Editing Dunite (section)

{{Short description|Ultramafic and ultrabasic rock from Earth's mantle which is made of the mineral olivine}}
{{Distinguish|Dunnite}}
[[File:Basanite&Dunite bomb.JPG|thumb|right|upright=1.2|Small [[volcanic bomb]] of (black) [[basanite]] with (green) dunite]]

'''Dunite''' ({{IPAc-en|ˈ|d|uː|n|aɪ|t|,_|ˈ|d|ʌ|n|aɪ|t}}), also known as '''olivinite''' (not to be confused with the mineral [[olivenite]]), is an [[intrusive rock|intrusive]] [[igneous rock]] of [[ultramafic]] composition and with [[phaneritic]] (coarse-grained) texture. The [[mineral]] assemblage is greater than 90% [[olivine]], with minor amounts of other minerals such as [[pyroxene]], [[chromite]], [[magnetite]], and [[pyrope]]. Dunite is the olivine-rich [[endmember]] of the [[peridotite]] group of mantle-derived rocks.

Dunite and other peridotite rocks are considered the major constituents of the [[Earth's mantle]] above a depth of about {{cvt|400|km|mi}}. Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of [[ophiolite]] sequences where slabs of mantle rock from a [[subduction zone]] have been thrust onto [[continental crust]] by [[obduction]] during continental or [[island 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 retrograde [[metamorphism]] in near-surface environments and is altered to [[serpentinite]] and [[soapstone]].

[[File:Peridotite Olivine-Orthopyroxene-Clinopyroxene Dunite highlighted.svg|thumb|upright=1.4|The dunite field is highlighted in green.]]

The type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, and [[xenolith]]s may represent the refractory residue left after the extraction of [[basalt]]ic [[magma]]s in the [[upper mantle (Earth)|upper mantle]]. However, a more likely method of dunite formation in mantle sections is by interaction between [[lherzolite]] or [[harzburgite]] 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 or [[picrite|picritic]] magma chambers. These "[[cumulate rock|cumulate]]" dunites typically occur in thick layers in [[layered intrusion]]s,  associated with cumulate layers of [[wehrlite]], olivine [[pyroxenite]], [[harzburgite]], and even [[chromitite]] (a cumulate rock consisting largely of chromite). Small layered intrusions may be of any geologic age, for example, the [[Triassic]] [[Palisades Sill]] in New York and the larger [[Eocene]] [[Skaergaard complex]] in Greenland. The largest layered mafic intrusions are tens of kilometers in size and almost all are [[Proterozoic]] in age, e.g. the [[Stillwater igneous complex]] (Montana), the [[Muskox intrusion]] (Canada), and the [[Great Dyke]] (Zimbabwe). Cumulate dunite may also be found in ophiolite complexes, associated with layers of wehrlite, pyroxenite, and [[gabbro]].

Dunite was named by the Austrian geologist [[Ferdinand von Hochstetter]] in 1859, after [[Dun Mountain]] near [[Nelson, New Zealand|Nelson]], [[New Zealand]].<ref>{{cite journal |url=http://sp.lyellcollection.org/content/287/1/375.abstract |last=Johnston |first=M. R. |title=Nineteenth-century observations of the Dun Mountain Ophiolite Belt, Nelson, New Zealand and trans-Tasman correlations |journal=Geological Society, London, Special Publications |publisher=[[Geological Society of London]] |year=2007 |volume=287 |issue=1 |pages=375–87|doi=10.1144/SP287.27 |bibcode=2007GSLSP.287..375J |s2cid=129776536 |url-access=subscription }}</ref> Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surface [[weathering]] that [[oxidation|oxidizes]] 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 the [[Dun Mountain Ophiolite Belt]].

A massive exposure of dunite in the United States can be found as [[Twin Sisters Mountain]], near [[Mount Baker]] in the northern [[Cascade Range]] of Washington. In Europe it occurs in the [[Troodos Mountains]] of [[Cyprus]]. In southern [[British Columbia]], Canada dunite rocks form the core of an ultramafic rock complex located near the small community of [[Tulameen]]. The rocks are locally enriched in [[platinum]] group metals, [[chromite]] and [[magnetite]].