HR: 0800h
AN: V41A-1420    [Abstracts]
TI: OH in Rutile: an Oxygen and Water Barometer
AU: * Johnson, E A
EM: johnsoel@ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences, 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences, 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AU: Antignano, A
EM: aantigna@ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences, 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AU: Tropper, P
V41A-1420 AF: University of California, Los Angeles, Department of Earth and Space Sciences, 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AB: Dehydration of the subducting lithosphere induces oxidation and partial melting in the mantle wedge above subduction zones, and storage of water in the form of hydroxyl in high-pressure mineral phases may be an important mechanism for transfer of water to the mantle. It is therefore important to quantify water content of fluids and oxygen fugacity in subduction zones, but these variables can be difficult to measure or infer in many rocks. This study investigates the possibility of determining oxygen fugacity or water activity based on OH concentration measurements in rutile. The solubility of OH in pure rutile has been determined using rutile grains from aqueous fluid solubility experiments (Tropper and Manning 2005, Am Min, 90, 502). In pure rutile, H⁺ is stoichiometrically incorporated into the structure via reduction of Ti⁴⁺ to Ti³⁺, resulting in a change in color from pale yellow to deep blue. Synthetic rutile crystals were equilibrated in pure H₂O or a H₂O-NaCl solution at 1-2 GPa and 600-1100°C. The runs were unbuffered with respect to oxygen fugacity but were close to the NNO buffer (Newton and Manning 2005, J Petr, 46, 701). Rutile OH concentrations were determined using FTIR spectroscopy and the calibration of Maldener (2001, Min Pet, 71, 21). At a constant pressure of 1 GPa, OH concentrations of rutile in equilibrium with pure H₂O increase exponentially from 600 to 1100°C. The data are fit with the equation [OH] = 17.7exp(4.00×10^-3T) (R=0.998), where [OH] is in ppm H₂O wt. and T is in °C. Increasing pressure from 1 to 2 GPa at 1100°C results in an increase in OH solubility from 1540 to 2220 ppm H₂O. OH solubility in rutile decreases from 2220 to 1290 ppm H₂O by lowering the water activity of the fluid from 1 to 0.49 at P = 2 GPa and T = 1100°C. Using the solubility data and the exchange reaction, Ti³⁺O(OH) + ¬O₂ = Ti⁴⁺O₂ + «H₂O, we calculate ΔH = -29.2 kJ/mol, ΔS = -6.54 J/mol, and ΔV = 0.80 cm³/mol for this reaction. Preliminary models indicate that OH in rutile is an effective barometer for water activity and may be a particularly sensitive indicator of oxygen fugacity for systems in which pressure and temperature of formation are constrained.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005