Peridotite

Peridotite (US: PERR-ih-doh-tyte, pə-RID-ə-) is a dense, phaneritic (coarse-grained) igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from 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 Earth's mantle. The compositions of peridotite nodules found in certain basalts are of special interest along with diamond pipes (kimberlite), because they provide samples of 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 Earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of 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; 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 including blue, brown, and red.

Classification

Igneous rocks rich in magnesium and iron with a color index greater than 90 are defined as ultramafic. Ultramafic rocks may be further classified by their relative proportions of olivine, orthopyroxene, clinopyroxene, and hornblende, which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of the total volume of these four mineral families in the rock. Peridotites are further classified as follows:

Dunite: more than 90% olivine Dunite is found as prominent veins in the peridotite layer of ophiolites, which are interpreted as slices of oceanic lithosphere (crust and upper mantle) thrust onto continents. Dunite also occurs as a cumulate in layered intrusions, where olivine crystallized out of a slowly cooling body of magma and accumulated on the floor of the magma body to form the lowest layer of the intrusion. Dunite almost always contains accessory chromite. Kimberlite: formed in volcanic pipes and at least 35% olivineKimberlite is a highly brecciated variant of peridotite formed in volcanic pipes and is known for being the host rock to diamonds. Unlike other forms of peridotite, kimberlite is quite rare. Pyroxene peridotite: From 40% to 90% olivine and less than 5% hornblende Harzburgite: less than 5% clinopyroxene Harzburgite makes up the bulk of the peridotite layer of ophiolites. It is interpreted as depleted mantle rock, from which basaltic magma has been extracted. It also forms as a cumulate in Type I layered intrusions, forming a layer just above the dunite layer. Harzburgite likely makes up most of the mantle lithosphere underneath continental cratons. Wehrlite: less than 5% orthopyroxene Wehrlite makes up part of the transition zone between the peridotite layer and overlying gabbro layer of ophiolites. In Type II layered intrusions, it takes the place of harzburgite as the layer just above the dunite layer. Lherzolite: intermediate content of clinopyroxene and orthopyroxene Lherzolite is thought to make up much of the upper mantle. It has almost exactly the composition of a mixture of three parts harzburgite and one part tholeiitic basalt (pyrolite) and is the likely source rock for basaltic magma. It is found as rare xenoliths in basalt, such as those of Kilbourne Hole in southern New Mexico, US, and at Oahu, Hawaii, US. Hornblende peridotite: From 40% to 90% olivine and less than 5% pyroxene Hornblende peridotite is found as rare xenoliths in andesites above subduction zones. They are direct evidence of alteration of mantle rock by fluids released by the subducting slab. Pyroxene hornblende peridotite: Intermediate between pyroxene peridotite and hornblende peridotite Pyroxene hornblende peridotite is found as rare xenoliths, such as those of Wilcza Góra in southwest Poland. Here it likely formed by alteration of mantle rock by carbonated hydrous silicic fluids associated with volcanism.

Composition

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