HR: 1340h
AN: MR23C-0074    [Abstracts]
TI: Grain-Scale Distribution of Hydrous Melts and Kinetics of Atomistic Transport Processes in Diopside-Anorthite-Albite Mixtures Determined in situ Using Electrical Impedance in High Pressure Laboratory Experiments
AU: * ter Heege, J
EM: Jan.terHeege@ruhr-uni-bochum.de
AF: Experimental Geophysics, Institute for Geology, Mineralogy & Geophysics Ruhr-University Bochum, Bochum, 44780 Germany
AU: Renner, J
EM: renner@geophysik.ruhr-uni-bochum.de
AF: Experimental Geophysics, Institute for Geology, Mineralogy & Geophysics Ruhr-University Bochum, Bochum, 44780 Germany
AB: The physical properties of hydrous partially molten systems at high pressure play an important role in melt migration phenomena in the deep Earth. The transport properties and rheology of melt-bearing porous media are directly linked to the grain-scale melt distribution as well as solid and melt viscosities. Abundance of partially melting, viscosity of the solid and melt phases, and melt topology are critically influenced by the presence of water. At high pressures, complete miscibility between silicate melts and hydrous fluids may occur, which changes the physical properties and topology of the fluid phase. However, experiments that investigate the physical properties of supercritical hydrous melts require in situ measurements at high pressure and few of such technically challenging studies exist. Electrical impedance measurements can be used to determine mobility and kinetics of charge carriers and also the geometrical distribution of phases (melt interconnectivity). Such information is crucial for estimates of the viscosity of hydrous melts and for modeling aggregate viscosities. We developed an experimental setup that can be used to measure electrical impedance at pressures up to 4 GPa and temperatures up to 1200°C in a solid medium piston-cylinder apparatus. Initially, we investigated changes in electrical impedance during pyrophyllite dehydration and calcite-aragonite phase transformation to compare results from our apparatus to previous studies. Then, we focused on changes in electrical impedance during partial melting in hydrous diopside-anorthite and diopside-albite mixtures of varying composition, produced by mixing water and synthetic glass powder of the relevant composition. The electrical impedance data for heating and subsequent cooling at constant pressure generally show three distinct regimes, marked by changes in the apparent activation energy for conduction (Q): (1) a low temperature regime, (2) an intermediate temperature regime, (3) a high temperature regime. For the samples investigated and pressures of 2-3 GPa, the transition between regime (1) and (2) occurs at ~620°C and is marked by an increase in Q from ~85 to ~285 kJ/mol for hydrous diopside-albite, at ~660°C and ~150 to ~235 kJ/mol for hydrous diopside-anorthite, and at ~780°C and 50 to 105 kJ/mol for pure diopside with water. The transition between regime (2) and (3) occurs between 740°C and 980°C depending on the composition of the sample. Microstructural investigation suggests that the transition between regime (1) and (2) is related to the nucleation of grains in the glasses and that the transition between regime (2) and (3) is related to hydrous partial melting, at least for hydrous diopside-anorthite and diopside-albite. Furthermore, the microstructures of the hydrous diopside-anorthite-albite mixtures suggest that batches of interconnected melt are formed that rapidly separate into a silicate melt phase and a hydrous fluid phase upon quenching.
UR: http://homepage.ruhr-uni-bochum.de/Jan.terHeege/
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
DE: 3914 Electrical properties
DE: 5109 Magnetic and electrical properties (0925)
DE: 5194 Instruments and techniques
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005