A revised estimation of the steady‐state geotherm for the continental lithosphere and its implication for mantle melting

Abstract

ABSTRACTAn analytical solution has been derived for the steady‐state geotherm of the continental lithosphere, using an empirical thermal conductivity model that incorporates the experimentally observed temperature and pressure effect. Based on recent global compilations of crustal thickness and heat flow data, a standard continental lithosphere is re‐defined by a global mean model with total crustal thickness of 40 km and surface heat flow of 55 mWm‐2 (within which 28 mWm‐2 is assumed to be derived from deep mantle source). The thickness of the continental lithosphere (≅125 km), consistent with previous models, is given by the depth at which the geotherm intersects the potential asthenosphere temperature of 1280°C. It is shown that the new steady‐state geotherm is much hotter than that based on the previously adopted model where material thermal conductivity is assumed to be constant (≅3.14 W/m/k) throughout the lithosphere. The consequence of this re‐evaluation of pre‐extension thermal structure in the lithosphere is that the minimum stretching factor required to cause the onset of dry mantle partial melting can be 15–20% lower than the previous estimate. Also, if minor amounts of water or other volatile element or dry basalt are present in the upper mantle, melting of the subcontirfental mantle is very likely to occur for any geotherms constructed using surface heat flows > 30 mWm‐2.