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1.
Density measurements have been carried out on the melt system diopside-anorthite from room temperature to 1600° C at 1 atm, and from 1400° C to 1800° C at pressures up to 20 Kb. The densities were determined based on the dilatometric curve and density at 22° C for lowtemperatures, the double-bob Archimedean method for high-temperatures at 1 atm, and on the sinking and floating spheres method for high-pressure conditions.The results at 1 atm indicate that the thermal expansion coefficient of the glassy state is almost constant, while that of the liquid state decreases with increasing temperature. Density decreased with increasing anorthite content for both glassy and liquid states. Melts in the liquid-state mix ideally with respect to volume, while the glassy state exhibits a maximum excess volume at Di30An70. Density-pressure relations clearly show a density reversion between diopsiderich and anorthite-rich melts; the anorthite-rich melt becomes denser than diopside-rich melt at pressures above 8 kb.The free volumes of both the liquid and glassy states decreased with increasing anorthite content.Isothermal compressibilities and the hard-sphere diameter have been calculated based on the hard-sphere liquid model using thermal expansion coefficients and surface tension data. Calculated compressibilities for diopside-rich melt (Di:>Di60) agreed well with the experimental data, while calculated and observed compressibilities for anorthite-rich melt did not. This evidence indicates that diopside melt may be regarded as a discrete-melt composed of small constituent units (about 10 Å in average diameter) and much interstitial space, while anorthite melt is a three-dimensional network melt with little interstitial space. The critical composition Di60An40 is similar to that of the eutectic and corresponds to breaks between composition and other physical properties. It is proposed that the composition may reflect a kind of critical state in the substitution of the continuous structure of anorthite melt for the discrete structure of diopside melt. The critical state may be interpreted based on the site-percolation theory.  相似文献   

2.
In situ X-ray viscometry of the silicate melts was carried out at high pressure and at high temperature. The viscosity of the silicate melts in the diopside(Di)–jadeite(Jd) system was determined in the pressure range from 1.88 GPa to 7.9 GPa and in the temperature range from 2,003 K to 2,173 K. The viscosity of the Di 25%–Jd 75% melt decreases continuously to 5.0 GPa, whereas the viscosity of the Di 50%–Jd 50% melt increases over 3.5 GPa. The viscosity of the Di50%–Jd 50% melt reaches a minimum around 3.5 GPa. Since the amounts of silicon in the two melts are the same, the difference in the pressure dependence of the viscosity may be controlled by another network-forming element, i.e., aluminum. The difference in the pressure dependence of the viscosities in the melts with two intermediate compositions in the Di–Jd system is estimated to be due to the difference in the melt structures at high pressures and high temperatures.  相似文献   

3.
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   

4.
Summary Density and viscosity measurements of three melts of volcanic rock composition (basalt and andesite) at low temperatures were carried out to understand the role of free volume in the viscous behavior of a magma and to estimate the flow unit in the melts. The data combined with literature data suggest the following conclusion: free-volume theory is not applicable to these silicate melts; the relation between viscosity and the inverse of free volume does not yield a straight line in a wide temperature range from the glass-transion temperature to 1550°C. However, two depolymerized melts, diopside and Oki-Dozen alkali basalt (OAB), yield almost linear relationships. Thus, the free-volume theory should hold to a fairly good approximation for these two melts. Based on this approximation, the radius of flow unit for diopside melt was calculated to be about 4.7 Å, and that for Oki-Dozen alkali basalt to be about 4.2 Å. The three-dimensional silicate anions which may correspond to the flow unit are Si14O35 14– and Si16O40 16– for diopside melt, and Si10O25 10– and Si12O30 12– for OAB melt. The temperature effect on the initial slope of the viscosity-pressure relation has also been examined in the frame of free-volume theory. It was concluded that the relative increase of the initial slope of the relation with increasing temperature might be caused by the increase of free volume.With 6 Figures  相似文献   

5.
Beginning of melting and subsolidus relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K2O-CaO-Al2O3-SiO2-H2O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al2O3-SiO2-H2O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al2O3-SiO2-H2O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al2SiO5 polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.  相似文献   

6.
To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO2 vapor. The depression of the temperature of the solidus of albite by CO2 decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO2 in NaAlSi3O8 liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO2 lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si3O8 composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO2 and CaO and depleted in Al2O3 relative to CaAl2Si2O8.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO2 component in the liquid ( ). We predict that this increases the ratio of molecular CO2/CO 3 2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO2 in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi3O8 liquids, which is in contrast to that in CaAl2Si2O8 liquid. Also, reaction of CO2 with NaAlSi3O8 liquid to form CO 3 2– that is complexed with Na+ must be accompanied by a change in Al3+ from network-former to network-modifier, as Na+ is no longer abailable to charge-balance Al3+ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO 3 2– with the excess Ca2+ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO2 increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO2-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO2 produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO 3 2– , resulting in a low CO2/ CO 3 2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744  相似文献   

7.
Kosmochlor (NaCrSi2O6) was synthesized by the flux method from melts along the join Na2O·2 SiO2-Na2O·Cr2O3·4 SiO2 at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi2O6-NaCrSi2O6 was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr2O3) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.  相似文献   

8.
Partial melting experiments on plagioclase (An60) and diopside have been carried out using pairs of large crystals to investigate textures and kinetics of melting. The experiments were done at one atmosphere pressure as a function of temperature (1,190–1,307° C) and time (1.5–192 h). Melting took place mainly at the plagioclase-diopside contact planes. Reaction zones composed of fine mixtures of calcic plagioclase and melt were developed from the surface of the plagioclase crystal inward. There exists a critical temperature, below which only a few % melting can occur over the duration of the experiments. This sluggish melting is caused by slow NaSi-CaAl diffusion in plagioclase, because the plagioclase crystal must change its composition to produce albite-rich cotectic melts. Diffusion in the solid also affects the chemical composition of the melts. During initial melting, potassium is preferentially extracted from plagioclase because K-Na diffusion in plagioclase is faster than that of NaSi-CaAl. This also causes a shift in the cotectic compositions. Above the critical temperature, on the other hand, melting is promoted by a metastable reaction in which the plagioclase composition does not change, and which produces melts with compositional gradients along the original An60-diopside tie line. The critical temperature is determined by the intersection of the cotectic and the An60-diopside tie line. Interdiffusion coefficients of plagioclase-diopside components in the melt are estimated from melting rates above the critical temperature by using a simplified steady-state diffusion model (e.g., 10–8 cm2/sec at 1,300° C).Many examples of reaction zones due to partial melting have been described as spongy or fingerprint-like textures in xenoliths. Metastable melting above the critical temperature is considered to take place in natural melting where there is a high degree of melting. However, we cannot exclude the possibility of disequilibrium created by sluggish melting controlled by diffusion in the minerals. If melting occurs close to the solidus, this process can be important even for partial melting in the upper mantle.  相似文献   

9.
Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°-1300°C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na2O-SiO2-H2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂XH2O, decreases with increasing total water content. At low water contents, the influence of H2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q4-species to form Q3 and Q2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q3-species react with water to form Q2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H-29Si cross polarization NMR data.  相似文献   

10.
The effect of silica concentration on the solubility of Al and Ti in diopsidic pyroxenes has been investigated at one atmosphere in the system diopside-CaTiAl2O6-SiO2 at temperatures between 1150–1420° C. The composition of pyroxene in the system diopside-CaTiAl2O6-SiO2 is influenced by the total SiO2 content. Near the join diopside-CaTiAl2O6, the pyroxene forms a solid solution with the CaTiAl2O6 molecule, and co-exists with perovskite for compositions greater than 11 weight percent CaTiAl2O6. Anorthite is an accompanying phase. With increasing total SiO2 content a series of mineralogical changes involving Ti-bearing phases occur. Sphene solid solution co-exists with diopside solid solution, anorthite, and perovskite in a small compositional range near the diopside-CaTiAl2O6 join. Additional total SiO2 results in the elimination of perovskite and a decrease of solid solution in the pyroxenes. With further increase in SiO2 content, tridymite appears and the pyroxene is approximately pure diopside. Rutile joins diopside, anorthite, sphene solid solution and tridymite over a broad compositional range in the Ti, Si-rich part of the system. These results demonstrate that increased silica concentration decreases the solubility of Al and Ti in diopsidic pyroxenes and controls the stability of co-existing Ti-bearing phases.  相似文献   

11.
We report results of experiments constraining oxygen isotope fractionations between CO2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000.lnαCO2-Na melilitic melt, is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na2O). We combine these data with the known reduced partition function ratio of CO2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts.  相似文献   

12.
The thermal expansion of supercooled liquids in the haplobasaltic (anorthite-diopside) system have been determined via methods of container-based dilatometry. The expansivity data obtained in this study agree well with estimates provided by previous dilatometric determinations in the system that have relied on alternative experimental strategies. The data have been combined with high-temperature, superliquidus determinations of melt density to obtain expressions for the volume-temperature (V-T) relationships of liquids in the anorthite-diopside system. The V-T data clearly indicate a nonlinear temperature dependence of volume for all melts investigated. The variation is most striking for diopside, where the coefficient of volume thermal expansion decreases ∼56% from temperatures near the glass transition to superliquidus temperatures. With increasing anorthite content, the degree of variation appears to decrease. An42Di58 exhibits a decrease of 39% of its coefficient of thermal expansion and An98Di02 of 33%, respectively. The expansivities obtained in this study cannot be reproduced by means of published models that are based on linear V-T relationships. They require instead a reanalysis of existing pressure-V-T equations of state models for silicate melts.  相似文献   

13.
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14.
CO2 solubility has a slight negative temperature dependence in olivine melilitite at 30 kb with 9% CO2 dissolved at 1,450 °C, 8.5% at 1,550 °C and 8.3% at 1,650° C. CO2 is dissolved as the carbonate molecule (CO 3 2– ) only. Feldspar melts (albite-anorthite) dissolve much less CO2 at 30 kb (around 2%) with a slight increase with increasing anorthite content. A CO2 absorption peak in infrared spectra of albite-rich glasses diappears in favour of the CO 3 2– peak with increasing anorthite content. It is inferred that CO2 was present as CO 3 2– in albite-rich melts also, but reverts to CO2 during quenching because of bonding differences related to Ca2+ and Na+ in the melts.  相似文献   

15.
 The speciation of water dissolved in glasses along the join NaAlSi3O8-KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040° C and 2 kbar. These melts have been isobarically and rapidly (200° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H2O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H2O] [O m ]/[OH]2 is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange. Received: 8 March 1995/Accepted: 6 June 1995  相似文献   

16.
Partitioning of Mg and Fe2+ between olivine and mafic melts has been determined experimentally for eight different synthetic compositions in the temperature range between 1335 and 1425°C at 0.1 MPa pressure and at fo2 ∼1 log unit below the quartz-fayalite-magnetite buffer. The partition coefficient [KD = (Fe2+/Mg)ol/(Fe2+/Mg)melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25-1.2), and then decreases with further depolymerization of melt (NBO/T from 1.2-2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in NBO/T ranges for basaltic-picritic melts (0.4-1.5) is nearly identical to that obtained in the present experiments. Because the present experiments were carried out at constant pressure (0.1 MPa) and in a relatively small temperature range (90°C), the observed variations of Mg and Fe2+ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of KD may depend on the relative proportions of four-, five-, and six-coordinated Mg2+ and Fe2+ in melt as a function of degree of NBO/T.  相似文献   

17.
Structural similarities between dry diopside melt and superhydrous albite melt (X w >0.5) — both lack three-dimensional silicate units — suggest that thermodynamic relations may be similar. A model based on that assumption successfully predicts diopside melting relations and H2O solubilities. For the model, the three partial differential equations describing solution of H2O in albite melt for X w >0.5 have been integrated for diopside melt from X w =0 to X w at least as large as 0.76, with two exceptions: an alternative partial differential equation for Henrian solution of H2O in dilute melts was applied for X w <0.20, and an alternative differential equation for the pressure dependence of a w at pressures below 2 kbar was developed. The latter alternative equation yields relatively small ¯Vw's at low pressures rather than the large ¯Vw's calculated from the equation from the albite system. Available experimental solubility data are not precise enough to offer a choice between the small-¯Vw and large-¯Vw equations. Integration of all the partial differential equations was constrained solely by the P and T of a single experimentally-determined point on the H2O-saturated solidus.Solubilities calculated by a Henrian-analogue solution model (a di=X di 2 ) from the experimental H2O saturated solidus lie outside experimental solubility constraints for dilute melts. On the other hand, a Henrian model (a di=Xdi) successfully predicts solubilities in dilute melts. The formulation of the Henrian model and magnitudes of model molar entropies of solution are consistent with the hypothesis that H2O dissolves in diopside melt as an essentially undissociated species with little ordering on melt structural sites. That species could in turn be consistently, if not uniquely, interpreted to be molecular H2O or a hydroxylation (OH–) complex formed from nonbridging oxygens.  相似文献   

18.
The structure of H2O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na2O•4SiO2-Na2O•4(NaAl)O2-H2O (5 and 10 mol% Al2O3, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H2O at nominal densities (from PVT properties of pure H2O) of 0.85 g/cm3 (NA10 experiments) and 0.86 g/cm3 (NA5 experiments) with the aluminosilicate + H2O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H2O (H2O°) and OH groups that form bonds with cations exist in all three phases. The OH/H2O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H2O, fH2O) with (OH/H2O°)melt > (OH/H2O°)fluid at all pressures and temperatures. Structural units of Q3, Q2, Q1, and Q0 type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q0 and Q1 increases and Q2 and Q3 decrease with fH2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of fH2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to fH2O.The melt structural data are used to describe relationships between activity of H2O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qn-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H2O content.  相似文献   

19.
The activity of a given mineral component in a silicate melt can be calculated from the compositions of coexisting melt and crystals, provided that 1) the component is an independently variable component of the crystal, and 2) appropriate thermodynamic data for the component are known. This approach is used to calibrate the compositional dependence of the activities of forsterite, fayalite, anorthite, and albite from experimental data on natural mafic-to-intermediate melts. The natural logarithms of the activities of forsterite and anorthite can be closely approximated as second-degree polynomial functions of the melt composition (r 2=0.99 and 0.97, respectively); corresponding fits for fayalite and albite are significantly poorer (r 2=0.81 and 0.87, respectively). The shapes of the fitted activity surfaces yield information about speciation in silicate melts. The activity models for forsterite and anorthite provide excellent geothermometers with standard deviations of temperature residuals of approximately 10° C. These geothermometers, when combined with the activity models for fayalite and albite, can be used to predict the temperature at which olivine or plagioclase will crystallize from a melt, along with the composition of the crystals.  相似文献   

20.
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   

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