HR: 0800h
AN: V41A-1357    [Abstracts]
TI: Halogen Chemical Diffusivities in Silicate Melts
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: Earth and Environmental Sciences, University of Munich,, Theresienstr. 41/III,, Munich, D-80333 Germany
AB: Halogens may exert a significant influence on the physico-chemical properties and the structure of silicate glasses and melts, as well as on their phase relations. Furthermore, the geochemistry of halogens from volcanic systems potentially provides valuable information on the nature and efficiency of the dagssing process in subduction zone volcanism. Knowledge of the transport properties of halogens in silicate melts is a necessary prerequisite in order to model the information contained in halogen concentrations of eruptive products and volcanic gases in terms of the potential influence of kinetics in controlling degassing. Towards this end, chemical diffusion of halogens (fluorine, iodine, chlorine and bromine) has been invesigated in melts in the system Na-Fe-Si-O-(F,Cl,Br,I) over a wide range of temperature (450 - 1400øC) using diffusion couple techniques. Halogens were added in the form of FeF3, FeCl3, FeI2 or FeBr3. Starting melts were fined by stirring for several hours at 1000-1100øC using a concentric cylinder viscometer. The synthesis temperature was restricted to 1100øC to limit the volatilization of halogens. Melted and doubly polished discs were then put into platinum tubes (5mm diameter), where the halogen-rich sample was located at the bottom, and sealed by welding. During the experiments the temperature was monitored with a thermocouple located at the vicinity of the capsule. Run durations were between 30 minutes and 1 hour. The recovered samples were analyzed using an electron microprobe in order to determine the diffusion profiles of the halogens. The experiments for the I-containing samples were conducted between 450 and 1025 øC and for a run duration of 30 to 45 min, for the Cl-containing samples between 800 and 1100øC and 45 to 60 min, while the Br-containing materials were investigated between 750 and 1000øC for 45 to 60 min. The preliminary results suggest a significant range of at least 3 orders of magnitude between the diffusion coefficients for F, Cl, Br and I at 1000øC. This raises the possibility of significant kinetic disequilibrium during foaming and rapid degassing of magma prior to and during eruption. The range also implies that the diffusion of halogens under these conditions in magma is intrinsic in nature and not controlled by melt viscosity.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting