HR: 0800h
AN: V31C-0621    [Abstracts]
TI: The Solubility of Kyanite in H2O-SiO2 Fluids at 800 C and 1 GPa: Implications for Al-Si Complexing in Subduction-Zone Fluids
AU: * Tropper, P
EM: Peter.Tropper@uibk.ac.at
AF: Faculty of Geo- and Atmospheric Sciences, Institute of Mineralogy and Petrography, University of Innsbruck, Innsbruck, A-6020 Austria
AU: * Tropper, P
EM: Peter.Tropper@uibk.ac.at
AF: Dept. of Earth and 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 and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: Aluminium has long been regarded as the major element that is least soluble in H₂O during metamorphic and metasomatic processes. However, widespread geological evidence such as aluminiumsilicate-bearing veins in eclogites suggests at least local Al mobility in aqueous fluids in high-P environments. In the absence of unusually high fluid fluxes, such observations imply high Al solubility in associated metamorphic fluids; however, the mechanism by which Al solubility is enhanced has not been established. We measured the solubility of kyanite in fluids of varying initial SiO₂/H₂O ratio at 800°C and 1 GPa in a piston-cylinder apparatus. Run times were 24 hours. Solubility was determined by the weight loss from natural kyanite crystals. Initial SiO_2,aq concentrations were varied between corundum and quartz saturation by adding small quartz crystals, which dissolve rapidly relative to kyanite to produce the desired initial SiO₂/H₂O ratio. Run products were slightly etched kyanite and white amorphous SiO₂-rich quench gel. Runs were carefully inspected to verify the absence of quartz or corundum. SiO₂ contents of less than 0.979 molal yielded corundum growth on the surface of the starting kyanite. The concentration of Al increases with increasing dissolved SiO₂ content from 0.0101 m_Al at 1.023 m_SiO2,aq to 0.0135 m_Al at 1.252 m_SiO2,aq, where m_i is molality of the subscripted species. The maximum dissolved Al is found at quartz saturation, where m_Al is 3.33 times greater than the concentration in equilibrium with corundum in pure H₂O. The large enhancement of Al concentration by Si at conditions in which the activity of aluminate complexes must decrease, and in the absence of significant departures from acid-base neutrality, is strong evidence for formation of Al-Si complexes. Previous experimental investigations on the solubility behaviour of silicates in supercritical fluids have shown that multi-nuclear complexing is an important mechanism in solubility processes at high P and T (e.g. Si-Si polymerization). Although, no data on the degree of Si-Al polymerization at high P-T yet exist, several studies at low P and T show that formation of Al-Si complexes can occur in crustal fluids. We suggest that such complexes are even more stable at high P and T, and may play an important role in material transport in subduction zones. Our experimental results indicate that the formation of kyanite + quartz-bearing veins, which are ubiquitous in regional metamorphic terranes, are the natural manifestation of the proposed Al-Si complexing processes.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005