HR: 0830h
AN: V41C-0304    [PDF]
TI: Latent heat release and temperature changes as a result of metastable (Mg,Fe)$_2$SiO$_4$ phase transitions
AU: * Marton, F C
EM: Fred.Marton@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universit„t Bayreuth, Bayreuth, D-95440 Germany
AU: Frost, D J
AF: Bayerisches Geoinstitut, Universit„t Bayreuth, Bayreuth, D-95440 Germany
AB: When olivine transforms to wadsleyite or ringwoodite, heat is liberated and the temperature of the mineral assemblage increases. Such temperature increases have an important bearing on a wide range of geophysical problems. In normal mantle, for example, the release of latent heat will affect the width of mantle seismic discontinuities. In subducting slabs, on the other hand, latent heat release can significantly reduce the amount of olivine that can persist metastably. The heat released from metastable (Mg,Fe)$_{2}$SiO$_{4}$ transitions may also cause local superheating in the slabs that could enhance shear instabilities and cause deep earthquakes. Thermodynamic calculations of such metastable reactions indicate that the temperature increases can be 200 K or more, depending on the pressure and temperature conditions, with larger $T$ increases occurring farther from equilibrium. Calculating these temperature changes at equilibrium is straight-forward, as the reactions are reversible and $\Delta G$ = 0, so the heat released $Q_{rev}$ = $\Delta H$ = $T \Delta S$. However, under metastable conditions the reactions are irreversible and $\Delta G \neq$ 0, thus the heat released $Q_{irr}$ = $\Delta H$ = $T \Delta S$ + $\Delta G$. Estimates of the heat released in geophysical processes have been calculated using constant values of the volume change (usually at $STP$) and ignoring the temperature dependence of $\Delta H$, which can lead to overestimations of the latent heat release. We have attempted to measure the latent heat released from metastable olivine transformations in a multianvil apparatus. A large 25mm multianvil assembly is used with a 3mm diameter hot pressed sample with a thermocouple inserted into a hole in the sample. The sample is thermally insulated and is adjacent, in the same thermal regime, to a second reference sample also with a thermocouple inserted. The assembly is pressurized with a 5000 tonne press to conditions well into the wadsleyite/ringwoodite stability field and then heated until the sample transforms. While the magnitude of the latent heat cannot be accurately determined, important information can be ascertained on the variation of latent heat release with increasing pressure away from the equilibrium boundary.
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting