共查询到20条相似文献，搜索用时 31 毫秒
1.
H. Taniguchi 《Contributions to Mineralogy and Petrology》1988,100(4):484-489
The surface tension between silicate melts and air has been measured for melt compositions lying on the diopside-anorthite (Di-An) join from 1300° C to 1580° C. It ranges from 300 dyne/cm to 400 dyne/cm, and decreases with increasing temperature, except for a pure diopside composition. At relatively high temperatures, the surface tension decreases as the anorthite content increases, whereas at lower temperature it is almost constant. These results suggest that diopside melt has a more discrete structure at higher temperatures, whereas, anorthite-bearing melts do not dissociate in the temperature range studied. They also suggest that the structure of both the surface and interior parts of the melt are almost identical at lower temperatures, but at higher temperatures, the surface part has a more polymerized structure with Al2O3 enrichment. The surface energy, obtained from the relationship between surface tension and temperature, increases from 294 erg/cm2 (Di composition) to 1013 erg/cm2 (Di40An60) with increasing anorthite content. 相似文献
2.
Alexandra Navrotsky David Ziegler Richard Oestrike Papu Maniar 《Contributions to Mineralogy and Petrology》1989,101(1):122-130
Transposed-temperature-drop calorimetry, using a Setaram HT 1500 calorimeter, was used to study directly the melting at 1773 K of mixtures of crystalline albite, anorthite, and diopside and of anorthite and forsterite. The enthalpy of albite at 1000–1773 K, starting with both crystalline and glassy samples, was also measured. The results confirm previously measured enthalpies of fusion of albite, diopside and anorthite (Stebbins et al. 1982, 1983; Richet and Bottinga 1984,1986). The new results use thermochemical cycles which completely avoid the glassy state by transforming crystals directly to melts. The enthalpy of fusion of forsterite is estimated to be 89±12 kJ/mol at 1773 K and 114±20 kJ/mol at its melting point of 2163 K. The data allow semiquantitative evaluation of heats of mixing in the molten silicates. Along the Ab-An join, enthalpies of mixing in the liquid at 773 K are the same or somewhat more negative than those in the glass at 986 K, whereas along Ab-Di and An-Di, enthalpies of mixing in the liquid are distinctly more positive than in the glass. These differences correlate with excess heat capacities in the liquids suggested by Stebbins et al. (1984). 相似文献
3.
Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar
volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess
term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium
in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz
(FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately
0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine
glassy lavas represents an excellent probe of the relative oxidation state of their source regions. 相似文献
4.
Dr. H. Taniguchi 《Mineralogy and Petrology》1985,34(2):117-130
Summary The relation between the species of silicate anions in a silicate melt and their infrared characteristic frequency is discussed. A simple relation approximated with a quadratic equation is established between the ratio of non-bridging oxygens to silicon atoms of a silicate anion and the characteristic frequency. Based on this relation, the silicate anions in some magmatic silicate melts were estimated. The results obtained in the present study agreed well qualitatively with those estimated by some researchers on the basis of other spectroscopic methods.The constituent silicate segments and the distribution of AlO4 tetrahedra in fully polymerized melts, albite, jadeite and anorthite, have been investigated. The polymerization degree of the silicate segments in the network seemed to decrease with the increase of AlO4 tetrahedron. It is also suggested, that AlO4 in the albite melt distributed more randomly, while those in anorthite melt had a tendency to form an aluminous segment.
With 6 Figures 相似文献
Infrarot-spektroskopische Untersuchungen der Silikatanionen-Strukturen einiger magmatischer Silikatschmelzen
Zusammenfassung Die Beziehungen zwischen den Spezies der Silikatanionen in den Silikatschmelzen und ihrer charakteristischen Infrarot-Frequenz wurde untersucht. Es gibt eine einfache Beziehung zwischen dem Verhältnis nicht brückenbildender Sauerstoffatome zu Si-Atomen der Silikatanionen und der charakteristischen Frequenz, die mit einer quadratischen Gleichung beschrieben werden kann. Auf Grund dieser Beziehung werden die Silikatanionen in einigen magmatischen Schmelzen abgeschätzt. Das Ergebnis dieser Untersuchung stimmt qualitativ mit den Werten überein, die von einigen Forschern durch andere spektroskopische Methoden erhalten wurden.Die Silikatsegmente und die Verteilung der AlO4-Tetraeder in vollkommen polymerisierten Schmelzen, in Albit, Jadeit und Anorthit wurden untersucht. Der Polymerisationsgrad der Silikatsegmente in den Netzwerken scheint mit der Zunahme der AlO4-Tetraeder geringer zu werden. Die Untersuchungläßt erkennen, daß AlO4-Tetraeder in Albit unregelmäßig verteilt sind, und in Anorthit die Neigung haben, ein Al-Segment zu bilden.
With 6 Figures 相似文献
5.
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
6.
Diamond crystallization from carbon solutions in compositionally variable melts of model eclogite with dolomite [CaMg(CO3)2], potassium carbonate (K2CO3), and multicomponent K-Na-Ca-Mg-Fe carbonates was studied at 7.0–8.5 GPa. Concentration barriers for the nucleation of the diamond were determined at a standard pressure of 8.5 GPa for variable proportions of silicate and carbonate components in the growth solutions. They correspond to 35, 65, and 40 wt % of silicate components for systems with dolomite, K2CO3, and carbonatites, respectively. At higher contents of silicates in silicate-carbonate melts, the nucleation of diamond phase ceases, but diamond crystallization on seed crystals continues and is accompanied by the spontaneous crystallization of thermodynamically unstable graphite. In melts of the albite (NaAlSi3O8)-K2CO3-C compositions, the concentration barrier of diamond nucleation at 8.5 GPa is up to 90–92 wt % of the albite component, and diamond growth on seeds was observed in albite-carbon melts. Using mineralogical and experimental data, we developed a model of mantle carbonate-silicate (carbonatite) melts as the main parental media for natural diamonds; it was shown that the composition of the silicate constituent of such parental melts is variable and corresponds to the mantle ultrabasic-basic series. With respect to concentration contributions and dominant role in the genesis of diamond in the Earth’s mantle, major (carbonate and silicate) and minor or admixture components were distinguished. The latter include both soluble in carbonate-silicate melts (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble components (sulfides, metals, and carbides). Both major and minor components may affect the position of the concentration barriers of diamond nucleation in natural parent media. 相似文献
7.
In this study, we used first-principles calculations based on density functional theory to investigate silicon and oxygen isotope fractionation factors among the most abundant major silicate minerals in granites, i.e., quartz and plagioclase (including albite and anorthite), and an important accessory mineral zircon. Combined with previous results of minerals commonly occurring in the crust and upper mantle (orthoenstatite, clinoenstatite, garnet, and olivine), our study reveals that the Si isotope fractionations in minerals are strongly correlated with SiO4 tetrahedron volume (or average Si–O bond length). The 30Si enrichment order follows the sequence of quartz > albite > anorthite > olivine ≈ zircon > enstatite > diopside, and the 18O enrichment follows the order of quartz > albite > anorthite > enstatite > zircon > olivine. Our calculation predicts that measurable fractionation of Si isotopes can occur among crustal silicate minerals during high-temperature geochemical processes. This work also allows us to evaluate Si isotope fractionation between minerals and silicate melts with variable compositions. Trajectory for δ30Si variation during fractional crystallization of silicate minerals was simulated with our calculated Si isotope fractionation factors between minerals and melts, suggesting the important roles of fractional crystallization to cause Si isotopic variations during magmatic differentiation. Our study also predicts that δ30Si data of ferroan anorthosites of the Moon can be explained by crystallization and aggregation of anorthite during lunar magma ocean processes. Finally, O and Si isotope fractionation factors between zircon and melts were estimated based on our calculation, which can be used to quantitatively account for O and Si isotope composition of zircons crystallized during magma differentiation. 相似文献
8.
Experimental determination of oxygen isotope fractionations between CO2 vapor and soda-melilite melt
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
9.
Distribution of Mg2+ between olivine and silicate melt,and its implications regarding melt structure
William P. Leeman 《Geochimica et cosmochimica acta》1978,42(6):789-800
Consideration of experimental data on the distribution of Mg2+ between olivine and silicate liquid clearly demonstrates that the distribution coefficient (KMg) is dependent upon variations in temperature, pressure and melt composition, largely because these variables control the solubility of Mg2+ in the melt phase. Attempts to minimize composition dependence of KMg, utilizing various activity-composition models for silicate melts, have been partially successful. Composition-related effects do not appear to be large, however, for melts of restricted range in composition (e.g., tholeiitic or lunar basalts) as long as the contents of alkalis and the alkali/alumina ratio are relatively small (on a molar basis). For such melts, KMg may be used as a reliable geothermometer. By analogy, these conclusions can be extended to the distribution of other divalent cation such as Fe2+, Mn2+, Ni2+ and Co2+. 相似文献
10.
Timothy P. Loomis 《Geochimica et cosmochimica acta》1979,43(11):1753-1759
Heat capacity models for anorthite (An) and albite (Ab) crystal and supercooled liquid, together with the assumption of ideal mixing of these components were used to derive average values of enthalpy and volume of melting from phase equilibrium data that are in significant disagreement with some recently published thermodynamic data. In an effort to find a means of calculating both liquidus temperature and crystal composition of plagioclase for a given hydrous melt composition, the activity models for feldspar components in hydrous melts and solubility calculations suggested by Burnham (1975) and the enthalpy data above were tested by comparing predicted and observed liquidi. These assumptions lead to satisfactory agreement in the systems Ab-An-H2O and Ab-Si4O8-H2O but liquidi in the system An-Si4O8-H2O and complex systems differ radically from those calculated. For hydrous complex melts an empirical model using the above solubility and activity assumptions was fit to experimental data on coexisting melt and plagioclase compositions. Despite the demonstrable theoretical limitations of the assumptions involved, this empirical model apparently balances inaccuracies and reproduces the original data with absolute mean errors for 66 experiments of 17°C and 5 mol% An. It is sufficiently precise for use in kinetic crystallization models and may be useful as a geothermometer in some applications; it is probably not sufficiently accurate to be used as a geobarometer. 相似文献
11.
The maximum limits of the assemblage albiteforsterite have been determined experimentally at high pressures and temperatures. At subsolidus temperatures, albite plus forsterite is replaced at high pressures by jadeitic clinopyroxene and enstatitic orthopyroxene. The boundary for this reaction lies within experimental uncertainity of that for jadeite=albite+nepheline. Melting of albite+forsterite at high pressures produces enstatite+liquid, which is different from the low-pressure eutectic behavior. Melting rates are very slow and several hundred hours are required to establish equilibrium near the solidus. The subsolidus boundary for albite plus forsterite lies near that for sanidine plus forsterite, but with a shallower slope which more closely matches that of anorthite plus forsterite. Both albite plus forsterite and anorthite plus forsterite are replaced at high pressures by an assemblage containing clinopyroxene plus orthopyroxene, unlike sanidine plus forsterite, which is replaced by a feldspathoid plus orthopyroxene. The presence of sodium enlarges the depth region over which plagioclase lherzolite can stably exist; it may also stabilize alkali feldspar plus olivine in crustal rocks. 相似文献
12.
In implementing into a molecular dynamics simulation code a simple interionic potential developed to describe the nine component system K2O-Na2O-CaO-MgO-FeO-Fe2O3-Al2O3-TiO2-SiO2 (KNCMFATS), it has been possible to reproduce satisfactorily a number of thermodynamic, structural and transport properties of a representative set of natural silicate melts. An important conclusion reached in this study is the good transferability of the potential from felsic to ultramafic compositions although this transferability becomes less accurate with high silica contents (rhyolitic composition and beyond) and with very iron-rich silicates (e.g. fayalite). A key feature of the simulation is to make the link between macroscopic properties of the melt and its microscopic structure and dynamics. We thus obtain a relationship between the molar volume of the melt, the number of network modifiers and the oxygen coordination number. The simulation also allows one to quantify the coordination environment around the cations as function of the melt composition. Furthermore, the electrical conductivity of the high temperature liquid is investigated. 相似文献
13.
Using a literature survey of analyses of high-Ca pyroxene and co-existing silicate melt pairs and analyses of low-Ca pyroxene-silicate melt pairs, we have performed a thermodynamic analysis of pyroxene-melt equilibria. Three sets of mixing model pairs have been considered, based on two mixing models for liquid silicate solutions and two for pyroxene solid solutions. A modified version of a model developed by Bottinga and Weill (1972) for the mixing properties of silicate melts, in which the melt is considered to be composed of independent network-forming and network-modifying quasi-lattices, more successfully accounts for variations in melt composition than does a model which considers the melt to be composed of simple oxides which mix ideally. An empirical model for the mixing properties of pyroxenes, in which the M1 and M2 sites are considered to be equivalent and are combined as a hypothetical ‘M’ site, is as successful in accounting for variations in pyroxene composition at high temperatures as an ideal multisite mixing model.Using a variety of pyroxene-melt relations, and combinations of the mixing models outlined above, we have developed several pyroxene-melt and low-Ca pyroxene-high-Ca pyroxene geothermometers which have internally-consistent precisions of approximately ±20°C (1σ). One of the two-pyroxene geothermometers has been used to calculate ‘quench’ temperatures for a number of eucrites. Computed temperatures are subsolidus, and are consistent with independent geothermometers and with petrographic observations. The equations may also be used to calculate the composition of pyroxene crystallizing from a silicate melt of known composition, with or without independent knowledge of temperature. Internally consistent precisions vary, but are approximately ± 3 mol% Fs, ± 5 mol% En, and ±4 mol% Wo (all 1σ). These equations may have application in modeling the evolution of mineral compositions during differentiation of basaltic magmas, particularly terrestrial layered intrusions and the lunar magma ocean. 相似文献
14.
J. Nicholls 《Contributions to Mineralogy and Petrology》1980,74(2):211-220
A symmetrical, strictly regular solution model is used to estimate H2O solubilities in silicate melts. The standard state chemical potential of dissolved H2O and the adjustable parameter in the activity coefficient are determined by least squares analyses of data on H2O solubility in silicate melts. The adjustable parameter in the expression for the activity coefficient (In) is a function only of the anhydrous melt composition and eleven values are provided for melts ranging in composition from picrite to rhyolite. At the 95% confidence level, the model should estimate H2O contents to within 4.8% of the amount present if the amount present is less than 10 wt.%. This compares to the reproducibility of 2.25% of the amount present for experimental determinations. To apply the model to rocks and magmas estimates ofT, P, and the fugacity of H2O are required.Variation of the H2O content of the melt changes the activity of other components. Knowledge of this variation removes the requirement that the fugacity of H2O be estimated. Application of the properties of exact differentials to the Gibbs function for the hydrous melt provides an expression relating the chemical potential of a feldspar component to the H2O content of the melt. This expression contains a second adjustable parameter which depends on the anhydrous melt composition. Using this second expression, the H2O content can be estimated ifT, P, and feldspar composition are known. Data are too meagre to evaluate the quantitative success of the second method. 相似文献
15.
The thermodynamic, structural and transport properties of natural silicate melts under pressure are investigated by molecular dynamics simulation with the help of a force field recently introduced by us [Guillot B. and Sator N. (2007) A computer simulation study of natural silicate melts. Part I: low pressure properties. Geochim. Cosmochim. Acta71, 1249-1265]. It is shown that the simulation reproduces accurately the bulk moduli of a large variety of silicate liquids as evaluated from ultrasonic studies. The equations of state (EOS) of the simulated melts are in good agreement with the density data on mid-ocean ridge basalt, komatiite, peridotite and fayalite as obtained either by sink/float experiments or by shock-wave compression. From the structural point of view it is shown that the population of [5]Al and [6]Al species increases rapidly upon initial compression (0-50 kbar) whereas for Si these highly coordinated species are found in a significant abundance (>5%) only above ∼50 kbar for [5]Si and ∼100-150 kbar for [6]Si. This increase of the coordination of network formers is not the only response of the melt structure to the densification: there is also a large redistribution of the T-O-T (T = Si, Al) bond angles with the pressure and noticeably upon initial compression in rhyolitic and basaltic liquids. Furthermore, a detailed analysis of the population of bridging oxygens (BO) and nonbridging oxygens (NBO) points out that the polymerization of the melt generally increases when the pressure increases, the magnitude of this polymerization enhancement being all the more important that the melt is depolymerized at low pressure. The role of triclusters (threefold coordinated oxygens to network former cations) is particularly emphasized in acidic and basaltic liquids. The pressure-induced redistribution of the oxygen atoms through the melt structure is also stressed. Finally, the simulation predicts a nonmonotonic behavior of the diffusivity of network former ions when the pressure increases, a feature with depends on the melt composition. This could have a counterpart in the electrical conductivity at sufficiently high temperature when the viscosity of the liquid is low. 相似文献
16.
D. C. Presnall Selena A. Dixon James R. Dixon T. H. O'Donnell N. L. Brenner R. L. Schrock D. W. Dycus 《Contributions to Mineralogy and Petrology》1978,66(2):203-220
In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814. 相似文献
17.
Hilmar V. Platen 《Contributions to Mineralogy and Petrology》1965,11(4):334-381
To elucidate the problem regarding the genesis of granites and migmatites, the influence of a) anorthite component, b) mafic components, and c) composition of the fluid phase on the crystallization of, granitic melts has been experimentally studied. As starting material, an obsidian with an average granitic composition has been selected and mixtures of this obsidian with anorthite or biotite have been made. H2O or diluted solutions of NH3, HF or HCl have been used as a fluid phase. The experiments were conducted under 2000 bars gas pressure and under isochemical conditions. At the outset of each experiment obsidian and the mixtures have been melted under 2000 bars pressure and then the temperature has been brought down stepwise, in order, to study the sequence and temperature of crystallization directly from the melt. The results of these experiments are plotted on a series of quartz-orthoclase-albite (Q-Ab-Or) melt diagrams, so that a better comparison with the different granitic systems and with the simple Q-Ab-Or-H20 system ofTuttle andBowen, (1958) can be achieved. The melt or crystallization diagrams of these complex granitic systems under study, represent projections of the Q-Ab-Or-An (An= anorthite) tetrahedron on to the base Q-Ab-Or. In all these triangular diagrams, there are three regions of crystallization: quartz, plagioclase and alkalifelspar. These three regions of crystallization are separated from each other by three cotectic lines; which meet at a point: the eutectic point. The sequence of crystallization of the leucocratic minerals; observed in the different granitic systems investigated experimentally here, can be understood with the help of such diagrams easily. From these experiments the following petrogenetically interesting results have been obtained:
- 1.The order of crystallization of granitic melts is to a great extent influenced: by the normative proportion of the Ab/An components and the composition of the fluid phase. (It is not the variation in the An content directly but the variation in the relative proportions of Ab/An, that causes a change in the order of crystallization.) In general, with decreasing Ab/An proportion in a rock or a melt, the amount of albite component in the.eutectic melt decrease,. while at the same time- the amount of quartz and orthoclase components increase. When the Ab/An proportion of a rock is more than 3.5, then the corresponding eutectic melt is of granitic composition; and when the proportion is lesser than 3.5 the eutectic melt is quartz and orthoclase rich and has aplitic composition. Calc-alkaligranites and alkaligranites have an average Ab/An proportion of 4.7 and 10.5, respectively. 相似文献
18.
Richard F. Wendlandt Wendy J. Harrison 《Contributions to Mineralogy and Petrology》1979,69(4):409-419
Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO2 vapor/carbonate melt and CO2 vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO2 vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO2 vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO2-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO2-rich fluids under both mantle and crustal conditions. 相似文献
19.
B.H.W.S. de Jong Charles M. Schramm Victor E. Parziale 《Geochimica et cosmochimica acta》1984,48(12):2619-2629
29Si MAS NMR experiments have been carried out to determine the silica species distribution (Q distribution) in albite, NaAlSi3O8, and anorthite, CaAl2Si2O8, composition glasses (designated albite and anorthite glass). Our results indicate that the Q distribution of albite glass contains all five possible silica species and shows a tendency towards high Q3 and Q4 concentrations, whereas anorthite glass does not contain Q4 and has a high Q0 concentration. Rationalizations are made in terms of the observed Q distributions to explain differences in devitrification behavior of these two glasses. 27Al MAS NMR data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass. 相似文献
20.
To assess the effect of the melt composition on bromine concentrations in magmas, we have investigated bromide solubility for water-saturated, iron-free silicic melts with variable Na+K/Al and Si/Al molar ratios (albite, haplogranite, rhyolite, and pantellerite). The experiments were performed in rapid quench cold-seal autoclaves over a range of pressure (1, 1.5, and 2 kbar) and temperature (900, 1000, and 1080 °C) with run durations from 5 to 7 days. A series of natural volcanic glasses and melt inclusions hosted in magmatic minerals were analysed together with the synthetic glasses by PIXE (proton-induced X-ray emission). The Br concentrations range from 5360 to 7850 ppm for albite, from 2800 to 3900 ppm for haplogranite, from 4300 to 5900 ppm for rhyolite, and from 9745 to 11,250 ppm for pantellerite. Br concentrations are negatively correlated with pressure in H2O-saturated silicic melts and vary with (Na+K)/Al molar ratio with a minimum value at the ratio close to unity. Br behaves similarly to chlorine for all of these melt compositions. The bromide solubility is similar in albitic and rhyolitic melts, which implies that Df/m is nearly the same for both compositions and is applicable for natural rhyolites as suggested in our previous study (Bureau et al., 2000). This means that the volcanic Br contribution to the atmosphere may be significant. In natural obsidian samples and MI hosted in quartz, olivine, and leucite, the Br concentration varies from < 3 to 28 ppm, with the highest concentrations in pantelleritic melts. We attribute the low Br concentrations of natural melts to a low initial abundance of this halogen in the Earth mantle. However, because Br behaves as an incompatible element before water exsolution, our results imply that magmas could contain much more dissolved Br before eruption and water degassing than the few ppm usually measured in volcanic rocks. Br behaviour during magma crystallisation is controlled by its partitioning into the H2O-rich fluid phase when this occurs. In addition, its potential high solubility in silicate melts makes it a very sensitive chemical tracer of magma contamination by seawater and Br-rich material. This infers that the investigation of Br behaviour in subduction-zone samples may help for a better understanding of volatiles cycling between the Earth reservoirs. 相似文献