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peridotite xenolith in a typical olivine-rich peridotite cut by a centimeter-thick layer of greenish-black pyroxenite, San Carlos, southwestern United States. Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

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