The Benfontein kimberlite sills; magmatic reactions and high intrusion temperatures

Abstract

The kimberlite sills at Benfontein, South Africa, were intruded as xenolith-poor, phenocryst-rich carbonated peridotite magmas. The dominant pre-intrusion phenocryst phases are olivine (Fo (sub 86-92) ), Mg-ilmenite ss (13.0-18.0 wt percent MgO, 0.5-3.0 wt percent Cr 2 O 3 ), Mg-Al-chromite, and REE-perovskite. Post-intrusion magmatic reaction mantling has encapsulated all pre-existing groundmass oxides in Mg-Al-titanomagnetite. The Benfontein sills appear to have been emplaced at much higher temperatures than are probably typical for most other kimberlites. In a high heat flow environment, comparable to that recorded by the Lesotho geotherm, under open system vapor composition conditions, an adiabatic diapiric upwelling of peridotitic magma may be capable of producing the high temperatures that apparently prevailed during the intrusion. The temperature of reaction for coupled Fe-based and C-based fO 2 buffers in the upper mantle (for example, EMOG-MH (hematite mol percent isopleths)) increases with decreasing pressure and at near-surface pressures approximates the temperature of intrusion for the phase compositions observed at Benfontein.--Modified journal abstract.