Ultramafic rock

Ultramafic (also referred to as ultrabasic) rocks are igneous and meta-igneous rocks with 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). The Earth's mantle is considered to be composed of ultramafic rocks.

Intrusive ultramafic rocks

Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers [Ballhaus, C.G. & Glikson, A.Y., 1995, Petrology of layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block, central Australia. AGSO Journal, 16/1&2: 69-90.] . Such cumulate rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include the dunites, peridotites and pyroxenites. Other rare varieties include troctolite which has a greater percentage of calcic plagioclase. These grade into the anorthosites. Gabbro and norite often occur in the upper portions of the layered ultramafic sequences. Hornblendite and, rarely phlogopitite are also found.

Volcanic ultramafic rocks

Volcanic ultramafic rocks are rare outside of the Archaean and are essentially restricted to the Neoproterozoic or earlier, although some boninite lavas currently erupted within back-arc basins (Manus Trough, Philippines) verge on being ultramafic. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on Io may be ultramafic, as evidenced by their temperatures which are higher than terrestrial mafic eruptions.

Examples include komatiite [Hill 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. ISBN 0-909869-55-3] and picritic basalt. 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 rocks

Technically ultrapotassic rocks and melilitic rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO. which can be considered "ultramafic".

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 dykes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lampproite occurrences occur as volcanic and subvolcanic diatremes and maars; 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 ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process to typical ultramafic rocks.

Carbonatites are rare high-carbonate, low-silica igneous rocks.

Metamorphic ultramafic rocks

Metamorphism of ultramafic rocks in the presence of water and/or carbon dioxide results in two main classes of metamorphic ultramafic rock; talc carbonate and serpentinite.

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 carbon dioxide.

When the metamorphic fluids in contact with the ultramafic rock have less than 10% CO2 the metamorphic reactions favor serpentinisation reactions, resulting in chlorite-serpentine-amphibole type assemblages.

Distribution in space and time

The majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. Ultramafic magmas in the Phanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.

Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones.

Ultramafic rocks and the regolith

Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as calcium, potassium and phosphorus) are exposed on the surface, the high metal content of the rocks creates unique vegetation. Examples are the ultramafic woodlands and ultramafic barrens of the Appalachian mountains and piedmont, the "wet maquis" of the New Caledonia rain forests, and the ultramafic forests of Mount Kinabalu and other peaks in Sabah, Malaysia. Vegetation is typically stunted, and is sometimes home to endemic species adapted to the metallic soils.

Often thick, magnesite-calcrete caprock, clayey 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, 710-735] [Schellmann, W. (1983): Geochemical principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, Sao Paulo, 119-135]

See also

* Ultramafic rock types: Peridotite, dunite, norite, essexite, komatiite.
* Cumulate rocks and rock types: chromitite, magnetite, anorthosite
* Ultramafic-associated ore deposits: Lateritic nickel ore deposits, kambalda type komatiitic nickel ore deposits, diamond
* Kimberlite, lamproite, lamprophyre
* Ophiolite
* Ultramafic to mafic layered intrusions
* Igneous differentiation, fractional crystallisation

References