Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr13a1697d&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR13A-1697
Other
3611 Thermodynamics (0766, 1011, 8411), 3612 Reactions And Phase Equilibria (1012, 8412), 3630 Experimental Mineralogy And Petrology, 5430 Interiors (8147), 5724 Interiors (8147)
Scientific paper
Ice VI and ice VII may be important in the interiors of Europa, Ganymede, Callisto and Titan. Oceans and interior pore waters in these bodies likely contain dissolved salts. To address the role of salt on ice VI and ice VII, we investigated phase equilibria in the system H2O -NaCl at 1 molal (5.5 wt%) NaCl in an externally heated diamond-anvil cell. Phase identifications were made by optical microscopy combined with Raman spectroscopy. Experiments were conducted at 22-150°C and up to 5 GPa by allowing the cell to thermally equilibrate at a given temperature and then varying pressure isothermally while observing phase changes. The liquidus curves of ice VI and ice VII in a 5.5 wt% NaCl solution were determined. Melting was observed from 22 to 80°C (ice VI) and from 35 to 150°C (ice VII). Both melting curves are steeper than the respective NaCl-free curves, indicating that the freezing-point depression at this bulk composition increases with pressure. The intersection of the two liquidus curves indicates that VI-VII-liquid triple point is shifted toward lower T and higher P relative to pure H2O. The 5.5 wt% NaCl bulk composition crystallizes into a single solid phase of NaCl-bearing ice VI or ice VII solid solution over the investigated T range (the subscript 'ss' indicates solid solution). Large single crystals of ice VIss or ice VIIss can also be grown by slow compression of the cell from near-liquidus conditions to the solidus. Raman spectra of these crystals clearly show zoning in these crystals. The zoning persists for days at 22°C, indicating relatively slow Na+ and Cl- diffusivity. The large depression of the freezing point in a 1 molal NaCl solution has important implications for the oceans and interiors of the icy satellites of Jupiter and Saturn. Salty fluids may remain stable to much greater depth than expected. This would promote extensive hydrothermal metamorphism of the silicate interiors. If not limited to ice VI and VII, this behavior may suppress formation of ices at the bottoms of deep oceans in Titan and the Galilean satellites. The observation that ices VI and VII form solid solutions with NaCl from 22 to 150°C is also important. The qualitative inference of low Na+ and Cl- diffusivity suggests that compositional gradients could persist over at least modest time scales in these ices. Moreover, the presence of NaCl in ice VI and VII will likely reduce their viscosity and increase electrical conductivity.
Daniel Isabelle
Manning Craig E.
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