HR: 0800h
AN: V31B-0614    [Abstracts]
TI: Dependence of Glass Transition Temperature on the Oxidation State of Ferrosilicate Melts
AU: Nichols, A R
EM: nichols@min.uni-muenchen.de
AF: University of Munich, Theresienstr. 41/3, Munich, 80333 Germany
AU: * Potuzak, M
EM: potuzak@min.uni-muenchen.de
AF: University of Munich, Theresienstr. 41/3, Munich, 80333 Germany
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: University of Munich, Theresienstr. 41/3, Munich, 80333 Germany
AB: Global models suggest that iron is one of the major fabric element of the terrestrial planet interiors and probably the most abundant transition metal at all. The presence of volcanic rocks containing iron has been confirmed on Earth, Moon, Mars and Venus. Based on our study we suggest that the oxidation state of iron is probably second only to volatiles in its influence on the physical properties of natural silicic magma. This is shown by examining the effect of the oxidation state on the glass transition temperature. The heat capacity of ferrosilicate melts has been measured using differential scanning calorimetry (DSC). Two different simple Fe-bearing systems have been studied: (i) anorthite-diopside eutectic composition (AnDi) with 10 wt% of Fe as a basalt analogue and (ii) sodium disilicate (NS2) with variable amount of Fe (up to 30 wt% Fe). Samples with different oxidation state of Fe have been prepared using the concentric cylinder method. The melt was reduced stepwise by flowing CO₂ or a mixture of CO₂-CO through the alumina muffle tube. A drop of liquid was quenched in water after each redox equilibrium step. The resulting glasses were analyzed by electron microprobe, and volumetric titration was employed to determine FeO. The high temperature viscosities rapidly decrease with increasing ferrous iron content. This decrease is non-linear function of iron reduction. Fragments of glass were analyzed by differential scanning calorimetry (DSC). Glasses were placed in a platinum crucible and heated through a range of temperatures from 298 to 1050 K, approximately 50 K above the glass transition. After initial heating at 5 K.min^-1, the samples were cooled and heated at matched rates of 20, 15, 10 and 5 K.min^-1. Glass transition temperatures (Tg) have been defined as a peak of the heat capacity curve (Cp). Tg decreases steadily with increasing ferrous content, the strongest affect is during the initial reduction of Fe. Reducing Fe by about 66% causes a drop in Tg of about 60 K. These changes are independent of heating/cooling rate. The equivalence of the activation energy of the melts obtained from DSC and low-T viscometry allow to predict viscosities of melts at temperature close to the Tg. The low-T viscosities are strongly ferric/ferrous dependent. These effects are similar to those of water. Therefore, like with the addition of water, the reduction of Fe can strongly influence the magma's physical properties and thus magma behaviour during crystallization, degassing, foaming and fragmentation.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005