A new approach to computing steady-state geotherms: The marginal stability condition

•CBL marginal stability condition is used instead of the heat flow boundary condition.•Predicted thickness of suboceanic thermal boundary layer agrees with LAB observations.•A MSC-geotherm deep segment is not affected by crustal heat production uncertainty.•Model explains a xenolith-based heat flow vs. age/lithosphere thickness correlation.

A necessary physical condition for steady-state mantle convection is the marginal stability of convective boundary layer (CBL) accommodating the transition from conductive lithosphere to convective mantle. We incorporate the marginal stability condition (MSC) of the CBL into the lithosphere thermal modeling using it instead of the heat flow boundary condition specified on the surface. For the oceanic region, the MSC-based approach allows to calculate rather than postulate the thickness of the oceanic lithosphere beneath old oceanic crust areas. The model allows to estimate the potential temperature and to predict the depth at which the suboceanic CBL base occurs. The latter agrees well with the seismologically observed lithosphere-asthenosphere boundary. In the continental region, the CBL is immediately adjacent to the base of the chemical boundary layer comprising a crust and a melt-depleted continental keel. A deep segment of the MSC-based continental geotherm is almost independent of the uncertainty of the crustal heat production in the sense that two geotherms corresponding to the same lithosphere thickness and potential temperature but different crustal heat production converge at depth. Besides, the solution may be additionally adjusted so that the calculated surface heat flow matches observations without affecting the properties of the deeper geotherm segment. The model predicts quantitative relations between the chemical boundary layer thickness, the potential temperature of convecting mantle, and the lithospheric geotherm. The predictions correlate with the lithospheric geotherms documented using the kimberlite xenolith/xenocryst thermobarometry.

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