260 IGNEOUS ROCKS/Komatiite Exploration Nedlands: University of Western Australia, Geology Department and Extension Service Gurney JJ, et al (eds.) (1999) The J B Dawson Volume: the P H Nixon Volume Proceedings of the 7th International Kimberlite Conference April 1998, Cape Town Red Roof Design Janse AJA (1995) A history of diamond sources in Africa: part I Gems and Gemmology 31: 228 255 Janse AJA (1998) A history of diamond sources in Africa: part II Gems and Gemmology 32: 30 Jaques AL, Lewis JD, and Smith CB (1986) The kimberlites and lamproites from Western Australia Geological Survey of Western Australia Bulletin 132: 268 Kornprobst T (ed.) (1984) Kimberlites and Related Rocks Proceedings of the 3rd International Kimberlite Confer ence Amsterdam, Oxford, New York, Tokyo: Elsevier Mitchell RH (1995) Kimberlites, Orangeites and Related Rocks New York: Plenum Press Mitchell RH and Bergman SC (1991) Petrology of Lam proites New York: Plenum Press Nixon PH (ed.) (1973) Lesotho Kimberlites Lesotho National Development Corporation Nixon PH (1980) The morphology and mineralogy of dia mond pipes In: Glover JE and Groves DI (eds.) Kimber lites and Diamonds, pp 32 47 Nedlands: University of Western Australia, Geology Department and Extension Service Nixon PH (1980) Regional diamond exploration: theory and practice In: Glover JE and Groves DI (eds.) Kimber lites and Diamonds, pp 64 75 Nedlands: University of Western Australia, Geology Department and Extension Service Sobolev NV (1977) Deep Seated Inclusions in Kimberlite and the Problem of the Upper Mantle Washington: American Geophysical Union Komatiite N T Arndt, LCEA, Grenoble, France C M Lesher, Laurentian University, ON, Canada ß 2005, Elsevier Ltd All Rights Reserved Introduction It is easy to explain roughly what a komatiite is but difficult to give a rigorous definition The simple definition, as given by Arndt and Nisbet in 1982; is that komatiite is an ultramafic volcanic rock A limit of 18% MgO separates komatiites from less magnesian volcanic rocks such as picrites, ankaramites, or magnesian basalts The term komatiitic basalt is applied to volcanic rocks containing less than 18% MgO that can be linked, using petrological, textural, or geochemical arguments, to komatiites The definition seems to include intrusive rocks; this was probably not intended Implicit to the definition of komatiite is the notion – difficult to prove – that komatiites crystallize from liquids that contained more than about 18% MgO Complications arise from the existence of other MgO-rich volcanic rocks that either formed through the accumulation of olivine from less magnesian liquids, or crystallized from magmas with geochemical characteristics quite unlike those of most komatiites An example of the first type is a phenocryst-charged basaltic liquid (a picrite according to some definitions); an example of the second is meimechite, a rare alkaline ultramafic lava (see Lava) with unusual major and trace element composition To distinguish komatiite from other types of highly magnesian volcanic rock, it is useful to include spinifex texture in the definition (Figure 1A, B) Spinifex, a texture characterized by the presence of large skeletal or dendritic crystals of olivine or pyroxene, is present in many, but not all komatiite flows A workable definition of komatiite should include the phrase ‘‘komatiite is an ultramafic volcanic rock containing spinifex or related to lavas containing this texture’’ With the last part of the definition, we can make allowance for the manner in which texture varies within komatiitic units For example, many komatiite flows have an upper spinifex-textured layer and a lower olivine-cumulate layer (Figure 1C); and other flows grade along strike from layered spinifex-textured portions to massive olivine-phyric units With the inclusion of the phrase about spinifex, the lower olivine-cumulate portions of layered flows or the olivine-phyric units can also be described as komatiite On the other hand, meimechites, picrites, and other rock types that contain no spinifex are excluded For further discussion, see the Further Reading section at the end of this article Because of the facility with which olivine fractionates or accumulates in low-viscosity ultrabasic liquids, the compositions of komatiite lavas vary considerably MgO contents range from 18% (the limit between komatiite and komatiitic basalt), to as high as 50% in the lower cumulate portions of layered flows (Figure 2) Elements that are immobile during the metamorphism and hydrothermal alteration, which affects all komatiites to a greater or lesser extent,