HR: 0800h
AN: V31A-1406    [Abstracts]
TI: Solubility of corundum and quartz in the system Al$_{2}$O$_{3}$-SiO$_{2}$-NaCl-H$_{2}$O at deep-crustal metamorphic conditions: 800 $\deg$C and 10 kbar
AU: * Newton, R C
EM: rcnewton@ucla.edu
AF: Dept. of Earth & Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth & Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: The solubility of corundum in H$_{2}$O-NaCl-SiO$_{2}$ fluids has been measured at 800 $\deg$C and 10 kbar. Solubility was determined by weight loss after equilibration of polished single crystals of synthetic corundum with fluids in welded Pt envelopes. Fluid compositions were X$_{NaCl}$=0 to 0.5 and SiO$_{2}$ up to quartz saturation at a given X$_{NaCl}$. The experiments were 1 to 7 days duration in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium. Kyanite and sillimanite are slightly more stable than quartz + corundum at these P-T conditions, but did not appear. The solubility in initially pure H$_{2}$O is very small: m$_{Al2O3}$ (mol Al$_{2}$O$_{3}$/kg H$_{2}$O) = 0.00135, but increases rapidly as the fourth root of X$_{NaCl}$ to 0.01 at X$_{NaCl}$=0.1 and 0.015 at halite saturation (X$_{NaCl}$\sim$0.6). Quenched solutions were pH neutral. The data were modeled with simple mixing models involving NaAl(OH)$_{4}$, Al(OH)$_{3}$, and any of their several dehydration and dissociation products, as well as Na$^{+}$, Cl$^{-}$, and H$_{2}$O. The best-fit reactions are: X$_{NaCl}$$<$0.3: Al$_{2}$O$_{3}$(cr) + Na$^{+}$ + H$_{2}$O = NaAlO(OH)$_{2}$ + AlO$^{+}$ X$_{NaCl}$$>$0.3: Al$_{2}$O$_{3}$(cr) + Na$^{+}$ = NaAlO$_{2}$ + AlO$^{+}$ Silica in solution further amplifies NaCl-enhancement of corundum solubility. Al$_{2}$O$_{3}$ molality at metastable quartz saturation rises rapidly from 0.006 at X$_{NaCl}$=0, to 0.038 at X$_{NaCl}$=0.1, and then more gradually to 0.045 at X$_{NaCl}$=0.3. Quenched solutions from SiO$_{2}$- and NaCl-rich compositions are acid (pH$\sim$1). The dissolution reaction, assuming a neutral NaAlSi species, must therefore be similar to: Al$_{2}$O$_{3}$(cr) + 2Na$^{+}$ + {\it x}Si(OH)$_{4}$ = 2NaAl Si$_{x/2}$O$_{x/2+y}$(OH)$_{x-2y+4)}$ + ({\it x}+2{\it y}-5)H$_{2}$O + 2H$^{+}$. Simple-mixing modeling suggests that the average Si/Al ratio ({\it x}/2) and number of non-bridging O$^{2-}$ ({\it y}) of the dominant Al solute species at 0.03$\leq$XNaCl$\leq$0.6 and quartz saturation are given by the formula: NaAlSi$_{2.75}$O$_{6.38}$(OH)$_{2.24}$, close to the NaAlSi$_{3}$O$_{8}$-H$_{2}$O join. Small amounts of spherical globules of aluminosilicate glass appeared close to quartz saturation at X$_{NaCl}$$\geq$0.1 It was not proven in our experiments whether a small field of stable silicate liquid intervenes near quartz saturation, or whether the melt phase was metastable. The great enhancement of Al$_{2}$O$_{3}$ solubility in the presence of NaCl and solute silica indicates that, under some conditions of high-grade metamorphism, Al$_{2}$O$_{3}$ is a substantially mobile component. NaCl solutions infiltrating peraluminous rocks may become quite acidic.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting