THERMAL CONDUCTIVITY AND THERMODYNAMICS OF MAJORITIC GARNETS: IMPLICATIONS FOR THE TRANSITION ZONE

Abstract

Thermal conductivity at room temperature and pressure (k0), and heat capacity and total entropy as functions of temperature are calculated for majorite-pyrope garnets from new and previous infrared reflectivity and Raman scattering data. The thermodynamic properties, including the effect of cation disorder, vary by less than 4% over this binary, whereas k0 changes by >400% due to cation disorder. For mantle garnet compositions (~70 mol% majorite), k0 is ~2.3 W/m K, which is ~25% k0 of ringwoodite, the other major mineral in Earth's transition zone (TZ). The dynamics of the TZ is governed by its two dominant minerals having disparate values for k and viscosity, which parallels the situation in the lower mantle. If the TZ has substantial (60-70%) proportions of majoritic garnet, as suggested by petrological models, then heat flow therein is inhibited relative to the upper and lower mantles. Sub-adiabatic temperatures are implied, which is consistent with steep velocity gradients in the TZ. Association of low thermal conductivity with high temperatures should contribute to the buoyancy of the TZ layer, weaken it, and promote transformation to garnet: such destablization through positive feedback possibly provides a mechanism for catastrophic overturns of the TZ and upper mantle.