HR: 14:55h
AN: MR33A-06    [Abstracts]
TI: Temperature-Dependent Thermal Diffusivity of Crustal Minerals, Rocks and Melts: Implications for Positive Thermal Feedback During Crustal Anatexis
AU: * Whittington, A G
EM: whittingtona@missouri.edu
AF: Geological Sciences, University of Missouri, Columbia, MO 65211, United States
AU: Hofmeister, A M
EM: hofmeist@levee.wustl.edu
AF: Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, United States
AU: Nabelek, P I
EM: nabelekp@missouri.edu
AF: Geological Sciences, University of Missouri, Columbia, MO 65211, United States
AU: Pertermann, M
EM: maik.pertermann@rice.edu
AF: Earth Science, Rice University, Houston, TX 77005, United States
AB: The thermal evolution of orogenic belts is governed by the rates of heat transfer by advection and conduction. The physical properties governing heat conduction are thermal diffusivity (κ) and conductivity (k = κρC_P), where ρ is density and C_P is heat capacity. Numerical models of orogenic belts typically assume constant values for κ (~10^-6 m²s^-1) and k (~4 Wm^{- 1}K^-1), but in fact κ, C_P, and hence k), are significantly temperature-dependent. Thermal diffusivity data for orthoclase crystal and glass, haplogranitic glass, leucogranite and garnet schist, were acquired at temperatures up to 1000 °C, using the laser-flash method (LFA). This isolates the phonon component of heat transfer from radiative transfer and avoids thermal contact losses. In all cases, κ decreases rapidly with increasing temperature, asymptotically approaching a high-temperature limit. Glasses and melts have lower κ than chemically equivalent crystalline materials. Dissolved water also has a depressing effect on κ (Hofmeister et al. 2006 Geophys. Res. Lett.), so that hydrous granitic liquids should be particularly efficient insulators. Crustal melting will therefore produce an insulating layer, which will retain heat. This positive feedback between melting and thermal insulation may promote increased melt fraction, and may provide a mechanism for leucogranite generation by shear heating. Steady-state thermal gradients are governed by the conductivity, k. Calculations for orthoclase crystal, glass and liquid indicate that k of crystals decreases with T, while k of glass increases with T. At 1000°C, k is ~2 Wm^-1K^-1 for the crystal, but only ~1.5 Wm^-1K^-1 for glass/melt. The low thermal conductivity of melts suggests that steady-state thermal gradients will be higher across partially molten layers than in unmelted crust.
DE: 3619 Magma genesis and partial melting (1037)
DE: 5134 Thermal properties
DE: 5139 Transport properties
DE: 8130 Heat generation and transport
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting