HR: 10:20h
AN: V12A-01    [Abstracts]
TI: Near-Solidus Solubility of Albite+Quartz in H2O at 1 GPa: Implications for Crustal Magmas and Fluids
AU: * Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095, United States
AU: Antignano, A
EM: aantigna@ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095, United States
AB: The solubility and melting of albite (Ab, NaAlSi₃O₈) ± quartz (Qz, SiO₂) in H₂O has been used to gain insight into the nature of magmas and metamorphic fluids in the continental crust. For example, the Ab+H₂O and Ab+Qz+H₂O solidi are widely used as guides to crustal melting. However, some studies suggest that Ab dissolves incongruently in H₂O at near-solidus conditions, which implies that these solidi are not likely to be encountered in nature. To address this problem, we carried out experiments on the stability and solubility of the assemblage Ab+Qz in H₂O at 1 GPa, 580-640°C. Starting materials were natural Amelia Ab and Brazilian Qz. Ab crystals were placed in a 1.6 mm OD Pt capsule, which was lightly crimped and then loaded with ultrapure H₂O and a quartz crystal into a 3.5 mm OD Pt capsule, which was sealed by arc welding. Experiments were conducted for 24 h in a piston-cylinder apparatus using 1" NaCl- graphite furnaces. Quenched run products were evaluated to assess stable assemblages, and solubilities were determined from crystal weight changes. Hydrous melt (L) became stable at 630±10°C. Paragonite (Pg, NaAl₃Si₃O₁₀(OH)₂) was present in all run products, indicating that Ab+Qz and L+Qz dissolve incongruently in H₂O, and that Ab+Qz+L+H₂O is metastable at 1 GPa. The bulk solubility of the assemblage Ab+Pg+Qz in H₂O doubles from 4.0 to 8.0 wt% as the solidus is approached (580- 630°C), with corresponding increases Si, Al, and Na molalities. Over the same T range, Na/Al and Na/Si (molar) respectively decrease (2.1 to 1.5) and increase (0.153 to 0.175), indicating that fluids are Na- and Si-rich peralkaline solutions far from Ab stoichiometry. At 580°C, measured Al solubility agrees well with that predicted from extrapolation of thermodynamic data for Al and Na-Al species; however, measured Al solubility at the solidus is much greater than predicted. Similar observations hold for Na and Si. This implies that there is a strong, pre-melting increase in the extent of polymerization of aqueous species in the narrow 50°C interval immediately below the solidus. Our results illustrate that phase relations in the system Ab-H₂O and Ab-Qz- H₂O are more complex than previously appreciated, and that polymeric aqueous species involving Na, Al, and Si are predominant in these model near-solidus and melt-saturated crustal fluids.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 3653 Fluid flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting