Peridotite is a dense, coarse-grained igneous rock, consisting mostly of the
minerals olivine and pyroxene. Peridotite is ultramafic, as the rock
contains less than 45% silica. It is high in magnesium, reflecting the high
proportions of magnesium-rich olivine, with appreciable iron. Peridotite is
derived from the Earth's mantle, either as solid blocks and fragments, or as
crystals accumulated from magmas that formed in the mantle. The compositions
of peridotites from these layered igneous complexes vary widely,
reflecting the relative proportions of pyroxenes, chromite, plagioclase, and
amphibole. Peridotite is the dominant rock of the
upper part of the Earth's mantle. The compositions of peridotite nodules found
in certain basalts and diamond pipes are of special interest, because they
provide samples of the Earth's mantle brought up from depths ranging from
about 30 km to 200 km or more. Some of the nodules preserve isotope ratios of
osmium and other elements that record processes that occurred when the earth
was formed, and so they are of special interest to paleogeologists because they
provide clues to the early composition of the Earth's mantle and the
complexities of the processes that occurred.
The word peridotite comes from the gemstone peridot, which consists of pale
green olivine. Classic peridotite is bright green with some specks of black,
although most hand samples tend to be darker green. Peridotitic outcrops
typically range from earthy bright yellow to dark green in color; this is
because olivine is easily weathered to iddingsite. While green and yellow are
the most common colors, peridotitic rocks may exhibit a wide range of colors
such as blue, brown, and red. Types of peridotite
Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1.
Wehrlite: mostly composed of olivine plus clinopyroxene.
Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low
proportions of basaltic ingredients. Lherzolite: most common form of
peridotite, mostly composed of olivine, orthopyroxene, and clinopyroxene, and
have relatively high proportions of basaltic ingredients. Partial fusion of
lherzolite and extraction of the melt fraction can leave a solid residue of
harzburgite. Composition
Olivine is a magnesium orthosilicate containing some iron with the variable
formula2SiO4; the pyroxenes are chain silicates having the variable
formula(Cr,Al,Fe+3,Mg,Mn,Ti,V)Si2O6 comprising a large number of different
minerals. Magnesium-rich olivine forms a large
proportion of peridotite, and so magnesium content is high. Layered
igneous complexes have much more varied compositions, depending on the fractions
of pyroxenes, chromite, plagioclase, and amphibole. Minor minerals and mineral
groups in peridotite include plagioclase, spinel, garnet, amphibole,
and phlogopite. In peridotite, plagioclase is stable at relatively low
pressures, aluminous spinel at higher pressures, and garnet at yet higher
pressures. Pyroxenites are related ultramafic
rocks, which are composed largely of orthopyroxene and/or clinopyroxene;
minerals that may be present in lesser abundance include olivine, garnet,
plagioclase, amphibole, and spinel. Distribution and location
Peridotite is the dominant rock of the Earth's mantle above a depth of about
400 km; below that depth, olivine is converted to the higher-pressure mineral
wadsleyite. Oceanic plates consist of up to about 100 km of peridotite covered by
a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro,
and minor sediments. The peridotite below the ocean crust, "abyssal
peridotite," is found on the walls of rifts in the deep sea floor. Oceanic
plates are usually subducted back into the mantle in subduction zones. However,
pieces can be emplaced into or overthrust on continental crust by a
process called obduction, rather than carried down into the mantle; the
emplacement may occur during orogenies, as during collisions of one continent
with another or with an island arc. The pieces of oceanic plates emplaced within
continental crust are referred to as ophiolites; typical ophiolites consist
mostly of peridotite plus associated rocks such as gabbro, pillow basalt,
diabase sill-and-dike complexes, and red chert. Other masses of peridotite have
been emplaced into mountain belts as solid masses but do not appear to be
related to ophiolites, and they have been called "orogenic peridotite
massifs" and "alpine peridotites." Peridotites also occur as fragments
carried up by magmas from the mantle. Among the rocks that commonly include
peridotite xenoliths are basalt and kimberlite. Certain volcanic rocks,
sometimes called komatiites, are so rich in olivine and pyroxene that they also
can be termed peridotite. Small pieces of peridotite have even been found in
lunar breccias. The rocks of the peridotite family are
uncommon at the surface and are highly unstable, because olivine reacts quickly
with water at typical temperatures of the upper crust and at the Earth's
surface. Many, if not most, surface outcrops have been at least partly
altered to serpentinite, a process in which the pyroxenes and olivines are
converted to green serpentine. This hydration reaction involves considerable
increase in volume with concurrent deformation of the original textures.
Serpentinites are mechanically weak and so flow readily within the earth.
Distinctive plant communities grow in soils developed on serpentinite, because
of the unusual composition of the underlying rock. One mineral in the
serpentine group, chrysotile, is a type of asbestos.
Morphology and texture Peridotites can take on a massive form
or may be in layers. Layered peridotites may form the base layers of gabbroic
complexes. Even though some layered peridotites appear to have no associated
gabbro, they probably once were part of such a complex. Three main textures are
found in peridotites: the first are well formed crystals of olivine surrounded by
other minerals. These olivine crystals probably settled out of the magma first.
Another texture is equal sized crystals with straight grain boundaries
intersecting at 120°. This may result when slow cooling allowed
recrystallization to minimize surface energy. A third texture has long
crystals with ragged curvilinear boundaries, a result of internal
deformation. Many peridotite occurrences have
characteristic textures. For example, peridotites with well-formed olivine
crystals occur mainly as layers in gabbroic complexes. "Alpine" peridotites
generally have irregular crystals that occur as more or less serpentinized
lenses bounded by faults in belts of folded mountains such as the Alpines,
the Pacific coast ranges, and in the Appalachian piedmont. Peridotite nodules
with irregular equigranular textures are often found in alkaline basalts and in
kimberlite pipes. Some peridotites, rich in amphibole, have a concentric layered
structure and form parts of plutons called Alaskan-type zoned ultramafic
complexes. Origin
Peridotites have two primary modes of origin, as mantle rocks formed during
the accretion and differentiation of the Earth, or as cumulate rocks formed by
precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas;
these magmas are ultimately derived from the upper mantle by partial melting of
mantle peridotites. Mantle peridotites are sampled as
alpine-type massifs in collisional mountain ranges or as xenoliths in
basalt or kimberlite. In all cases these rocks are pyrometamorphic and represent
either fertile mantle or partially depleted mantle. Alpine peridotites may
be either of the ophiolite association and representing the uppermost mantle
below ocean basins, or masses of subcontinental mantle emplaced along
thrust faults in mountain belts. Layered peridotites are igneous
sediments and form by mechanical accumulation of dense olivine crystals.
Some peridotite forms by precipitation and collection of cumulate olivine and
pyroxene from mantle-derived magmas, such as those of basalt composition.
Peridotites associated with Alaskan-type ultramafic complexes are cumulates that
probably formed in the root zones of volcanoes. Cumulate peridotites are also
formed in komatiite lava flows. Basaltic magma is formed from
lherzolites in the mantle. Once the magma moves out it leaves harzburgites
behind in the mantle. However early formed crystals in basaltic magma can
also form harzburgite within the mantle. Associated rocks
Komatiites are high degree partial melts of peridotite.
Eclogite, a rock similar to basalt in composition, is composed primarily of
sodic clinopyroxene and garnet. Eclogite is associated with peridotite in some
xenolith occurrences; it also occurs with peridotite in rocks metamorphosed
at high pressures during processes related to subduction.
Economic geology According to a new study released in
November 2008, peridotite may potentially be used in a low-cost, safe
and permanent method of capturing and storing atmospheric CO2 as part of
climate change-related greenhouse gas sequestration. It was already known that
peridotite reacts with CO2 to form a solid carbonate-like limestone or marble
mineral; and the study concluded that this process can be speeded up a million
times or more by simple drilling and hydraulic fracturing to allow injection
of the CO2 into the subsurface peridotite formation.
Peridotite is named for the gemstone peridot, a glassy green gem mined in
Asia and Arizona. Some peridotite is mined for ornamental stone.
Peridotite that has been hydrated at low temperatures forms serpentinite, which
may include chrysotile asbestos and talc.
Layered intrusions with cumulate peridotite are typically associated with
sulfide or chromite ores. Sulfides associated with peridotites form nickel
ores and platinoid metals; most of the platinum used in the world today is
mined from the Bushveld Igneous Complex in South Africa and the Great Dyke of
Zimbabwe. The chromite bands found in peridotites are the world's major source
of chromium. References
Further reading Alfred T. Anderson, Jr., 2002.
"Peridotite", AccessScience@McGraw-Hill, DOI 10.1036/1097-8542.498300.
Harvey Blatt and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and
Metamorphic, 2nd ed., Freeman, ISBN 0-7167-2438-3
J.-L. Bodinier and M. Godard, 2004, Orogenic, Ophiolitic, and Abyssal
Peridotites, in The Mantle and Core, Treatise on Geochemistry v. 2,
Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
External links Media related to Peridotite at Wikimedia
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