"Geophysical implications of new measurements and
models of thermal transport from crust to core"

Dr. Anne Hofmeister

Washington University in St. Louis

 

Contact-free, laser-flash analysis (LFA) accurately (±2%) measures lattice thermal diffusivity (D) at high temperature (T). Conventional measurements of minerals underestimate D by ~20% near 298 K due to interface resistance, although simultaneously existing direct radiative transfer artificially elevates D as T rises. Available pressure (P) determinations possess these and other problems.  However, reproduced values of dD/dP agree with the damped harmonic oscillator model. Data on rocks have these errors and additional problems such as compression of pore space. LFA results on minerals, glasses, melts and rocks are internally consistent and show that heat transport is generally more efficient than currently viewed. The new data pertain to diverse processes in the earth. For example, melts impede transport of heat, providing feedback loops and runaway melting in orogenic belts as well as mid-ocean ridges.

As regards the deep Earth, models combined with new LFA data on perovskite compounds show that lattice thermal conductivity (klat) is high and independent of T, increasing from 7.5 to 30 W/m-K (±25%) across the lower mantle (LM) due to compression. Diffusive radiative transfer is estimated from a recent model: For expected fine grain-size, spectral characteristics do not play a strong role, indicating that krad increases from ~1 to ~5 W/m-K across the LM, estimated from olivine spectra.

Although greater accuracy through improved measurements is needed, these results demonstrate that the LM is an efficient conductor of heat. Even a low, adiabatic temperature gradient can carry the power inferred to run the dynamo, suggesting that mantle convection is limited to above 670 km.