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Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous rock and meta-igneous rocks with a very low silica content (less than 45%), generally >18% magnesium oxide, high FeO, low potassium, and are usually composed of greater than 90% mafic (dark colored, high magnesium and iron content). Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as and ultrapotassic igneous rocks.
Examples include komatiiteHill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. Physical volcanology of komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia., Geological Society of Australia. and Picrite. Komatiites can be host to ore deposits of nickel.Lesher, C.M., Arndt, N.T., and Groves, D.I., 1984, Genesis of komatiite-associated nickel sulfide deposits at Kambalda, Western Australia: A distal volcanic model, in Buchanan, D.L., and Jones, M.J. (Editors), Sulphide Deposits in Mafic and Ultramafic Rocks, Institution of Mining and Metallurgy, London, p. 70–80.
Ultrapotassic, ultramafic igneous rocks such as lamprophyre, lamproite and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved.
Most of these rocks occur as dikes, , or , and very rarely, intrusions. Most kimberlite and lamproite occurrences occur as volcanic and subvolcanic diatremes and ; lavas are virtually unknown.
Vents of Proterozoic lamproite (Argyle diamond mine), and Cenozoic lamproite (Gaussberg, Antarctica) are known, as are vents of Devonian lamprophyre (Scotland). Kimberlite pipes in Canada, Russia and South Africa have incompletely preserved tephra and agglomerate facies.
These are generally diatreme events and as such are not lava flows although tephra and Volcanic ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process than typical ultramafic rocks.
Talc carbonation reactions occur in ultramafic rocks at lower greenschist through to granulite facies metamorphism when the rock in question is subjected to metamorphism and the metamorphic fluid has more than 10% molar proportion of CO2 (carbon dioxide).
When such metamorphic fluids have less than 10% molar proportion of CO2, reactions favor serpentinisation, resulting in chlorite group-Serpentine group-amphibole type assemblages.
Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obduction onto continental crust along and above subduction zones.
Often thick, magnesite-calcrete caprock, laterite and duricrust forms over ultramafic rocks in tropical and subtropical environments. Particular floral assemblages associated with highly nickeliferous ultramafic rocks are indicative tools for mineral exploration.
Weathered ultramafic rocks may form lateritic nickel ore deposits.Golightly, J.P. (1981): Nickeliferous Laterite Deposits. Economic Geology 75, pp. 710–735Schellmann, W. (1983): Geochemical principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, São Paulo, pp. 119–135
Lichen communities on ultramafic rocks show distinctive characteristics, including the unusual co-presence of species that typically grow on either acidic rock or calcium-rich rocks, due to the rocks' unique chemical composition. While some lichen species appear to be characteristic of ultramafic environments within specific geographical regions, very few species are found exclusively on these rocks. Studies have shown that lichen communities on ultramafic rocks can be more diverse than those on adjacent mafic rocks, with some localities showing notably higher species counts on serpentinites compared to other rock types. These communities often display xerophyte characteristics and may include species with disjunct distribution patterns. The weathering action of lichens on ultramafic rocks can promote biogeochemistry processes, including the complete depletion of magnesium from serpentine minerals beneath lichen thallus and the formation of secondary minerals common in serpentine soils.
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