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1.
The concentrations and behavior of oxygen and oxide ion were studied in silicate melts of composition CaO · MgO · xSiO2 (1.25 ≤ x ≤ 3) in the temperature range 1425 to 1575°C by cyclic voltammetry and chronopotentiometry. Electroreduction of oxygen is a reversible, 2 electron process involving dissociated oxygen atoms. The Henry's Law constant for O2 in molten diopside (CaO · MgO · 2SiO2) is 0.023 ± 0.004 mole/l atm at 1450°C. The diffusion coefficient for molecular oxygen in diopside melt is 4.5 ± .5 × 10?6 cm2/sec at 1450°C and the activation energy of diffusion is 80 ± 2 kcal/mole. Oxide ions produced by electroreduction of oxygen, rapidly dissociate silicate polymers, causing the concentration of free oxide ions in diopside melt to be buffered at a low level (4.7 ± .8 × 10?5 mole/l). The concentration of free oxide ion increases at higher proportions of metal oxides but remains at this value in more silicic melts. The rate of formation of oxide ions by polymerization in diopside melt is 0.021 ± .007 mole/l sec. Thermodynamic parameters (the standard free energy, enthalpy and entropy) for the oxidation of Ni, Co, and Zn in diopside melt in equilibrium with gaseous oxygen agree with those for solid oxide systems. The platinum reference electrode in molten diopside is a reversible, oxygen electrode.  相似文献   

2.
Kinetic effects on trace element partitioning have been measured for anorthite, forsterite, and diopside grown from synthetic compositions doped with REE. A seeding technique allowed determination of crystal growth rates and partitioning information was obtained from electron microprobe analyses. Compositional deviations from equilibrium values were sought in the crystals and as gradients in the quenched liquids adjacent to the crystals. The principal result is that large deviations in trace element distribution coefficients from equilibrium values do not occur because of a compensating effect. Rapid growth depletes the melt adjacent to the crystal in the elements of which the crystal is composed, leading to different values for apparent distribution coefficients. However, as the boundary layer melt becomes depleted in the components of the crystal, growth slows and the size of the compositional perturbations decreases. Crystals grown at very high rates (e.g., > 0.2 μm/sec for diopside) tended to be too small for accurate microprobe analyses, but are probably not compositionally extreme since the melts adjacent to the crystals did not acquire sizable compositional gradients. At moderately high growth rates (e.g., 0.02 μm/sec), crystals form in the presence of boundary layer compositions perturbed by as much as 10% from bulk melt values and, in diopside, attain concentrations for excluded trace elements about 70% higher than equilibrium values for crystals plus bulk melt. At the slower growth rates typical of igneous systems, kinetic effects on trace element partitioning are probably negligible.  相似文献   

3.
Gem-quality chrysolite (peridot) from a phlogopite deposit related to the Kovdor ultrabasic-alkaline massif in the Kola Peninsula, Russia, was studied using a variety of techniques (optical mineralogical microscopy, chemical, Mössbauer spectroscopy, and photoluminescence) to determine its chemical composition, the Fe2+/Fe3+ ratio, refraction indexes, density, as well as to examine inclusions in it. Much attention was devoted to the microprobe identification of crystalline inclusions in the host chrysolite (apatite, tetraferriphlogopite, amphibole, and magnetite), its exsolution products (diopside and magnetite), and the daughter phases of melt inclusions in this mineral (which were subdivided into primary and secondary genetic types). The daughter phases of these melt inclusions are silicates (forsterite, diopside, tetraferriphlogopite, clinohumite, and serpentine), various carbonates (Ca-dominated carbonates are characteristic of the primary inclusions, whereas Mg-rich carbonates were found only in the secondary inclusions), magnetite, djerfisherite (alkali sulfide), and Ba-Sr-REE carbonates. The presence of melt inclusions testifies to a magnatic genesis of the gem, and the simultaneous occurrence of these inclusions with crystalline inclusions can be used as an additional identification feature of gem chrysolite from the Kovdor Massif.  相似文献   

4.
Calcium- and aluminum-rich inclusions (CAIs), occurring in chondritic meteorites and considered the oldest materials in the solar system, can provide critical information about the environment and time scale of creation of planetary materials. However, interpretation of the trace element and isotope compositions of CAIs, particularly the light elements Li, Be, and B, is hampered by the lack of constraint on melilite-melt and spinel-melt partition coefficients. We determined melilite-melt and spinel-melt partition coefficients for 21 elements by performing controlled cooling rate (2 °C/h) experiments at 1 atmosphere pressure in sealed platinum capsules using a synthetic type B CAI melt. Trace element concentrations were measured by secondary ion mass spectrometry (SIMS) and/or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Melilites vary only slightly in composition, ranging from Åk31-43. Results for the partitioning of trace elements between melilite and melt in three experiments and between spinel and melt in two experiments show that partition coefficients are independent of trace element concentration, are in good agreement for different analytical techniques (SIMS and LA-ICP-MS), and are in agreement with previous measurements in the literature. Partition coefficients between intermediate composition melilites and CAI melt are the following: Li, 0.5; Be, 1.0; B, 0.22; Rb, 0.012; Sr, 0.68; Zr, 0.004; Nb, 0.003; Cs, 0.002; Ba, 0.018; La, 0.056; Nd, 0.065; Sm, 0.073; Eu, 0.67; Er, 0.037; Yb, 0.018; Hf, 0.001; Ta, 0.003; Pb, 0.15; U, 0.001; Th, 0.002. Site size energetics analysis is used to assess isovalent partitioning into the different cation sites. The Young’s modulus deduced from +2 cations partitioning into the melilite X site agrees well with the bulk modulus of melilite based on X-ray diffraction methods. The changes in light element partitioning as melilite composition varies are predicted and used in several models of fractional crystallization to evaluate if the observed Li, Be, and B systematics in Allende CAI 3529-41 are consistent with crystallization from a melt. Models of crystallization agree reasonably well with observed light element variations in areas previously interpreted to be unperturbed by secondary processes [Chaussidon, M., Robert, F., McKeegan, K.D., 2006. Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived 10Be and for the possible presence of the short-lived nuclide 7Be in the early solar system. Geochim. Cosmochim. Acta70, 224-245], indicating that the trends of light elements could reflect fractional crystallization of a melt. In contrast, areas interpreted to have been affected by alteration processes are not consistent with crystallization models.  相似文献   

5.
This experimental study examines the mineral/melt partitioning of incompatible trace elements among high-Ca clinopyroxene, garnet, and hydrous silicate melt at upper mantle pressure and temperature conditions. Experiments were performed at pressures of 1.2 and 1.6 GPa and temperatures of 1,185 to 1,370 °C. Experimentally produced silicate melts contain up to 6.3 wt% dissolved H 2O, and are saturated with an upper mantle peridotite mineral assemblage of olivine+orthopyroxene+clinopyroxene+spinel or garnet. Clinopyroxene/melt and garnet/melt partition coefficients were measured for Li, B, K, Sr, Y, Zr, Nb, and select rare earth elements by secondary ion mass spectrometry. A comparison of our experimental results for trivalent cations (REEs and Y) with the results from calculations carried out using the Wood-Blundy partitioning model indicates that H 2O dissolved in the silicate melt has a discernible effect on trace element partitioning. Experiments carried out at 1.2 GPa, 1,315 °C and 1.6 GPa, 1,370 °C produced clinopyroxene containing 15.0 and 13.9 wt% CaO, respectively, coexisting with silicate melts containing ~1–2 wt% H 2O. Partition coefficients measured in these experiments are consistent with the Wood-Blundy model. However, partition coefficients determined in an experiment carried out at 1.2 GPa and 1,185 °C, which produced clinopyroxene containing 19.3 wt% CaO coexisting with a high-H 2O (6.26±0.10 wt%) silicate melt, are significantly smaller than predicted by the Wood-Blundy model. Accounting for the depolymerized structure of the H 2O-rich melt eliminates the mismatch between experimental result and model prediction. Therefore, the increased Ca 2+ content of clinopyroxene at low-temperature, hydrous conditions does not enhance compatibility to the extent indicated by results from anhydrous experiments, and models used to predict mineral/melt partition coefficients during hydrous peridotite partial melting in the sub-arc mantle must take into account the effects of H 2O on the structure of silicate melts.  相似文献   

6.
The partitioning behavior of cerium, europium, gadolinium and ytterbium between an aqueous “vapor” phase and water saturated silicate melt have been experimentally examined using a new experimental approach employing radioactive tracers and a double-capsule technique. Equilibrium was established by reversing the partition coefficient1 and by betatrack autoradiography. Aqueous solution compositions were varied by adding different amounts of chloride and in some cases fluoride or carbon dioxide. The H2O contents of the Spruce Pine pegmatite melts were varied by conducting experiments at 4.0 kb, 800°C and at 1.25 kb, 800°C. A jadeite-nepheline composition (75 wt% Jadeite) also was employed at 4.0 kb, 800°C.The chloride experiments (Spruce Pine 4 kb, 800°C) show a linear relationship between the cube of the chloride molality and the partition coefficients of the trivalent rare earths. Europium, under the experimental fO2 conditions (quartz-fayalite-magnetite buffer), varied linearly as the fifth power of the chloride molality. At the chloride molalities examined (<1.1 mC1), all the rare earths partitioned preferentially into the melt phase (KPRE <1). Relative to pure water, the presence of chloride and fluoride fon increased the partitioning of the individual rare earths into the vapor phase, while carbon dioxide did not. Europium anomalies were recorded 1n all experiments, particularly those involving the Spruce P1ne melt at 4.0 kb and 800°C which displayed a large positive europium anomaly at all chloride molalities. Furthermore, a relative fractionation of the trivalent rare earths was also observed in these experiments, such that KPCe>KPGd>KPYb. The smaller ytterbium ion was consistently concentrated in the melt phase relative to the other rare earths in all experiments on the Spruce Pine composition. Experiments on the jadeite-nepheline composition showed no relative fractionation and a positive europium anomaly. The 1.25 kb experiment on the Spruce Pine composition showed a negative europium anomaly in plots of KpRE vs. REE.The overall rare earth partitioning at a constant chloride molality (mCl = .914) was such that KPSP(1.25 kb) > KPSP(4.0 kb) > KPJd-Ne(4.0 kb), where SP = Spruce Pine, Jd-Ne = jadeitenepheli Using the model of Burnnam (1975), It is suggested that the trivalent rare earth partitioning is related to the cube of the melt octahedral site concentration; a property which 1n hydrous melts 1s dependent on melt composition and hydroxyl molality. Excellent agreement was found for the Spruce Pine melt, whereas the jadeite-nepheline melt gave apparent hydroxyl molalities which were too high for the measured partition coefficient. Additional octahedral sites are proposed for this unusual composition perhaps due to some aluminum in 6-fold coordination. The apparent compositional variation of europium partitioning at a constant oxygen fugacity is believed to be related to both the octahedral melt site concentration for trlvalent europium and an 8-coordinated site concentration for divalent europium. Any parameter which affects the numbers of these sites (PH2O, melt composition) will affect the rare earth partitioning. The observed dependency of the partition coefficient on the structural state of the melt could be as significant as its dependency on crystalline structural constraints. Furthermore, since PH2O can drastically effect the melt structural state, its effects could be reflected in melt/crystal partition coefficients.  相似文献   

7.
Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust. This study provides an extensive set of mineral trace element data obtained by LA-ICP-MS analyses of mafic and intermediate granulites from Central Finland. Mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the REE and ilmenite for the HFSE. Coherent mineral/mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts. Mineral trace element data were used for the formulation of a set of D mineral/melt partition coefficients that is applicable for trace element modelling under lower crustal conditions. D mineral/melt were derived by the application of predictive models and using observed constant mineral/mineral ratios. The comparison of the calculated D mineral/melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral/melt partitioning. The D values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions. They provide a first coherent set of D values for Sc, V, Cr and Ni between clinopyroxene, amphibole, garnet, orthopyroxene, ilmenite and melt. In addition, they emphasize the strong impact that ilmenite exerts on the distribution of Nb and Ta.  相似文献   

8.
We have determined the partition coefficients of a large number of trace elements between CaTiO3 perovskite and anhydrous silicate melts at atmospheric pressure and 3 GPa. Determination of the concentration limits of Henrys law behaviour in the CaO-Al2O3–SiO2–TiO2 system reveals that the incorporation of rare earth elements (REE) and tetravalent large ion lithophile elements (LILE4+ such as U and Th) at the Ca-site of CaTiO3 perovskite occurs with charge compensation through Ca-vacancy formation rather than by coupled substitution of Al for Ti. When melt composition is varied, we find that partition coefficients for REE and Th are strong functions of the CaO content of the melt. The observed trends are in excellent agreement with those predicted from the Ca-vacancy model. Given that they adopt the same crystal structure and have similar trace element partitioning behaviour, CaTiO3 perovskite and the deep mantle phase CaSiO3 perovskite can be considered analogous to one another. When the analogy is pursued in detail, we find that partitioning into both phases follows the composition-dependence predicted by the Ca-vacancy model. Thus, substitution of REE, U4+ and Th into CaSiO3 in the lower mantle also occurs with Ca-vacancy formation to balance charge. Furthermore when 2+, 3+ and 4+ partition coefficients for both phases are plotted as functions of CaO melt content, the trends for CaSiO3 and CaTiO3 appear to be continuous. This surprising result means that partitioning into Ca-perovskite is independent of pressure and temperature and also of whether or not the host is CaSiO3 or CaTiO3. One implication is that CaSiO3 crystallising from a peridotitic magma ocean may have partition coefficients for Th and U up to about 400. Crystallisation and sequestration of as little as 0.25 volume% of this phase in the lower mantle early in earth history would make a significant contribution to current mantle heat production.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

9.
Isobaric and isothermal experiments were performed to investigate the effect of melt composition on the partitioning of trace elements between titanite (CaTiSiO5) and a range of different silicate melts. Titanite-melt partition coefficients for 18 trace elements were determined by secondary ion mass spectrometry (SIMS) analyses of experimental run products. The partition coefficients for the rare earth elements and for Th, Nb, and Ta reveal a strong influence of melt composition on partition coefficients, whereas partition coefficients for other studied monovalent, divalent and most quadrivalent (i.e., Zr, Hf) cations are not significantly affected by melt composition. The present data show that the influence of melt composition may not be neglected when modelling trace element partitioning.It is argued that it is mainly the change of coordination number and the regularity of the coordination space of trace elements in the melt structure that controls partition coefficients in our experiments. Furthermore, our data also show that the substitution mechanism by which trace elements are incorporated into titanite crystals may be of additional importance in this context.  相似文献   

10.
Microscopic distribution and speciation of Na, Mg, Ni and Cs in a simulated (inactive) nuclear waste glass were studied using micro X-ray fluorescence (μ-XRF) and micro X-ray absorption spectroscopy (μ-XAS), after aqueous leaching during 12 years at 90 °C. Na and Mg are major constituents of the glass that can be used to determine the progress of the glass corrosion process and the nature of secondary alteration phases. Ni and Cs represent dose determining long-lived radionuclides (59Ni, 135Cs) in vitrified nuclear waste.The Na-Mg μ-XRF maps revealed that the core regions of the glass fragments are apparently unaltered and compositionally homogeneous, whereas rims and interstitial spaces are enriched with Mg-rich smectite formed during the leaching process. The micro X-ray absorption near edge structure (μ-XANES) spectra collected at the Mg K-edge in the altered zones show three sharp resonances typical for crystalline Mg-silicates. These resonances are distinctive of Mg occupying undistorted octahedral positions. In contrast, the μ-XANES spectra collected in the core zones of the glass fragments lack this resonance pattern and are identical to the spectra measured on the pristine (unleached) MW glass.Micro extended X-ray absorption fine structure (μ-EXAFS) and μ-XANES analyses at the Ni K-edge revealed three distinct Ni(II) species: (a) Ni uniformly distributed in the glass matrix, (b) micro-inclusions with high Ni concentrations and (c) Ni associated to the Mg-clay. The comparison with reference spectra of unleached MW and other Ni-bearing silicate glasses indicated that species (a) represents the original coordinative environment of Ni in the glass. The μ-EXAFS analyses revealed that species (b) is structural Ni in trevorite (NiFe2O4), which probably formed through unmixing processes during the cooling of the glass melt. The μ-EXAFS of species (c) could be successfully modeled assuming specific adsorption or incorporation of Ni into the lattice of trioctahedral Mg-clay minerals. Alternative models assuming other elements (Ni, Al, Fe) in addition to Mg in the second shell could not be fitted successfully.Aqueous concentration data were used to calculate the speciation of the leaching solutions. Saturation index (SI) calculations indicate undersaturation with respect to NiCO3 and NiSO4·7H2O, but oversaturation with respect to β-Ni(OH)2. The latter result is probably due to the omission of Ni borate and Ni silicate complexes in the speciation calculations, for which formation constants are not available. With the help of estimation techniques, we could infer that such complexes would dominate the Ni speciation and consequently reduce the SI below the saturation of β-Ni(OH)2.The μ-XRF maps show that Cs is uniformly distributed in the MW glass, since no region with high Cs concentration could be detected. The Cs LIII-edge μ-XAS spectra were all very similar independently of the degree of alteration, indicating similar coordination environments of Cs in the core regions of the glass as well as in the secondary clays. These spectra largely differ from that measured for pollucite (a potential secondary Cs-phase in altered glasses) implying that the coordination environments of Cs in the MW glass and in pollucite are fundamentally different.The present study shows that μ-XRF and μ-XAS are essential tools in determining the fate and the retention mechanisms of radionuclides released from nuclear waste during aqueous alteration. Our spectroscopic analyses allowed us to exclude formation of specific Ni and Cs secondary solids (e.g. nepouite, β-Ni(OH)2, pollucite) during the aqueous alteration. Ni and Cs are instead distributed as trace elements in the alteration phases formed by major elements during the leaching process. Our results imply that solid solution and/or adsorption equilibria, rather than pure phase solubility equilibria, are the adequate chemical models to determine Ni and Cs aqueous concentrations in performance assessments for radioactive waste repositories.  相似文献   

11.
We propose a theory for crystal-melt trace element partitioning that considers the energetic consequences of crystal-lattice strain, of multi-component major-element silicate liquid mixing, and of trace-element activity coefficients in melts. We demonstrate application of the theory using newly determined partition coefficients for Ca, Mg, Sr, and Ba between pure anorthite and seven CMAS liquid compositions at 1330 °C and 1 atm. By selecting a range of melt compositions in equilibrium with a common crystal composition at equal liquidus temperature and pressure, we have isolated the contribution of melt composition to divalent trace element partitioning in this simple system. The partitioning data are fit to Onuma curves with parameterizations that can be thermodynamically rationalized in terms of the melt major element activity product (aAl2O3)(aSiO2)2 and lattice strain theory modeling. Residuals between observed partition coefficients and the lattice strain plus major oxide melt activity model are then attributed to non-ideality of trace constituents in the liquids. The activity coefficients of the trace species in the melt are found to vary systematically with composition. Accounting for the major and trace element thermodynamics in the melt allows a good fit in which the parameters of the crystal-lattice strain model are independent of melt composition.  相似文献   

12.
The partitioning of Ni between olivine and silicate melt has been investigated experimentally at atmospheric pressure in air. Beta track autoradiography using 63Ni and direct microprobe analysis of polished run products were employed. At constant temperature and bulk composition, the olivineliquid partition coefficient for Ni in the system Di70Fo25Qtz5 remains independent of concentration from approximately 10 ppm to 40,000 ppm Ni in olivine. Similar experiments by Mysen (1979) in the system Jd80Fo20 utilizing beta track autoradiography alone indicated that Henry's Law was followed only in the concentration interval of approx. 10–1000 ppm Ni in olivine. Above 1000 ppm, olivine/liquid partition coefficients decreased monotonically to about half of the value observed below 1000 ppm. We have performed experiments in the system Jd80Fo20, but are unable to replicate Mysen's results. While in agreement with Mysen below 1000 ppm Ni in olivine, we do not observe the decrease in partition coefficient value at higher concentrations. We conclude from our reversed experiments that, at constant temperature and bulk composition, the olivineliquid partition coefficient for Ni in the system Jd80Fo20 remains independent of concentration from approx. 10–60,000 ppm Ni in olivine. Attempts to resolve these differing conclusions by changing experimental techniques have been unsuccessful.  相似文献   

13.
The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal strategies have been employed, one in which the sample is brought to volumetric equilibrium by long-duration dwells at low temperatures (817 °C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures (890–913 °C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature range of 817–913 °C of 84.4 ± 2.8 × 10−4 cm3/mol K. The expansivity is 65% higher than that obtained for diopside melt obtained at superliquidus temperatures using the double bob Archimedean method. Combined fitting of the new low temperature, volume–temperature data from the present study and the superliquidus data from the literature has been performed. The combined fit yields the following equations for the volume–temperature relationship of diopside liquid (T=temperature in °C):
The standard error of the fit using both equations reproduces the volume–temperature data for diopside liquid within experimental error. This result reconciles the disparate values of expansivity measured at low temperatures in the supercooled state and at superliquidus temperatures and confirms the temperature-dependence of the expansivity of diopside liquid. Comparison with previous low temperature estimates of melt volume and expansivity are discussed in light of these new results. Received: 18 November 1999 / Accepted: 24 January 2000  相似文献   

14.
Partitioning of manganese between forsterite and silicate liquid   总被引：1，自引：0，他引：1  
Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na2O-Al2O3-SiO2 (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated Dmnfo?Liq) depends on liquid composition as well as temperature: at 1350°C, DMnFo?Liqranges from 0.60 (basic melt, SiO2 = 47wt%) to 1.24 (acidic melt, SiO2 = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition.The effects of melt composition and temperature on DMnfo?Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of DmnFo?Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting DMnFo?Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In DMnFo?Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn2+ ions in acidic melts than in basic melts.Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.  相似文献   

15.
The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific (N = 7), the Atlantic (N = 10) and the Indian (N = 11) oceanic ridges and the Red Sea (N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (Xsulf), the achievement of equilibrium between sulfide melt and silicate melts (Reff), and the respective proportions between fractional and batch crystallization processes (Sb) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼103) to Ir (∼105). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.  相似文献   

16.
Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E a = 332 kJ mol-1, and the apparent rate constant, k = 1041.2 mol diopside cm-2 s-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m-2) than the pits nucleated at defects (39 to 65 mJ m-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (Eb-homogeneous = 2.59 × 10-16 mJ K-1) were lower than the pits nucleated at defects (Eb-defect assisted = 8.44 × 10-16 mJ K-1).  相似文献   

17.
Additions of the low occurrence stable isotopes 61Ni, 65Cu, and 68Zn were used as tracers to determine the exchange kinetics of metals between dissolved and particulate forms in laboratory studies of natural water and suspended sediments from South San Francisco Bay, CA. Dissolved metal isotope additions were made so that the isotope ratios (rather than total metal partitioning) were significantly altered from initial ambient conditions. Dissolved metal concentrations were determined using an organic ligand sequential extraction technique followed by analysis with high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS). Exchangeable particulate concentrations were extracted using a 20% acetic acid leach followed by determination using HR-ICPMS. Equilibrium and kinetic sorption parameters were quantified according to a general model for trace metal partitioning assuming pseudo-first-order kinetics. Partition coefficients (KD) were tracked as a function of time over the fortnight experiment. For Ni, Cu, and Zn the initial ambient KD values were found to be 103.65, 103.88, and 104.52 L kg−1, respectively. As a result of the dissolved metal isotope additions, the partition coefficients for all three metals dropped and then increased back to near ambient KD values after 14 days. Curve-fitting concentration versus time profiles from both dissolved and exchangeable particulate data sets allowed determination of kinetic rate constants. The best estimates of forward and backward kinetic rate constants for Ni, Cu, and Zn respectively are k′f = 0.03, 0.07, 0.12 d−1 and kb = 0.13, 0.12, 0.15 d−1. These results predict that sorption equilibria in South Bay should be reached on the order of a month for Ni, on the order of 3 weeks for Cu, and on the order of 2 weeks for Zn. Together, the dissolved and exchangeable particulate data indicate more sluggish sorption kinetics for Ni than for Cu and Zn and suggest that different chemical forms control the speciation of these three metals in South Bay. Order of magnitude metal sorption exchange rates were estimated using these kinetic results. These calculations indicate that sorption exchange between dissolved and suspended particulate phases can cause dynamic internal cycling of these metals in South San Francisco Bay.  相似文献   

18.
Partition coefficients (zircon/meltDM) for rare earth elements (REE) (La, Ce, Nd, Sm, Dy, Er and Yb) and other trace elements (Ba, Rb, B, Sr, Ti, Y and Nb) between zircon and melt have been calculated from secondary ion mass spectrometric (SIMS) analyses of zircon/melt inclusion pairs. The melt inclusion-mineral (MIM) technique shows that DREE increase in compatibility with increasing atomic number, similar to results of previous studies. However, DREE determined using the MIM technique are, in general, lower than previously reported values. Calculated DREE indicate that light REE with atomic numbers less than Sm are incompatible in zircon and become more incompatible with decreasing atomic number. This behavior is in contrast to most previously published results which indicate D > 1 and define a flat partitioning pattern for elements from La through Sm. The partition coefficients for the heavy REE determined using the MIM technique are lower than previously published results by factors of ≈15 to 20 but follow a similar trend. These differences are thought to reflect the effects of mineral and/or glass contaminants in samples from earlier studies which employed bulk analysis techniques.DREE determined using the MIM technique agree well with values predicted using the equations of Brice (1975), which are based on the size and elasticity of crystallographic sites. The presence of Ce4+ in the melt results in elevated DCe compared to neighboring REE due to the similar valence and size of Ce4+ and Zr4+. Predicted zircon/meltD values for Ce4+ and Ce3+ indicate that the Ce4+/Ce3+ ratios of the melt ranged from about 10−3 to 10−2. Partition coefficients for other trace elements determined in this study increase in compatibility in the order Ba < Rb < B < Sr < Ti < Y < Nb, with Ba, Rb, B and Sr showing incompatible behavior (DM < 1.0), and Ti, Y and Nb showing compatible behavior (DM > 1.0).The effect of partition coefficients on melt evolution during petrogenetic modeling was examined using partition coefficients determined in this study and compared to trends obtained using published partition coefficients. The lower DREE determined in this study result in smaller REE bulk distribution coefficients, for a given mineral assemblage, compared to those calculated using previously reported values. As an example, fractional crystallization of an assemblage composed of 35% hornblende, 64.5% plagioclase and 0.5% zircon produces a melt that becomes increasingly more enriched in Yb using the DYb from this study. Using DYb from Fujimaki (1986) results in a melt that becomes progressively depleted in Yb during crystallization.  相似文献   

19.
Diffusion coefficients of Co2+ and Ni2+ in synthetic single crystal forsterite along the c-axis were determined in the temperature ranges, 700–1200?°C and 800–1300?°C, respectively. The synthesized forsterite specimens were coated with thin evaporated films of CoO and NiO on the c-surface and annealed for diffusion experiments. The short penetration distance of diffusing ions in forsterite was measured by secondary ion mass spectrometry using the depth profile method. The diffusion coefficients of Co (700–1200?°C) and Ni (800–1300?°C) are given by: and The observed diffusion coefficient values show good linear relationships in Arrhenius plots and the activation energy values obtained agree well with the previous values, although the diffusion coefficient values observed at the high temperature end of the experimental range deviate from the previous values. These results indicate that Co and Ni diffuse in olivine with a single mechanism within the temperature range observed, possibly with an extrinsic in nature as in the case of Mg tracer diffusion observed by Chakraborty et?al. 1994 and of Fe-Mg interdiffusion by Chakraborty.  相似文献   

20.
Distribution coefficients have been experimentally determined for the partitioning of nickel, cobalt and manganese between calcium-rich clinopyroxenes and coexisting silicate liquids. Temperatures ranged from 1110–1360°C and oxygen fugacities in the furnaces were controlled by gas mixtures at one atmosphere total pressure. Bulk compositions used include synthetic compositions in the system albite-anorthite-diopside and a natural basalt. Charges were doped with a few percent transition metal oxides and analyzed by electron microprobe. Measured clinopyroxene/liquid distribution coefficients range from 1.5–14 for Ni, 0.5–2.0 for Co and 0.3–1.2 for Mn. Diopside/liquid distribution coefficients for nickel are shown to be independent of Ni content over a range of from 3 ppm to 3 wt.% Ni in the liquid and to increase with decreasing temperature. From analyses of pyroxenes grown from experimental charges differing only in the amounts of transition metals present, nickel and cobalt are shown to occupy the M1 site of diopside while manganese occupies both M1 and M2.Ordinary weight ratio distribution coefficients are strongly dependent on liquid composition as well as temperature. For example, experiments on synthetic Ab-An-Di compositions give clinopyroxene/liquid distribution coefficients higher by about a factor of five than those from experiments at the same temperature on a natural basalt. For Ni and Co, which occupy only the M1 site of clinopyroxene, an equilibrium constant can be defined in terms of activities of components in the liquid and solid phases. Activities of components in the solid are approximated by their mole fractions. An activity/concentration model based on the viscosity model of BOTTINGA and WEILL (1972) is used for the liquid. This model approximates the activity of silica as its mole fraction among the network-forming components SiO2, TiO2, KAlO2, NaAlO2 and Ca0.5AlO2.Activities of network modifiers such as CaO are approximated as their mole fractions among the network-modifying components CaO, MgO, FeO, FeO1.5, etc. When these estimated activities are used in the expression for the equilibrium constant, the effects of compositional differences on trace element distribution coefficients can be understood and the results of experiments on synthetic and natural compositions reconciled.  相似文献   

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