HR: 11:35h
AN: V51L-06    [PDF]
TI: Influence of halogens (F, Cl, Br, I) on lava rheology.
AU: Hess, K
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Courtial, P
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Potuzak, M
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AB: Degassing of halogens from magma is an important constraint on the nature and efficiency of volatile degassing in subvolcanic systems. Fundamental information on the transport properties of halogens in melts and the melts containing halogens are required for the quantification of models dealing with disequilibrium degassing based on halogens abundances in quenched volcanic glasses. The shear viscosities of melts in the system Na-Fe-Si-O-Br-I have been determined over a wide range of temperatures (1200-400 $\deg$C) at 1 atm pressure in air. The compositions are based on the addition of Fe2O3, FeBr3 and FeI2 to a base melt composition corresponding to sodium disilicate (Na2Si2O5). Viscosities were determined using concentric cylinder and micropenetration methods and measurements span the range of 100.5 to 1011 Pa s. The chemical compositions of these melts have been analysed after viscometry determinations. The iron is fully oxidized under the conditions of the viscometry. Although Bromine and iodine are highly volatile elements in silicate melts, levels of bromine and iodine over 1 wt.% have been stabilized in these melts, presumably assisted by the presence of ferric iron. Although some volatilisation occurs during the original synthesis of these samples none occurs during viscometry. The anionic substitutions Br2O-1 and I2O-1 have very small influences on the viscosity of these melts. These effects are similar to the influence of Cl2O-1 substitution. As a consequence, minor to major element abundances of Br and I in strongly peralkaline undersaturated volcanic rocks are not likely to have a significant influence on melt viscosity. These results lead us to infer low values of diffusivity for Br and I in volcanic melts. Thus disequilibrium degassing involving Br and I is highly likely.Implications of these these putative diffusivities of Br and I in explosive volcanism will be discussed.
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting