The Experts below are selected from a list of 29676 Experts worldwide ranked by ideXlab platform
Roger H. Mitchell - One of the best experts on this subject based on the ideXlab platform.
-
Ultra High Pressure–Temperature Melting Experiments on an SiO2-Rich Lamproite from Smoky Butte, Montana: Derivation of Siliceous Lamproite Magmas from Enriched Sources Deep in the Continental Mantle
Journal of Petrology, 1997Co-Authors: A. D. Edgar, Roger H. MitchellAbstract:Liquidus to near-solidus phase relations between 0·8 and 12·0 GPa INTRODUCTION and 900 and 1200°C were determined for an SiO2-rich lamproite Kimberlite, orangeite (formerly Group 2 kimberlite; Mitfrom Smoky Butte, Montana. At 7–10 GPa, assemblages between chell, 1995a) and lamproite are considered to be the Liquidus and solidus are clinopyroxene+garnet, garnet+ products of the deepest levels of continental alkaline clinopyroxene+coesite, garnet+clinopyroxene+coesite+rutile+ magmatism, on the basis of the occurrence of diamond K–Ti silicate, and garnet+clinopyroxene+K–Ti silicate+ in some of these rocks and the presence of entrained phlogopite+coesite+rutile. At 12 GPa, comparable assemblages mantle-derived xenoliths. Of these rock types, the lamare garnet, garnet+clinopyroxene+coesite, and garnet+ proites, consisting of two end-member varieties—olivine clinopyroxene+K silicate+coesite+rutile. At 6–12 GPa, K–Ti lamproite and leucite lamproite—are more enriched in silicates and K–Ba phosphate compounds occur between 1200 and incompatible elements and large ion lithophile elements 1400°C. At 12 GPa, K silicate replaces the K–Ti silicate compound (LILE) than kimberlite and orangeite (see Mitchell & present at lower pressures. At >6 GPa, clinopyroxene and garnet Bergman, 1991; Mitchell, 1995a). The geochemical charincorporate K in their structures, particularly at lower temperatures. acteristics of lamproites suggest that their mantle source Near-Liquidus phase relations of the Smoky Butte lamproite are comregions must be enriched in K, light rare earth elements pared with those of previously investigated lamproite of a similar (LREE), Ti, Ba, P, F and other incompatible elements. composition. It is considered unlikely that near-Liquidus garnetite Geochemical and high-temperature–pressure exmineral assemblages at 7–12 GPa could reflect the source of these perimental studies (Arima & Edgar, 1983; Foley, 1989, magmas. A more likely source for these SiO2-rich lamproitic magmas 1990; Edgar et al., 1992; Edgar & Vukadinovic, 1993; is an assemblage comparable with the near-solidus assemblage of Sato & Ito, 1994; Mitchell, 1995b) demonstrate, on the garnet+clinopyroxene+phlogopite and minor K-, Ti-, Ba-enriched basis of near-Liquidus mineral assemblages, that lamcompounds. On partial melting such an assemblage would yield a proitic liquids cannot be in equilibrium with a garnetcomposition comparable with that of the SiO2-rich magma from Smoky bearing lherzolite mantle source. In contrast, Sato & Ito Butte. SiO2-rich lamproite magmas may be primary melts from an (1994) have concluded that a garnet harzburgite from enriched source from deep in the lithospheric mantle. which phlogopite had already melted out was a possible
-
ultra high pressure temperature melting experiments on an sio2 rich lamproite from smoky butte montana derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle
Journal of Petrology, 1997Co-Authors: A. D. Edgar, Roger H. MitchellAbstract:Liquidus to near-solidus phase relations between 0·8 and 12·0 GPa INTRODUCTION and 900 and 1200°C were determined for an SiO2-rich lamproite Kimberlite, orangeite (formerly Group 2 kimberlite; Mitfrom Smoky Butte, Montana. At 7–10 GPa, assemblages between chell, 1995a) and lamproite are considered to be the Liquidus and solidus are clinopyroxene+garnet, garnet+ products of the deepest levels of continental alkaline clinopyroxene+coesite, garnet+clinopyroxene+coesite+rutile+ magmatism, on the basis of the occurrence of diamond K–Ti silicate, and garnet+clinopyroxene+K–Ti silicate+ in some of these rocks and the presence of entrained phlogopite+coesite+rutile. At 12 GPa, comparable assemblages mantle-derived xenoliths. Of these rock types, the lamare garnet, garnet+clinopyroxene+coesite, and garnet+ proites, consisting of two end-member varieties—olivine clinopyroxene+K silicate+coesite+rutile. At 6–12 GPa, K–Ti lamproite and leucite lamproite—are more enriched in silicates and K–Ba phosphate compounds occur between 1200 and incompatible elements and large ion lithophile elements 1400°C. At 12 GPa, K silicate replaces the K–Ti silicate compound (LILE) than kimberlite and orangeite (see Mitchell & present at lower pressures. At >6 GPa, clinopyroxene and garnet Bergman, 1991; Mitchell, 1995a). The geochemical charincorporate K in their structures, particularly at lower temperatures. acteristics of lamproites suggest that their mantle source Near-Liquidus phase relations of the Smoky Butte lamproite are comregions must be enriched in K, light rare earth elements pared with those of previously investigated lamproite of a similar (LREE), Ti, Ba, P, F and other incompatible elements. composition. It is considered unlikely that near-Liquidus garnetite Geochemical and high-temperature–pressure exmineral assemblages at 7–12 GPa could reflect the source of these perimental studies (Arima & Edgar, 1983; Foley, 1989, magmas. A more likely source for these SiO2-rich lamproitic magmas 1990; Edgar et al., 1992; Edgar & Vukadinovic, 1993; is an assemblage comparable with the near-solidus assemblage of Sato & Ito, 1994; Mitchell, 1995b) demonstrate, on the garnet+clinopyroxene+phlogopite and minor K-, Ti-, Ba-enriched basis of near-Liquidus mineral assemblages, that lamcompounds. On partial melting such an assemblage would yield a proitic liquids cannot be in equilibrium with a garnetcomposition comparable with that of the SiO2-rich magma from Smoky bearing lherzolite mantle source. In contrast, Sato & Ito Butte. SiO2-rich lamproite magmas may be primary melts from an (1994) have concluded that a garnet harzburgite from enriched source from deep in the lithospheric mantle. which phlogopite had already melted out was a possible
A. D. Edgar - One of the best experts on this subject based on the ideXlab platform.
-
Ultra High Pressure–Temperature Melting Experiments on an SiO2-Rich Lamproite from Smoky Butte, Montana: Derivation of Siliceous Lamproite Magmas from Enriched Sources Deep in the Continental Mantle
Journal of Petrology, 1997Co-Authors: A. D. Edgar, Roger H. MitchellAbstract:Liquidus to near-solidus phase relations between 0·8 and 12·0 GPa INTRODUCTION and 900 and 1200°C were determined for an SiO2-rich lamproite Kimberlite, orangeite (formerly Group 2 kimberlite; Mitfrom Smoky Butte, Montana. At 7–10 GPa, assemblages between chell, 1995a) and lamproite are considered to be the Liquidus and solidus are clinopyroxene+garnet, garnet+ products of the deepest levels of continental alkaline clinopyroxene+coesite, garnet+clinopyroxene+coesite+rutile+ magmatism, on the basis of the occurrence of diamond K–Ti silicate, and garnet+clinopyroxene+K–Ti silicate+ in some of these rocks and the presence of entrained phlogopite+coesite+rutile. At 12 GPa, comparable assemblages mantle-derived xenoliths. Of these rock types, the lamare garnet, garnet+clinopyroxene+coesite, and garnet+ proites, consisting of two end-member varieties—olivine clinopyroxene+K silicate+coesite+rutile. At 6–12 GPa, K–Ti lamproite and leucite lamproite—are more enriched in silicates and K–Ba phosphate compounds occur between 1200 and incompatible elements and large ion lithophile elements 1400°C. At 12 GPa, K silicate replaces the K–Ti silicate compound (LILE) than kimberlite and orangeite (see Mitchell & present at lower pressures. At >6 GPa, clinopyroxene and garnet Bergman, 1991; Mitchell, 1995a). The geochemical charincorporate K in their structures, particularly at lower temperatures. acteristics of lamproites suggest that their mantle source Near-Liquidus phase relations of the Smoky Butte lamproite are comregions must be enriched in K, light rare earth elements pared with those of previously investigated lamproite of a similar (LREE), Ti, Ba, P, F and other incompatible elements. composition. It is considered unlikely that near-Liquidus garnetite Geochemical and high-temperature–pressure exmineral assemblages at 7–12 GPa could reflect the source of these perimental studies (Arima & Edgar, 1983; Foley, 1989, magmas. A more likely source for these SiO2-rich lamproitic magmas 1990; Edgar et al., 1992; Edgar & Vukadinovic, 1993; is an assemblage comparable with the near-solidus assemblage of Sato & Ito, 1994; Mitchell, 1995b) demonstrate, on the garnet+clinopyroxene+phlogopite and minor K-, Ti-, Ba-enriched basis of near-Liquidus mineral assemblages, that lamcompounds. On partial melting such an assemblage would yield a proitic liquids cannot be in equilibrium with a garnetcomposition comparable with that of the SiO2-rich magma from Smoky bearing lherzolite mantle source. In contrast, Sato & Ito Butte. SiO2-rich lamproite magmas may be primary melts from an (1994) have concluded that a garnet harzburgite from enriched source from deep in the lithospheric mantle. which phlogopite had already melted out was a possible
-
ultra high pressure temperature melting experiments on an sio2 rich lamproite from smoky butte montana derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle
Journal of Petrology, 1997Co-Authors: A. D. Edgar, Roger H. MitchellAbstract:Liquidus to near-solidus phase relations between 0·8 and 12·0 GPa INTRODUCTION and 900 and 1200°C were determined for an SiO2-rich lamproite Kimberlite, orangeite (formerly Group 2 kimberlite; Mitfrom Smoky Butte, Montana. At 7–10 GPa, assemblages between chell, 1995a) and lamproite are considered to be the Liquidus and solidus are clinopyroxene+garnet, garnet+ products of the deepest levels of continental alkaline clinopyroxene+coesite, garnet+clinopyroxene+coesite+rutile+ magmatism, on the basis of the occurrence of diamond K–Ti silicate, and garnet+clinopyroxene+K–Ti silicate+ in some of these rocks and the presence of entrained phlogopite+coesite+rutile. At 12 GPa, comparable assemblages mantle-derived xenoliths. Of these rock types, the lamare garnet, garnet+clinopyroxene+coesite, and garnet+ proites, consisting of two end-member varieties—olivine clinopyroxene+K silicate+coesite+rutile. At 6–12 GPa, K–Ti lamproite and leucite lamproite—are more enriched in silicates and K–Ba phosphate compounds occur between 1200 and incompatible elements and large ion lithophile elements 1400°C. At 12 GPa, K silicate replaces the K–Ti silicate compound (LILE) than kimberlite and orangeite (see Mitchell & present at lower pressures. At >6 GPa, clinopyroxene and garnet Bergman, 1991; Mitchell, 1995a). The geochemical charincorporate K in their structures, particularly at lower temperatures. acteristics of lamproites suggest that their mantle source Near-Liquidus phase relations of the Smoky Butte lamproite are comregions must be enriched in K, light rare earth elements pared with those of previously investigated lamproite of a similar (LREE), Ti, Ba, P, F and other incompatible elements. composition. It is considered unlikely that near-Liquidus garnetite Geochemical and high-temperature–pressure exmineral assemblages at 7–12 GPa could reflect the source of these perimental studies (Arima & Edgar, 1983; Foley, 1989, magmas. A more likely source for these SiO2-rich lamproitic magmas 1990; Edgar et al., 1992; Edgar & Vukadinovic, 1993; is an assemblage comparable with the near-solidus assemblage of Sato & Ito, 1994; Mitchell, 1995b) demonstrate, on the garnet+clinopyroxene+phlogopite and minor K-, Ti-, Ba-enriched basis of near-Liquidus mineral assemblages, that lamcompounds. On partial melting such an assemblage would yield a proitic liquids cannot be in equilibrium with a garnetcomposition comparable with that of the SiO2-rich magma from Smoky bearing lherzolite mantle source. In contrast, Sato & Ito Butte. SiO2-rich lamproite magmas may be primary melts from an (1994) have concluded that a garnet harzburgite from enriched source from deep in the lithospheric mantle. which phlogopite had already melted out was a possible
Timothy L. Grove - One of the best experts on this subject based on the ideXlab platform.
-
experimental investigations of the role of h2o in calc alkaline differentiation and subduction zone magmatism
Contributions to Mineralogy and Petrology, 1993Co-Authors: Thomas W. Sisson, Timothy L. GroveAbstract:Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H2O-saturated with fO2 buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the Liquidus of wet high-alumina basalts because H2O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na2O content of the bulk composition as well as to H2O content, with the result that hornblende can form as a near Liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na2O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na2O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An90) forms from basaltic melts with high H2O contents but cannot form from dry melts with normal are Na2O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H2O contents. In sum, moderate pressure H2O-saturated phase relations show that magmatic H2O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.
-
High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano, northern California
Contributions to Mineralogy and Petrology, 1991Co-Authors: Karen S. Bartels, Rosamond J. Kinzler, Timothy L. GroveAbstract:Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the Liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near Liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K_2O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.
P. Wyllie - One of the best experts on this subject based on the ideXlab platform.
-
Melts in the mantle modeled in the system CaO-MgO-SiO_2-CO_2 at 2.7 GPa
Contributions to Mineralogy and Petrology, 2000Co-Authors: W. L. Huang, P. WyllieAbstract:The effect of CO_2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO_2-CO_2 at 2.7 GPa. The experiments provide isotherms for the vapor-saturated Liquidus surface, bracket piercing points for field boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics and peritectics). CO_2-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 °C above the solidus, with a transition to silicate liquids (>80% silicate) within ∼75 °C across a plateau on the Liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate Liquidus field boundary, so the carbonate Liquidus field is therefore a forbidden volume for liquid magmas. This confirms the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed for the carbonation and melting reactions for mineral assemblages corresponding to lherzolite, harzburgite, websterite and wehrlite, with carbonate, CO_2 vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing compositions of eutectic and peritectic liquids on the Liquidus surface. At an invariant point Q (∼2.8 GPa/1230 °C), all peridotite assemblages coexist with a calcite-dolomite solid solution (75 ± 5% CaCO_3) and a dolomitic carbonatite melt [57% CaCO_3 (CC), 33% MgCO_3 (MC), 10% CaMgSi_2O_6 (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic) at pressures below Q (∼70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q will begin to crystallize, releasing CO_2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.
Craig E Manning - One of the best experts on this subject based on the ideXlab platform.
-
hydrous albite magmas at lower crustal pressure new results on Liquidus h2o content solubility and h2o activity in the system naalsi3o8 h2o nacl at 1 0 gpa
Contributions to Mineralogy and Petrology, 2016Co-Authors: A R Makhluf, R C Newton, Craig E ManningAbstract:The system albite–H2O serves as an important model for the generation of granitic magmas, yet relatively few experimental investigations have focused on phase relations at high pressure. This study reports new experimental results, at 1.0 GPa and 690–1050 °C, on the temperature and liquid composition at vapor-saturated melting, the H2O content of undersaturated silicate liquids in equilibrium with albite, the solubility of albite in H2O–NaCl fluids immediately below the solidus, and the activity of H2O in hydrous NaAlSi3O8 liquids along the Liquidus. Albite melts and dissolves congruently at all temperatures and salinities. In the NaCl-absent system, the temperature of vapor-saturated melting of low albite, confirmed by X-ray diffraction, is 695 ± 5 °C and the liquid composition is 18.14 ± 1.35 wt% H2O. The temperature dependence of the fluid-undersaturated Liquidus curve in the system NaAlSi3O8–H2O varies with H2O wt% (\(w_{{{\text{H}}_{2} {\text{O}}}}\)) according to $$T = - 2.0331 \times 10^{ - 3} w_{{{\text{H}}_{2} {\text{O}}}}^{3} + 1.6497w_{{{\text{H}}_{2} {\text{O}}}}^{2} {-} 58.963w_{{{\text{H}}_{2} {\text{O}}}} + 1235.5\,^{ \circ } {\text{C}}$$ indicating positive curvature in temperature-composition coordinates and a dry melting temperature of ~1235 °C. At 690 °C, immediately below the solidus, albite solubility decreases drastically with NaCl content of the fluid phase, from 8.8 ± 0.6 wt% in the NaCl-free fluid to ˂2 % at NaCl concentration of only 10 mol%. Experiments determining the activity of H2O (\(a_{{{\text{H}}_{2} {\text{O}}}}\)) in liquids at vapor-saturated melting exploited low Cl solubility in liquids and low albite solubility in the presence of H2O–NaCl fluids. The maximum Cl content of quenched glasses, only 0.95 wt%, and very low albite solubility together make possible H2O activity measurement in melts equilibrated with NaCl–H2O solutions. When combined with activity data for H2O–NaCl fluids, experimentally determined \(a_{{{\text{H}}_{2} {\text{O}}}}\) along the Liquidus is described by $$T = - {469.16a_{{H_{2} O}}^{L}}^{{\frac{1}{2}}} {-} 93.382a_{{H_{2} O}}^{L} + 1235.5\;^\circ C.$$ At 1 GPa, H2O activities in hydrous albitic melts in H2O–NaCl fluids agree with those in the presence of H2O–CO2 fluids, provided appropriate fluid mixing data are used. Our results constrain an asymmetric regular solution model for the mixing of Na1/8Al1/8Si3/8O and H2O, yielding Margules parameters of W Ab = 25.56 ± 0.54 and \(W_{{{\text{H}}_{2} {\text{O}}}} = \, 10.50\; \pm \;0.74{\text{ kJ}}/{\text{mol}}\). The results imply critical mixing of liquid and vapor at ~1100 °C. Using existing speciation models, our results imply that H2O dissolved in hydrous albitic liquids is almost entirely in the form of molecular H2O (OH− concentration is negligible) at temperatures near vapor-saturated melting. The results provide a more complete experimental foundation for modeling the system NaAlSi3O8–H2O at high pressure.
-
Hydrous albite magmas at lower crustal pressure: new results on Liquidus H_2O content, solubility, and H_2O activity in the system NaAlSi_3O_8–H_2O–NaCl at 1.0 GPa
Contributions to Mineralogy and Petrology, 2016Co-Authors: A R Makhluf, R C Newton, Craig E ManningAbstract:The system albite–H_2O serves as an important model for the generation of granitic magmas, yet relatively few experimental investigations have focused on phase relations at high pressure. This study reports new experimental results, at 1.0 GPa and 690–1050 °C, on the temperature and liquid composition at vapor-saturated melting, the H_2O content of undersaturated silicate liquids in equilibrium with albite, the solubility of albite in H_2O–NaCl fluids immediately below the solidus, and the activity of H_2O in hydrous NaAlSi_3O_8 liquids along the Liquidus. Albite melts and dissolves congruently at all temperatures and salinities. In the NaCl-absent system, the temperature of vapor-saturated melting of low albite, confirmed by X-ray diffraction, is 695 ± 5 °C and the liquid composition is 18.14 ± 1.35 wt% H_2O. The temperature dependence of the fluid-undersaturated Liquidus curve in the system NaAlSi_3O_8–H_2O varies with H_2O wt% ( $$w_{{{\text{H}}_{2} {\text{O}}}}$$ w H 2 O ) according to $$T = - 2.0331 \times 10^{ - 3} w_{{{\text{H}}_{2} {\text{O}}}}^{3} + 1.6497w_{{{\text{H}}_{2} {\text{O}}}}^{2} {-} 58.963w_{{{\text{H}}_{2} {\text{O}}}} + 1235.5\,^{ \circ } {\text{C}}$$ T = - 2.0331 × 10 - 3 w H 2 O 3 + 1.6497 w H 2 O 2 - 58.963 w H 2 O + 1235.5 ∘ C indicating positive curvature in temperature-composition coordinates and a dry melting temperature of ~1235 °C. At 690 °C, immediately below the solidus, albite solubility decreases drastically with NaCl content of the fluid phase, from 8.8 ± 0.6 wt% in the NaCl-free fluid to ˂2 % at NaCl concentration of only 10 mol%. Experiments determining the activity of H_2O ( $$a_{{{\text{H}}_{2} {\text{O}}}}$$ a H 2 O ) in liquids at vapor-saturated melting exploited low Cl solubility in liquids and low albite solubility in the presence of H_2O–NaCl fluids. The maximum Cl content of quenched glasses, only 0.95 wt%, and very low albite solubility together make possible H_2O activity measurement in melts equilibrated with NaCl–H_2O solutions. When combined with activity data for H_2O–NaCl fluids, experimentally determined $$a_{{{\text{H}}_{2} {\text{O}}}}$$ a H 2 O along the Liquidus is described by $$T = - {469.16a_{{H_{2} O}}^{L}}^{{\frac{1}{2}}} {-} 93.382a_{{H_{2} O}}^{L} + 1235.5\;^\circ C.$$ T = - 469.16 a H 2 O L 1 2 - 93.382 a H 2 O L + 1235.5 ∘ C . At 1 GPa, H_2O activities in hydrous albitic melts in H_2O–NaCl fluids agree with those in the presence of H_2O–CO_2 fluids, provided appropriate fluid mixing data are used. Our results constrain an asymmetric regular solution model for the mixing of Na_1/8Al_1/8Si_3/8O and H_2O, yielding Margules parameters of W _Ab = 25.56 ± 0.54 and $$W_{{{\text{H}}_{2} {\text{O}}}} = \, 10.50\; \pm \;0.74{\text{ kJ}}/{\text{mol}}$$ W H 2 O = 10.50 ± 0.74 kJ / mol . The results imply critical mixing of liquid and vapor at ~1100 °C. Using existing speciation models, our results imply that H_2O dissolved in hydrous albitic liquids is almost entirely in the form of molecular H_2O (OH^− concentration is negligible) at temperatures near vapor-saturated melting. The results provide a more complete experimental foundation for modeling the system NaAlSi_3O_8–H_2O at high pressure.