Phase Diagram for Ammonia-Water Mixtures at High Pressures: Implications for Icy Satellites

Abstract

The (NH_3)x(H_20)_(1.x) phase diagram for 0 ≤ X ≤ 0.50 has been reexamined at temperatures from 125 K to 400 K and at pressures to 6.0 GPa using diamond-anvil cells. By electroplating the gasket materials with gold, complicated reactions between sample solutions and gasket materials, which affected earlier studies, have been avoided. Sample pressures were determined using the ruby-luminescence technique, and phase assignments were made using optical characterization. Phase assignments were confirmed by Raman spectroscopy. At room temperature the stable phases observed were fluid, high pressure ices (VI and VII), and ammonia monohydrate, NH_3·H_2O. The Ice VI and Ice VII liquidi at 295 K were extrapolated to intersect at X = 0.26 ± 0.01 and 2.1 GPa. At room temperature, the eutectic for Ice VII and NH_3·H_2O was observed at 3.3 ± 0.2 GPa, and extrapolation of the room temperature liquidus indicates that the cotectic composition is near X= 0.45. Near X= 0.33. the stable phases were high pressure ices (VI, VII, and VIII), NH_3·H_2O, and another phase tentatively identified as ammonia dihydrate, NH_3·2H_2O. At this composition, the Ice VI liquidus and the congruent melting curve of NH_3·2H_2O interesect at 1.8 ± 0.2 GPa and 252 ± 5 K, and the Ice VII liquidus is approximately linear with a slope of 0.016 ± 0.002 GPa K^(-1. To within the uncertainty of the experiment, the Ice VI liquidus continues smoothly from the Ice VII liquidus. The quadruple point among NH_3·H_2O, NH_3·H2O , Ice VI, and fluid is located at 250 ± 5 K and 1.9 ± 0.3 GPa, with the accompanying double cotectic at a composition of X= 0.36 ± O.Ql. The eutectic for NH_3·H_2O and Ice VII is approximately linear with a slope of 0.033 ± 0.003 GPa K^(-1). We have applied these data to the interior of Titan in a manner similar to the analysis of Lunine and Stevenson (1987). The main implication of these results is that Titan is likely to have a thicker NH_3·H_2O ocean than previously suspected, because the stability field of NH_3·2H_2O is smaller than previously supposed. Implications for methane and ammonia volcanism on Titan are briefly discussed. The experimentally observed reactivity between the liquid and iron (for example) may also have implications for planetary and satellite evolution.