The emplacement of class 1 kimberlites

Abstract Class 1 kimberlites are distinct from Class 2 and Class 3, in that they are characterized by three zones, namely; root, diatreme and crater. Class 2 and 3 kimberlites do not form diatreme zones filled with diatreme-facies rocks. Consequently these kimberlites probably form by different processes. Within the root zones of Class 1 kimberlites, subvolcanic contact breccias occur at particular horizons within the immediate wall rocks. These could be formed by hydraulic fracturing caused by crystallization-induced exsolution of juvenile volatiles within the cooling hypabyssal kimberlite. This exsolution may subsequently also lead to the formation of transitional-facies kimberlites in the uppermost parts of extended columns or plugs of hypabyssal kimberlite. From such settings, break through to surface and explosive eruption is thought to occur and pipes having characteristic diatremes slope angles of 82° and 500–700 m deep craters are formed. Through these processes these kimberlites undergo fundamental textural changes to produce a variety of rocks from hypabyssal-, through transitional-, to diatreme- and crater-facies kimberlites.