This range of Jo,, from one log unit above Nickel-Nickel Oxide (NNO + 1) to two log units below the Iron-Quartz-Fayalite (IQF-2) buffers, spans the range relevant for crystal/liquid processes in terrestrial planets and meteorite parent bodies. There is no effect of oxygen fugacity on D$/@" over the range of fo, from 10-5.5 to 10 m9.4. Over the range of j& from 10-9.4 to 10 m'".3, D$/@" shows an apparent decrease from a value of 6.8 at log fo, =-9.4 to a value of 5.5 at log fo, =-10.3, a factor of 1.2. At the lowest value of fo, = 1O-12.6 the value for DNi o'm is 3 4 a factor of 2 lower than that measured under oxidizing conditions. Experimentally produced melts are metal saturated over the range of fo, from 10-'".3 to 10-'2.6. Partition coefficients for Co are constant (D~~'g'" = 2.8) over the range of fo, from 1O-66 to 10-'06. The Co partition coefficient decreases to a value of 1.65 at log fo, =-12.2. The variations in Dg','"'" are examined using equilibrium constants that account for the effects of variable silicate melt composition on Ni partitioning. Measured values of Dg?*" and values estimated from equilibrium constants are identical for the experiments carried out under oxidizing conditions. The predicted values of D$\'*" for the low fo, experiments are higher than the measured values. These changes in Ni and Co partitioning behavior at low so, may indicate that Ni and Co are present as Ni*+ and Ni" and Co2+ and Coo in the silicate melt. We estimate that approximately 30% of the Ni may be present as Ni" in the low fo, experiments.
Experimental study of the influence of SiO2 on the solubility of cobalt and iron in silicate melts
Petrology, 2007
The solubility of cobalt and iron in silicate melts with variable SiO 2 content was experimentally determined under controlled oxygen fugacity. It was shown that, independent of temperature and oxygen fugacity, the solubility of the two metals reaches a maximum (minimum of CoO and FeO activity coefficients) in melts of intermediate compositions. The analysis of available published data demonstrated that the γ MeO values of at least four metals (Ni, Co, Fe, and Cr) dissolving in melts as divalent oxides show a minimum in melts with ≈ 57 ± 2 mol %. The position of the minimum is essentially independent of the element, melt temperature, and oxide concentration (from a few ppm to 13 wt %). The extremes of iron solubility ( γ FeO ) in Fe-rich MgOfree melts may shift toward significantly lower values, although this inference requires additional experimental verification. Using a numerical example, some problems were discussed in the use of experimental data obtained in different laboratories for the development of a general model for the γ MeO dependence on melt composition.
Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts
Geochimica et Cosmochimica Acta, 2020
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Na2O solubility in CaO–MgO–SiO2 melts
Geochimica et Cosmochimica Acta, 2011
The sodium solubility in silicate melts in the CaO-MgO-SiO 2 (CMS) system at 1400°C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na (g) evaporation from a Na 2 O-xSiO 2 melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa 2 O (sample) = aNa 2 O (source) . This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na 2 O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO 2 < 100; in wt%), we found that Na 2 O solubility is conveniently modeled as a linear function of the optical basicity (K) calculated on a Na-free basis melt composition. In our experiments, cNa 2 O (sample) ranges from 7 Â 10 À7 to 5 Â 10 À6 , indicating a strongly non-ideal behavior of Na 2 O solubility in the studied CMS melts (cNa 2 O (sample) ( 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na 2 O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na 2 O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q 4 .
Geochimica et Cosmochimica Acta, 1993
Distribution coefficients for Fe and Ni between sulfide and silicate melts have been determined as functions of Co: and fs2. Experiments were carried out in a piston-cylinder apparatus at 1450°C and 8 kbars. In a system at sulfide saturation the J~, Co2 and aveo are interdependent; therefore, it was possible to vary foJfs~ systematically by changing the bulk Fe content of the starting materials. This was accomplished in two ways, first by varying the sulfide to silicate mass ratio, and second by synthesizing silicate starting materials of differing FeO contents. The fo~/J~ was further constrained by the choice of graphite capsule material combined with pre-oxidation of the starting silicate. This produced melts which were saturated or nearly saturated in CO -S vapor. The fo~ and J~: were computed from the independent relationships of fo~/fs2 with vapor and silicate melt compositions. The molar distribution coefficients of Ni and Fe, D(Nimol) (=X Nis ~v r<~o~ and D(Femol), exhibit sulf IA sil ] strong dependencies on the molar S concentration of the sulfide melt and FeO content of the silicate melt. D(Nimol) ranges from 1458 to 8857 for sulfide melt S concentrations of 45.8 to 51.1 mol%, respectively. Negative correlations of log D(Nimol) and log D(Femol) with log (foJfs~) are consistent with reactions between sulfide and oxide components of these elements in both melts and indicate that the controlling equilibrium is MOsil + 1/252 = MSsulf + 1/202, (M = metal). The different values of D(Niw~) reported in the literature can be rationalized in terms of differences in fo:/fs: of the experiments. It is suggested that all chalcophile elements, including PGEs, behave similarly. In particular, it is demonstrated that D(Irwt) and D(Pdw0 vary directly with the S content of the sulfide melt and indirectly with Co: in published experiments. For the possible range in foJfs2 of sulfide-saturated magmas in nature, variations in Ds for Ni and the other chalcophile elements of more than one order of magnitude may exist.
2002
The thermodynamic theory describing the partitioning of trace elements between crystals and silicate melt implies that partition coefficients should depend on the major-element composition of the melt from two different causes, namely (1) the activity coefficient of the trace-element oxide component in the melt, and (2) the activities of all the major-element components needed to balance the trace-element substitution in the crystal (the ''stoichiometric control''). Partition coefficients are also expected to vary with the composition of the crystal, and temperature and pressure. Because these variables cannot be controlled independently in direct crystal/melt partitioning studies, it has not been possible to disentangle their effects, or to determine their relative importance. In order to explore the effects of melt composition on activity coefficients of trace-element oxide components, the activity coefficients of five such components, MoO 2 , MoO 3 , FeO, NiO and CoO, were measured in 18 different melt compositions in the system CaO -MgO -Al 2 O 3 -SiO 2 plus one composition in CaO -MgO -Al 2 O 3 -SiO 2 -TiO 2 at 1400 jC, by equilibration with the metal under controlled oxygen fugacity. MoO 2 and MoO 3 are expected to have geochemical properties similar to the High Field Strength Elements (HFSEs). The activity coefficients of MoO 2 and MoO 3 vary by factors of 20 and 60, respectively, over the range of compositions investigated. Their variation is highly correlated, and mainly depends on the amount of CaO in the melt, suggesting the influence of CaMoO 3 and CaMoO 4 complexes. The analogy between Mo and HFSEs implies that melt composition can be expected to have an important influence on HFSE partition coefficients. The activity coefficients of FeO, NiO and CoO vary by a factor of two over the same range of melt compositions, but show no simple dependence on any particular major-element oxide component. However, the activity coefficients of all three components are very highly correlated with each other. This means that the effect of melt composition can be largely eliminated if the ratios of two activity coefficients are taken, as, for example, when two-element distribution coefficients are used. D Chemical Geology 186 of melting (e.g., batch or fractional), the degree of melting, and the nature and extent of any subsequent fractional crystallization. Well-known equations exist relating the concentrations of a trace element M in melt and residue to the crystal/melt partition coefficients, D M melt/crystal , and the degree of melting, F, both for batch melting and fractional melting (e.g., . Increasingly, mathematical models for more complicated scenarios are being developed (e.g., . In all this modelling, the assumption has been made that values of D M melt/crystal can be treated as constants. The same assumption has also made in Rare Earth Element (REE) inversion modelling . To what extent this assumption is reasonable, or whether it is wishful thinking, is difficult to judge, for there has been little in the way of empirical evidence as to how values of D M melt/crystal
Behavior of molybdenum in alkali silicate melts: Effects of excess SiO2 and CO2
Lithos, 1983
The effects of excess SiO2 and CO~ on the solubility of molybdetfite in hydrous sodium disilica~.e melts were experimentally determined at 680 bars and 650°C. The molybdenite solubility decreases with increasing SiO2 and CO2. Under the experimental conditions, the MoS2 content of the vapor-saturated liquid decreases from 10 wt.% to 2.5 w!.% at SlOe saturation, la the presence of CO,, the solubility decreases to 4.6 wt.% MoS:: and becemes negligible at high Pco 2. These results are explained as deriving from the increased polymerization a~:~d hence aecreased NBO/Si ratio of the melt with increasing SiOz content and CO2, respectively. Sulfi~.r dissolves principally as SO£ 2 at the relatively high fo2 of the experiments. Consequently, the effect of sulfur is to lower the Mo solubility by effectively decreasing the NBO/Si ratio of the melt. Sulfur saturation is. therefore, likely to be a limiting factor in the Mo content of alkali silicate melts because of the chalcophile affinities of molybdenum.