System NaAlSi Research Articles

An experimental study on the pressure dependence of viscosity in silicate melts

The effect of pressure on melt viscosity was investigated for five compositions along the join An(CaAl(2)Si(2)O(8))-Di(CaMgSi(2)O(6)) and four alkali silicates containing lithium, sodium, and potassium in constant ratio of approximately 1:1:1, but alkali-silica ratios are varying. The experiments were performed in an internally heated gas pressure vessel at pressures from 50 to 400 MPa in the viscosity range from 10(8) to 10(11.5) Pas using parallel plate viscometry. The polymerized An composition shows a negative pressure dependence of viscosity while the other, more depolymerized compositions of the join An-Di have neutral to positive pressure coefficients. The alkali silicates display neutral to slightly positive pressure coefficients for melt viscosity. These findings in the high viscosity range of 10(8)-10(11) Pas, where pressure appears to be more efficient than in low viscous melts at high temperature, are consistent with previous results on the viscosity of polymerized to depolymerized melts in the system NaAlSi(3)O(8)-CaMgSi(2)O(6) by Behrens and Schulze [H. Behrens and F. Schulze, Am. Mineral. 88, 1351 (2003)]. Thus we confirm that the sign of the pressure coefficient for viscosity is mainly related to the degree of melt polymerization in silicate and aluminosilicate melts.

Pressure dependence of melt viscosity in the system NaAlSi3O8-CaMgSi2O6

The effect of pressure on melt viscosity was investigated in the system NaAlSi 3 O 8 -CaMgSi 2 O 6 (Ab-Di) at pressures from 0.1 to 400 MPa using a parallel plate viscometer. The new measurements in the high viscosity range of 10 8.5 -10 11 Pa·s are consistent with previous data obtained at higher temperature (lower viscosity) in that the pressure dependence changes from positive for the polymerized Ab melt to negative for the depolymerized Di melt. However, a pressure independent viscosity is observed at much higher Ab contents at low temperature (70-90 mol%) than at high temperature (<30 mol%, Brearley et al. 1986). Hence, the sign of the pressure dependence of viscosity changes from positive to negative with increasing temperature for intermediate compositions along the join. The apparent activation volume (V α ), which is derived from the pressure derivative of log viscosity using an Arrhenian relationship, decreases systematically with temperature for Di-rich compositions, e.g., for the Di melt from 52.0 ± 2.7 cm 3 /mol at 1020 K to 24.8 ± 6.6 cm 3 /mol at 1057 K. To model the P-T dependence of viscosity, we propose a modification of the commonly used equation based on Adam-Gibbs theory as where A AG , B AG , and C AG are fit parameters and S conf is the configurational entropy. The parameter C AG can be interpreted as the volume change of a structural unit during transition from one stable configuration to another one. The new approach excellently reproduces the experimental viscosity data for Di melts over a wide range of temperature. A modified Vogel-Fulcher-Tamman equation also describes the data with a similar reproducibility, but one additional fit parameter is needed. Experimental viscosity data for Ab melts scatter too much at given P-T to be fitted by a unique model. To calculate viscosities of stoichiometric Ab melt at elevated pressure and temperature, we suggest that the C AG parameter determined in our study be combined with published viscosity equations for ambient pressure

PH changes in peralkaline late-magmatic fluids

The 1.15-Ga-old Ilimaussaq intrusive complex in South Greenland shows an extensive fractionation trend from alkaline augite syenite to various varieties of strongly peralkaline, agpaitic nepheline syenites. The peralkaline nepheline-bearing syenites crystallized between ca. 900 and 450 °C at 1 kbar and they are cut by late-magmatic hydrothermal veins with nepheline-absent assemblages of albite + aegirine + analcime ± sodalite ± Na–Be-silicates (tugtupite, chkalovite, sorensenite) ± ussingite (NaAlSi3O8*NaOH). Based on fluid inclusions and phase equilibria, these veins crystallized between 300 and 500 °C at 1 kbar. Textures indicate that the hydrothermal veins at least partly replaced earlier Ilimaussaq rocks. The occurrence of ussingite and tugtupite suggests that the late-magmatic fluids had strongly basic pH values. Speciation calculations show that the pH in fluids of the system Na–Al–Si–O–H–Cl mainly depends on the Na/Cl ratio and, to a lesser degree, on salinity and temperature. If the Na/Cl ratio is greater than 1, pH (at 400 °C and 1 kbar, where neutrality is about at pH 5) lies between 7 and 12. Because Na/Cl tends to decrease in the final stages of magmatism and during crystallization of the vein assemblage, pH of late-magmatic fluids generally should become more acidic, and only two processes can increase Na/Cl and, thus, pH: dissolution of a Cl-poor or Cl-free Na silicate or unmixing of an HCl-enriched vapour phase. Field observations and microtextures suggest that replacement reactions are responsible for the change to basic pH at least in some alteration assemblages. While replacement of 1 mol nepheline by 1 mol analcime would not alter the pH, the volume-conserving reaction \(1.85\;{\rm Ne} + 2.3\;{\rm H}_{\rm 2} {\rm O} + 0.19\;{\rm H}_{\rm 4} {\rm SiO}_4 = 1.02\;{\rm Anl} + 0.83\,\;{\rm Na}^ + + 0.83\;{\rm Al(OH)}_4^ - \) can be used to model the replacement process quantitatively, provided it occurred in a more or less closed system. Progress of this reaction leads to successively increasing pH of the fluid during fluid–rock interaction and stabilizes minerals such as ussingite and tugtupite. Transferring the two processes to a larger scale, it is proposed that the extreme 'hyper-agpaitic' assemblages at Ilimaussaq or at the Kola peninsula, which include copious amounts of very basic, water-soluble minerals such as trona, villiaumite or thermonatrite, are formed either in this way by autometasomatic reactions of late-magmatic fluids or melts (or supercritical fluid-melt-mixtures) with earlier crystallized rocks of the same plutonic complex or by large-scale vapour unmixing in the very final stages of magmatism.

Heat capacities of haplogranitic glasses and liquids

The heat capacities of 27 glasses have been determined from room temperature to temperatures corresponding to supercooled liquid behavior. The investigated compositions are based on a haplogranite (near the 2-kbar pH 2O minimum melt composition in the system NaAlSi 3O 8-KAlSi 3O 8-SiO 2) to which alkali and alkaline earth oxides (Li 2O, Na 2O, K 2O, Rb 2O, Cs 2O, MgO, CaO, SrO, and BaO) have been added individually. Where comparison is possible, our data below the glass transition are consistent with predictive models previously proposed in the literature. In addition, the partial molar heat capacity of Li 2O in silicate glasses is determined from our data. Extrapolated glassy and relaxed liquid enthalpies intersect at a temperature defined as the limiting fictive temperature (T f′). At this temperature, the glassy heat capacity of all the studied compositions is close to the theoretical upper limit of 3R per gram-atom, where R is the gas constant. For samples cooled at 5 K/min, liquid viscosity of all samples is 10 12.56±0.43 Pa s at T f′. For other cooling rates, this result implies that log η(T f′) = 11.5 − log Q, where η(T f′) is the viscosity at the limiting fictive temperature and Q is the cooling rate (K/s). Liquid heat capacity is found to generally increase with addition of all oxides, although the details of the variations are obscured by the fact that experimental uncertainties are of a similar magnitude to variations in heat capacity caused by compositional change. On the other hand, the “configurational heat capacity” (C p conf(T f′)), defined as the difference between the fully relaxed liquid heat capacity and the glassy heat capacity at the limiting fictive temperature, shows much less dispersion as a function of composition. Its variation is a nonlinear function of composition, with little, if any, change for additions of oxide less than 10 mol%, but increasing values for greater additions of oxides. By use of previously determined liquid expansivities, we calculate that volume changes account for ∼15% of the configurational heat capacity. We conclude that liquid heat capacity should be considered as the sum of a vibrational contribution, of value close to 3R per gram-atom, and a configurational contribution related to liquid structure, rather than trying to define a single partial molar heat capacity for each liquid oxide component.

A thermodynamic model for hydrous silicate melts in the system NaAlSi 3O 8–KAlSi 3O 8–Si 4O 8–H 2O

Computation of crystal–liquid equilibria in hydrous silicate systems requires a model of the free energy of the hydrous liquid that defines the activity of the melt components at given temperature, pressure and composition. We present in this study a parametrization of the free energy of the liquid in the haplogranite system NaAlSi 3O 8–KAlSi 3O 8–Si 4O 8–H 2O based on the Margules approach. The excess free energy of the multicomponent melt is approximated from the binaries with the Kohler extrapolation method. Model parameters have been fitted to phase equilibrium data by mathematical programming techniques. A small but complex excess function of the anhydrous melt composition is necessary to reproduce reported liquidus phase relations. Using partial molar C p data from the literature for the H 2O melt component and a simple polynomial approximation for the molar volume, standard state enthalpy and entropy were refined close to −287 kJ/mol and 67.2 J/K mol, respectively. Calculated crystal–liquid phase relations are in good agreement with measurements to 5 kbar, and the modelled melt–fluid coexistence surface yields a valuable first order approximation of the H 2O solubility at near liquidus temperatures. Thermodynamic assessment of solubility and liquidus data suggests that H 2O mixing differs considerably in feldspar melts and in silica melts. Si 4O 8–H 2O mixing contributes to a very minor degree to the haplogranite system.

Viscosities of granitic (sensu lato) melts: Influence of the anorthite component

The viscosities of a series of granitic ( sensu lato ) melts have been determined in the range of 10 3 to 10 12 Pa·s. The anhydrous melt compositions are based on the addition of 10, 20, 50, and 75 wt% of the anorthite component (CaAl 2 Si 2 O 8 ) to a haplogranitic melt (HPG8) whose composition lies near the 2 kbar water-saturated minimum melt composition in the system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 . Melts with 10 and 20 wt% normative anorthite were subjected to high-pressure hydration syntheses using a piston-cylinder apparatus to generate water contents up to 2 wt%. Viscosities were determined for the anhydrous melts using the concentric cylinder method in the viscosity range of 10 2 to 10 5 Pa·s, and for both anhydrous and hydrated melts in the range of 10 9 to 10 12 Pa·s. The results for the temperature dependence of viscosity in the anhydrous system indicate that the influence on melt viscosity, caused by the addition of normative anorthite to the haplogranitic melt composition, is strongly temperature-dependent. Viscosity-temperature relationships of the melts become much more non-Arrhenian with addition of normative anorthite. The addition of water to melts with 10 and 20 wt% normative anorthite results in strong nonlinear decreases in viscosity. In the high viscosity range, the results for hydrous melts with 10% normative anorthite are adequately reproduced using the calcalkaline melt viscosity model of Hess and Dingwell (1996), whereas those for hydrous melts with 20 wt% normative anorthite are higher than the model predictions by amounts that depend on the water content. It appears that, at the higher temperatures anticipated for intermediate granitic magmatism, the calcalkaline model can adequately deal with up to 15 wt% normative anorthite in the melt composition in the range of temperatures relevant for intermediate magmas in nature.

Does organic acid adsorption affect alkali-feldspar dissolution rates?

Consideration of alkali-feldspar dissolution rates 1 The term alkali-feldspar is used in the present study to denote those feldspars that dissolve via aluminum deficient precursor complexes [Oelkers, E.H., Schott, J., Devidal, J.L., 1994. The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta, 58 (1994) 2011–2024.], which includes feldspars in the system NaAlSi 3O 8–KAlSi 3O 8, as well as plagioclases of compositions ∼An 80 or less [Oelkers, E.H., Schott, J., 1995b. Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis. Geochim. Cosmochim. Acta, 59: (1995) 5039–5053]. 1 taken from the literature suggests that in the absence of organic ligands and at far from equilibrium conditions they can be represented by r += k( a 3 H + / a Al +3 ) n , where r + refers to the far from equilibrium dissolution rate, k designates a rate constant, a i stands for the activity of the subscripted aqueous species, and n refers to a stoichiometric coefficient. Dissolution rates of these minerals have also been observed to increase substantially with increasing aqueous organic acid concentration. To assess if this latter effect is the result of either the change in aqueous aluminum activity due to aqueous aluminum–organic acid anion complex formation or organic acid anion adsorption to the mineral surface, dissolution rates predicted using the above equation are compared to measured alkali feldspar dissolution rates. A close correspondence between these predictions and the experimental data are found, providing strong evidence that organic acid anion adsorption does not affect alkali-feldspar dissolution rates.

Viscosity data for hydrous peraluminous granitic melts; comparison with a metaluminous model

We performed 27 viscosity determinations on dry and water-bearing peraluminous haplogranitic melts. The dry melt compositions cover the range of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition of Al 2 O 3 to a haplogranitic melt (HPG8) whose composition is near that of the projection of the 2 kbar H 2 O-saturated minimum melt composition into the system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 . The H 2 O contents of the hydrous melts were analyzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosity determinations were performed using a modified micropenetration method in the viscosity range of 10 10 to 10 11 Pa-s, at 1 atm pressure, and in the temperature ranges of 880-940 degrees C and 470-640 degrees C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the viscosity, which remains nearly constant despite further addition of Al 2 O 3 . Thus a viscosity maximum is inferred for dry slightly peraluminous granitic melts. The hydrous melt viscosity data were compared with the recent calculational model of Hess and Dingwell (1996), which was based on and designed for metaluminous melt viscosities. That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metaluminous melts.

Fluorine in aluminosilicate systems: Phase relations in the system NaAlSi 3O 8-CaAl 2Si 2O 8-F 2O −1

The liquidus phase equilibria of 0.9 NaAlSi 3O 8-0.1CaAl 2Si 2O 8-F 2O −1 was determined to 10 kbar for a composition with molar F (F + O) = 0.095 . The temperature of the liquidus is depressed by ~200°C at 1 bar, 2 kbar, and 10 kbar, relative to the volatile-free liquidus. Additional experiments were carried out at 2 kbar to examine the effects of increasing F content on phase equilibria. Increasing molar F (F + O) to 0.156 at fixed pressure decreases the temperature of the liquidus ~350°C from the temperature of the volatile-free liquidus. At constant F (F + O) (=0.095), X An of the plagioclase coexisting with the liquid decreased from An 30 at 1 atm to An 22 at 2 kbar and to An 17 at 10 kbar. Increasing fluorine content of the liquid at 2 kbar decreased the An content of the feldspar to a value of An 13 in equilibrium with the liquid with F (F + O) = 0.156 . Thermodynamic calculations demonstrate that the activity of NaAlSi 3O 8 in the liquid decreases with increasing fluorine content and concomitant decreasing temperature, but to a lesser extent than would be expected by the magnitude of the decrease in temperature. In contrast, the activity of CaAl 2Si 2O 8 decreases to a greater extent than is attributable solely to the effect of temperature. The effect of fluorine is in marked contrast to that of H 2O, which has little effect on the composition of the plagioclase in equilibrium with a melt of given (anhydrous) composition. Because of this, the degree of plagioclase fractionation in a fluorine-bearing, anhydrous system would be in considerable excess of estimates based solely on the fluorine-free anhydrous albite-anorthite system or the H 2O-saturated albite-anorthite-H 2O system. Rigorous application of these results to fractionation in late-stage, felsic systems will require additional experiments on the combined effects of H 2O and F in Ca-bearing granitic systems.

The mixing properties of melts and glasses in the system NaAlSi 3O 8-KAlSi 3O 8: Comparison of experimental data obtained by knudsen cell mass spectrometry and solution calorimetry

The apparent differences between heats of mixing determined by solution calorimetric measurements on quenched samples in the system NaAlSi 3O 8-KAlSi 3O 8 and heats of mixing determined at high temperatures by Knudsen cell mass spectrometry are caused by thermal effects related to the glass transition. When these effects are allowed for, the calorimetric data and Knudsen cell measurements are in good agreement.

Determination of activities in silicate melts by Knudsen cell mass spectrometry—I. The system NaAlSi 3O 8-KAlSi 3O 8

The mixing properties of aluminosilicate melts in the pseudobinary system NaAlSi 3O 8-KAlSi 3O 8 have been determined by measuring the compositions of their saturated vapours by hightemperature Knudsen cell mass spectrometry. The melts mix close to ideally over most of the composition range with small positive deviations from ideality for K-rich compositions. These may be related to incipient partial ordering of melt constituents into leucite-like and SiO 2-like structures above the feldspar liquidus.

Water and magmas; a mixing model

A model for the mixing of H 2O and silicate melts has been derived from the experimentally determined effects of H 2O on the viscosity (fluidity), volumes, electrical conductivities, and especially the thermodynamic properties of hydrous aluminosilicate melts. It involves primarily the reaction of H 2O with those O −2 ions of the melt that are shared (bridging) between adjacent (Al, Si)O 4 tetrahedra to produce OH − ions. However, in those melts that contain trivalent ions in tetrahedral coordination, such as the Al 3+ ion in feldspathic melts, the model further involves exchange of a proton from H 2O with a non-tetrahedrally coordinated cation that must be present to balance the net charge on the AlO 4 group. This cation exchange reaction, which goes essentially to completion, results in dissociation of the H 2O and is limited only by the availability of H 2O and the number of exchangeable cations per mole of aluminosilicate. In the system NaAlSi 3O 8-H 2O, upon which this thermodynamic model is based, there is 1 mole of exchangeable cations (Na +) per mole (GFW) of NaAlSi 3O 8, consequently ion exchange occurs for H 2O contents up to a 1:1 mole ratio ( X m w = mole fraction H 2 O = 0.5). For mole fractions of H 2O greater than 0.5, no further exchange can occur and the reaction with additional bridging oxygens of the melt produces 2 moles of associated OH − ions per mole of H 2O dissolved. These reactions lead to a linear dependence of the thermodynamic activity of H 2O ( a m w ) on the square of its mole fraction ( X m w ) for values of X m w , up to 0.5 and an exponential dependence on X m w at higher H 2O contents. Thus, for values of X m w ↬ 0.5, a m w = k(X m w) 2 , where k is a Henry's law constant for the dissociated solute. Extension of the thermodynamic model for NaAlSi 3O 8-H 2O to predict H 2O solubilities and other behavior of compositionally more complex aluminosilicate melts (magmas) requires placing these melts on an equimolal basis with NaAlSi 3O 8. This is readily accomplished using chemical analyses of quenched glasses by normalizing to the stoichiometric requirements of NaAlSi 3O 8, first in terms of equal numbers of exchangeable cations for mole fractions of H 2O up to 0.5 and secondly in terms of 8 moles of oxygen for higher H 2O contents. Chemical analyses of three igneous-rock glasses, ranging in composition from tholeiitic basalt to lithium-rich pegmatite, were thus recast and the experimental H 2O solubilities were computed on this equimolal basis. The resulting equimolal solubilities are all the same, within experimental error, as the solubility of H 2O in NaAlSi 3O 8 melt calculated from the thermodynamic relations. The equivalence of equimolal solubilities implies that the Henry's law constant ( k), which is a function of temperature and pressure, is independent of aluminosilicate composition over a wide range. Moreover, as a consequence of the Gibbs-Duhem relation and the properties of exact differentials, it is clear that the silicate components of the melt, properly defined, mix ideally. Thus, a relatively simple mixing model for H 2O in silicate melts has led to a quantitative thermodynamic model for magmas that has far-reaching consequences in igneous petrogenesis.

Crystallization of Alkali Feldspar and Quartz in the Haplogranite System NaAlSi3O8-KAlSi3O8-SiO2-H2O at 4 Kb

Results of experimental studies in the system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O, at 4 kb and from 650° to 1000°C, have been used to generate composition paths for liquid and crystal fractions as functions of temperature and bulk composition. In both the experimental work and the analyses of crystallization, attention has been devoted mainly to silicate liquid that is saturated with quartz and an alkali feldspar but unsaturated with respect to an aqueous vapor phase. Such liquid can be represented on a compound T-X surface that slopes, with decreasing H 2 O content in the system, toward SiO 2 for NaAlSi 3 O 8 -rich compositions and away from SiO 2 for KAlSi 3 O 8 -rich compositions. Compositions of the crystal fraction that separates from the liquid under equilibrium conditions in the temperature range 700° to 655°C have been calculated for an array of bulk compositions. Equilibrium crystallization history is characterized by numerous shifts and some reversals along paths that describe the composition of the crystal fraction. The various paths demonstrate that intensive parameters such as temperature, pressure, and H 2 O fugacity cannot be uniquely determined from the composition of a specific fraction. Paths also have been outlined, for a variety of bulk compositions, from the maximum temperature for separation of quartz ± alkali feldspar to the solidus temperature of 655° C under conditions of perfect fractional crystallization in which successive precipitates are immediately isolated from the silicate liquid. These crystal fraction paths are marked by discontinuities that indicate abrupt, discrete changes in composition, and with falling temperature, they tend to focus toward a compositional area centering about 20 NaAlSi 3 O 8 , 45 KAlSi 3 O 8 , 35 SiO 2 . The melting of crystalline assemblages that represent the haplogranite system can be considered as the inverse of equilibrium crystallization, fractional crystallization, or some combination of these idealized processes. Thus the composition path for an equilibrium crystal fraction can also serve for the tracing of compositional changes in a residue formed during equilibrium melting. A residue of given composition can be produced at constant elevated pressure under conditions representing many different combinations of temperature and bulk composition. The experimentally determined complexities of crystallization are reasonable indications of what can occur under natural plutonic conditions. They are potentially most useful, however, for the testing of genetic models based upon detailed studies of rocks in a geologic and petrographic context. They also lead to the conclusions that (1) the bulk composition of a granitic plutonite is not by itself sufficient for estimating conditions under which the rock was formed and (2) experimental data obtained for the haplogranite system under conditions that include the presence of an aqueous fluid phase ( P H 2 O ≈ P fluid = P tot ) are highly restricted in their pertinence to the crystallization histories of most granitic plutonites.

The role of H 2 O in silicate melts; II, Thermodynamic and phase relations in the system NaAlSi 3 O 8 -H 2 O to 10 kilobars, 700 degrees to 1100 degrees C

The role of H 2 O in silicate melts; II, Thermodynamic and phase relations in the system NaAlSi 3 O 8 -H 2 O to 10 kilobars, 700 degrees to 1100 degrees C

Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith

Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust.

Melting relations of NaAlSi 3 O 8 to 30 Kb in the presence of H 2 O:CO 2 = 50:50 vapor

Synthetic albite was reacted with oxalic acid dihydrate under oxidizing conditions in an internally-heated pressure vessel from 1 to 4 kb and in a piston-cylinder apparatus from 10 to 30 kb. Breakdown of the oxalic acid in subsolidus runs produced a vapor with H_2O:CO_2 = 50:50 in moles (X^v_(H_2O) = 0.5, neglecting solubility of solids in the vapor). The curve determined for the beginning of melting under these conditions defines a contour on the divariant solidus surface for the system NaAlSi_3O_8-H_2O-CO_2 for X^v_(H_2O) = 0.5, between the published limits for X^v_(H_2O) = 1.0 and X^v_(H_2O) = O. Addition of 50 mole percent of relatively insoluble CO_2 to NaAlSi_3O_8-H_2O at 15 kb total pressure raises the solidus temperature from 660° to 830°C. The results give no indication of increased solubility of CO_2 in NaAlSi_3O_8 liquids at pressures above 15 kb, in contrast with published results for a basalt liquid.

The role of H 2 O in silicate melts; I, P-V-T relations in the system NaAlSi 3 O 8 -H 2 O to 10 kilobars and 1000 degrees C

The role of H 2 O in silicate melts; I, P-V-T relations in the system NaAlSi 3 O 8 -H 2 O to 10 kilobars and 1000 degrees C

Melting Relationships in the System NaAlSi3O8-NaCl-H2O at One Kilobar Pressure, with Petrological Applications

Phase relationships in the system NaAlSi_3O_8-NaCl-H_2O appear to be ternary between 850° C and 950° C at 1 kbar pressure. A two-phase field containing NaCl-rich liquid and H_2O-rich vapor extends from the system NaCl-H_2O into the ternary system, although this field was not intersected by the joins investigated. In the system NaAlSi_3O_8-H_2O a wide miscibility gap occurs between liquid and vapor; in the system NaAlS_2O_8-NaCl a wide miscibility gap extends between silicate-rich liquid and NaCl-rich liquid at temperatures above 1,100° C at 1 bar pressure and also by inference at 1 kbar pressure. Both miscibility gaps are connected through the ternary system, separating a silicate liquid from a fluid phase with composition close to the join NaCl-H_2O and containing a small, unknown proportion of dissolved silicates. A temperature minimum is present on the ternary liquidus at 873 ± 5° C, representing the reaction: Albite+Fluid (H_2O-Nacl) ⇔ Liquid_(Albite) (1) The liquid composition, in terms of the anhydrous components, is approximately 99.7 wt percent NaAlSi_3O_8, 0.3 wt percent NaCl; its H_2O content is about 10 wt percent. It is expected that at lower pressures two additional reactions will occur: Albite+Vapor=Liquid_(Albite)+Liquid_(Nacl) (2) and Albite+Liquid_(Nacl)+Vapor; ⇔ Liquid_(Albite). (3) The results contrast with those in the system NaAlSi_3O_8-NaF-H_2O and confirm previous conclusions from the systems NaAlSi_3O_8-HCL-H_2O and NaAlSi_3O_8-HF-H_2O. Whereas fluoride (or NaF) tends to remain in the liquid (magma), chlorine (or NaCl) passes preferentially into the vapor or fluid phase. The solubility of H2O in a silicate melt increases when a small quantity of chlorine is present. Coexistence of H_2O-rich liquid inclusions and NaCl-rich liquid inclusions in crystalline phases in igneous rocks may indicate low-pressure conditions (<1 kbar) during capture of the inclusions.

Water pressures during differentiation and crystallization of some ash-flow magmas from southern Nevada

Water pressures between approximately 500 and 1,200 bars during differentiation and crystallization of source magmas for 6 Tertiary rhyolitic ash-flow sheets from southern Nevada are indicated by plots of 82 chemical analyses in the experimentally determined system NaAlSi ~3~ O ~8~ -KAlSi ~3~ O ~8~ -SiO ~2~ -H ~2~ O. These pressures are considerably lower than most previously estimated for rhyolitic magmas. The water pressures also provide some basis for estimating water content, temperature, and depth of the magmas during differentiation and crystallization.

Melting-relations in the system NaA1Si 3 O 8 –NaA1SiO 4 –NaFeSi 2 O 6 –CaMgSi 2 O 6 –H 2 O, and their bearing on the genesis of alkaline undersaturated rocks

Experimental data are presented that constitute a basis for a more detailed treatment at present under investigation, involving the addition of the alkali pyroxene end-members acmite (NaFeSi 2 O 6 ), diopside (CaMgSi 2 O 6 ), and hedenbergite (CaFeSi 2 O 6 ) to the silica- and potash-poor portion of ‘petrogeny’s residua system’. Phase relations have been determined in the systems NaAlSi 3 O 8 –NaFeSi 2 O 6 , NaAlSiO 4 –NaFeSi 2 O 6 , NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 at 1000 kg/cm 2 water vapour pressure; the partial pressure of oxygen was controlled in most of the experiments by a nickel–nickel oxide buffer assemblage. The composition of the ‘low melting-point’ in the system NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 is located at Ab 15 Ne 30 Ac 55 . Little agreement was found between this point and the density distribution plot of normative Ab+Or, Ne+Ks, Ac+Di in a total of 256 volcanic and plutonic alkaline undersaturated rocks. However, with the addition of known amounts of diopside to the system NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 , the determined phase relations show a close agreement with the normative plots for both alkaline and peralkaline undersaturated rocks. The possible genesis of alkaline rocks by processes involving crystal ⇋ liquid equilibrium is discussed in the light of this evidence.