Geophysical implications of reduction in thermal conductivity due to hydration

Laser‐flash measurements show that hydration lowers the lattice component of thermal diffusivity (D) for calcic and Fe‐Mg garnets, and glasses. Hydration depresses D above the glass transition, suggesting that melts respond likewise. For garnets, which have a wide range of total water and OH− contents (X), ∂(lnD)/∂X = −0.003%/ppm H2O. For olivine and quartz, D is constant, but thermal conductivity (k = ρCPD) decreases because hydration lowers density (ρ) but negligibly changes heat capacity (CP). This finding is geophysically significant: specifically, it suggests that hydration leads to heat retention, promoting convective instabilities and melting.