Ultramafic rocks (also referred to asultrabasic rocks, although the terms are not wholly equivalent) areigneous andmeta-igneous rocks with a very lowsilica content (less than 45%), generally >18%MgO, highFeO, lowpotassium, and are usually composed of greater than 90%maficminerals (dark colored, highmagnesium andiron content). TheEarth'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 ascarbonatites andultrapotassic igneous rocks.
Intrusive ultramafic rocks are often found in large, layeredultramafic intrusions wheredifferentiated rock types often occur in layers.[1] Suchcumulate rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include thedunites,peridotites andpyroxenites. Other rare varieties includetroctolite which has a greater percentage of calcic plagioclase. These grade into theanorthosites.Gabbro andnorite often occur in the upper portions of the layered ultramafic sequences.Hornblendite and, rarelyphlogopite, are also found.
Volcanic ultramafic rocks are rare outside of theArchaean and are essentially restricted to theNeoproterozoic or earlier.Subvolcanic ultramafic rocks anddykes persist longer, but are also rare. There is evidence of ultramafic rocks elsewhere in the solar system.
Examples includekomatiite[2] andpicritic basalt. Komatiites can be host toore deposits ofnickel.[3]
Ultramafictuff is extremely rare. It has a characteristic abundance ofolivine orserpentine and a scarcity or absence offeldspar andquartz. Rare occurrences may include unusual surface deposits ofmaars ofkimberlites in thediamond fields of southern Africa and other regions.
Technicallyultrapotassic rocks andmelilitic 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 aslamprophyre,lamproite andkimberlite 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 asdikes,diatremes,lopoliths orlaccoliths, and very rarely, intrusions. Most kimberlite and lamproite occurrences occur asvolcanic and subvolcanic diatremes andmaars; lavas are virtually unknown.
Vents ofProterozoic lamproite (Argyle diamond mine), andCenozoic lamproite (Gaussberg,Antarctica) are known, as are vents ofDevonian lamprophyre (Scotland). Kimberlite pipes in Canada, Russia and South Africa have incompletely preservedtephra andagglomeratefacies.
These are generallydiatreme events and as such are not lava flows although tephra andash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramaficchemistry via a different process than typical ultramafic rocks.
Metamorphism of ultramafic rocks in the presence ofwater and/orcarbon dioxide results in two main classes of metamorphic ultramafic rock;talc carbonate andserpentinite.
Talc carbonation reactions occur in ultramafic rocks at lowergreenschist through togranulite 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 inchlorite-serpentine-amphibole type assemblages.
The majority of ultramafic rocks are exposed inorogenic belts, and predominate inArchaean andProterozoic terranes. Ultramafic magmas in thePhanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.[citation needed]
Many surface exposures of ultramafic rocks occur inophiolite complexes where deep mantle-derived rocks have beenobducted ontocontinental crust along and abovesubduction zones.
Serpentine soil is a magnesium rich, calcium, potassium and phosphorus poor soil that develops on theregolith derived from ultramafic rocks. Ultramafic rocks also contain elevated amounts of chromium and nickel which may be toxic to plants. As a result, a distinctive type ofvegetation develops on these soils. Examples are the ultramaficwoodlands and barrens of theAppalachianmountains andpiedmont, the "wetmaquis" of theNew Caledonia rain forests, and the ultramaficforests ofMount Kinabalu and other peaks inSabah,Malaysia. Vegetation is typically stunted, and sometimes includesendemic species adapted to the soils.
Often thick,magnesite-calcretecaprock,laterite andduricrust forms over ultramafic rocks intropical andsubtropical environments. Particularfloral assemblages associated with highly nickeliferous ultramafic rocks are indicative tools formineral exploration.
Weathered ultramafic rocks may formlateritic nickel ore deposits.[4][5]
Lichen communities on ultramafic rocks show distinctive characteristics, including the unusual co-presence of species that typically grow on eitheracidic 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 displayxerophytic characteristics and may include species withdisjunct distribution patterns. The weathering action of lichens on ultramafic rocks can promotebiogeochemical processes, including the complete depletion of magnesium from serpentine minerals beneath lichenthalli and the formation ofsecondary minerals common in serpentine soils.[6]
Ultramafic lava may have been detected onIo, a moon ofJupiter, becauseheat-mapping of Io's surface found ultra-hot areas with temperatures in excess of 1,200 °C (2,190 °F). Themagma immediately below these hot spots is probably about 200 °C (360 °F) hotter, based on surface-to-subsurface temperature differences observed for lava on Earth. A temperature of 1,400 °C (2,550 °F) is thought to indicate the presence of ultramafic magma.[7][better source needed]
Mercury appears to have ultramafic volcanic rock.[8]
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