Komatiite is a rare rock because volcanic material that comes direct from the mantle is not common, although some examples can be found in ancient Archean rocks . La komatiite est une roche volcanique ultramafique (ou ultrabasique) à olivine et pyroxène.
Spinifex is a spectacular bladed olivine or pyroxene texture that characterizes komatiite, a rock almost exclusively restricted to the Archean; varioles are cm-scale leucocratic globular structures abundant in many Archean basalts. Nevertheless, komatiites account for about 10-14% of the world’s annual nickel production. Nature rarely has sharp boundaries and the classification and naming of rocks often impose what appears to be sharp boundary names onto a continuous spectrum. They were deposited in a layered system with from the top of the flow down: They formed large, mostly submerged oceanic plateaus (Polat and Kerrich, 2000). Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content).
Komatiites and variolitic basalts are widespread in Archean volcanic sequences. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material. The moon went from completely molten to a separate anorthosite crust and olivine/pyroxene at the base, equivalent to the Hadean earth. Komatiites are excellent indicators of the early composition and development of the mantle. The 3 major groups of the Ventersdorp Supergroup represent three unconformity bound units (Table 1): The basal, c. 1.5-2 km thick, flood basalts and komatiites are summarized as the Klipriviersberg Group.
In volcanic environments which permit magma mixing, such as the Kurrajong lava lake, localized mixing of chromite-saturated melts at different temperatures can give rise to local supersaturation of chromite, and development of thin chromite-rich ‘seams’ analogous to those in layered intrusions (Irvine, 1975). First discovered in the continental shield of Africa in the late 1960s, komatiites are considered to be the extrusive equivalent of peridotites; i.e., the metamorphosed remnants of Archean and Proterozoic high Mg, low Si, high temperature and low viscosity lava flows.
Dacite: a common volcanic rock composed of quartz and sodic plagioclase, along with small amounts of biotite and/or hornblende, and/or pyroxene. Proportions of komatiites in volcanic packages of greenstone belts as a function of age (modified after de Wit and Ashwal 1997, and Condie 2001). Geochemistry of komatiites from the Tipasjärvi, Kuhmo, Suomussalmi, Ilomantsi and Tulppio greenstone belts, Finland: Implications for tectonic setting and Ni sulphide prospectivity contrast, komatiites in the Kalgoorlie Terrane erupted at c. 2.7 Ga (Nelson, 1997) and are typically associated with felsic volcanic rocks or tholeiitic basalts (Barnes and Fiorentini, 2012); the majority of them have Al-undepleted compositions (Barnes, 2006). In this standard usage, “komatiite” is the ultramafic equivalent of the mafic “basalt.” However, it is widely recognized that lavas of mafic composition exist, usually intercalated with true komatiites containing spinifex textures and showing clear evidence of derivation from true komatiite parent magmas.