Dunite (/ˈduːnaɪt,ˈdʌnaɪt/), also known asolivinite (not to be confused with the mineralolivenite), is anintrusiveigneous rock ofultramafic composition and withphaneritic (coarse-grained) texture. Themineral assemblage is greater than 90%olivine, with minor amounts of other minerals such aspyroxene,chromite,magnetite, andpyrope. Dunite is the olivine-richendmember of theperidotite group of mantle-derived rocks.
Dunite and other peridotite rocks are considered the major constituents of theEarth's mantle above a depth of about 400 km (250 mi). Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base ofophiolite sequences where slabs of mantle rock from asubduction zone have been thrust ontocontinental crust byobduction during continental orisland arc collisions (orogeny). It is also found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny. Dunite typically undergoes retrogrademetamorphism in near-surface environments and is altered toserpentinite andsoapstone.
The type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, andxenoliths may represent the refractory residue left after the extraction ofbasalticmagmas in theupper mantle. However, a more likely method of dunite formation in mantle sections is by interaction betweenlherzolite orharzburgite and percolating silicate melts, which dissolve orthopyroxene from the surrounding rock, leaving a progressively olivine-enriched residue.
Dunite may also form by the accumulation of olivine crystals on the floor of large basaltic orpicritic magma chambers. These "cumulate" dunites typically occur in thick layers inlayered intrusions, associated with cumulate layers ofwehrlite, olivinepyroxenite,harzburgite, and evenchromitite (a cumulate rock consisting largely of chromite). Small layered intrusions may be of any geologic age, for example, theTriassicPalisades Sill in New York and the largerEoceneSkaergaard complex in Greenland. The largest layered mafic intrusions are tens of kilometers in size and almost all areProterozoic in age, e.g. theStillwater igneous complex (Montana), theMuskox intrusion (Canada), and theGreat Dyke (Zimbabwe). Cumulate dunite may also be found in ophiolite complexes, associated with layers of wehrlite, pyroxenite, andgabbro.
Dunite was named by the Austrian geologistFerdinand von Hochstetter in 1859, afterDun Mountain nearNelson,New Zealand.[1] Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surfaceweathering thatoxidizes the iron in olivine in temperate climates (weathering in tropical climates creates a deep red soil). The dunite from Dun Mountain is part of the ultramafic section of theDun Mountain Ophiolite Belt.
A massive exposure of dunite in the United States can be found asTwin Sisters Mountain, nearMount Baker in the northernCascade Range of Washington. In Europe it occurs in theTroodos Mountains ofCyprus. In southernBritish Columbia, Canada dunite rocks form the core of an ultramafic rock complex located near the small community ofTulameen. The rocks are locally enriched inplatinum group metals,chromite andmagnetite.
Dunite could be used tosequester CO2 and help mitigateglobal climate change via accelerated chemical rockweathering. This would involve the mining of dunite rocks in quarries followed bycrushing and grinding to create fine ground rock that would react with the atmosphericcarbon dioxide. The resulting products aremagnesite andsilica which could be commercialized.[2][3]
