CURRENT RESEARCH ACTIVITIES

Solubility behavior of water in haploandesitic melts at high pressure and high temperature

The solubility of H2O in alkali aluminosilicate melts in the pressure-temperature regime of the Earth's upper mantle is a linear or near linear positive function of pressure (16-18 mol % H2O/GPa) at constant temperature and negative near linear function of temperature (1-2 mol % H2O/100°C) at constant pressure. The solubility is negatively correlated with Al2O3 content of the melts.
The partial molar volume of H2O in the melts, , is derived from the solubility. The ranges between 7.8 and 12.8 cm3/mol, and decreases with increasing Al2O3 content. The /T ranges from -7.1±0.810-3 to -5.6±1.310-3 cm3/mol °C with this negative temperature-dependence diminishing slightly as the melts become more aluminous. The is 0.5-1.0 cm3/mol higher in potassic melts as compared with sodium aluminosilicate melts with equivalent Al2O3 and SiO2 content.

Fig. 1. Solubility of H2O in Na-aluminosilicate melts as a function of temperature and pressure for compositions shown

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Fig. 2. Partial molar volume of H2O in melts for compositions shown.

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Magma-water interactions in shallow magma chambers

The experimental data on partial molar volume in alkali aluminosilicate melts were combined with published partial molar volume information for anhydrous oxides in silicate melts to estimate density distribution in shallow, water-rich dacite magma chambers associated with explosive volcanism. For a chamber of constant bulk composition during a comparatively short explosive event such as that of Mount Pinatubo June 1991 or Mount St. Helens in May 1980, the average density of the magma after eruption is ~ 3 % higher than before the eruption occurred. Furthermore, the H2O-distribution in the chamber after eruption is such that the magma is gravitationally unstable with the most dense magma near the top of the magma chamber. As much as a 4 % density difference between the top and the bottom of large magma chambers may result.

Density difference between H2O-bearing (H2O contents shown) and anhydrous melts in the system Na2O-Al2O3-
SiO2-H2O as a function of pressure (upper panel) and calculated vertical density distribution in shallow dacite magma chamber before and after cataclysmic volcanic eruption (lower two panelts).

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Alkali aluminosilicate-saturated aqueous fluids in the Earth's upper mantle

The silicate solubility in aqueous fluid is in the 3-20 mol% range for NS4 (Na2Si4O9), 2-13 mol % for NS4A3 (NS4+3 mol % Al2O3), and 1.5-8 mol % for NS4A6 (NS4+6 mol % Al2O3) 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:

Xsilicate (mol%) = 1.9 - 1.3XAl2O3 (mol %) + 0.008T(°C) - 13P(GPa) + 7.3P2.

Partial molar volume of H2O in the silicate-saturated fluids, , is in the range 17-25 cm3/mol. Compared with the molar volume of pure H2O, V°H2O, the values are 10-15 % lower than the V°H2O--values 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.

For all compositions, the is a linear and negative function of pressure with b [=-1/Vo( /P)T] ~ 0.125 GPa-1 for NS4. The bNS4A3 increases from 0.141 to 0.172 in the 1000°-1300°C range, whereas bNS4A6 increases from 0.193 to 0.213 GPa-1.

The is a positive and linear function of temperature with thermal expansion coefficients in the range 3-8x10-4 K-1. This thermal expansivity resembles that of pure H2O although the exact values and the range in values for pure H2O differ from 1/Vo( /T)P silicate-saturated aqueous solution..

Fig. 1 - Silicate solubility as a function of pressure and temperature for compositions shown.

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Fig. 2 - Molar volume of silicate-saturated aqueous fluids as a function of temperature and pressure.

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Energetics of H2O exsolution during explosive volcanisms

Experimentally obtained partial molar volumes of H2O in silicate melts and silicate-saturated aqueous solutions have combined with published data to estimate the energy release during exsolution of H2O from H2O-saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions (~0.2 GPa and 800°-1000°C). About 1.9x109 ergs/g H2O were obtained at 800°C and 0.2 GPa. This value decreases by about 10% between 0.2 GPa and ambient pressure. Thus, the exact pressure during such eruptions is not needed to evaluate the effect of exsolved H2O on the energy budget. The energy contribution from exsolved H2O appears to be 2-4 % of the total energy budget of such volcanic eruptions.

Energy release from H2O exsolved (per g exsolved H2O) from H2O-saturated magma in typical shallow dacitic magma chamber (upper panel), and fraction of total energy release resulting from exsolved H2O in recent explosive eruptions (lower panel).

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