HR: 0800h
AN: V31D-0685    [Abstracts]
TI: Iron Diffusivity in Water Saturated Rhyolite Melt
AU: * Simon, A C
EM: adam.simon@unlv.edu
AF: Dept. of Geoscience, High Pressure Science and Engineering Center, UNLV, Las Vegas, NV 89154-4010, United States
AU: Bell, A
EM: bella19@unlv.nevada.edu
AF: Dept. of Geoscience, High Pressure Science and Engineering Center, UNLV, Las Vegas, NV 89154-4010, United States
AB: We have quantified experimentally the bulk chemical diffusivity of iron and the solubility of magnetite in peraluminous, water-saturated rhyolite melt at 100 MPa and 800°C by performing experiments in which we equilibrated a single crystal of magnetite with water-saturated rhyolite melt. The oxygen fugacity of each run was buffered at nickel-nickel oxide (NNO) and the assemblage was saturated with a 1.8 wt. % NaCl eq. NaCl-KCl- FeCl2-HCl-H2O volatile phase. The experimental charge contained a cylinder of magnetite (activity Fe3O4=1), cored from a single crystal of magnetite and placed at the base of a gold capsule, synthetic rhyolite glass placed above the magnetite cylinder and aqueous vapor which occupied the remaining capsule volume. The concentration profiles of FeO (and Na2O, K2O, Al2O3, SiO2 and Cl) in the quenched melt (i.e., glass) were measured over a distance of 400 microns beginning at the magnetite-rhyolite interface and moving orthogonally away from this interface into the glass until the concentration of iron fell below the limit of detection. Diffusion profiles were fit by inverting the measured concentrations of iron in the melt through the error function and solving for the diffusion coefficient assuming a stationary planar boundary; the near-intersection of the error function regression with the origin justifies this assumption. The calculated bulk chemical diffusivity for iron in H2O- saturated rhyolite is 4 E-10 cm2 sec-1; this measured diffusivity is consistent, albeit one-half to one order of magnitude lower than data for other divalent elements (Ca, Mg, Sn) in rhyolite. The Co value used to fit the diffusion profiles is consistent with published data for the equilibrium concentration of iron in rhyolite melt and, thus, the data yield the solubility of iron in water-saturated rhyolite melt. The aluminum saturation index (ASI) of the melt, hence concentrations of Na2O, K2O and Al2O3, remains essentially constant in the melt across the entire measured diffusion length indicating that the bulk diffusivity of iron is not affected by coupled diffusion with these major elements. The chlorine concentration in the melt, however, increases markedly toward the magnetite-glass interface. This finding suggests that iron and chlorine are associated strongly in the melt and that the presence of iron in the melt, owing to magnetite dissolution increases significantly the chlorine "solubility" in the melt. The new results constrain the growth and dissolution rates of iron-bearing minerals during the evolution of hydrous felsic melt, including magma mixing, and the apparent association of iron and chlorine in the melt provides important constraints on the mass transfer of iron, chlorine and other metals, to an exsolved volatile phase and how this impacts the acidity, hence metal-scavenging potential, of the volatile phase.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting