Diamond-the ultimate mantle mineral

摘要

At room temperature diamond is metastable, technically requiring a pressure of about 1.3 GPa (13 kbar) for full stability. This fact should not be a cause for concern, because the youngest economically diamondiferous kimberlite (the volcanic rock which brings diamond to the surface), at Mwadui, in Tanzania, is 20 m.y. (see Stachel and coworkers) and the diamonds from Premier mine in South Africa, which are particularly beautiful, have an eruption age of 1180 m.y.The majority of diamonds form in the keels of cratons, ancient landmasses whose ages often exceed 2500 m.y. Diamond formation is possible because these ancient land-masses are exceptionally thick (up to 200 km) and the isotherms within them are initially perturbed to greater depths. Towards the base of the craton this relationship causes the stability field of diamond to be breached. Typical minimum P-T conditions are 4.5 GPa and 1100 deg C. From the syngenetic silicate, oxide and sulphide inclusions in diamond formed under these conditions two principal growth environments can be identified. These environments are named after the minerals associated with two upper mantle rock types, peridotite, hence peridotitic (P-type), and eclogite, hence eclogitic (B-type). Other minor environments also exist, such as websteritic (W-type). The mineralogy and chemical compositions of these parageneses are well documented; a review by the first author also provides information on inclusion abundances within diamonds from mines around the world, inclusion geothermobarometry, as well as carbon isotopes and nitrogen aggregation systematics (see also Navon).