X-Pot : Chengcheng

Thermal conductivity of MORB-melt bearing olivine aggregate and its implication on heat transfer at the core-mantle boundary

Résumé :

Subducting basaltic crust into deep mantle results in the continuous formation of the MORB-rich components at the core-mantle boundary (CMB). Seismic low-velocity regions indicate such crustal components may undergo partial melting forming melt-rich zones adjacent to the CMB. Such partially molten regions are expected to affect the core heat-flux and develop thermal instabilities at the base of the lower mantle. However, the thermal properties of the MORB melt-bearing silicate system at high pressure are rarely studied and poorly constrained. Here we conduct thermal diffusivity measurement on San Carlos olivine aggregate with various volume fractions of MORB glass (0, 0.1, 0.5, 1, 4, 8 vol %) at 1 GPa and to 1600 K using multi-anvil apparatus. All the thermal diffusivity profiles show a gradual decrease with temperature from 400 to 1200 K consistent with the phonon processes. When melting occurs, an obvious drop in thermal diffusivity is observed. However, the influence of MORB on thermal diffusivity of olivine aggregate does not follow a simple trend. Our result shows that thermal conductivity decreases significantly when the MORB fraction is below 1 vol. %, which is manifested by a sharp drop (40-70%) in thermal diffusivity above 1400 K. However, the thermal conductivity of the melt-bearing sample decreases when the melt fraction exceeds 1 vol. %. This crucial observation suggests that the low-degree melting (< 1 vol. %) at the CMB suppresses the heat transfer from core to the lower mantle.