THE AMMONIA-WATER PHASE DIAGRAM AND PHASE VOLUMES TO 4 KBARS

Overview. As part of a continuing study [I] we have measured the densities of liquids and placed limits on the densities of solids in the ammonia-water system under conditions corresponding to the interiors of icy satellites. Our goal is to determine the high-pressure phase diagram of the NH3-H20 system and the densities of the equilibrium phases so as to constrain models of cryomagmatic processes. The apparatus and techniques have been described at previous conferences [I], and recent modifications are explained below. Our results for the thermal expansion of ammonia-water liquids agree with the measurements by Croft et al. at 1 a m [2]. Our preliminary pressure-melting curves of ammonia dihydrate and ammonia monohydrate are consistent with the Clapeyron equation's predictions. Boone and Nicol's 131 measured monohydrate melting curve agrees with ours but their dihydrate melting curve does not and its slope is thermodynamically inconsistent. We observe that (1) the peritectic melting point at low pressures shifts to more water-rich compositions with increasing pressure (at least to 2 kbars), (2) the melting behavior of ammonia dihydrate changes from incongruent to congruent melting at P -200 bars, and (3) the peritecticfeutectic liquid is slightly less dense than the compositionally equivalent solid along the pressure-melting curve to -2 kbars; above this pressure, where ice I1 becomes stable, the eutectic solid is much denser than the liquid. The small density contrast at pressures < 2 kbars suggests that there should be a tendency toward igneous intrusion of NH3-H20 liquids in icy satellites and that fractional crystallization should not be as important in NH3-H20 magmas in icy satellites as this process is in silicate magmas in Earth. Recent modifications to the exweriment. A newly fabricated pressure vessel uses a miniature silicon diode thermometer that extends into the middle of the sample. This system has a much faster thermal response than our old thermometer, thus improving the accuracy of measured transition temperatures and enabling us to distinguish easily between exothermic and endothermic reactions. We recently fabricated sample capsules that remain flexible below the ammonia-water freezing point, but they are slightly permeable to the pressure-transmitting fluid we want to use. When perfected, these capsules should allow us to measure the densities and phase transitions in completely solidified samples. Phase equilibria. Figure 1 shows isobaric phase boundaries in the ammonia-water system at pressures of 1 am, 1000 bars, 2000 bars, and 3000 bars. Ammonia dihydrate melts congruently above 200 bars and incongruently at lower pressures. The low-pressure peritectic (X = 32.6 wt % NH3, T = 176.16 K, P = 1 atm) shifts at higher pressures to a more water-rich eutectic (e.g., X = 28.6% NH3, T = 176.95 K, P = 1800 bars). The ammonia monohydrate liquidus is still preliminary, but our data suggests that at pressures above about 1200 bars the stable eutectic melting point may occur between monohydrate and water ice. The temperature-pressure curves of eutecticlperitectic melting in ice-bearing NH3-H20 mixtures are shown in Figure 2. The slope of the melting curve of pure ammonia dihydrate and of dihydrate-ice mixtures is consistent with the Clapeyron equation, especially when compressibility is considered (Fig. 3). Our observations and the predictions of the Clapeyron equation are inconsistent with an earlier determination of the melting curve of diiydrate [31. The slope of their dihydrate melting curve corresponds nicely with the slope of the melting curve of ammonia monohydrate reported by those researchers and as calculated from the Clapeyron equation. Hence, we suggest that they may have mistaken ammonia monohydrate for the dihydrate. However, unlike Boone and Nicol [31, who examined compositions as ammonia-rich as the monohydrate, we have not yet explored the region to the ammonia-rich side of the dihydrate composition, so our phase diagrams and interpretations are still preliminary. Densitv reiationshi~s and wlanetolo~ical im~lications. Density measurements on amrnoniawater liquids at low pressures (20-50 bars) confm the measurements of thermal expansivity made by Croft et al. [21. We have not yet succeeded in obtaining densities for the solid phases in the ammonia-water system, although the signs of the density differentials of the solids with respect to their equilibrium liquids are consistent with the known densities of these phases at 1 atm. We have used our volume data for the liquids and the volume data and models of solid densities from [21 to compile a figure showing some important density relations along the eutecticlperitectic melting curve from 1 to 1800 bars (Fig. 4). The densities are not actually expected to fall along straight lines, but for simplicity they were calculated as such. Throughout this pressure range there is no qualitative change in the calculated relative densities of the phases. However, the apparent change of slope from positive to negative in the pressure-melting curve of dihydrate (Fig. 2) implies that the densities of dihydrate and its congruent liquid cross at this pressure; we consider this a tentative result until we obtain more data. Figure 4 shows that the eutectic liquid has a density that is consiste~~tly ess than that of its own solid at any given pressure up to 2 kbars. Above approximately this pressure, ice changes to ice 11, so that the solidliquid density contrast increases further. These relationships indicate that gravitationally driven volcanic extrusion of ammonia-water liquid can occur through a crust of the same composition.