Evidence for a very thick Kaapvaal craton root: Implications for equilibrium fossil geotherms in the early continental lithosphere

The presence of diamonds Archaean age within the cratonic lithosphere is surprising because their formation implies a cool early mantle despite Earth's hotter state. There remains widely (albeit not exclusively) held concept that prevailing geothermal gradient cratons was much different from today. This ‘Archaean paradox’ supported by pressure and/or temperature (PT) estimates potentially ancient diamond inclusion (DI) falling close to modern geotherms. Solutions paradox include more effective heat loss mechanisms (e.g. pipes), deeper lithospheric roots or forming in disequilibrium Here we demonstrate effects thermally evolving Earth on equilibrium geotherms Kaapvaal craton South Africa. We modelled present-day conditions backward time 3.0 Ga at thermal adjusting three dominant variables: (i) time-corrected crustal production; (ii) secularly cooling potential (Tp) underlying convecting mantle; and, for illustrative purposes, (iii) variable depth lithosphere-asthenosphere boundary (LAB) flow LAB. Computed model were compared DI PT data craton, as well recorded xenoliths age. It demonstrated cannot be reconciled with nor those Proterozoic if either has remained constant thickness basal flux since Ga. Rather, provided reflect equilibration along fossil geotherms, it required substantially thicker than what preserved inferred former could have reached up ∼350 km Tp encapsulation high and they are If, however, asthenosphere modest formed later LAB depths shallower (∼250-275 km), but still present day. Although only necessary constraints this exercise, fundamental nature radioactive production rate mean our findings imply continental general attained its maximal vertical extent been variably eroded weakened time.