Home – Home – Jornals – Russian Geology and Geophysics 2004 number 9

The thermal conditions under which thermochemical plumes formed and ascended were analyzed. A thermochemical plume forms at the boundary of two layers, with heat flow coming from the lower layer. Locally, a chemical dope is supplied, which lowers the melting point near the base of the upper layer. As soon as the melting point becomes lower than the temperature at the boundary, the upper layer begins to melt and the plume ascends.
The physicochemical conditions at the core-mantle boundary at which a thermochemical plume forms were explored. Plausible reactions at core-mantle boundary involving H₂ and CH₄ are presented, which produce compounds reducing the melting point of the lower mantle. Probable composition and eutectic temperatures resulting from the reactions at the core-mantle boundary have been assessed.
General criteria and related equations have been deduced for heat and mass transfer of a thermochemical plume. Heat and mass transfer was analyzed near the base and roof of the plume. Formulas have been derived for the plume source heating power, source diameter, and mass flux of a chemical dope. The velocity and time of plume ascent as well as ultimate height of the plume have been determined. Basic equations have been derived to calculate the main plume parameters. Using these equations, we have determined patterns of temperature distribution and dope concentrations in the plume channel. We have also estimated plausible concentrations of a chemical dope at the base of the plume and a decrease in melting point owing to the dope. The minimum diameter, mean velocity and time of the plume ascent have been calculated, with physical properties of the lower mantle taken into account.