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 MgAl2O4, nearly ZnAl2O4 "hercynites" [(Mg,Fe 2+1) (Al, Fe3)2O4], and 2 magnetites (nearly Fe3O4) were characterized using optical spectroscopy and electron microprobe analysis. The magnetic transition in Fe3O4 is manifest as a lambda curve in 1/D, but otherwise, D decreases with increasing T and approaches a constant (Dsat) 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 Dsat 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-Mg2SiO4 and ringwoodite. Pressure derivatives are provided by relationships such as ∂ln (klat)/∂P = ∂ln(KT)/∂P. Our results show that the phonon contribution to heat transport in Earth's transition zone is high, particularly for large proportions of ringwoodite.