peridotite

Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Weathering alters the appearance of peridotite from its fresh, green tinge to a brownish appearance due to the formation of iron oxides. 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. Peridote outcrops include layered complexes, sills, irregular masses, dikes, and volcanic pipes, and peridotite is the primary reservoir of all naturally occurring diamonds and chromium ore, and peridotites are also a frequent host of platinum metals and talc deposits. In the Kimberley district of South Africa, the peridotite is typically referred to as kimberlite, and contains the only diamonds known to occur in the matrix of their original formation. 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.

links: images: hand-specimens: pale peridotite; weathered peridotite from Ronda in southern Spain; more typical color of peridotite, 2; dark; irregular grains of olivine and pyroxenes in peridotite; peridotite in basalt; greenish brown peridotite; green colored peridotite; peridotite from Pyrénées of France; close-up: slickensides on serpetinized peridotite; peridotite, closer; peridote surface; compare olivine in dunite; purple pyrope porphyroblasts in a peridotite with yellow-brown groundmass of weathered olivine, Otrøy, Western Gneiss Region, Caledonides, Norway; spinel harzburgite xenolith in peridotite nodule; peridotite (olivine + pyroxene) nodule in vesicular basalt; Minéraux Péridotite; Matsoko Coarse Peridotite, and Diamondiferous Peridotite; purple garnet peridotite from Greenland; thin-sections: peridotite fsu, 2; peroditite, Italy; peridotite, 2; kinked pyroxene in peridotite xenoliths in basaltic lava, Montferrier, France; peridotite comprising irregular interlocking crystals of olivine (bright colours) and magnesium-rich pyroxene (large grey crystals); xp ts; jp; mantle xenolith; serpentinized peridotite with a vein of chrysotile; Lame mince Péridotite; formations: exposed peridotite in the world's oldest ophiolite complex at Isua in Greenland (along with oldest sedimentary rocks, oldest banded iron formation, and oldest pillow lavas (wp) – the oldest rocks are the Acasta Gneisses in Canada, and the oldest mineral is Zircon in Australia); Red Hills, NZ, and Little Red Hill, West Otago, 2 miles South of Red Mountain, is a possible infaulted mantle peridotite, as is Mt Richards, and Red Mountain ultramafics; dunite vein in the serpentine (altered peridotite) for which the Lizard in Cornwall is famous; Péridotite incluse dans basalte, 2; peridotite in Norway; Serpentinized peridotite of the Kluane Ranges Mafic-Ultramafic Suite (TrK), Quill Creek Mafic-Ultramafic Complex; serpentinized mud volcano of altered peridotite in roadcut near Lake Berryessa, in northern CA; peridotite block, Shell Beach; the ophiolite sequence at the Smith River area is the site of one of the world's largest exposures of serpentinized peridotite; wispy pyroxene schlieren in pyroxene-dunite separating massive metasomatic dunite pipe (hosting Hall orebody), from surrounding layered websterite-peridotite; peridotite, Tahtalidag (NE continuation of Ala Dag range), Turkey; peridotite Tablelands, and Trout River Pond Trail, Gros Morne – the barren Tablelands (between Trout River and Woody Point in Gros Morne National Park) look more like Arizona than forested Newfoundland due to the ultramafic rock - peridotite - which makes up the Tablelands. Peridotite lacks the usual nutrients required to sustain most plant life, hence its barren appearance. The rock is very low in calcium, very high in magnesium, and has toxic amounts of heavy metals. Peridotite is also high in iron, which accounts for its brownish colour; Humboldt Massif with peridotite mantle rock exposures, and peridotite outcrop, southern New Caledonia, and pyroxenites in mantle peridotites; garnet peroditite in outcrop Gfohl Unit, southern part of the Bohemian Massif, Czech Republic, which mainly consists of acidic granulite and migmatitic gneiss; ASTER, Trodos Ophiolite, Cyprus; webpages: JosephineOphiolite; The Composition and Homogeneity of the Earth's Mantle; Coulée du volcan des Baumes, Cône volcanique des Baumes, Brèche volcanique, Quaternaire (1,5 MA), Volcanisme intra-continental