Ultramafic rock

Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are

Intrusive ultramafic rocks are often found in large, layered

Ultramafic tuff is extremely rare. It has a characteristic abundance of olivine or serpentine and a scarcity or absence of feldspar and quartz. Rare occurrences may include unusual surface deposits of maars of kimberlites in the diamond fields of southern Africa and other regions.

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

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 than typical ultramafic 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 CO₂ (carbon dioxide).

When such metamorphic fluids have less than 10% molar proportion of CO₂, reactions favor serpentinisation, resulting in

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.

Soil and regolith

Often thick,

Weathered ultramafic rocks may form lateritic nickel ore deposits.^[4][5]

Other celestial bodies

Io

Ultramafic lava may have been detected on

Mercury

Mercury appears to have ultramafic volcanic rock.^[7]

See also

• Chromitite
• Magnetite
• Kambalda type komatiitic nickel ore deposits
• Fractional crystallization (geology)
• Volcanism on Io, a moon of Jupiter

References