Palaeogeothermal gradients derived from xenoliths in kimberlite

Abstract

GREAT problems exist in the determination of an ancient geothermal gradient. An attempt has been made by Boyd¹, based primarily upon the equilibration conditions of coexisting pyroxenes in garnet lherzolite xenoliths from the Thaba Putsoa, Letseng and Mothae kimberlite intrusions in northern Lesotho. He derived the equilibration temperature of the xenoliths from the amount of enstatite in solid solution in the diopside², and estimated the pressure from the amount of Al₂O₃ in the orthopyroxene³. He argued that, for a group of lherzolite xenoliths, a line joining a series of pyroxene-derived pressure/temperature points should define the geothermal gradient in the mantle below northern Lesotho during the Cretaceous when the xenoliths were entrained by the kimberlite during its ascent. He found a Cretaceous gradient similar to that in present-day shield areas between depths of 100 and 150 km, though from 150 to 200 km depth it was considerably steeper. Moreover, whereas the lherzolites outlining the shallower ‘normal’ part of the gradient are of granular texture, those outlining the high-temperature, steeper and deeper segment exhibit a range of deformation and recrystallisation textures (the porphyroclastic and mosaic textures in the classification scheme of Bouiller and Nicolas⁴).