Peridotite

Peridotite
Name	Peridotite
Type	Ultramafic intrusive rock
Composition	Olivine, pyroxene, amphibole, spinel, garnet
Texture	Coarse-grained
Origin	Mantle-derived, cumulate

Peridotite

Peridotite is a dense, coarse-grained ultramafic intrusive rock composed predominantly of olivine and pyroxenes, central to studies of the Earth's mantle, magmatism, and tectonic processes. It forms the keystone of discussions linking mantle convection, plate tectonics, and ore genesis, appearing in research from the Mid-Atlantic Ridge to the Sierra Nevada (United States), and informing models used by institutions such as the United States Geological Survey and the British Geological Survey. Scholars associated with the Geological Society of America, the Mineralogical Society of America, and universities like Harvard University, University of Cambridge, and Stanford University frequently publish peridotite studies in journals tied to the American Geophysical Union and the European Geosciences Union.

Overview and Definition

Peridotite is defined petrographically and chemically as an ultramafic igneous rock with olivine >40% and low silica content, classified within schemes endorsed by the International Union of Geological Sciences and used in fieldwork by the Geological Survey of Canada. It is often contrasted with mafic suites noted in literature from the Deccan Traps, Colombia River Basalts and Siberian Traps, and appears in tectonic syntheses involving regions such as the Alps, Himalayas, and the East African Rift. Historical fieldwork by figures connected to the Royal Society and explorers such as those on voyages like the HMS Challenger (1872–1876) contributed to early mantle-rock descriptions.

Composition and Mineralogy

Typical mineral assemblages include forsteritic olivine, orthopyroxene (enstatite), clinopyroxene (diopside), and accessory spinel, garnet, and amphibole; these assemblages are analyzed using techniques standardized by laboratories at the Massachusetts Institute of Technology, ETH Zurich, and Max Planck Institute for Chemistry. Trace-element and isotope work referencing groups like the Smithsonian Institution and the Woods Hole Oceanographic Institution employ methods similar to those used in studies of Mount St. Helens and Kilauea to resolve mantle source characteristics. Phase equilibria experiments reported in proceedings from the American Mineralogist and the Journal of Petrology connect peridotite mineralogy to pressure-temperature conditions observed beneath regions such as the Icelandic hotspot and the Nazca Plate margin.

Formation and Geologic Setting

Peridotite forms by processes including partial melting, melt extraction, cumulate crystallization, and tectonic emplacement; canonical field examples are documented from the Upper Mantle sections exposed at ophiolites like the Semail Ophiolite, the Troodos Ophiolite, and the Zambales Ophiolite. It is central to models of lithospheric mantle beneath cratons such as the Canadian Shield, Kaapvaal Craton, and Baltic Shield, and it occurs in settings related to subduction zones like the Cascadia subduction zone and continental collide zones exemplified by the Tethys Ocean closure. Geodynamic interpretations by researchers affiliated with the Lamont–Doherty Earth Observatory and the California Institute of Technology integrate peridotite data with seismic studies from projects like the USArray and the European Plate Observing System.

Physical and Chemical Properties

Peridotite displays high density, low viscosity when partially molten, and variable magnetic susceptibility; physical properties are constrained by experimental work at facilities including the Argonne National Laboratory and the Oak Ridge National Laboratory. Major-element chemistry often shows low SiO2 and high MgO contents comparable to compositions reported from the Lherz Massif and the Ronda peridotites, while isotopic systems (Sr-Nd-Pb-Hf) used by researchers at the Scripps Institution of Oceanography and the GEOMAR Helmholtz Centre tie compositions to mantle reservoirs invoked in studies of the Tuvalu mantle plume and the Iceland plume. Reactions such as serpentinization documented in studies from the MAR (Mid-Atlantic Ridge) alter physical properties and host hydrogen-producing processes relevant to habitats studied by teams from the Monterey Bay Aquarium Research Institute.

Economic Importance and Uses

Peridotite-hosted processes create economically important deposits, including magmatic nickel-copper-platinum group element sulfide ores investigated by mining companies in regions like the Bushveld Complex, the Stillwater Complex, and the Voisey's Bay mine. Carbonation of peridotite is studied as a method for carbon sequestration in projects funded by agencies such as the European Commission and the U.S. Department of Energy, with field trials in locations like the Ocean Drilling Program sites and pilot projects near the Caribbean and Iceland. Gem-quality olivine ("peridot") from places such as Zabargad Island and San Carlos (Arizona) garners interest from museums like the Natural History Museum, London and the Smithsonian National Museum of Natural History.

Distribution and Occurrence

Peridotite occurs in xenoliths carried by alkaline volcanics in provinces like the Eifel volcanic fields, the Sierra de San Carlos, and the Tenerife volcanic province, and in ophiolitic complexes across the Mediterranean region, Central Asia, and the Caribbean arc. Significant mantle exposures and massifs such as the Lherz Massif, Reykjanes Peninsula, and the Dora Maira massif are subjects of study by consortia including the International Continental Scientific Drilling Program and universities like University of Tokyo and University of California, Berkeley. Exploration programs by entities like BHP and Rio Tinto have targeted peridotite-hosted mineralization in locales including the Pilbara and Norilsk.

Petrogenesis and Metamorphism

Petrogenetic models invoke partial melting degrees, melt-rock interaction, and metasomatism influenced by fluids from slabs studied in contexts like the Mariana Trench and the Japan Trench. Metamorphic transformations such as garnetization and spinel–garnet transitions documented from the Himalaya and the Alboran Sea region are integral to thermobarometry approaches developed at institutions like the University of Oxford and the University of Melbourne. Research collaborations spanning the International Union of Geodesy and Geophysics and the European Research Council apply geochemical fingerprints and thermodynamic modeling to link peridotite evolution to mantle dynamics beneath features such as the Azores and the Galápagos Islands.

Category:Igneous rocks