Thermal diffusivity of aluminous spinels and magnetite at elevated temperature with implications for heat transport in Earth's transition zone

Abstract

The phonon component of thermal diffusivity ( D ) from 12 single crystals in the spinel family was measured at temperatures ( T ) of up to ~2000 K, using laser-flash analysis. Synthetic disordered spinel, 4 gemstones near MgAl 2 O 4 , nearly ZnAl 2 O 4 , 4 “hercynites” [(Mg,Fe 2+ )(Al,Fe 3+ ) 2 O 4 ], and 2 magnetites (nearly Fe 3 O 4 ) were characterized using optical spectroscopy and electron microprobe analysis. The magnetic transition in Fe 3 O 4 is manifest as a lambda curve in 1/ D , but otherwise, D decreases with increasing T and approaches a constant ( D sat ) at high T . Part of the decrease in D as T increases results from disordering above ~700 K: these two effects were distinguished by making multiple heating runs. At 298 K, D decreases strongly as either cation substitution or Mg-Al disorder increases, whereas D sat is moderately perturbed by substitutions. For both ordered and (equilibrium) disordered spinels and hercynites, the temperature dependence of 1/ D is best described by low-order polynomial fits. For spinel, combining our data with previous cryogenic studies of thermal conductivity ( k ) constrains the T dependence of D and k from ~0 K to melting. The response of D to disorder, impurity content, and cation mass for the aluminates is used to constrain D ( T ) for γ-Mg 2 SiO 4 and ringwoodite. Pressure derivatives are provided by relationships such as ∂ln( k lat )/∂ P = ∂ln( K T )/∂ P . Our results show that the phonon contribution to heat transport in Earth’s transition zone is high, particularly for large proportions of ringwoodite.