HR: 0800h
AN: V31C-0618    [Abstracts]
TI: Interaction of Corundum, Wollastonite and Quartz With H2O-NaCl Solutions at 800 C and 10 Kbar
AU: * Newton, R C
EM: rcnewton@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095-1567 United States
AB: Aqueous fluids are potentially important transport agents in subduction zones and other high-P metamorphic environments. Recent studies indicate that at high P and T, the solubilities of major rock-forming elements are strongly enhanced by the formation of metal-chloride complexes, metal-hydroxide complexes and polynuclear metal-hydroxide clusters. However, the relative abundances of these species and the energetics of their interactions in high-pressure environments remains largely unknown. We measured the solubilities of corundum (Al₂O₃) and wollastonite (CaSiO₃) at 800 °C and 10 kbar in H₂O-NaCl solutions to halite saturation (X_NaCl = 0.6) . Both minerals show marked enhancement of solubility with increasing salinity. Al₂O₃ mol fraction rises rapidly to X_NaCl = 0.1, and then declines slowly towards halite saturation. Quenched experimental fluids have neutral pH. Modeling based on ideal solution of ions and molecules leads to a simple dissolution reaction and corresponding molality (m=mol/kg H₂O) expression: Al₂O₃(cor) + Na⁺ + 3H₂O = NaAl(OH)₄ + Al(OH)²⁺ and m_Al2O3 = [0.0232(a_NaCl)^1/4(a_H2O)^3/2+0.00123][1+2X_NaCl/(1-X_NaCl)] where H₂O and NaCl activities are given by a_H2O = (2-X_NaCl)/(2+X_NaCl) and a_NaCl = 4(X_NaCl)²/(1 + X_NaCl)². Wollastonite solubility in NaCl solutions is accurately described by: m_CaSiO3 = 0.6734X_NaCl + 0.1183(X_NaCl)^1/2 + 0.0204. There is a roughly 50-fold enhancement of dissolved wollastonite at halite saturation. Quenched experimental fluids are strongly basic (pH=11). A consistent dissolution reaction must therefore be similar to: CaSiO₃(wo) + Na⁺ + Cl^- = CaCl⁺ + OH^- + HNaSiO₃ Quartz solubility declines monotonically from m_SiO2 = 1.248 in pure H₂O to 0.20 at halite saturation. Quenched fluids are neutral, indicating that quartz does not react with solvent NaCl. The only salinity control on solubility is decrease of H₂O activity. The simple dissolution behaviors to be deduced from measurements on these minerals suggest that fluid-rock interaction in deep-crust/upper mantle metamorphic processes may be generally understandable in terms of quasi-ideal mixing models.
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