IGNEOUS ROCKS/Kimberlite 255 and 25% are super-deep (ferropericlase, magnesiumperovskite, nickel) The super-deep suite of minerals represents the mantle at depths greater than 670 km Upper Mantle Configuration Figure 12 Temperature ranges of various inclusions and ‘nodules’, normal peridotites, and the megacrysts suite (Repro duced from Dawson JB (1980) Kimberlites and Their Xenoliths Berlin, Heidelberg, New York: Springer Verlag.) Union), which are coarse assemblages of garnet and dark green spinel They are interpreted as early dense cumulates of an aluminous mantle under pressures equivalent to a depth of 70 km Megacrysts Single large crystals (more than cm) are common in kimberlites; they are alternatively referred to as discrete nodules Two series are recognized: chromerich and chrome-poor Such megacrysts may contain other minerals There are also diopside, mica, and zircon megacrysts It has been suggested that some megacrysts are genetically linked to the kimberlite, but such a relationship has been rejected by some authors, who consider the megacrysts to be accidental mantle-derived inclusions The minerals found in kimberlites and their origins are summarized in Table Studies of Diamonds and Their Inclusions Nowadays many studies are carried out on diamonds in kimberlites and the inclusions within them, in an attempt to determine the pressure and temperature (i.e mantle depth) at which they were formed In a study of the diamonds from Pipe DO-27, Lac de Gras, Slave Craton, North-west Territories, Canada, a high proportion were found to be cubo-octahedral stones with resorption characteristics Syngenetic inclusions are of three suites: 25% are peridotitic (garnet, clinopyroxene, sanidine, sulphide); 50% are eclogitic (garnet, olivine, orthopyroxene, sulphide); The Earth’s upper mantle is petrologically complex Xenoliths and xenocrysts from kimberlites provide a wealth of high-temperature and high-pressure rock types therefrom, but the problem is to delineate where in the mantle they originated and how they relate to each other genetically Two main zones – the harzburgite zone and the garnet lherzolite zone – can be recognized, but these exhibit large- and small-scale heterogeneities, represented by pyroxenites, eclogites, and the MARID suite of rocks These differences probably represent both lateral and vertical heterogeneities in the mantle Shearing and metasomatism are also evident; brittle and plastic deformation accompany magma rise and open up channels Convective processes may also operate A tentative model for the upper mantle and crust derived from studies of kimberlite xenoliths and xenocrysts is shown in Figure 13 The super-deep inclusions in diamonds, mentioned above, represent a further zone below 670 km This field of research is on going, and each kimberlite conference produces new evidence and models, without necessarily clarifying the picture, such are the contradictions and differing interpretations of the evidence Diamond Provenance There are several models for the origin of the diamonds in kimberlites Since they are found within eclogites of established mantle provenance, it is reasonable to suppose that they come from the mantle, where high pressures and temperatures occur Current models of diamond formation differ mainly in the source of the carbon Juvenile methane or other hydrocarbons, oxidized during ascent through the upper mantle or at the lower boundary of the lithosphere, is favoured by one school This is supported by the presence of peridotite inclusions in some diamonds Other models invoke crystallization from kimberlite liquids or from ultrabasic melts during the formation of cratonic roots Another set of models introduces the carbon to the mantle by subduction processes, the carbon being not juvenile and possibly biogenic in origin This model is supported by the existence of diamonds containing inclusions of the eclogite suite It seems likely, however, that several diamond-forming processes operate Regardless of the model favoured, it is generally accepted that most diamonds are xenocrysts in the transporting kimberlite magma Disruption and disaggregation of diamond-bearing horizons by the passage of magma