HR: 0800h
AN: V21A-0588    [Abstracts]
TI: Effect of fluid composition on monazite solubility, coarsening and crystal size distribution at 1.0 GPa and $1000\deg$C
AU: * Ayers, J C
EM: john.c.ayers@vanderbilt.edu
AF: Dept. of Earth & Environmental Sciences, Vanderbilt University, VU Station B #350105 2301 Vanderbilt Pl., Nashville, TN 37235-0105 United States
AU: Bryant, D L
EM: derek.l.bryant@vanderbilt.edu
AF: Dept. of Earth & Environmental Sciences, Vanderbilt University, VU Station B #350105 2301 Vanderbilt Pl., Nashville, TN 37235-0105 United States
AU: Giles, K
EM: kevin.giles@vanderbilt.edu
AF: Dept. of Earth & Environmental Sciences, Vanderbilt University, VU Station B #350105 2301 Vanderbilt Pl., Nashville, TN 37235-0105 United States
AB: The dependence of monazite aqueous solubility and coarsening rate on fluid composition was evaluated by performing experiments in a piston cylinder apparatus at 1.0 GPa and $1000\deg$C. Single crystal weight loss measurements in H2O $\pm$ HCl, NaOH show that the aqueous solubility of monazite increases when the deviation of pH from neutrality at $25\deg$C increases above six units. Monazite is amphoteric, with a U-shaped solubility curve demonstrating high solubility in acidic and alkaline fluids, similar to ambient conditions. Results of growth experiments show that the rate of monazite coarsening in quartzite + fluid is also strongly sensitive to fluid composition. Addition of 2 molal NaCl to H2O completely halted growth, presumably by ion adsorption on monazite surfaces. No modification of the initial crystal size distribution was evident. The strong dependence of monazite coarsening rate on fluid composition suggests that the size of crystals produced by metamorphic coarsening may not be a reliable indicator of duration of metamorphism. Results also suggest that adsorption of ions on mineral surfaces may be an important phenomenon even at high temperatures. Although measurable growth by Ostwald ripening $\pm$ coalescence occurs in 2 m HCl, and monazite solubility is higher in 2 m HCl than in pure H2O, the limited data do not conclusively show a higher growth rate in 2 m HCl than in H2O. Monazite coarsening in the presence of aqueous fluids will produce dateable metamorphic rims unless growth is inhibited by dissolved salts. In the growth experiments the volumetric phase proportion of monazite is constant, so CSD plots measured at different times rotate around a point at small size to form a CSD fan. For growth processes such as Ostwald ripening and coalescence that do not affect the volumetric phase proportion of a phase, CSD slopes and intercepts are not independent, so CSD diagrams cannot be used to characterize the growth process.
UR: http://www.cas.vanderbilt.edu/ayers/research.htm
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting