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Alkali aluminosilicate-saturated aqueous fluids in the earth’s upper mantle
Geochimica et Cosmochimica Acta, Vol. 64, No. 24, pp. 4243– 4256, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00
Pergamon
PII S0016-7037(00)00498-1
Alkali aluminosilicate-saturated aqueous fluids in the earth’s upper mantle BJORN O. MYSEN1,* and KEVIN WHEELER2 1
Geophysical Laboratory and Center for High-Pressure Research (CHiPR), Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington, DC 20015, USA 2 Department of Geological Sciences, Brown University, Providence, RI 02912, USA (Received January 6, 2000; accepted in revised from July 6, 2000)
Abstract—The solubility in aqueous fluids of peralkaline alkali aluminosilicate components has been determined in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively. Compositions were along the haploandesite join, Na2Si4O9–Na2(NaAl)4O9, with 0, 3, and 6 mol.% Al2O3, denoted NS4Ax, where x represents the mol.% Al2O3. The silicate solubility in aqueous fluid is in the 3 to 20 mol.% range for NS4, 2 to 13 mol.% for NS4A3, and 1.5– 8 mol.% for NS4A6 with a linear and positive temperature-dependence and a nonlinear and positive pressure-dependence. The silicate solubility decreases with increasing Al2O3 content. From stepwise regression, the pressure-, temperature-, and Al2O3-dependence of the solubility can be described with the expression: X silicate (mol.%) ⫽ 1.9 ⫺ 1.3X Al2O3 (mol.%) ⫹ 0.008T共⬚C) ⫹ 13P共GPa) ⫹ 7.3P 2 . fluid fluid Partial molar volume of H2O in the silicate-saturated fluids, V H , is in the range 17 to 25 cm3/mol. The V H 2O 2O was calculated with the assumption that the activity of dissolved silicate in aqueous fluid equals the mol fluid fraction of dissolved silicate. Compared with the molar volume of pure H2O, V ⬚H2O, the V H values are 10 2O to 15% lower at 0.8 GPa and as much as 15% higher than V ⬚H2O at 2.0 GPa. This volume difference diminishes as the system become more aluminous. fluid fluid For all compositions, the V H is a linear and negative function of pressure with  [⫽ ⫺1/V 0 (⭸V H /⭸P) T ] 2O 2O ⫺1 ⬃ 0.125 GPa for NS4. The NS4A3 increases from 0.141 to 0.172 in the 1000° to 1300°C range, whereas NS4A6 increases from 0.193 to 0.213 GPa⫺1. fluid The V H is a positive and linear function of temperature with thermal expansion coefficients in the range 2O 3– 8 ⫻ 10⫺4 K⫺1. This thermal expansivity resembles that of pure H2O although the exact values and the fluid range in values for pure H2O differ from 1/V 0 (⭸V H /⭸T) P of silicate-saturated aqueous solution. Copyright 2O © 2000 Elsevier Science Ltd
bearing melt, in turn, serves as a transport agent by which subducted H2O may be recycled back into the overlying crust. Evidence for interaction between rocks and aqueous fluids in the crust and the upper mantle of the earth is, therefore, extensive over a range of pressures and temperatures. Characterization of the solubility behavior of silicate components in aqueous fluids as a function of pressure, temperature, and bulk chemical composition is needed to describe these interaction processes. Available experimental data, particularly in the pressure-temperature regime of the upper mantle, however, are limited. It is known, though, that silicate solubility is positively correlated with fluid density (Manning, 1994) and because fluid density correlates with pressure (e.g., Haar et al., 1984) silica solubility in aqueous fluid increases with increasing pressure. In fact, recent preliminary experimental data obtained in the hydrothermal diamond anvil cell, indicate that there may be complete miscibility between aqueous fluids and at least some alkali silicate melts in the pressure-temperature regime of the upper mantle (Shen and Keppler, 1997). Furthermore, it has been suggested the solution behavior of silicate in aqueous fluids might be a distinct function of pressure as suggested, for example, by Fujii et al. (1996) for the system MgO–SiO2–H2O. At pressures ⬍ 3 GPa, the fluid in that study contained only silica, whereas at high pressure MgO became significantly soluble.
1. INTRODUCTION
Aqueous fluids are important agents for materials transport in the earth’s crust and upper mantle. High-grade metamorphism in deep continental crust can involve dehydration of hydrous phases with concomitant alteration of major- and trace-element patterns in high-temperature/-pressure metamorphic rocks (e.g., Rollinson and Windley, 1980; Fowler, 1984; Whitehouse, 1989). In subduction zones near convergent plate boundaries there is evidence for transport of major, minor, and trace elements from the subducting slab to the overlying mantle wedge (e.g., Riter and Smith, 1996; McInnes, 1996; Brenan et al., 1995). Alteration of the bulk chemistry of peridotite in the overlying mantle wedge via ingress of aqueous fluid can result in formation of biotite and K-richterite (Luth, 1997; Konzett and Ulmer, 1999). Orthopyroxene overgrowth on olivine, generated by reaction with silicate-rich fluids, has been observed in natural samples (Riter and Smith, 1996) and in laboratory experiments (Iizuka and Mysen, 1998). Ultimately, water transported into the peridotite wedge above subducting lithosphere may trigger partial melting (e.g., Kushiro, 1990). This H2O-
* Author to whom correspondence should be addressed (mysen@ gl.ciw.edu). 4243
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Temperature is an important factor as well. The silicate solubility increases with increasing temperature as the silicateH2O solvus shrinks (Kennedy et al., 1962; Manning, 1994; Mysen, 1998). Bulk chemical composition also affects silicate solubility at upper mantle pressures and temperatures (e.g., Mysen, 1998; Mysen and Acton, 1999). A composition effect has also been demonstrated in lower-pressure experiments in systems such as SiO2–KOH–H2O and Al2O3–KOH–H2O (Anderson and Burnham, 1967; Pascal and Anderson, 1989) and chloride-bearing SiO2–Al2O3–H2O systems (Anderson and Burnham, 1967). In light of the above considerations, it is evident that systematic experimental studies on the interaction between silicates and aqueous fluids at high pressure and high temperature are needed. Major important chemical components in upper mantle subduction zones and in the crust are alkalis, alumina and silica. An experimental study was carried out in the system Na2O–Al2O3–SiO2–H2O, therefore, in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively. 2. EXPERIMENTAL METHODS Starting compositions were glasses along the join Na2Si4O9– Na2(NaAl)4O9. Three compositions were studied (0, 3, and 6 mol.% Al2O3). These are denoted NS4 (0 mol.% Al2O3), NS4A3 (3 mol.% Al2O3), and NS4A6 (6 mol.% Al2O3). Because Na-charge-balanced Al3⫹ most likely is tetrahedrally coordinated in these melts (e.g., Mysen, 1999), anhydrous melts retain their NBO/T-value near 0.5 as Al/(Al ⫹ Si) is increased. This value of NBO/T is typical for average andesitic melts (Mysen, 1990), a magmatic liquid common in subduction zone environments. Anhydrous glasses were made from mixtures of spectroscopically pure Na2CO3, Al2O3, and SiO2 ground under alcohol for about 1 h, decarbonated during slow heating (⬃1.5°C/min), melted at 1250°C at 0.1 MPa for 60 minutes, and then quenched to glass. The glasses were crushed to ⬃20-m grain size and stored at 110°C when not in use. High-pressure and high-temperature experiments were conducted in the solid-media, high-pressure apparatus (Boyd and England, 1960). The samples were sealed in 3-mm outer diameter Pt containers and subjected to experimental pressure and temperature conditions in 0.75°-diameter furnace assemblies (Kushiro, 1976). Temperatures were measured with Pt-Pt90Rh10 thermocouples with no correction for pressure on their emf. Pressure was calibrated against the melting point of NaCl and the calcite-aragonite transformation. The uncertainties were ⫾10°C and ⫾0.1 GPa, respectively. The starting glasses (0.7– 4.0 mg of glass crushed to about 20-m grain size) were loaded together with double-distilled, deionized H2O (10 –35 l depending on desired H2O content) into the 3 mm outer diameter by 7- to 8-mm-long Pt containers and welded shut while submerged in liquid N2. Water was loaded with a microsyringe. The exact amount of H2O added was determined from the weight of H2O. The weighing accuracy for both glass powder and H2O is ⫾0.02 mg. With the ⬃20-m grain size of the glass starting material, the maximum transport distance required for H2O equilibration is less than 20 m. The duration of the experiments ranged from 300 min at the highest temperature (1300°C) to 3 days at 1000°C. Even for a 300-min experimental duration, from the relationship, x ⫽ (4Dt)1/2 ( x ⫽ diffusion distance, D ⫽ diffusion constant, and t ⫽ time), 20-m diffusion distance requires a diffusion constant of ⬃5 ⫻ 10⫺14 cm2/s. This value is many orders of magnitude smaller than the diffusion constant for H2O in silicate melts in the 1000 to 1300°C temperature interval of the present experiments (see Watson, 1994, for review of experimental data). Thus, the experimental run durations were more than sufficient to reach equilibrium. The composition of silicate-saturated aqueous fluids could not be quenched with the ⬃100°C/s quenching rate in the present experimental configuration. Quenched, silicate-saturated aqueous fluid appear as trains of silicate-containing bubbles (Fig. 1) ranging up to ⬃10-m diameter (depending on composition and temperature). Raman spectra
Fig. 1. Raman spectra of quenched fluid material and quenched hydrous glass of NS4A3 composition quenched from 1200°C at 1.3 GPa. Also shown is the Raman spectrum of the anhydrous NS4A3 glass starting material. Microphotograph shows bubbles of quenched silicate-saturated fluid.
of this quench material, obtained with a confocal Dilor XY microRaman spectrometer, testifies to the silica-rich nature of the quenched material in the bubbles (Fig. 1). The composition of the residual aqueous fluid after precipitation of hydrous amorphous silicate during quenching does not, therefore, represent the composition of the fluid during an experiment. Instrumental analysis of quenched fluids cannot, therefore, be used to determine the silicate solubility in these fluids (“silicate solubility” refers to both Al-free and Al-bearing compositions). Instead, phase equilibrium measurements that rely on optical examination (using a petrographic microscope) of the run products was employed to determine whether a
Fig. 2. Microphotographs of experimental charges immediately below (97 wt.% H2O) and immediately above (98 wt.% H2O) the corundum ⫹ fluid and fluid-only phase boundary for composition NS4A6 at 1200°C and 0.8 GPa.
Silicate-saturated fluids
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Fig. 3. Experimental data used to delineate the silicate solubility in fluid in the systems NS4-H2O (a), NS4A3 (b), and NS4A6 (c).
specific composition-contained fluid only (fluid bubble quench, Figs. 1 and 2), melt ⫹ fluid, melt ⫹ crystals ⫹ fluid, or crystals ⫹ fluid. The boundary between the field of fluid only and fluid ⫹ melt defines the
silicate solubility when no crystalline phases were detected (NS4 and NS4A3 composition; solid lines in Fig. 3a,b). The quench bubbles from aqueous fluid (which contain quenched silicate material) have signifi-
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Fig. 4. Silicate solubility in aqueous fluids as a function of pressure and temperature for compositions NS4, NS4A3, and NS4A6. Open symbols represent solubilities of KS4, KS4A3, and KS4A6 compositions from the system K2O–Al2O3– SiO2–H2O (data from Mysen and Acton, 1999).
cantly lower index of refraction (n ⫽ 1.45–1.5) than silicate glass (quenched melt) (refractive index, n ⬃ 1.55). This difference in index of refraction makes distinction between quenched melt and quenched aqueous fluid straightforward during examination of the experimental charges in a petrographic microscope. In the most Al-rich composition, NS4A6, there exist phase fields of melt ⫹ fluid, melt ⫹ corundum ⫹ fluid, corundum ⫹ fluid, and fluid only depending on silicate/H2O ratio (Fig. 3c). When corundum is present (see Fig. 2 for example), it is readily detected typically as large (up to ⬎10 m), euhedral crystals with high birefringence. The phase boundary between corundum ⫹ fluid and fluid only represents the silicate solubility under these conditions (examples of experimental charges used to bracket this boundary at 1200°C and 0.8 GPa are shown in Figure 2. See Figure 3c for the pressure and temperature range where the boundary between corundum ⫹ fluid and fluid only, shown with a thick solid line in Figure 3c, defines the NS4A6 solubility in aqueous fluid). With this technique, the experimental brackets that separate the melt ⫹ fluid and fluid only fields or between fluid ⫹ corundum and
fluid only are ⬍1 wt.% H2O wide. The exact uncertainty for each experimental condition is given together with the experimental results. The solubility behavior of the alkali silicate and alkali aluminosilicate components in aqueous fluids in the pressure and temperature range of interest most likely is incongruent. Such behavior was observed in the analog system K2O–Al2O3–SiO2–H2O in the same pressure and temperature range as those of the present experiments where both the K/Al and K/Si rations in aqueous fluids differed from those in coexisting H2O-saturated silicate melt (Mysen and Acton, 1999). Incongruent solution behavior has also been observed in the system NaAlSi3O8–H2O (Stalder et al., 1998). In the present experiments, the appearance of corundum coexisting with aqueous fluid in H2O-rich Al-bearing compositions is also consistent with incongruent solubility behavior in the aqueous fluid. Therefore, the systems are not strictly binary as the melt and fluid compositions change across the individual solvii that separate phase fields of melt only and aqueous fluid only. At the univariant fluid N melt ⫹ fluid and/or fluid N fluid ⫹ corundum boundary, the silicate or aluminosilicate components in the fluid are in stoichiometric proportions. Hence, this boundary defines the solubility
Silicate-saturated fluids
Fig. 5. Silicate solubility in aqueous fluid as a function of Al2O3 content of the system. The rate of change of silicate solubility in fluid with bulk Al2O3 of the system, (⭸X silicate/⭸X Al2O3) is shown for 1000°C. Open symbols represent solubilities of KS4, KS4A3, and KS4A6 compositions from the system K2O–Al2O3–SiO2–H2O (data from Mysen and Acton, 1999).
of the individual silicate or aluminosilicate compositions in aqueous fluid whether or not, in aqueous fluid coexisting with melt or corundum where total silicate content exceeds the silicate solubility in the fluid, the proportions of silicate components are not stoichiometric. 3. RESULTS
The silicate solubility in aqueous fluid as a function of temperature, pressure, and Al2O3 content is shown in Figures 4 and 5, where the mol.% solubility was calculated on the basis of one oxygen. The silicate solubility was obtained by fitting a third-order polynomial to the brackets at each temperature at each pressure for each composition from the experimental data in Figure 3A,B,C. In the latter figure, the heavy solid lines represent those fits. The thin lines in Figure 3C represent phase boundaries between two- and three-phase fields. Regression coefficients for the fits to the data summarized in
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Figures 4 and 5 are given in Table 1. Also shown in Figures 4 and 5 (as open symbols) are selected silicate solubility data for the equivalent compositions in the system K2O–Al2O3–SiO2– H2O (from Mysen and Acton, 1999). Data on silicate solubility for other compositions in the pressure- and temperature range of the present study are rare. By extrapolating the algorithm for silica solubility in H2O reported by Manning (1994) to 0.8 GPa and 1000°C (note that this pressure and temperature is outside the pressure and temperature range of the data used to derive that algorithm), the silica solubility is about 8 mol.% at that pressure and temperature. This solubility about 2.8 times that of Na2Si4O9 (NS4) at the same pressure and temperature, about 6 times greater than that of NS4A3 (3 mol.% Al2O3) and about 12 times that of NS4A6 (6 mol.% Al2O3). Although extrapolation of the Manning (1994) algorithm to pressure-temperature conditions outside those for which the algorithm was calibrated should be conducted with caution, it would appear that the silica content of the system is an important factor in governing the solubility in aqueous fluids. For all compositions, the silicate solubility is a positive and non-linear function of pressure (Fig. 4). This positive pressure correlation qualitatively is in accord with observations for other systems such as SiO2 (see Manning, 1994, and references therein), NaAlSi3O8 (Davis, 1972; Stalder et al., 1998), KAlSi3O8 (Morey and Hesselgesser, 1951), and MgO–SiO2– H2O (Fujii et al., 1996). For all compositions the pressuredependence becomes more pronounced as the pressure increases (Fig. 4). The pressure-derivative of the solubility, (⭸X silicate/⭸P) T , decreases as the Al2O3 content of the system increases (Table 1). For the NS4 composition (Na2Si4O9), the pressure derivative might increase slightly with increasing temperature, whereas for the Al-bearing compositions, there is no discernible effect of temperature on (⭸X silicate/⭸P) T (Table 1). Compared with the results for the equivalent compositions in the system K2O–Al2O3–SiO2–H2O (more limited data set, from Mysen and Acton, 1999), the silicate solubility in the Na2O– Al2O3–SiO2–H2O system is generally slightly smaller for the most Al-rich composition (KS4A6 vs. NS4A6), whereas for 0 and 3 mol.% Al2O3, the silicate solubility in the aqueous fluid in the Na2O–Al2O3–SiO2–H2O system is slightly higher than in the K2O–Al2O3–SiO2–H2O system at the same pressure and temperature. In general, (⭸X silicate/⭸P) T is greater for the Na2O–Al2O3–SiO2–H2O system than for the K2O–Al2O3– SiO2–H2O system (e.g., see Fig. 4). The silicate solubility in aqueous fluid increases linearly with increasing temperature for all compositions (Fig. 4; see also Table 1). This positive correlation with temperature is consistent with observations from other systems [Pascal and Anderson, 1989; Manning, 1994 (and references therein), Morey and Hesselgesser, 1951; Davis, 1972; Shen and Keppler, 1997]. The temperature derivative of the silicate solubility, (⭸X silicate/⭸T) P , tends to increase with increasing pressure (Table 1; see also Fig. 4). The (⭸X silicate/⭸T) P for equivalent compositions in the system K2O–Al2O3–SiO2–H2O in the same pressure and temperature range is also positive (Mysen and Acton, 1999). The (⭸X silicate/⭸T) P in the K2O–Al2O3–SiO2– H2O system is, however, smaller than for equivalent compositions in the Na2O–Al2O3–SiO2–H2O system.
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Table 1. Temperature-, (⭸X silicate/⭸T) P , and pressure-, (⭸X silicate/⭸P) T , and composition, (⭸X silicate/⭸X Al2O3)P,T dependence of silicate solubility in aqueous fluids. (⭸X silicate/⭸T) P ⫻ 10 3 (mol.%/°C) Pressure (GPa) 0.8 1.05 1.3 1.65 2.0 Temp. (°C) 1000 1100 1200 1300
NS4 9.0 (0.6) 9.6 (0.2) 10.1 (0.4) 8.0 (0.6) 14.6 (0.7) ⫺16.6 ⫺19.6 ⫺20.7 ⫺22.2
(0.3) (3.4) (4.7) (5.8)
⫹ ⫹ ⫹ ⫹
NS4 19.2 22.2 23.4 24.4
(0.1) (1.2) (1.7) (2.0)
⫻ ⫻ ⫻ ⫻
NS4A3 8.9 (0.7) 9.4 (1.4) 7.9 (0.3) 10.1 (0.7) 7.1 (0.5) (⭸X silicate/⭸P) T (mol.%/GPa) NS4A3 P ⫺17.1 (5.7) ⫹ 17.8 (2.0) P ⫺15.9 (4.4) ⫹ 16.8 (1.5) P ⫺15.7 (3.7) ⫹ 16.6 (1.3) P ⫺14.8 (4.1) ⫹ 15.8 (1.4)
NS4A6 3.7 (0.3) 3.3 (0.2) 5.2 (0.6) 8.6 (0.9) 10.3 (0.7) ⫻ ⫻ ⫻ ⫻
P P P P
⫺2.9 ⫺3.6 ⫺3.7 ⫺3.2
(2.0) (2.1) (1.0) (0.3)
NS4A6 ⫹ 3.8 (0.7) ⫹ 4.6 (0.8) ⫹ 5.2 (0.4) ⫹ 5.4 (0.1)
⫻ ⫻ ⫻ ⫻
P P P P
(⭸X silicate/⭸X Al2O3) (mol.%/mol.% Al2O3) Temp. (°C) 1000 1100 1200 1300
0.8 GPa ⫺0.72 ⫹ 0.58X Al2O3 ⫺0.69 ⫹ 0.04X Al2O ⫺0.58 ⫹ 0.009XAl2O ⫺0.49 ⫺ 0.03X Al2O
1.05 GPa ⫺0.63 ⫹ 0.04X Al2O3 ⫺0.73 ⫹ 0.04X Al2O ⫺0.71 ⫹ 0.025X Al2O ⫺0.34 ⫺ 0.055X Al2O
1.3 GPa ⫺0.97 ⫹ 0.6X Al2O3 ⫺1.05 ⫹ 0.06X Al2O ⫺1.07 ⫹ 0.05X Al2O ⫺1.19 ⫹ 0.06X Al2O
Increasing Al2O3 results in decreasing silicate solubility for all compositions and at all pressures and temperatures investigated (Fig. 5). The rate of decrease, (⭸X silicate/⭸X Al2O3) P,T , varies slightly with alumina content (Table 1), and increases significantly with increasing pressure (Fig. 5). Similar behavior was noted by Mysen and Acton (1999) for silicate solubility in aqueous fluids in the K2O–Al2O3–SiO2–H2O system. From stepwise regression of the data, the solubility of silicate components in aqueous fluids in the Na2O–Al2O3–SiO2–H2O system in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively, can be expressed with the equation: X silicate ⫽ 1.9共3.5兲 ⫺ 1.3共0.9兲 ⫻ X Al2O3 ⫹ 0.008共0.002兲 ⫻ T ⫹ 13共4兲 ⫻ P ⫹ 7.3共1.5兲 ⫻ P 2.
(1)
In Eqn. 1, X Al2O3 and X silicate are in mol.%, T is °C, and P is GPa. R 2 ⫽ 0.89. 4. DISCUSSION
4.1. Volume Relations The solubility data for silicate in aqueous fluid may be used to derive partial molar volumes of H2O in the fluid. The free energy of solution of silicate at fixed temperature as a function of pressure is:
冕
a Hfluid 2O ⌬G T 共P兲 ⫽ 0 ⫽ ⌬G T 共1 bar兲 ⫹ RT ln ⬚ ⫹ f H 2O
P
V Hfluid dP, 2O
1
(2) fluid where a H is activity of H2O in the fluid, f ⬚H2O is the fugacity 2O of pure H2O (data from Haar et al., 1984, were used in the fluid present calculations), and V H is the partial molar volume of 2O H2O in the fluid. The slope of the (P ⫺ 1)/RT vs. ln( f ⬚H2O/ fluid XH ) obtained from isothermal solubility data at several pres2O fluid sures equals V H at given temperature T. By using Eqn. 2 for 2O
1.65 GPa ⫺1.77 ⫹ 0.09X Al2O3 ⫺1.59 ⫹ 0.06X Al2O ⫺1.51 ⫹ 0.05X Al2O ⫺1.36 ⫹ 0.03X Al2O
2.0 GPa ⫺1.25 ⫺ 0.13XAl2O3 ⫺1.94 ⫺ 0.04X Al2O 02.21 ⫺ 0.004X Al2O ⫺2.60 ⫹ 0.05X Al2O
fluid this purpose, it is assumed that a H (activity of H2O) can be 2O fluid substituted with X H2O (mol fraction of H2O). As these are comparatively dilute aqueous solutions, this assumption appears reasonable. fluid The relationships between (P ⫺ 1)/RT and ln( f ⬚H2O/X H ) 2O display slight but distinct curvature in the 0.8 to 2.0 GPa pressure range for all compositions and at all four temperatures (Fig. 6; see Table 2 for regression coefficients). Therefore, fluid fluid provided that the assumption of a H ⫽ XH is appropriate, 2O 2O the partial molar volume of H2O in these silicate-saturated aqueous fluids depends somewhat on pressure. Within the error fluid of the data, this pressure-dependence of V H is linear and 2O negative (Table 3; see also Fig. 7). This behavior contrasts with the distinctly nonlinear behavior of the molar volume of pure H2O as a function of pressure in the same pressure- and temperature-regime (data from Haar et al., 1984; shown as dashed lines in Fig. 7). For the Al-free NS4 composition, the partial molar volume of H2O ranges between 19 and 24.5 cm3/mol in the pressure and temperature range under consideration. In the lower-pressure range (P ⬍ 1.3 GPa), this volume is less than the molar volume of pure H2O. The inverse relationship holds at the highest pressure (Fig. 7). The compressibility of H2O in NS4saturated aqueous fluid is less, therefore, than that of pure H2O. fluid The pressure-dependence of V H in NS4-saturated fluids is 2O essentially independent of temperature with  [⫽ ⫺1/ fluid V 0 (⭸V H /⭸P)] ⫽ 0.125 GPa⫺1, which corresponds to a bulk 2O modulus of 8 GPa. The bulk modulus of pure H2O, on the other hand increases with increasing pressure from about 3.5 at 0.8 GPa to about 10 at 2.0 GPa. fluid The relationship between V H and pressure for the Al2O bearing compositions is also linear and negative, but for these fluid compositions the pressure-dependence of V H is also a func2O fluid tion of temperature so that (⭸V H2O/⭸P) T increases with increasing temperature (Table 3). For the NS4A3 composition, this trend translates to an increase in  from 0.141 GPa⫺1 at 1000°C to 0.172 GPa⫺1 at 1300°C (Table 3). For NS4A6-
Silicate-saturated fluids
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fluid Table 2. Coefficients for polynomial fit, ln( f ⬚H2O/X H ) ⫽ a ⫹ 2O b[(P ⫺ 1)/RT] ⫹ c[(P ⫺ 1)/RT] 2 .
NS4 in fluid Temperature (°C) 1000 1100 1200 1300 Temperature (°C) 1000 1100 1200 1300 Temperature (°C) 1000 1100 1200 1300
fluid Fig. 6. Relationship between ln( f ⬚H2O/X H ) and (P ⫺ 1)/RT, 2O fluid where f ⬚H2O is the fugacity of H2O (from Haar et al., 1984), X H is the 2O mol fraction of H2O in silicate-saturated aqueous fluid, P is pressure (bar), R is the gas constant, and T is temperature (K).
saturated aqueous fluid  increases from 0.193 GPa⫺1 to 0.213 GPa⫺1 in the same temperature range. fluid The difference between V H and the molar volume of pure 2O H2O in the Al-bearing systems is smaller than for Al-free NS4 and decreases with increasing Al-content. This behavior probably is a direct result of the fact that the silicate solubility decreases with increasing Al-content. One would expect the partial molar volume of H2O to approach the molar volume of pure H2O as the silicate content of the solution decreases. fluid At pressures near 0.8 GPa, the V H in all three composi2O
a b 7.59 ⫾ 0.05 24.7 ⫾ 0.7 7.71 ⫾ 0.07 25.6 ⫾ 1.2 7.81 ⫾ 0.09 26.6 ⫾ 1.6 7.89 ⫾ 0.09 27.6 ⫾ 1.8 NS4A3 in fluid a b 7.58 ⫾ 0.08 24.8 ⫾ 1.2 7.67 ⫾ 0.07 26.3 ⫾ 1.2 7.76 ⫾ 0.08 27.6 ⫾ 1.4 7.82 ⫾ 0.08 29.9 ⫾ 1.5 NS4A6 in fluid a b 7.49 ⫾ 0.05 26.5 ⫾ 0.9 7.59 ⫾ 0.05 27.9 ⫾ 0.9 7.67 ⫾ 0.06 29.3 ⫾ 1.0 7.73 ⫾ 0.05 30.8 ⫾ 0.9
c ⫺16.5 ⫾ 2.7 ⫺18.0 ⫾ 4.7 ⫺20.2 ⫾ 6.9 ⫺22.3 ⫾ 8.4 c ⫺18.3 ⫾ 4.8 ⫺22.5 ⫾ 5.0 ⫺27.0 ⫾ 6.0 ⫺32.2 ⫾ 7.0 c ⫺26.8 ⫾ 3.2 ⫺31.6 ⫾ 3.7 ⫺36.6 ⫾ 4.4 ⫺42.2 ⫾ 4.2
tions (NS4, NS4A3, and NS4A6) are essentially the same fluid (22–25 cm3/mol, depending on temperature), but because V H 2O in the Al-bearing compositions (NS4A3 and NS4A6) decreases fluid more rapidly with increasing pressure than V H for the Al-free 2O composition, NS4, at the highest pressures under study the fluid V H in NS4A3- and NS4A6-saturated aqueous fluids is 2O fluid smaller than V H in NS4-saturated fluid. In short, the partial 2O molar volume of H2O in silicate-saturated fluids in the system decreases with increasing Al2O3 content even though the silicate solubility actually decreases. Notably, the partial molar volume of H2O in Na2O–Al2O3–SiO2–H2O melts also decreases with increasing Al2O3 content (Mysen and Wheeler, 2000) as does the partial molar volume of H2O in K2O–Al2O3– SiO2–H2O melts (Mysen and Acton, 1999). fluid The V H of silicate-saturated aqueous fluids increase lin2O early with increasing temperature (Fig. 8; see also Table 4). Qualitatively similar behavior was inferred from the silicate solubility data in the system K2O–Al2O3–SiO2–H2O (Mysen and Acton, 1999). The thermal expansivity of pure H2O is also constant, at constant pressure, for pure H2O (dashed lines in Fig. 8). Whereas for Al-free composition (NS4), there is no discernible pressure effect on thermal expansion, for both composition NS4A3 and NS4A6 the thermal expansivity decreases with increasing pressure (Table 4), an effect qualitatively similar to that of pure H2O. This closer similarity of volume behavior in NS4A3, NS4A6, and pure H2O may result from the decreasing silicate solubility with increasing Al content. In other words, as the Al2O3 concentration of the system increases, the aqueous fluid becomes more dilute and its behavior is likely more closely to resemble that of pure H2O. 4.2. Density Relations In the pressure-temperature regime under investigation, aqueous fluids can dissolve as much as 32 wt.% (about 20 mol.%) silicate components (NS4 composition at 1300°C and 2.0 GPa; see Fig. 3A). Solution of this silicate may affect the density of the aqueous fluid. Because the solubility depends significantly on pressure, temperature, and composition, the density of silicate-saturated aqueous fluid may then depend on
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B. O. Mysen and K. Wheeler Table 3. Regression coefficients for pressure-dependent partial molar volume of H2O in silicate-saturated aqueous fluid. fluid V H ⫽ a ⫹ bP(GPa) 2O
NS4
a b r2  G
1000°C 24.73 ⫾ 0.01 ⫺3.165 ⫾ 0.008 0.999 0.127 ⫾ 0.005 7.9 ⫾ 0.3
1100°C 25.52 ⫾ 0.01 ⫺3.165 ⫾ 0.008 0.999 0.124 ⫾ 0.005 8.1 ⫾ 0.3
1200°C 26.60 ⫾ 0.05 ⫺3.34 ⫾ 0.03 0.999 0.126 ⫾ 0.0010 7.9 ⫾ 0.1
1300°C 27.57 ⫾ 0.04 ⫺3.43 ⫾ 0.03 0.999 0.124 ⫾ 0.0011 8.1 ⫾ 0.1
1200°C 27.55 ⫾ 0.04 ⫺4.43 ⫾ 0.03 0.999 0.161 ⫾ 0.001 6.21 ⫾ 0.04
1300°C 29.01 ⫾ 0.05 ⫺5.00 ⫾ 0.03 0.999 0.172 ⫾ 0.001 5.81 ⫾ 0.03
1200°C 29.30 ⫾ 0.06 ⫺6.06 ⫾ 0.04 0.999 0.206 ⫾ 0.001 4.85 ⫾ 0.02
1300°C 30.83 ⫾ 0.04 ⫺6.57 ⫾ 0.03 0.999 0.213 ⫾ 0.001 4.69 ⫾ 0.02
NS4A3
a b r2  G
1000°C 24.80 ⫾ 0.05 ⫺3.50 ⫾ 0.04 0.999 0.141 ⫾ 0.002 7.09 ⫾ 0.10
1100°C 26.30 ⫾ 0.06 ⫺4.06 ⫾ 0.04 0.999 0.154 ⫾ 0.002 6.5 ⫾ 0.1 NS4A6
a b r2  G
1000°C 26.44 ⫾ 0.06 ⫺5.10 ⫾ 0.04 0.999 0.193 ⫾ 0.002 5.18 ⫾ 0.05
1100°C 27.85 ⫾ 0.04 ⫺5.57 ⫾ 0.03 0.999 0.200 ⫾ 0.001 5.00 ⫾ 0.03
fluid  ⫽ ⫺1/V 0 (⭸V H /⭸T) P , G ⫽ 1/  . 2O
fluid Fig. 7. Partial molar volume of H2O in silicate-saturated aqueous fluid, V H , as a function of pressure. Molar volume 2O of pure H2O, V ⬚H2O is shown as dashed lines (data from Haar et al., 1984).
Silicate-saturated fluids
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fluid Fig. 8. Partial molar volume of H2O in silicate-saturated aqueous fluid, V H , as a function of temperature. Molar volume 2O fluid of pure H2O, V ⬚H2O is shown as dashed lines (data from Haar et al., 1984). Open symbols: V H for silicate-saturated 2O aqueous fluid in the system K2O–Al2O3–SiO2–H2O for compositions indicated (data from Mysen and Acton, 1999).
pressure, temperature, and composition in ways that could differ from that of pure H2O. This is in part because the molecular weight of silicate-saturated fluids differs from that of pure H2O and in part because the partial molar volume of H2O in silicate-saturated aqueous fluids differs from that of pure H2O (see Figs. 7 and 8). In addition, the partial molar volume of the silicate components very likely differs from that of H2O itself so that the volumes may also contribute to changes of the molar volume and, therefore, density of silicate-saturated aqueous fluid. The partial molar volume of oxide components in silicatesaturated aqueous fluids and their pressure and temperature derivatives are not known. From in situ, high-temperature/highpressure Raman spectroscopy of coexisting H2O-saturated silicate melt and silicate-saturated aqueous fluid in the system K2O–SiO2–H2O, Mysen (1998) concluded that the types of silicate complexes in silicate-saturated aqueous fluids and water-saturated silicate melts resemble one another. One may suggest, therefore, that the partial molar volumes of the oxide components in the silicate-saturated fluids may not differ greatly from those in silicate melts. If so, by using the oxide volume data of Lange and Carmichael (1987) and Kress and Carmichael (1991) the molar volumes of silicate-saturated fluids shown in Figure 9 result. In that figure, the partial molar volume of H2O in the fluids is shown as dashed lines. For the Al-bearing compositions (NS4A3 and NS4A6), the solubility of silicate components is insufficient to cause significant changes in the molar volume (within uncertainty in the data) compared with the partial molar volume of H2O in these
fluid fluids. In other words, V H and V fluid are indistinguishable 2O within experimental uncertainty. For the NS4 composition, which has the highest silicate solubility, a difference between the molar volume of the silicate-saturated aqueous fluid and the partial molar volume of H2O is discernible although the difference is less than 0.5 cm3/mol even at the highest silicate content (at 2.0 GPa and 1300°C). As would be expected, the smaller the silicate content, the smaller is the difference. In general, the differences are so small, however (⬍2%), that at least in the pressure-temperature regime of the present experiments, the molar volume of the silicate-saturated fluid, V fluid, could be approximated with the partial molar volume of H2O in that fluid fluid, V H , without introducing significant errors. 2O The density of the fluids, therefore, is:
fluid ⫽
M fluid M fluid ⬃ fluid . V fluid V H 2O
(3)
The resulting pressure- and temperature-dependence of the fluid density, fluid, governed by the composition-, pressurefluid and temperature-dependence of V H and the molecular weight 2O of the fluid, M fluid, is shown in Figure 10. The density of silicate-saturated aqueous fluids is a distinct non-linear function of pressure (Fig. 10). This behavior results from the strong positive and nonlinear solubility of silicate in fluid the fluids with pressure (Fig. 4), whereas V H is a linear 2O function of pressure (Fig. 7). Further, whereas the pressuredependence of the density of pure H2O decreases with increasing pressure (e.g., Haar et al., 1984; Brodholt and Wood, 1993),
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B. O. Mysen and K. Wheeler Table 4. Regression coefficients for temperature-dependent partial molar volume of H2O in silicate-saturated aqueous fluid. fluid V H ⫽ a ⫹ bT(°C) 2O
NS4 0.8 GPa
1.05 GPa
1.3 GPa
1.65 GPa
2.0 GPa
a b r2 ␣ ⫻ 10 4
13.5 ⫾ 0.2 0.0087 ⫾ 0.0002 0.999 6.4 ⫾ 0.2
12.7 ⫾ 0.2 0.0087 ⫾ 0.0002 0.999 6.9 ⫾ 0.2
12.2 ⫾ 0.2 0.0084 ⫾ 0.0001 0.999 6.9 ⫾ 0.1
11.5 ⫾ 0.2 0.0080 ⫾ 0.0002 0.998 7.0 ⫾ 0.2
10.8 ⫾ 0.2 0.9976 ⫾ 0.0001 0.999 7.0 ⫾ 0.2
0.8 GPa a b r2 ␣ ⫻ 10 4
1.05 GPa 12.1 ⫾ 0.3 0.0099 ⫾ 0.0003 0.999 8.2 ⫾ 0.3
1.3 GPa 12.1 ⫾ 0.2 0.0090 ⫾ 0.0004 0.998 7.4 ⫾ 0.4
2.0 GPa 13.0 ⫾ 0.2 0.0060 ⫾ 0.0002 0.998 4.6 ⫾ 0.2
13.7 ⫾ 0.2 0.0041 ⫾ 0.0003 0.999 3.0 ⫾ 0.2
2.0 GPa 11.4 ⫾ 0.2 0.0066 ⫾ 0.0001 0.999 5.8 ⫾ 0.1
11.6 ⫾ 0.2 0.0047 ⫾ 0.0002 0.999 4.1 ⫾ 0.2
NS4A3 1.65 GPa 12.9 ⫾ 0.2 0.0074 ⫾ 0.0001 0.999 5.7 ⫾ 0.1 NS4A6 0.8 GPa a b r2 ␣ ⫻ 10 4
1.05 GPa 11.7 ⫾ 0.2 0.0107 ⫾ 0.0002 0.999 9.2 ⫾ 0.2
1.3 GPa 11.8 ⫾ 0.2 0.0093 ⫾ 0.0002 0.999 7.9 ⫾ 0.2
1.65 GPa 11.5 ⫾ 0.2 0.0083 ⫾ 0.0002 0.999 7.2 ⫾ 0.2
fluid ␣ ⫽ 1/V 0 (⭸V H /⭸T) P . 2O
the pressure-dependence of the density of silicate-saturated aqueous fluids increases with increasing pressure. This latter difference is a result of the increasing silicate solubility with increasing pressure. Because of the different (⭸V ⬚H2O/⭸P) T and fluid (⭸V H /⭸P) T (Fig. 7), the density difference between that of 2O silicate-saturated aqueous fluids and that of pure H2O passes through a minimum near 1.5 GPa (Fig. 10). This minimum becomes less pronounced as the system becomes more aluminous because the silicate solubility decreases with increasing Al2O3 content (Fig. 10; see also Fig. 5). The density of silicate-saturated aqueous fluids shows negative and approximately linear correlation with temperature as is also the case for pure H2O (Haar et al., 1984; see also Table 5), but the (⭸ °H2O/⭸T) P is always more negative than (⭸fluid/
⭸T) P of silicate-saturated aqueous fluid. The expansivity of the silicate-saturated aqueous fluids approaches, however, that of pure H2O as the system becomes more Al-rich. This relationship is a direct consequence of the decreased silicate solubility with increasing Al2O3 content (Fig. 5). Several isochores of silicate-saturated aqueous fluids in the Na2O–Al2O3–SiO2–H2O system are compared with those of pure H2O in the same pressure- and temperature-range in Figure 11. It is evident from those results that the isochores generally have a more gentle ⭸P/⭸T slope than do those of pure H2O (dashed lines in Fig. 11). This difference is principally a function of the fact that the density of the silicate-saturated aqueous fluids are less sensitive to temperature than that of pure H2O.
Table 5. Regression coefficients for temperature-dependent density of silicate-saturated aqueous fluid.
H2O ⫽ a ⫹ bT(°C) NS4 0.8 GPa
1.05 GPa
a b ⫻ 10 5 r2
1.04 ⫾ 0.01 ⫺19.5 ⫾ 0.9 0.999
1.085 ⫾ 0.007 ⫺20.5 ⫾ 0.6 0.998
1.14 ⫾ 0.01 ⫺21.0 ⫾ 1.1 0.993 NS4A3
1.30 ⫾ 0.02 ⫺25.8 ⫾ 1.9 0.989
1.378 ⫾ 0.002 ⫺17.7 ⫾ 0.2 0.999
a b r2
0.8 GPa 1.06 ⫾ 0.03 ⫺23.6 ⫾ 2.4 0.980
1.05 GPa 1.10 ⫾ 0.02 ⫺22.5 ⫾ 2.1 0.983
1.3 GPa 1.13 ⫾ 0.01 ⫺20.9 ⫾ 0.9 0.996 NS4A6
1.65 GPa 1.16 ⫾ 0.02 ⫺15.8 ⫾ 1.3 0.987
2.0 GPa 1.29 ⫾ 0.03 ⫺13.2 ⫾ 2.2 0.47
a b r2 ⭸ ⬚H2O/⭸T ⫻ 10 5
0.8 GPa 1.12 ⫾ 0.02 ⫺30.7 ⫾ 1.5 0.995 ⫺32.7
1.05 GPa 1.169 ⫾ 0.006 ⫺30.3 ⫾ 0.5 0.999 ⫺31.0
1.3 GPa 1.211 ⫾ 0.007 ⫺28.6 ⫾ 0.7 0.999 ⫺29.0
1.65 GPa 1.25 ⫾ 0.02 ⫺23.1 ⫾ 1.8 0.988 ⫺26.7
2.0 GPa 1.32 ⫾ 0.01 ⫺16.1 ⫾ 0.9 0.3 ⫺24.0
Data for pure H2O from Haar et al. (1984).
1.3 GPa
1.65 GPa
2.0 GPa
Silicate-saturated fluids
Fig. 9. Molar volume of silicate-saturated aqueous fluids as a function of pressure for compositions as indicated. Open fluid symbols and dashed lines: Partial molar volume of H2O, V H , for the same compositions. 2O
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Fig. 10. Density of silicate-saturated aqueous fluids in the systems indicated as a function of pressure. Dashed lines show the density of pure H2O (data from Haar et al., 1984). Also shown is the density difference between silicate-saturated aqueous fluid and pure H2O for the three systems as a function of pressure at temperatures indicated.
Silicate-saturated fluids
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Fig. 11. Calculated isochores for silicate-saturated aqueous fluids compared with those for pure H2O (data from Haar et al., 1984). Acknowledgments—This research was conducted with partial support from National Science Foundation Grants EAR-9901886 and REU9619551. Associate editor: D. B. Dingwell REFERENCES Anderson G. M. and Burnham C. W. (1967) Reactions of quartz and corundum with aqueous chloride and hydroxide solutions at high temperatures and pressures. Am. J. Sci. 265, 12–27.
Boyd F. R. and England J. L. (1960) Apparatus for phase equilibrium measurements at pressures up to 50 kilobars and temperatures up to 1750°C. J. Geophys. Res. 65, 741–748. Brenan J. M., Shaw H. F., Ryerson F. J., and Phinney D. L. (1995) Mineral-aqueous fluid partitioning of trace elements at 900° and 2.0 GPa: Constraints on the trace element geochemistry of mantle and deep crustal fluids. Geochim. Cosmochim. Acta 59, 3331– 3350. Davis N. F. (1972) Experimental studies in the system sodium alumina trisilicate-water. Part I. The apparent solubility of albite in super-
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critical water. Geosciences. State College, PA: The Pennsylvania State University. Fowler M. B. (1984) Large-ion lithophile element mobility in the lower continental crust; mineralogy and geochemistry of the hornblendegranulite subfacies at Gruinard Bay and its relationships with amphibolite-facies and granulite-facies end members. In Metamorphic Studies (ed. M. Brown), 141, p. 1076. London, UK: Geological Society of London. Fujii T., Mibe K., and Masuda A. (1996) Composition of fluid coexisting with olivine and pyroxene at high pressure: The role of water on the differentiation of the mantle. Misasa Seminar on Evolutionary Processes of Earth and Planetary Materials. Misasa, Japan: Okayama University Institute for the Study of the Earth’s Interior. Haar L., Gallagher J. S., and Kelly G. S. (1984) Steam Tables. Thermodynamic and transport properties and computer programs for vapor and liquid states of water in SI units. New York: Hemisphere Publishing Corporation. Iizuka Y. and Mysen B. O. (1998) Experimental study on dehydration and silica metasomatism in the subduction zones. Terra Nova 10, 28. Kennedy G. C., Wasserburg G. J., Heard H. C., and Newton R. C. (1962) The upper three-phase region in the system SiO2-H2O. Am. J. Sci. 260, 501–521. Konzett J. and Ulmer P. (1999) The stability of hydrous potassic phases in lherzolitic mantle—an experimental study to 9.5 GPa in simplified and natural bulk compositions. J. Petrol., in press. Kress V. C. and Carmichael I. S. E. (1991) The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states. Contr. Mineral. Petrol. 108, 92–92. Kushiro I. (1976) A new furnace assembly with a small temperature gradient in solid-media, high-pressure apparatus. Carnegie Instn. Washington Year Book 75, 832– 833. Kushiro I. (1990) Partial melting of mantle wedge and evolution of island arc crust. J. Geophys. Res. 95, 15929 –15939. Lange R. A. and Carmichael I. S. E. (1987) Densities of Na2O-K2OCaO-MgO-Fe2O3-Al2O3-TiO2-SiO2 liquids: New-measurements and derived partial molar properties. Geochim. Cosmochim. Acta 51, 2931–2946. Luth R. W. (1997) Experimental study of the system phologopitediopside from 3.5 to 17 GPa. Am. Mineral. 62, 1198 –1209. Manning C. E. (1994) The solubility of quartz in H2O in the lower crust and upper mantle. Geochim. Cosmochim. Acta 58, 4831– 4840.
McInnes B. (1996) Fluid-peridotite interactions in mantle wedge xenoliths. EOS, Trans. Am. Geophys. Union 77, 282. Morey G. W. and Hesselgesser J. M. (1951) The solubility of quartz and some other substances in superheated steam at high pressures. Am. Soc. Mech. Eng. Trans. 73, 865. Mysen B. O. (1990) Relationships between melt structure and petrologic processes. Earth-Sci. Rev. 27, 261–365. Mysen B. O. (1998) Interaction between aqueous fluid and silicate melt in the pressure and temperature regime of the Earth’s crust and upper mantle. N. Nb. Mineral. 172, 227–244. Mysen B. O. (1999) Structure and properties of magmatic liquids: From haplobasalt to haploandesite. Geochim. Cosmochim. Acta 63, 95–112. Mysen B. O. and Acton M. (1999) Water in H2O-saturated magmafluid systems: Solubility behavior in K2O-Al2O3-SiO2-H2O to 2.0 GPa and 1300°C. Geochim. Cosmochim. Acta 63, 3799 –3815. Mysen B. O. and Wheeler K. (2000) Solubility behavior of water in haploandesitic melts at high pressure and high temperature. Am. Mineral., in press. Pascal M. L. and Anderson G. M. (1989) Speciation of Al, Si, and K in supercritical solutions: Experimental study and interpretation. Geochim. Cosmochim. Acta 53, 1843–1856. Riter J. C. A. and Smith D. (1996) Xenolith constraints on the thermal history of the of the mantle beneath the Colorado Plateau. Geology 24, 267–279. Rollinson H. R. and Windley B. F. (1980) Selective elemental depletion during metamorphism of archaean granulites, Scourie, NW Scotland. Contrib. Mineral. Pet. 72, 257–263. Shen A. H. and Keppler H. (1997) Direct observation of complete miscibility in the albite-H2O system. Nature 385, 710 –712. Stalder R., Ulmer P., Thompson A. B., and Gunther D. (1998) Experimental determination of second critical endpoints in fluid/melt systems. Mineral. Mag. 62A, 1441–1442. Whitehouse M. J. (1989) Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian Complex of north-west Scotland; implications for Pb-Pb dating. Geochimica et Cosmochimica Acta 53, 717–724. Watson E. B. (1994) Diffusion in volatile-bearing magmas. In Volatiles in Magmas (eds. M. R. Carroll, and J. R. Holloway), pp. 371– 411. Washington, D.C.: Mineralogical Society of America.
Pergamon
PII S0016-7037(00)00498-1
Alkali aluminosilicate-saturated aqueous fluids in the earth’s upper mantle BJORN O. MYSEN1,* and KEVIN WHEELER2 1
Geophysical Laboratory and Center for High-Pressure Research (CHiPR), Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington, DC 20015, USA 2 Department of Geological Sciences, Brown University, Providence, RI 02912, USA (Received January 6, 2000; accepted in revised from July 6, 2000)
Abstract—The solubility in aqueous fluids of peralkaline alkali aluminosilicate components has been determined in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively. Compositions were along the haploandesite join, Na2Si4O9–Na2(NaAl)4O9, with 0, 3, and 6 mol.% Al2O3, denoted NS4Ax, where x represents the mol.% Al2O3. The silicate solubility in aqueous fluid is in the 3 to 20 mol.% range for NS4, 2 to 13 mol.% for NS4A3, and 1.5– 8 mol.% for NS4A6 with a linear and positive temperature-dependence and a nonlinear and positive pressure-dependence. The silicate solubility decreases with increasing Al2O3 content. From stepwise regression, the pressure-, temperature-, and Al2O3-dependence of the solubility can be described with the expression: X silicate (mol.%) ⫽ 1.9 ⫺ 1.3X Al2O3 (mol.%) ⫹ 0.008T共⬚C) ⫹ 13P共GPa) ⫹ 7.3P 2 . fluid fluid Partial molar volume of H2O in the silicate-saturated fluids, V H , is in the range 17 to 25 cm3/mol. The V H 2O 2O was calculated with the assumption that the activity of dissolved silicate in aqueous fluid equals the mol fluid fraction of dissolved silicate. Compared with the molar volume of pure H2O, V ⬚H2O, the V H values are 10 2O to 15% lower at 0.8 GPa and as much as 15% higher than V ⬚H2O at 2.0 GPa. This volume difference diminishes as the system become more aluminous. fluid fluid For all compositions, the V H is a linear and negative function of pressure with  [⫽ ⫺1/V 0 (⭸V H /⭸P) T ] 2O 2O ⫺1 ⬃ 0.125 GPa for NS4. The NS4A3 increases from 0.141 to 0.172 in the 1000° to 1300°C range, whereas NS4A6 increases from 0.193 to 0.213 GPa⫺1. fluid The V H is a positive and linear function of temperature with thermal expansion coefficients in the range 2O 3– 8 ⫻ 10⫺4 K⫺1. This thermal expansivity resembles that of pure H2O although the exact values and the fluid range in values for pure H2O differ from 1/V 0 (⭸V H /⭸T) P of silicate-saturated aqueous solution. Copyright 2O © 2000 Elsevier Science Ltd
bearing melt, in turn, serves as a transport agent by which subducted H2O may be recycled back into the overlying crust. Evidence for interaction between rocks and aqueous fluids in the crust and the upper mantle of the earth is, therefore, extensive over a range of pressures and temperatures. Characterization of the solubility behavior of silicate components in aqueous fluids as a function of pressure, temperature, and bulk chemical composition is needed to describe these interaction processes. Available experimental data, particularly in the pressure-temperature regime of the upper mantle, however, are limited. It is known, though, that silicate solubility is positively correlated with fluid density (Manning, 1994) and because fluid density correlates with pressure (e.g., Haar et al., 1984) silica solubility in aqueous fluid increases with increasing pressure. In fact, recent preliminary experimental data obtained in the hydrothermal diamond anvil cell, indicate that there may be complete miscibility between aqueous fluids and at least some alkali silicate melts in the pressure-temperature regime of the upper mantle (Shen and Keppler, 1997). Furthermore, it has been suggested the solution behavior of silicate in aqueous fluids might be a distinct function of pressure as suggested, for example, by Fujii et al. (1996) for the system MgO–SiO2–H2O. At pressures ⬍ 3 GPa, the fluid in that study contained only silica, whereas at high pressure MgO became significantly soluble.
1. INTRODUCTION
Aqueous fluids are important agents for materials transport in the earth’s crust and upper mantle. High-grade metamorphism in deep continental crust can involve dehydration of hydrous phases with concomitant alteration of major- and trace-element patterns in high-temperature/-pressure metamorphic rocks (e.g., Rollinson and Windley, 1980; Fowler, 1984; Whitehouse, 1989). In subduction zones near convergent plate boundaries there is evidence for transport of major, minor, and trace elements from the subducting slab to the overlying mantle wedge (e.g., Riter and Smith, 1996; McInnes, 1996; Brenan et al., 1995). Alteration of the bulk chemistry of peridotite in the overlying mantle wedge via ingress of aqueous fluid can result in formation of biotite and K-richterite (Luth, 1997; Konzett and Ulmer, 1999). Orthopyroxene overgrowth on olivine, generated by reaction with silicate-rich fluids, has been observed in natural samples (Riter and Smith, 1996) and in laboratory experiments (Iizuka and Mysen, 1998). Ultimately, water transported into the peridotite wedge above subducting lithosphere may trigger partial melting (e.g., Kushiro, 1990). This H2O-
* Author to whom correspondence should be addressed (mysen@ gl.ciw.edu). 4243
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Temperature is an important factor as well. The silicate solubility increases with increasing temperature as the silicateH2O solvus shrinks (Kennedy et al., 1962; Manning, 1994; Mysen, 1998). Bulk chemical composition also affects silicate solubility at upper mantle pressures and temperatures (e.g., Mysen, 1998; Mysen and Acton, 1999). A composition effect has also been demonstrated in lower-pressure experiments in systems such as SiO2–KOH–H2O and Al2O3–KOH–H2O (Anderson and Burnham, 1967; Pascal and Anderson, 1989) and chloride-bearing SiO2–Al2O3–H2O systems (Anderson and Burnham, 1967). In light of the above considerations, it is evident that systematic experimental studies on the interaction between silicates and aqueous fluids at high pressure and high temperature are needed. Major important chemical components in upper mantle subduction zones and in the crust are alkalis, alumina and silica. An experimental study was carried out in the system Na2O–Al2O3–SiO2–H2O, therefore, in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively. 2. EXPERIMENTAL METHODS Starting compositions were glasses along the join Na2Si4O9– Na2(NaAl)4O9. Three compositions were studied (0, 3, and 6 mol.% Al2O3). These are denoted NS4 (0 mol.% Al2O3), NS4A3 (3 mol.% Al2O3), and NS4A6 (6 mol.% Al2O3). Because Na-charge-balanced Al3⫹ most likely is tetrahedrally coordinated in these melts (e.g., Mysen, 1999), anhydrous melts retain their NBO/T-value near 0.5 as Al/(Al ⫹ Si) is increased. This value of NBO/T is typical for average andesitic melts (Mysen, 1990), a magmatic liquid common in subduction zone environments. Anhydrous glasses were made from mixtures of spectroscopically pure Na2CO3, Al2O3, and SiO2 ground under alcohol for about 1 h, decarbonated during slow heating (⬃1.5°C/min), melted at 1250°C at 0.1 MPa for 60 minutes, and then quenched to glass. The glasses were crushed to ⬃20-m grain size and stored at 110°C when not in use. High-pressure and high-temperature experiments were conducted in the solid-media, high-pressure apparatus (Boyd and England, 1960). The samples were sealed in 3-mm outer diameter Pt containers and subjected to experimental pressure and temperature conditions in 0.75°-diameter furnace assemblies (Kushiro, 1976). Temperatures were measured with Pt-Pt90Rh10 thermocouples with no correction for pressure on their emf. Pressure was calibrated against the melting point of NaCl and the calcite-aragonite transformation. The uncertainties were ⫾10°C and ⫾0.1 GPa, respectively. The starting glasses (0.7– 4.0 mg of glass crushed to about 20-m grain size) were loaded together with double-distilled, deionized H2O (10 –35 l depending on desired H2O content) into the 3 mm outer diameter by 7- to 8-mm-long Pt containers and welded shut while submerged in liquid N2. Water was loaded with a microsyringe. The exact amount of H2O added was determined from the weight of H2O. The weighing accuracy for both glass powder and H2O is ⫾0.02 mg. With the ⬃20-m grain size of the glass starting material, the maximum transport distance required for H2O equilibration is less than 20 m. The duration of the experiments ranged from 300 min at the highest temperature (1300°C) to 3 days at 1000°C. Even for a 300-min experimental duration, from the relationship, x ⫽ (4Dt)1/2 ( x ⫽ diffusion distance, D ⫽ diffusion constant, and t ⫽ time), 20-m diffusion distance requires a diffusion constant of ⬃5 ⫻ 10⫺14 cm2/s. This value is many orders of magnitude smaller than the diffusion constant for H2O in silicate melts in the 1000 to 1300°C temperature interval of the present experiments (see Watson, 1994, for review of experimental data). Thus, the experimental run durations were more than sufficient to reach equilibrium. The composition of silicate-saturated aqueous fluids could not be quenched with the ⬃100°C/s quenching rate in the present experimental configuration. Quenched, silicate-saturated aqueous fluid appear as trains of silicate-containing bubbles (Fig. 1) ranging up to ⬃10-m diameter (depending on composition and temperature). Raman spectra
Fig. 1. Raman spectra of quenched fluid material and quenched hydrous glass of NS4A3 composition quenched from 1200°C at 1.3 GPa. Also shown is the Raman spectrum of the anhydrous NS4A3 glass starting material. Microphotograph shows bubbles of quenched silicate-saturated fluid.
of this quench material, obtained with a confocal Dilor XY microRaman spectrometer, testifies to the silica-rich nature of the quenched material in the bubbles (Fig. 1). The composition of the residual aqueous fluid after precipitation of hydrous amorphous silicate during quenching does not, therefore, represent the composition of the fluid during an experiment. Instrumental analysis of quenched fluids cannot, therefore, be used to determine the silicate solubility in these fluids (“silicate solubility” refers to both Al-free and Al-bearing compositions). Instead, phase equilibrium measurements that rely on optical examination (using a petrographic microscope) of the run products was employed to determine whether a
Fig. 2. Microphotographs of experimental charges immediately below (97 wt.% H2O) and immediately above (98 wt.% H2O) the corundum ⫹ fluid and fluid-only phase boundary for composition NS4A6 at 1200°C and 0.8 GPa.
Silicate-saturated fluids
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Fig. 3. Experimental data used to delineate the silicate solubility in fluid in the systems NS4-H2O (a), NS4A3 (b), and NS4A6 (c).
specific composition-contained fluid only (fluid bubble quench, Figs. 1 and 2), melt ⫹ fluid, melt ⫹ crystals ⫹ fluid, or crystals ⫹ fluid. The boundary between the field of fluid only and fluid ⫹ melt defines the
silicate solubility when no crystalline phases were detected (NS4 and NS4A3 composition; solid lines in Fig. 3a,b). The quench bubbles from aqueous fluid (which contain quenched silicate material) have signifi-
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Fig. 4. Silicate solubility in aqueous fluids as a function of pressure and temperature for compositions NS4, NS4A3, and NS4A6. Open symbols represent solubilities of KS4, KS4A3, and KS4A6 compositions from the system K2O–Al2O3– SiO2–H2O (data from Mysen and Acton, 1999).
cantly lower index of refraction (n ⫽ 1.45–1.5) than silicate glass (quenched melt) (refractive index, n ⬃ 1.55). This difference in index of refraction makes distinction between quenched melt and quenched aqueous fluid straightforward during examination of the experimental charges in a petrographic microscope. In the most Al-rich composition, NS4A6, there exist phase fields of melt ⫹ fluid, melt ⫹ corundum ⫹ fluid, corundum ⫹ fluid, and fluid only depending on silicate/H2O ratio (Fig. 3c). When corundum is present (see Fig. 2 for example), it is readily detected typically as large (up to ⬎10 m), euhedral crystals with high birefringence. The phase boundary between corundum ⫹ fluid and fluid only represents the silicate solubility under these conditions (examples of experimental charges used to bracket this boundary at 1200°C and 0.8 GPa are shown in Figure 2. See Figure 3c for the pressure and temperature range where the boundary between corundum ⫹ fluid and fluid only, shown with a thick solid line in Figure 3c, defines the NS4A6 solubility in aqueous fluid). With this technique, the experimental brackets that separate the melt ⫹ fluid and fluid only fields or between fluid ⫹ corundum and
fluid only are ⬍1 wt.% H2O wide. The exact uncertainty for each experimental condition is given together with the experimental results. The solubility behavior of the alkali silicate and alkali aluminosilicate components in aqueous fluids in the pressure and temperature range of interest most likely is incongruent. Such behavior was observed in the analog system K2O–Al2O3–SiO2–H2O in the same pressure and temperature range as those of the present experiments where both the K/Al and K/Si rations in aqueous fluids differed from those in coexisting H2O-saturated silicate melt (Mysen and Acton, 1999). Incongruent solution behavior has also been observed in the system NaAlSi3O8–H2O (Stalder et al., 1998). In the present experiments, the appearance of corundum coexisting with aqueous fluid in H2O-rich Al-bearing compositions is also consistent with incongruent solubility behavior in the aqueous fluid. Therefore, the systems are not strictly binary as the melt and fluid compositions change across the individual solvii that separate phase fields of melt only and aqueous fluid only. At the univariant fluid N melt ⫹ fluid and/or fluid N fluid ⫹ corundum boundary, the silicate or aluminosilicate components in the fluid are in stoichiometric proportions. Hence, this boundary defines the solubility
Silicate-saturated fluids
Fig. 5. Silicate solubility in aqueous fluid as a function of Al2O3 content of the system. The rate of change of silicate solubility in fluid with bulk Al2O3 of the system, (⭸X silicate/⭸X Al2O3) is shown for 1000°C. Open symbols represent solubilities of KS4, KS4A3, and KS4A6 compositions from the system K2O–Al2O3–SiO2–H2O (data from Mysen and Acton, 1999).
of the individual silicate or aluminosilicate compositions in aqueous fluid whether or not, in aqueous fluid coexisting with melt or corundum where total silicate content exceeds the silicate solubility in the fluid, the proportions of silicate components are not stoichiometric. 3. RESULTS
The silicate solubility in aqueous fluid as a function of temperature, pressure, and Al2O3 content is shown in Figures 4 and 5, where the mol.% solubility was calculated on the basis of one oxygen. The silicate solubility was obtained by fitting a third-order polynomial to the brackets at each temperature at each pressure for each composition from the experimental data in Figure 3A,B,C. In the latter figure, the heavy solid lines represent those fits. The thin lines in Figure 3C represent phase boundaries between two- and three-phase fields. Regression coefficients for the fits to the data summarized in
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Figures 4 and 5 are given in Table 1. Also shown in Figures 4 and 5 (as open symbols) are selected silicate solubility data for the equivalent compositions in the system K2O–Al2O3–SiO2– H2O (from Mysen and Acton, 1999). Data on silicate solubility for other compositions in the pressure- and temperature range of the present study are rare. By extrapolating the algorithm for silica solubility in H2O reported by Manning (1994) to 0.8 GPa and 1000°C (note that this pressure and temperature is outside the pressure and temperature range of the data used to derive that algorithm), the silica solubility is about 8 mol.% at that pressure and temperature. This solubility about 2.8 times that of Na2Si4O9 (NS4) at the same pressure and temperature, about 6 times greater than that of NS4A3 (3 mol.% Al2O3) and about 12 times that of NS4A6 (6 mol.% Al2O3). Although extrapolation of the Manning (1994) algorithm to pressure-temperature conditions outside those for which the algorithm was calibrated should be conducted with caution, it would appear that the silica content of the system is an important factor in governing the solubility in aqueous fluids. For all compositions, the silicate solubility is a positive and non-linear function of pressure (Fig. 4). This positive pressure correlation qualitatively is in accord with observations for other systems such as SiO2 (see Manning, 1994, and references therein), NaAlSi3O8 (Davis, 1972; Stalder et al., 1998), KAlSi3O8 (Morey and Hesselgesser, 1951), and MgO–SiO2– H2O (Fujii et al., 1996). For all compositions the pressuredependence becomes more pronounced as the pressure increases (Fig. 4). The pressure-derivative of the solubility, (⭸X silicate/⭸P) T , decreases as the Al2O3 content of the system increases (Table 1). For the NS4 composition (Na2Si4O9), the pressure derivative might increase slightly with increasing temperature, whereas for the Al-bearing compositions, there is no discernible effect of temperature on (⭸X silicate/⭸P) T (Table 1). Compared with the results for the equivalent compositions in the system K2O–Al2O3–SiO2–H2O (more limited data set, from Mysen and Acton, 1999), the silicate solubility in the Na2O– Al2O3–SiO2–H2O system is generally slightly smaller for the most Al-rich composition (KS4A6 vs. NS4A6), whereas for 0 and 3 mol.% Al2O3, the silicate solubility in the aqueous fluid in the Na2O–Al2O3–SiO2–H2O system is slightly higher than in the K2O–Al2O3–SiO2–H2O system at the same pressure and temperature. In general, (⭸X silicate/⭸P) T is greater for the Na2O–Al2O3–SiO2–H2O system than for the K2O–Al2O3– SiO2–H2O system (e.g., see Fig. 4). The silicate solubility in aqueous fluid increases linearly with increasing temperature for all compositions (Fig. 4; see also Table 1). This positive correlation with temperature is consistent with observations from other systems [Pascal and Anderson, 1989; Manning, 1994 (and references therein), Morey and Hesselgesser, 1951; Davis, 1972; Shen and Keppler, 1997]. The temperature derivative of the silicate solubility, (⭸X silicate/⭸T) P , tends to increase with increasing pressure (Table 1; see also Fig. 4). The (⭸X silicate/⭸T) P for equivalent compositions in the system K2O–Al2O3–SiO2–H2O in the same pressure and temperature range is also positive (Mysen and Acton, 1999). The (⭸X silicate/⭸T) P in the K2O–Al2O3–SiO2– H2O system is, however, smaller than for equivalent compositions in the Na2O–Al2O3–SiO2–H2O system.
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Table 1. Temperature-, (⭸X silicate/⭸T) P , and pressure-, (⭸X silicate/⭸P) T , and composition, (⭸X silicate/⭸X Al2O3)P,T dependence of silicate solubility in aqueous fluids. (⭸X silicate/⭸T) P ⫻ 10 3 (mol.%/°C) Pressure (GPa) 0.8 1.05 1.3 1.65 2.0 Temp. (°C) 1000 1100 1200 1300
NS4 9.0 (0.6) 9.6 (0.2) 10.1 (0.4) 8.0 (0.6) 14.6 (0.7) ⫺16.6 ⫺19.6 ⫺20.7 ⫺22.2
(0.3) (3.4) (4.7) (5.8)
⫹ ⫹ ⫹ ⫹
NS4 19.2 22.2 23.4 24.4
(0.1) (1.2) (1.7) (2.0)
⫻ ⫻ ⫻ ⫻
NS4A3 8.9 (0.7) 9.4 (1.4) 7.9 (0.3) 10.1 (0.7) 7.1 (0.5) (⭸X silicate/⭸P) T (mol.%/GPa) NS4A3 P ⫺17.1 (5.7) ⫹ 17.8 (2.0) P ⫺15.9 (4.4) ⫹ 16.8 (1.5) P ⫺15.7 (3.7) ⫹ 16.6 (1.3) P ⫺14.8 (4.1) ⫹ 15.8 (1.4)
NS4A6 3.7 (0.3) 3.3 (0.2) 5.2 (0.6) 8.6 (0.9) 10.3 (0.7) ⫻ ⫻ ⫻ ⫻
P P P P
⫺2.9 ⫺3.6 ⫺3.7 ⫺3.2
(2.0) (2.1) (1.0) (0.3)
NS4A6 ⫹ 3.8 (0.7) ⫹ 4.6 (0.8) ⫹ 5.2 (0.4) ⫹ 5.4 (0.1)
⫻ ⫻ ⫻ ⫻
P P P P
(⭸X silicate/⭸X Al2O3) (mol.%/mol.% Al2O3) Temp. (°C) 1000 1100 1200 1300
0.8 GPa ⫺0.72 ⫹ 0.58X Al2O3 ⫺0.69 ⫹ 0.04X Al2O ⫺0.58 ⫹ 0.009XAl2O ⫺0.49 ⫺ 0.03X Al2O
1.05 GPa ⫺0.63 ⫹ 0.04X Al2O3 ⫺0.73 ⫹ 0.04X Al2O ⫺0.71 ⫹ 0.025X Al2O ⫺0.34 ⫺ 0.055X Al2O
1.3 GPa ⫺0.97 ⫹ 0.6X Al2O3 ⫺1.05 ⫹ 0.06X Al2O ⫺1.07 ⫹ 0.05X Al2O ⫺1.19 ⫹ 0.06X Al2O
Increasing Al2O3 results in decreasing silicate solubility for all compositions and at all pressures and temperatures investigated (Fig. 5). The rate of decrease, (⭸X silicate/⭸X Al2O3) P,T , varies slightly with alumina content (Table 1), and increases significantly with increasing pressure (Fig. 5). Similar behavior was noted by Mysen and Acton (1999) for silicate solubility in aqueous fluids in the K2O–Al2O3–SiO2–H2O system. From stepwise regression of the data, the solubility of silicate components in aqueous fluids in the Na2O–Al2O3–SiO2–H2O system in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively, can be expressed with the equation: X silicate ⫽ 1.9共3.5兲 ⫺ 1.3共0.9兲 ⫻ X Al2O3 ⫹ 0.008共0.002兲 ⫻ T ⫹ 13共4兲 ⫻ P ⫹ 7.3共1.5兲 ⫻ P 2.
(1)
In Eqn. 1, X Al2O3 and X silicate are in mol.%, T is °C, and P is GPa. R 2 ⫽ 0.89. 4. DISCUSSION
4.1. Volume Relations The solubility data for silicate in aqueous fluid may be used to derive partial molar volumes of H2O in the fluid. The free energy of solution of silicate at fixed temperature as a function of pressure is:
冕
a Hfluid 2O ⌬G T 共P兲 ⫽ 0 ⫽ ⌬G T 共1 bar兲 ⫹ RT ln ⬚ ⫹ f H 2O
P
V Hfluid dP, 2O
1
(2) fluid where a H is activity of H2O in the fluid, f ⬚H2O is the fugacity 2O of pure H2O (data from Haar et al., 1984, were used in the fluid present calculations), and V H is the partial molar volume of 2O H2O in the fluid. The slope of the (P ⫺ 1)/RT vs. ln( f ⬚H2O/ fluid XH ) obtained from isothermal solubility data at several pres2O fluid sures equals V H at given temperature T. By using Eqn. 2 for 2O
1.65 GPa ⫺1.77 ⫹ 0.09X Al2O3 ⫺1.59 ⫹ 0.06X Al2O ⫺1.51 ⫹ 0.05X Al2O ⫺1.36 ⫹ 0.03X Al2O
2.0 GPa ⫺1.25 ⫺ 0.13XAl2O3 ⫺1.94 ⫺ 0.04X Al2O 02.21 ⫺ 0.004X Al2O ⫺2.60 ⫹ 0.05X Al2O
fluid this purpose, it is assumed that a H (activity of H2O) can be 2O fluid substituted with X H2O (mol fraction of H2O). As these are comparatively dilute aqueous solutions, this assumption appears reasonable. fluid The relationships between (P ⫺ 1)/RT and ln( f ⬚H2O/X H ) 2O display slight but distinct curvature in the 0.8 to 2.0 GPa pressure range for all compositions and at all four temperatures (Fig. 6; see Table 2 for regression coefficients). Therefore, fluid fluid provided that the assumption of a H ⫽ XH is appropriate, 2O 2O the partial molar volume of H2O in these silicate-saturated aqueous fluids depends somewhat on pressure. Within the error fluid of the data, this pressure-dependence of V H is linear and 2O negative (Table 3; see also Fig. 7). This behavior contrasts with the distinctly nonlinear behavior of the molar volume of pure H2O as a function of pressure in the same pressure- and temperature-regime (data from Haar et al., 1984; shown as dashed lines in Fig. 7). For the Al-free NS4 composition, the partial molar volume of H2O ranges between 19 and 24.5 cm3/mol in the pressure and temperature range under consideration. In the lower-pressure range (P ⬍ 1.3 GPa), this volume is less than the molar volume of pure H2O. The inverse relationship holds at the highest pressure (Fig. 7). The compressibility of H2O in NS4saturated aqueous fluid is less, therefore, than that of pure H2O. fluid The pressure-dependence of V H in NS4-saturated fluids is 2O essentially independent of temperature with  [⫽ ⫺1/ fluid V 0 (⭸V H /⭸P)] ⫽ 0.125 GPa⫺1, which corresponds to a bulk 2O modulus of 8 GPa. The bulk modulus of pure H2O, on the other hand increases with increasing pressure from about 3.5 at 0.8 GPa to about 10 at 2.0 GPa. fluid The relationship between V H and pressure for the Al2O bearing compositions is also linear and negative, but for these fluid compositions the pressure-dependence of V H is also a func2O fluid tion of temperature so that (⭸V H2O/⭸P) T increases with increasing temperature (Table 3). For the NS4A3 composition, this trend translates to an increase in  from 0.141 GPa⫺1 at 1000°C to 0.172 GPa⫺1 at 1300°C (Table 3). For NS4A6-
Silicate-saturated fluids
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fluid Table 2. Coefficients for polynomial fit, ln( f ⬚H2O/X H ) ⫽ a ⫹ 2O b[(P ⫺ 1)/RT] ⫹ c[(P ⫺ 1)/RT] 2 .
NS4 in fluid Temperature (°C) 1000 1100 1200 1300 Temperature (°C) 1000 1100 1200 1300 Temperature (°C) 1000 1100 1200 1300
fluid Fig. 6. Relationship between ln( f ⬚H2O/X H ) and (P ⫺ 1)/RT, 2O fluid where f ⬚H2O is the fugacity of H2O (from Haar et al., 1984), X H is the 2O mol fraction of H2O in silicate-saturated aqueous fluid, P is pressure (bar), R is the gas constant, and T is temperature (K).
saturated aqueous fluid  increases from 0.193 GPa⫺1 to 0.213 GPa⫺1 in the same temperature range. fluid The difference between V H and the molar volume of pure 2O H2O in the Al-bearing systems is smaller than for Al-free NS4 and decreases with increasing Al-content. This behavior probably is a direct result of the fact that the silicate solubility decreases with increasing Al-content. One would expect the partial molar volume of H2O to approach the molar volume of pure H2O as the silicate content of the solution decreases. fluid At pressures near 0.8 GPa, the V H in all three composi2O
a b 7.59 ⫾ 0.05 24.7 ⫾ 0.7 7.71 ⫾ 0.07 25.6 ⫾ 1.2 7.81 ⫾ 0.09 26.6 ⫾ 1.6 7.89 ⫾ 0.09 27.6 ⫾ 1.8 NS4A3 in fluid a b 7.58 ⫾ 0.08 24.8 ⫾ 1.2 7.67 ⫾ 0.07 26.3 ⫾ 1.2 7.76 ⫾ 0.08 27.6 ⫾ 1.4 7.82 ⫾ 0.08 29.9 ⫾ 1.5 NS4A6 in fluid a b 7.49 ⫾ 0.05 26.5 ⫾ 0.9 7.59 ⫾ 0.05 27.9 ⫾ 0.9 7.67 ⫾ 0.06 29.3 ⫾ 1.0 7.73 ⫾ 0.05 30.8 ⫾ 0.9
c ⫺16.5 ⫾ 2.7 ⫺18.0 ⫾ 4.7 ⫺20.2 ⫾ 6.9 ⫺22.3 ⫾ 8.4 c ⫺18.3 ⫾ 4.8 ⫺22.5 ⫾ 5.0 ⫺27.0 ⫾ 6.0 ⫺32.2 ⫾ 7.0 c ⫺26.8 ⫾ 3.2 ⫺31.6 ⫾ 3.7 ⫺36.6 ⫾ 4.4 ⫺42.2 ⫾ 4.2
tions (NS4, NS4A3, and NS4A6) are essentially the same fluid (22–25 cm3/mol, depending on temperature), but because V H 2O in the Al-bearing compositions (NS4A3 and NS4A6) decreases fluid more rapidly with increasing pressure than V H for the Al-free 2O composition, NS4, at the highest pressures under study the fluid V H in NS4A3- and NS4A6-saturated aqueous fluids is 2O fluid smaller than V H in NS4-saturated fluid. In short, the partial 2O molar volume of H2O in silicate-saturated fluids in the system decreases with increasing Al2O3 content even though the silicate solubility actually decreases. Notably, the partial molar volume of H2O in Na2O–Al2O3–SiO2–H2O melts also decreases with increasing Al2O3 content (Mysen and Wheeler, 2000) as does the partial molar volume of H2O in K2O–Al2O3– SiO2–H2O melts (Mysen and Acton, 1999). fluid The V H of silicate-saturated aqueous fluids increase lin2O early with increasing temperature (Fig. 8; see also Table 4). Qualitatively similar behavior was inferred from the silicate solubility data in the system K2O–Al2O3–SiO2–H2O (Mysen and Acton, 1999). The thermal expansivity of pure H2O is also constant, at constant pressure, for pure H2O (dashed lines in Fig. 8). Whereas for Al-free composition (NS4), there is no discernible pressure effect on thermal expansion, for both composition NS4A3 and NS4A6 the thermal expansivity decreases with increasing pressure (Table 4), an effect qualitatively similar to that of pure H2O. This closer similarity of volume behavior in NS4A3, NS4A6, and pure H2O may result from the decreasing silicate solubility with increasing Al content. In other words, as the Al2O3 concentration of the system increases, the aqueous fluid becomes more dilute and its behavior is likely more closely to resemble that of pure H2O. 4.2. Density Relations In the pressure-temperature regime under investigation, aqueous fluids can dissolve as much as 32 wt.% (about 20 mol.%) silicate components (NS4 composition at 1300°C and 2.0 GPa; see Fig. 3A). Solution of this silicate may affect the density of the aqueous fluid. Because the solubility depends significantly on pressure, temperature, and composition, the density of silicate-saturated aqueous fluid may then depend on
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B. O. Mysen and K. Wheeler Table 3. Regression coefficients for pressure-dependent partial molar volume of H2O in silicate-saturated aqueous fluid. fluid V H ⫽ a ⫹ bP(GPa) 2O
NS4
a b r2  G
1000°C 24.73 ⫾ 0.01 ⫺3.165 ⫾ 0.008 0.999 0.127 ⫾ 0.005 7.9 ⫾ 0.3
1100°C 25.52 ⫾ 0.01 ⫺3.165 ⫾ 0.008 0.999 0.124 ⫾ 0.005 8.1 ⫾ 0.3
1200°C 26.60 ⫾ 0.05 ⫺3.34 ⫾ 0.03 0.999 0.126 ⫾ 0.0010 7.9 ⫾ 0.1
1300°C 27.57 ⫾ 0.04 ⫺3.43 ⫾ 0.03 0.999 0.124 ⫾ 0.0011 8.1 ⫾ 0.1
1200°C 27.55 ⫾ 0.04 ⫺4.43 ⫾ 0.03 0.999 0.161 ⫾ 0.001 6.21 ⫾ 0.04
1300°C 29.01 ⫾ 0.05 ⫺5.00 ⫾ 0.03 0.999 0.172 ⫾ 0.001 5.81 ⫾ 0.03
1200°C 29.30 ⫾ 0.06 ⫺6.06 ⫾ 0.04 0.999 0.206 ⫾ 0.001 4.85 ⫾ 0.02
1300°C 30.83 ⫾ 0.04 ⫺6.57 ⫾ 0.03 0.999 0.213 ⫾ 0.001 4.69 ⫾ 0.02
NS4A3
a b r2  G
1000°C 24.80 ⫾ 0.05 ⫺3.50 ⫾ 0.04 0.999 0.141 ⫾ 0.002 7.09 ⫾ 0.10
1100°C 26.30 ⫾ 0.06 ⫺4.06 ⫾ 0.04 0.999 0.154 ⫾ 0.002 6.5 ⫾ 0.1 NS4A6
a b r2  G
1000°C 26.44 ⫾ 0.06 ⫺5.10 ⫾ 0.04 0.999 0.193 ⫾ 0.002 5.18 ⫾ 0.05
1100°C 27.85 ⫾ 0.04 ⫺5.57 ⫾ 0.03 0.999 0.200 ⫾ 0.001 5.00 ⫾ 0.03
fluid  ⫽ ⫺1/V 0 (⭸V H /⭸T) P , G ⫽ 1/  . 2O
fluid Fig. 7. Partial molar volume of H2O in silicate-saturated aqueous fluid, V H , as a function of pressure. Molar volume 2O of pure H2O, V ⬚H2O is shown as dashed lines (data from Haar et al., 1984).
Silicate-saturated fluids
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fluid Fig. 8. Partial molar volume of H2O in silicate-saturated aqueous fluid, V H , as a function of temperature. Molar volume 2O fluid of pure H2O, V ⬚H2O is shown as dashed lines (data from Haar et al., 1984). Open symbols: V H for silicate-saturated 2O aqueous fluid in the system K2O–Al2O3–SiO2–H2O for compositions indicated (data from Mysen and Acton, 1999).
pressure, temperature, and composition in ways that could differ from that of pure H2O. This is in part because the molecular weight of silicate-saturated fluids differs from that of pure H2O and in part because the partial molar volume of H2O in silicate-saturated aqueous fluids differs from that of pure H2O (see Figs. 7 and 8). In addition, the partial molar volume of the silicate components very likely differs from that of H2O itself so that the volumes may also contribute to changes of the molar volume and, therefore, density of silicate-saturated aqueous fluid. The partial molar volume of oxide components in silicatesaturated aqueous fluids and their pressure and temperature derivatives are not known. From in situ, high-temperature/highpressure Raman spectroscopy of coexisting H2O-saturated silicate melt and silicate-saturated aqueous fluid in the system K2O–SiO2–H2O, Mysen (1998) concluded that the types of silicate complexes in silicate-saturated aqueous fluids and water-saturated silicate melts resemble one another. One may suggest, therefore, that the partial molar volumes of the oxide components in the silicate-saturated fluids may not differ greatly from those in silicate melts. If so, by using the oxide volume data of Lange and Carmichael (1987) and Kress and Carmichael (1991) the molar volumes of silicate-saturated fluids shown in Figure 9 result. In that figure, the partial molar volume of H2O in the fluids is shown as dashed lines. For the Al-bearing compositions (NS4A3 and NS4A6), the solubility of silicate components is insufficient to cause significant changes in the molar volume (within uncertainty in the data) compared with the partial molar volume of H2O in these
fluid fluids. In other words, V H and V fluid are indistinguishable 2O within experimental uncertainty. For the NS4 composition, which has the highest silicate solubility, a difference between the molar volume of the silicate-saturated aqueous fluid and the partial molar volume of H2O is discernible although the difference is less than 0.5 cm3/mol even at the highest silicate content (at 2.0 GPa and 1300°C). As would be expected, the smaller the silicate content, the smaller is the difference. In general, the differences are so small, however (⬍2%), that at least in the pressure-temperature regime of the present experiments, the molar volume of the silicate-saturated fluid, V fluid, could be approximated with the partial molar volume of H2O in that fluid fluid, V H , without introducing significant errors. 2O The density of the fluids, therefore, is:
fluid ⫽
M fluid M fluid ⬃ fluid . V fluid V H 2O
(3)
The resulting pressure- and temperature-dependence of the fluid density, fluid, governed by the composition-, pressurefluid and temperature-dependence of V H and the molecular weight 2O of the fluid, M fluid, is shown in Figure 10. The density of silicate-saturated aqueous fluids is a distinct non-linear function of pressure (Fig. 10). This behavior results from the strong positive and nonlinear solubility of silicate in fluid the fluids with pressure (Fig. 4), whereas V H is a linear 2O function of pressure (Fig. 7). Further, whereas the pressuredependence of the density of pure H2O decreases with increasing pressure (e.g., Haar et al., 1984; Brodholt and Wood, 1993),
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B. O. Mysen and K. Wheeler Table 4. Regression coefficients for temperature-dependent partial molar volume of H2O in silicate-saturated aqueous fluid. fluid V H ⫽ a ⫹ bT(°C) 2O
NS4 0.8 GPa
1.05 GPa
1.3 GPa
1.65 GPa
2.0 GPa
a b r2 ␣ ⫻ 10 4
13.5 ⫾ 0.2 0.0087 ⫾ 0.0002 0.999 6.4 ⫾ 0.2
12.7 ⫾ 0.2 0.0087 ⫾ 0.0002 0.999 6.9 ⫾ 0.2
12.2 ⫾ 0.2 0.0084 ⫾ 0.0001 0.999 6.9 ⫾ 0.1
11.5 ⫾ 0.2 0.0080 ⫾ 0.0002 0.998 7.0 ⫾ 0.2
10.8 ⫾ 0.2 0.9976 ⫾ 0.0001 0.999 7.0 ⫾ 0.2
0.8 GPa a b r2 ␣ ⫻ 10 4
1.05 GPa 12.1 ⫾ 0.3 0.0099 ⫾ 0.0003 0.999 8.2 ⫾ 0.3
1.3 GPa 12.1 ⫾ 0.2 0.0090 ⫾ 0.0004 0.998 7.4 ⫾ 0.4
2.0 GPa 13.0 ⫾ 0.2 0.0060 ⫾ 0.0002 0.998 4.6 ⫾ 0.2
13.7 ⫾ 0.2 0.0041 ⫾ 0.0003 0.999 3.0 ⫾ 0.2
2.0 GPa 11.4 ⫾ 0.2 0.0066 ⫾ 0.0001 0.999 5.8 ⫾ 0.1
11.6 ⫾ 0.2 0.0047 ⫾ 0.0002 0.999 4.1 ⫾ 0.2
NS4A3 1.65 GPa 12.9 ⫾ 0.2 0.0074 ⫾ 0.0001 0.999 5.7 ⫾ 0.1 NS4A6 0.8 GPa a b r2 ␣ ⫻ 10 4
1.05 GPa 11.7 ⫾ 0.2 0.0107 ⫾ 0.0002 0.999 9.2 ⫾ 0.2
1.3 GPa 11.8 ⫾ 0.2 0.0093 ⫾ 0.0002 0.999 7.9 ⫾ 0.2
1.65 GPa 11.5 ⫾ 0.2 0.0083 ⫾ 0.0002 0.999 7.2 ⫾ 0.2
fluid ␣ ⫽ 1/V 0 (⭸V H /⭸T) P . 2O
the pressure-dependence of the density of silicate-saturated aqueous fluids increases with increasing pressure. This latter difference is a result of the increasing silicate solubility with increasing pressure. Because of the different (⭸V ⬚H2O/⭸P) T and fluid (⭸V H /⭸P) T (Fig. 7), the density difference between that of 2O silicate-saturated aqueous fluids and that of pure H2O passes through a minimum near 1.5 GPa (Fig. 10). This minimum becomes less pronounced as the system becomes more aluminous because the silicate solubility decreases with increasing Al2O3 content (Fig. 10; see also Fig. 5). The density of silicate-saturated aqueous fluids shows negative and approximately linear correlation with temperature as is also the case for pure H2O (Haar et al., 1984; see also Table 5), but the (⭸ °H2O/⭸T) P is always more negative than (⭸fluid/
⭸T) P of silicate-saturated aqueous fluid. The expansivity of the silicate-saturated aqueous fluids approaches, however, that of pure H2O as the system becomes more Al-rich. This relationship is a direct consequence of the decreased silicate solubility with increasing Al2O3 content (Fig. 5). Several isochores of silicate-saturated aqueous fluids in the Na2O–Al2O3–SiO2–H2O system are compared with those of pure H2O in the same pressure- and temperature-range in Figure 11. It is evident from those results that the isochores generally have a more gentle ⭸P/⭸T slope than do those of pure H2O (dashed lines in Fig. 11). This difference is principally a function of the fact that the density of the silicate-saturated aqueous fluids are less sensitive to temperature than that of pure H2O.
Table 5. Regression coefficients for temperature-dependent density of silicate-saturated aqueous fluid.
H2O ⫽ a ⫹ bT(°C) NS4 0.8 GPa
1.05 GPa
a b ⫻ 10 5 r2
1.04 ⫾ 0.01 ⫺19.5 ⫾ 0.9 0.999
1.085 ⫾ 0.007 ⫺20.5 ⫾ 0.6 0.998
1.14 ⫾ 0.01 ⫺21.0 ⫾ 1.1 0.993 NS4A3
1.30 ⫾ 0.02 ⫺25.8 ⫾ 1.9 0.989
1.378 ⫾ 0.002 ⫺17.7 ⫾ 0.2 0.999
a b r2
0.8 GPa 1.06 ⫾ 0.03 ⫺23.6 ⫾ 2.4 0.980
1.05 GPa 1.10 ⫾ 0.02 ⫺22.5 ⫾ 2.1 0.983
1.3 GPa 1.13 ⫾ 0.01 ⫺20.9 ⫾ 0.9 0.996 NS4A6
1.65 GPa 1.16 ⫾ 0.02 ⫺15.8 ⫾ 1.3 0.987
2.0 GPa 1.29 ⫾ 0.03 ⫺13.2 ⫾ 2.2 0.47
a b r2 ⭸ ⬚H2O/⭸T ⫻ 10 5
0.8 GPa 1.12 ⫾ 0.02 ⫺30.7 ⫾ 1.5 0.995 ⫺32.7
1.05 GPa 1.169 ⫾ 0.006 ⫺30.3 ⫾ 0.5 0.999 ⫺31.0
1.3 GPa 1.211 ⫾ 0.007 ⫺28.6 ⫾ 0.7 0.999 ⫺29.0
1.65 GPa 1.25 ⫾ 0.02 ⫺23.1 ⫾ 1.8 0.988 ⫺26.7
2.0 GPa 1.32 ⫾ 0.01 ⫺16.1 ⫾ 0.9 0.3 ⫺24.0
Data for pure H2O from Haar et al. (1984).
1.3 GPa
1.65 GPa
2.0 GPa
Silicate-saturated fluids
Fig. 9. Molar volume of silicate-saturated aqueous fluids as a function of pressure for compositions as indicated. Open fluid symbols and dashed lines: Partial molar volume of H2O, V H , for the same compositions. 2O
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Fig. 10. Density of silicate-saturated aqueous fluids in the systems indicated as a function of pressure. Dashed lines show the density of pure H2O (data from Haar et al., 1984). Also shown is the density difference between silicate-saturated aqueous fluid and pure H2O for the three systems as a function of pressure at temperatures indicated.
Silicate-saturated fluids
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Fig. 11. Calculated isochores for silicate-saturated aqueous fluids compared with those for pure H2O (data from Haar et al., 1984). Acknowledgments—This research was conducted with partial support from National Science Foundation Grants EAR-9901886 and REU9619551. Associate editor: D. B. Dingwell REFERENCES Anderson G. M. and Burnham C. W. (1967) Reactions of quartz and corundum with aqueous chloride and hydroxide solutions at high temperatures and pressures. Am. J. Sci. 265, 12–27.
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