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1.
In this experimental study, we examine the mineral-melt partitioning of major and trace elements between clinopyroxene and CO2-rich kimberlitic melts at a pressure of 6 GPa and temperatures of 1410°C and 1430°C. The melts produced contain ∼ 28 wt% dissolved CO2, and are saturated with olivine and clinopyroxene. To assess the effects of temperature, crystal and melt compositions on trace element partitioning, experiments were performed in the model CaO-MgO-Al2O3-SiO2-CO2 system. Our results reveal that all the elements studied, except Al, Mg, Si, and Ga, are incompatible in clinopyroxene. Partition coefficients show a considerable range in magnitude, from ∼ 10−3 for DU and DBa to ∼ 2.5 for DSi. The two experimental runs show similar overall partitioning patterns with the D values being lower at 1430°C. Rare earth elements display a wide range of partition coefficients, DLa (0.012-0.026) being approximately one order of magnitude lower than DLu (0.18-0.23). Partition coefficients for the 2+ and 3+ cations entering the M2-site exhibit a near-parabolic dependence on radius of the incorporated cations as predicted from the lattice strain model. This underlines the contribution made by the crystal structure toward controlling the distribution of trace elements. Using data obtained in this study combined with that in the published literature, we also discuss the effects that other important parameters, namely, melt composition, pressure, and temperature, could have on partitioning.Our partition coefficients have been used to model the generation of the Group I (GI) kimberlites from South Africa. The numerical modeling shows that kimberlitic melts can be produced by ∼0.5% melting of a MORB-type depleted source that has been enriched by small-degree melts originating from a similar depleted source. This result suggests that the source of GI kimberlites may be located at the lithosphere-asthenosphere transition. Percolation of small degree melts from the asthenosphere would essentially create a metasomatic horizon near the bottom of the non-convecting sublithospheric mantle. Accumulation of such small degree melts together with the presence of volatiles and conductive heating would trigger melting of the ambient mantle and subsequently lead to eruption of kimberlitic melts. Additionally, our model shows that the GI source can be generated by metasomatism of a 2 Ga old MORB source ca. 1 Ga ago. Assuming that MORB-type mantle is the most depleted source of magmas on earth, then this is the oldest age at which the GI source could have existed. However, this age most likely reflects the average age of a series of metasomatic events than that of a single event.  相似文献   

2.
The energetics of multicomponent diffusion in molten CaO-Al2O3-SiO2 (CAS) were examined experimentally at 1440 to 1650°C and 0.5 to 2 GPa. Two melt compositions were investigated: a haplodacitic melt (25 wt.% CaO, 15% Al2O3, and 60% SiO2) and a haplobasaltic melt (35% CaO, 20% Al2O3, and 45% SiO2). Diffusion matrices were measured in a mass-fixed frame of reference with simple oxides as end-member components and Al2O3 as a dependent variable. Chemical diffusion in molten CAS shows clear evidence of diffusive coupling among the components. The diffusive flux of SiO2 is significantly enhanced whenever there is a large CaO gradient that is oriented in a direction opposite to the SiO2 gradient. This coupling effect is more pronounced in the haplodacitic melt and is likely to be significant in natural magmas of rhyolitic to andesitic compositions. The relative magnitude of coupled chemical diffusion is not very sensitive to changes in temperature and pressure.To a good approximation, the measured diffusion matrices follow well-defined Arrhenius relationships with pressure and reciprocal temperature. Typically, a change in temperature of 100°C results in a relative change in the elements of diffusion matrix of 50 to 100%, whereas a change in pressure of 1 GPa introduces a relative change in elements of diffusion matrix of 4 to 6% for the haplobasalt, and less than 5% for the haplodacite. At a pressure of 1 GPa, the ratios between the major and minor eigenvalues of the diffusion matrix λ1/λ2 are not very sensitive to temperature variations, with an average of 5.5 ± 0.2 for the haplobasalt and 3.7 ± 0.6 for the haplodacite. The activation energies for the major and minor eigenvalues of the diffusion matrix are 215 ± 12 and 240 ± 21 kJ mol−1, respectively, for the haplodacite and 192 ± 8 and 217 ± 14 kJ mol−1 for the haplobasalt. These values are comparable to the activation energies for self-diffusion of calcium and silicon at the same melt compositions and pressure. At a fixed temperature of 1500°C, the ratios λ1/λ2 increase with the increase of pressure, with λ1/λ2 varying from 2.5 to 4.1 (0.5 to 1.3 GPa) for the haplodacite and 4 to 6.5 (0.5 to 2.0 GPa) for the haplobasalt. The activation volumes for the major and minor eigenvalues of the diffusion matrix are 0.31 ± 0.44 and 2.3 ± 0.8 cm3 mol−1, respectively, for the haplodacite and −1.48 ± 0.18 and −0.42 ± 0.24 cm3 mol−1 for the haplobasalt. These values are quite different from the activation volumes for self-diffusion of calcium and silicon at the same melt compositions and temperature. These differences in activation volumes between the two melts likely result from a difference in the structure and thermodynamic properties of the melt between the two compositions (e.g., partial molar volume).Applications of the measured diffusion matrices to quartz crystal dissolution in molten CAS reveal that the activation energy and activation volume for quartz dissolution are almost identical to the activation energy and activation volume for diffusion of the minor or slower eigencomponent of the diffusion matrix. This suggests that the diffusion rate of slow eigencomponent is the rate-limiting factor in isothermal crystal dissolution, a conclusion that is likely to be valid for crystal growth and dissolution in natural magmas when diffusion in liquid is the rate-limiting factor.  相似文献   

3.
An end member of the tourmaline series with a structural formula □(Mg2Al)Al6(BO3)3[Si6O18](OH)4 has been synthesized in the system MgO-Al2O3-B2O3-SiO2-H2O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B2O3, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B2O3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al2O3-B2O3-SiO2-H2O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions.The (3a)-vacant tourmaline has the space group R3m with a =15.90 A?, c = 7.115 A?, and V = 1557.0 A?3. However, these values vary at room temperature with the pressure-temperature conditions of synthesis by ±0.015 A? in a, ±0.010 A? in c, and ±4.0 A?3 in V, probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are no = 1.631(2), nE = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca.  相似文献   

4.
The rate of spinel (MgAl2O4) growth at the interface between MgO and Al2O3 was investigated systematically at temperatures of 1200° to ∼2000°C and pressures between 1.0 and 4.0 GPa with a solid-media, piston-cylinder apparatus. As reported in previous 1-atm studies, the thickness (ΔX) of the spinel layer increases linearly with the square root of time for experiments differing only in duration, irrespective of pressure-temperature (P-T) conditions. The reaction rate constant (k = ΔX2/2t) is log-linear in 1/T and also in pressure. The apparent activation energy of 410 kJ/mol is independent of pressure; the apparent activation volume increases systematically with increasing temperature. Electron microprobe traverses across the spinel layer reveal a significant Al excess and charge-compensating Mg deficit near the spinel/corundum interface. This nonstoichiometry is promoted by high temperatures (>1500°C), suppressed by high pressures and varies linearly across the spinel to a near-stoichiometric composition at the interface with periclase. The Al and Mg composition gradients can be used to extract interdiffusion coefficients for Al ↔ Mg exchange through the spinel, which are described by D?=2.5×10−6 exp(−28200/T) m2sThese diffusivities differ substantially from the reaction rate constant k, reflecting the fact that k is a combination of the diffusivity and the reaction potential as indicated by the difference in spinel composition across the spinel layer (i.e., coexisting with corundum vs. coexisting with periclase). A simple model can be used to separate the two effects and show that the reaction potential (i.e., the MgO-Al2O3 phase diagram) is sensitive to changes in both temperature and pressure, whereas the governing diffusivity depends only on temperature.  相似文献   

5.
Five hundred eighty-five viscosity measurements on 40 melt compositions from the ternary system CaMgSi2O6 (Di)-CaAl2Si2O8 (An)-NaAlSi3O8 (Ab) have been compiled to create an experimental database spanning a wide range of temperatures (660-2175°C). The melts within this ternary system show near-Arrhenian to strongly non-Arrhenian properties, and in this regard are comparable to natural melts. The database is used to produce a chemical model for the compositional and temperature dependence of melt viscosity in the Di-An-Ab system. We use the Vogel-Fulcher-Tammann equation (VFT: log η = A + B/(T − C)) to account for the temperature dependence of melt viscosity. We also assume that all silicate melts converge to a common viscosity at high temperature. Thus, A is independent of composition, and all compositional dependence resides in the parameters B and C. The best estimate for A is −5.06, which implies a high-temperature limit to viscosity of 10-5.06 Pa s. The compositional dependence of B and C is expressed by 12 coefficients (bi=1,2.6, cj=1,2..6) representing linear (e.g., bi=1:3) and higher order, nonlinear (e.g., bi=4:6) contributions. Our results suggest a near-linear compositional dependence for B (<10% nonlinear) and C (<7% nonlinear). We use the model to predict model VFT functions and to demonstrate the systematic variations in viscosity due to changes in melt composition. Despite the near linear compositional dependence of B and C, the model reproduces the pronounced nonlinearities shown by the original data, including the crossing of VFT functions for different melt compositions. We also calculate values of Tg for melts across the Di-An-Ab ternary system and show that intermediate melt compositions have Tg values that are depressed by up to 100°C relative to the end-members Di-An-Ab. Our non-Arrhenian viscosity model accurately reproduces the original database, allows for continuous variations in rheological properties, and has a demonstrated capacity for extrapolation beyond the original data.  相似文献   

6.
K. K. Podlesskii 《Petrology》2010,18(4):350-368
Consistent thermodynamic data on the properties of pure mineral end members and the mixing properties of solid solutions in the system FeO-MgO-Al2O3-SiO2 were employed to simulate phase relations of sapphirine, garnet, spinel, orthopyroxene, cordierite, quartz, Al silicates, and corundum. Compositional variations of the solid solutions with temperature notably modify the topology of the P-T diagrams, which differ from the petrogenetic grids widely used in the literature. It is worth noting that the evaluation of P-T metamorphic conditions based on reaction relations in sapphirine-bearing assemblages cannot be so far considered reliable enough. The lower stability limit of the sapphirine + quartz assemblage in the system in question is possibly located at much lower P-T parameters: at least 835°C and ∼6 kbar. The sapphirine + kyanite assemblage can be stable at temperatures below 860°C and a pressure of ∼11 kbar, and the stability field of the sapphirine + olivine assemblage is narrow and constrained to temperatures no higher than ∼800°C.  相似文献   

7.
Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al2O3-SiO2 (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.  相似文献   

8.
The first natural occurrence of a high-pressure polymorph of chromite has been discovered in the shock-metamorphosed Suizhou meteorite. The composition of this high-pressure polymorph is identical to that of the precursor chromite. The Raman spectrum of this polymorph is distinct from that of chromite. Synchrotron X-ray diffraction analysis revealed that this polymorph has an orthorhombic CaTi2O4- type structure. The cell parameters are: a = 9.462(6) Å, b = 9.562(9) Å, c = 2.916(1) Å, V = 263.8(4) Å3 (Z=4), space group = Bbmm, and the density = 5.63 g/cm3 (the numbers in parentheses are standard deviations on the last significant digits). This polymorph is 11.5% denser than chromite. The P-T conditions for the phase transformation from chromite to the CaTi2O4-structured polymorph are estimated at 20-23 GPa and 1800 to 2000 °C, respectively. This dense CaTi2O4-structured FeCr2O4 phase could be a host phase for Cr, Al, Fe, Mg and Mn and other metallic elements in the deep Earth.  相似文献   

9.
Enthalpies of solution in 2PbO· B2O3 at 712°C have been measured for glasses in the systems albite anorthite diopside, NaAlO2-SiO2, Ca0.5AlO2-SiO2 and albite-anorthite-quartz. The systems albite-anorthite and diopside-anorthite show substantial negative enthalpies of mixing, albite-diopside shows significant positive heats of mixing. For compositions up to NaAlO2 = 0.42 (which includes the subsystem albite-silica) the system NaAlO2-SiO2 shows essentially zero heats of mixing. A negative ternary excess heat of mixing is found in the plagioclase-rich portion of the albite-anorthite-diopside system. The join Si4O8-CaAl2Si2O8 shows small but significant heats of mixing. In albite-anorthite-quartz. ternary glasses, the ternary excess enthalpy of mixing is positive.Based on available heat capacity data and appropriate consideration of the glass transition, the enthalpy of the crystal-glass transition (vitrification) is a serious underestimate of the enthalpy of the crystal-liquid transition (fusion) especially when the melting point, Tf, is many hundreds of degrees higher than the glass transition temperature, Tg. On the other hand, the same heat capacity data suggest that the enthalpies of mixing in albite-anorthite-diopside liquids are calculated to be quite similar to those in the glasses. The enthalpies of mixing observed in general support the structural models proposed by Taylor and Brown (1979a, b) and others for the structure of aluminosilicate glasses.  相似文献   

10.
Activity coefficients of oxide components in the system CaO-MgO-Al2O3-SiO2 (CMAS) were calculated with the model of Berman (Berman R. G., “A thermodynamic model for multicomponent melts with application to the system CaO-MgO-Al2O3-SiO2,” Ph.D. dissertation, University of British Columbia, 1983) and used to explore large-scale relationships among these variables and between them and the liquid composition. On the basis of Berman’s model, the natural logarithm of the activity coefficient of MgO, ln(γMgOLiq), and ln(γMgOLiq/γSiO2Liq) are nearly linear functions of ln(γCaOLiq). All three of these variables are simple functions of the optical basicity Λ with which they display minima near Λ ∼ 0.54 that are generated by liquids with low ratios of nonbridging to tetrahedral oxygens (NBO/T) (<0.3) and a mole fraction ratio, XSiO2Liq/XAl2O3Liq, in the range 4 to 20. Variations in ln(γCaOLiq) at constant Λ near the minimum are due mostly to liquids with (XCaOLiq + XMgOLiq)/XAl2O3Liq < 1. The correlations with optical basicity imply that the electron donor power is an important factor in determining the thermodynamic properties of aluminosilicate liquids.For a constant NBO/T, ln(γCaOLiq/γAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) form curves in terms of XSiO2Liq/XAl2O3Liq. The same liquids that generate minima in the Λ plots are also associated with minima in ln(γCaOLiqγAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) as a function of XSiO2Liq/XAl2O3Liq. In addition, there are maxima or sharp changes in slope for NBO/T > 0.3, which occur for XSiO2Liq/XAl2O3Liq ranging from ∼0 to ∼6 and increase with increasing NBO/T. The systematic variations in activity coefficients as a function of composition and optical basicity reflect underlying shifts in speciation as the composition of the liquid is changed. On the basis of correlations among the activity coefficients, it is likely that the use of CaO, an exchange component such as SiMg−1 and two of MgO, CaAl2O4, or MgAl2O4 would yield significant savings in the number of parameters required to model the excess free energy surface of liquids over large portions of CMAS relative to the use of oxide end members.Systematic behavior of thermodynamic properties extends to small amounts of other elements dissolved in otherwise CMAS liquids. For example, ln(XFe2+Liq/XFe3+Liq) at constant oxygen fugacity is linearly correlated with ln(γCaOLiq). Similarly, ln(CS), where CS is the sulfide capacity is linearly correlated at constant temperature with each of the optical basicity, ln(aCaOLiq) and ln(γCaOLiq), although the correlation for the latter breaks down for low values of Λ. The well-known systematic behavior of sulfide capacity as a function of optical basicity for systems inside as well as outside CMAS suggests that ln(γCaSLiq) is also a simple function of optical basicity and that the relationships observed among the activity coefficients in CMAS may hold for more complex systems.  相似文献   

11.
The enthalpies of solution of several synthetic garnets on the join Mg3Al2Si3O12-Ca3Al2Si3O12 (pyrope-grossular) and of several synthetic clinopyroxenes on the join CaMgSi2O6-CaAl2SiO6 (diopside-Ca-Tschermak's molecule) were measured in a melt of composition 2PbO · B2O3 at 970 K. The determinations were made with sufficient precision so that thermochemical characterizations of the solid solutions could be achieved.The pyrope-grossular solutions show positive enthalpies of mixing. The non-ideality in the range 0–30 mole % grossular is relatively the largest and is in good agreement with the predictions of Ganguly and Kennedy (1974) based largely on cation partitioning of natural high grade metamorphic garnets with biotite, and with the deductions of Hensenet al. (1975) based on measurement of the compositions of synthetic pyrope-rich garnets equilibrated with anorthite, Al2SiO5 and quartz. However, the garnets show smaller excess enthalpies at higher grossular contents. This would lead to an asymmetric solvus with a critical temperature lower than predicted by the symmetrical regular solution model of Ganguly and Kennedy (1974). The composition-dependent non-ideality can be understood by simple ionic size considerations in solid substitution and is analogous to the situations for the calcite-dolomite and enstatite-diopside solvi.The heats of solution of pyropes crystallized in the range 1000–1500°C were all the same, within the precision of measurement, and thus we have found no evidence for temperature-dependent cation disordering as a possible explanation of the high entropy of pyrope, as suggested by Charluet al. (1975). Positional disorder of dodecahedral Mg is a more probable reason.The diopside-CaTs join is also non-ideal, with the larger positive enthalpy deviations near the diopside end. The calorimetric data in the magnesian range are consistemt with the model for completely disordered tetrahedral Si and Al which results from the free energy derivations of wood (1975) based on syntheses of diopside-rich aluminous pyroxenes in the presence of anorthite and quartz. At higher Al concentrations the calorimetric data seem more consistent with the ‘local charge-balance’ model of Wood (1975).No evidence for temperature-dependent disorder was found for either the diopside or CaTs end-members.  相似文献   

12.
The Pitzer’s interaction parameters, λN–M, involving the Mth cationic Al species Al3+ or AlOH2+ or AlO+ and the Nth neutral species SiO2(aq) (at temperatures of 25–300 °C) or CO2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λN–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λN–M for both SiO2(aq) and CO2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO2(aq)–Al3+ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO2 solubilities in concentrated solutions of AlCl3, 0.272 ± 0.010 (2σ), and Al2(SO4)3, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO2, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study.  相似文献   

13.
Corundum (α-Al2O3) solubility was measured in 0.1-molal CaCl2 solutions from 400 to 600°C between 0.6 and 2.0 kbar. The Al molality at 2 kbar increases from 3.1 × 10−4 at 400°C to 12.7 × 10−4 at 600°C. At 1 kbar, the solubility increases from 1.5 × 10−4m at 400°C to 3.4 × 10−4m at 600°C. These molalities are somewhat less than corundum solubility in pure H2O (Walther, 1997) at 400°C but somewhat greater at 600°C. The distribution of species was computed considering the Al species Al(OH)30 and Al(OH)4−, consistent with the solubility of corundum in pure H2O of Walther (1997) and association constants reported in the literature. The calculated solubility was greater than that measured except at 600°C and 2.0 kbar, indicating that neutral-charged species interactions are probably important.A Setchénow model for neutral species resulted in poor fitting of the measured values at 1.0 kbar. This suggests that Al(OH)30 has a greater stability relative to Al(OH)4− than given by the models of Pokrovskii and Helgeson (1995) or Diakonov et al. (1996). The significantly lower Al molalities in CaCl2 relative to those in NaCl solutions at the same concentration confirm the suggestions of Walther (2001) and others that NaAl(OH)40 rather than an Al-Cl complex must be significant in supercritical NaCl solutions to give the observed increase in corundum solubility with increasing NaCl concentrations.  相似文献   

14.
The diffusion profile method has been employed to measure tin diffusion coefficients and SnO2 solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log ƒO2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn2+ species, whereas at oxidized conditions SnO2 is much less soluble, and tin is present dominantly as a Sn4+ species. There is a strong melt composition control on SnO2 solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log ƒO2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO2 for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO2 solubility is 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn02 solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn4+/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from 0.4 at log ƒO2 of FMQ + 3.49 to 0.01 at FMQ - 0.57. Over the same log foZ range the Sn4+/ΣSn ratio for the A.S.I. 0.6 composition ranges from 0.98 to 0.4 and for the A.S.I. 1.25 composition, from 0.8 to 0.02.Tin diffusivity is dependent on both fO2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10−7.5 cm2/sec for the peraluminous compositions and 10−8.2 cm2/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10−8.2 and 10−9.0 cm2/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn2+ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn4+ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn2+ and Sn4+ is the same, but the bond strengths are different. At fixed fO2 the faster diffusivity in the peraluminous compositions reflects the lower Sn4+/Sn2+ ratio. The fact the Sn4+/Sn2+ ratio in melts varies greatly with ƒO2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.  相似文献   

15.
The stability of coexisting orthopyroxene, sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O as a function of pressure, mainly at 1,000° C, and at oxygen fugacities defined mostly by the hematite-magnetite buffer. The upper stability of the assemblage is terminated at 17 kbars, 1,000° C, by the reaction opx+Al-silicate gar+qz, proceeding toward lower pressures with increasing Fe/(Fe+Mg) ratio in the system. The lower stability is controlled by the reaction opx+sill+qz cord, which occurs at 11 kbars in the iron-free system but is lowered to 9 kbars with increasing Fe/(Fe+Mg). Spinel solid solutions are stabilized, besides quartz, up to 14 kbars in favour of garnet in the iron-rich part of the system (Fe/(Fe+Mg)0.30). Ferric-ferrous ratios in orthopyroxene are increasing with increasing ferro-magnesian ratio. At least part of the generally observed increase in Al content with Fe2+ in orthopyroxene is not due to an increased solubility of the MgAlAlSiO6 component but rather of a MgFe3+AlSiO6 component. The data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz.  相似文献   

16.
Yavapaiite, KFe(SO4)2, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO4)2, ΔH°f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol−1.K−1 and ΔG°f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe2O3 + H2O has been calculated, at pH2O = 1 atm, the phase boundary lies at 219 ± 2°C.  相似文献   

17.
A method is shown for calculating vapor pressures over a CMAS droplet in a gas of any composition. It is applied to the problem of the evolution of the chemical and Mg and Si isotopic composition of a completely molten droplet having the composition of a likely refractory inclusion precursor during its evaporation into the complementary, i.e. modified solar, gas from which it originally condensed, a more realistic model than previous calculations in which the ambient gas is pure H2(g). Because the loss rate of Mg is greater than that of Si, the vapor pressure of Mg(g) falls and its ambient pressure rises faster than those of SiO(g) during isothermal evaporation, causing the flux of Mg(g) to approach zero faster and MgO to approach its equilibrium concentration sooner than SiO2. As time passes, δ25Mg and δ29Si increase in the droplet and decrease in the ambient gas. The net flux of each isotope crossing the droplet/gas interface is the difference between its outgoing and incoming flux. δ25Mg and δ29Si of this instantaneous gas become higher, first overtaking their values in the ambient gas, causing them to increase with time, and later overtaking their values in the droplet itself, causing them to decrease with time, ultimately reaching their equilibrium values. If the system is cooling during evaporation and if mass transfer ceases at the solidus temperature, 1500 K, final MgO and SiO2 contents of the droplet are slightly higher in modified solar gas than in pure H2(g), and the difference increases with decreasing cooling rate and increasing ambient pressure. During cooling under some conditions, net fluxes of evaporating species become negative, causing reversal of the evaporation process into a condensation process, an increase in the MgO and/or SiO2 content of the droplet with time, and an increase in their final concentrations with increasing ambient pressure and/or dust/gas ratio. At cooling rates <∼3 K/h, closed-system evaporation at Ptot ∼ 10−3 bar in a modified solar gas, or at lower pressure in systems with enhanced dust/gas ratio, can yield the same δ25Mg in a residual CMAS droplet for vastly different evaporated fractions of Mg. The δ25Mg of a refractory residue may thus be insufficient to determine the extent of Mg loss from its precursor. Evaporation of Mg into an Mg-bearing ambient gas causes δ26Mg and δ25Mg of the residual droplet to fall below values expected from Rayleigh fractionation for the amount of 24Mg evaporated, with the degree of departure increasing with increasing fraction evaporated and ambient pressure of Mg. δ26Mg and δ25Mg do not depart proportionately from Rayleigh fractionation curves, with δ25Mg being less than expected on the basis of δ26Mg by up to ∼1.2‰. Such departures from Rayleigh fractionation could be used in principle to distinguish heavily from lightly evaporated residues with the same δ25Mg.  相似文献   

18.
The solubility of baddeleyite (ZrO2) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400 °C and pressures up to 700 bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)3° and ZrF2(OH)2°. Formation constants determined for these species (Zr4+ + nF− + mOH− = ZrFn(OH)m°) range from 43.7 at 100 °C to 46.41 at 400 °C for ZrF(OH)3°, and from 37.25 at 100 °C to 43.88 at 400 °C for ZrF2(OH)2°.Although the solubility of ZrO2 is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF3+-ZrF62−). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for the concentration of this metal at conditions commonly encountered in fluoride-rich natural hydrothermal systems.  相似文献   

19.
Enthalpies of solution of synthetic clinopyroxenes on the join CaMgSi2O6-Mg2Si2O6 have been measured in a melt of composition Pb2B2O5 at 970 K. Most of the measurements were made on samples crystallized at 1600°–1700°C and 30 kbar pressure, which covered the range 0–78 mole per cent Mg2Si2O6, and whose X-ray patterns could be satisfactorily indexed on the diopside (C2/c) structure. For the reaction: Mg2Si2O6→-Mg2Si2O6 enstatite diopside the present data, in conjunction with previous and new measurements on Mg2Si2O6 enstatite, determine ΔH° ~ 2 kcal and WH (regular solution parameter) ~ 7 kcal. These values are in good agreement with those deduced by Saxena and Nehru (1975) from a study of high temperature, high pressure phase equilibrium data under the assumption that the excess entropy of mixing is small, but, in light of the recent theoretical treatment of Navrotsky and Loucks (1977, Phys. Chem. Min.1, 109–127), the meanings of these parameters may be ambiguous.Heat of solution measurements on Ca-rich binary diopsides made by annealing glasses at 1358°C in air gave slighter higher values than the higher temperature high pressure samples. This may be evidence for some (Ca, Mg) disorder of the sort postulated by Navrotsky and Loucks (1977, Phys. Chem. Min.1, 109–127), although no differences in heat of solution dependent on synthesis temperature in the range 1350°–1700°C could be found in stoichiometric CaMgSi2O6.  相似文献   

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