The Realm of Ultrahigh-Pressure Metamorphism

The discovery of diamond and coesite in crustal rocks is compelling evidence that continental material has experienced pressures that can only be achieved at mantle depths. At least 20 terranes of unequivocal continental crust containing diamond or coesite are now recognized around the globe; their study constitutes a new field in petrology called ultrahigh-pressure metamorphism. The idea that continents do not subduct has given way to the notion that Earth has been sufficiently cool since the Cryogenian (~850 Ma) to allow density changes to drive continental crust into the mantle during collision. Some of this crust is exhumed to the surface, some pools at the Moho, and the rest sinks into the mantle. In this issue, microscopic observations, phase-equilibrium modeling, geochronology, and geodynamic modeling track the journey of crustal rocks to the mantle and back to Earth's surface. * Buoyancy : the upward force exerted on the crust by the mantle; the term also refers to the difference in density between the mantle and the oceanic or continental crust because density determines the amount of displacement (floating or sinking) of the crust with respect to the mantle. Coesite : a high-pressure polymorph of SiO2. Coesite is more dense than quartz and is stable at depths greater than 90 km ( P > 2.6 GPa) at 600 °C. Coesite is the main index mineral of UHP metamorphism, but it can also form by shock from a meteorite impact. Delamination : a process whereby rocks at the base of the crust become dense, perhaps by overthickening or gravitational instability, and sink into the mantle. These so-called mantle drips have been imaged seismically and produced in geodynamic models. Diamond : a dense, high-pressure polymorph of carbon, in which each carbon atom is bonded to four other carbon atoms in a rigid tetrahedron, resulting in its cubic symmetry. Diamond is also an index mineral of UHP metamorphism. Eclogite : a metamorphosed mafic rock that contains garnet and omphacite, but not plagioclase. Other common minerals in eclogite are kyanite, quartz/coesite, phengite, epidote-group minerals, magnetite, rutile, and zircon. Exhumation : the movement of a buried rock towards Earth's surface. Exhumation is distinct from uplift, which is simply the increase in elevation of the surface. Garnet peridotite : an ultramafic metamorphic rock composed of garnet + olivine ± pyroxene, sometimes called orogenic peridotite because such rocks are found in collisional orogens. Garnet peridotites are commonly derived from the sublithospheric mantle, but they can also represent minor ultramafic cumulates associated with crustal mafic intrusions. High-pressure (HP) metamorphism : metamorphism at pressures above the calcite to aragonite transition. Na pyroxenes are also stable in this field. HP rocks occupy the blueschist, eclogite, and HP granulite facies. Metamorphic facies : the collection of mineral assemblages from rocks of all bulk compositions that crystallize at the same pressure and temperature conditions. The greenschist, blueschist, amphibolite, granulite, and eclogite facies boundaries are established based on the mineral assemblages in metamorphosed mafic rocks. Omphacite : a dense, Na-rich clinopyroxene, with a minimum of 20% jadeite component. Jadeite is the pure Na pyroxene (NaAlSi2O6) end-member. Pseudomorph : the result of the replacement of one mineral by another, where the new mineral assumes the form of the original. Relamination : a process whereby continental rocks that have reached mantle depths—or their melt products—are returned to the base of the crust. Stishovite : the second high-pressure polymorph of SiO2, formed by the conversion of coesite at mantle depths in excess of 200 km (~7 GPa) at 600 °C or 300 km (~10 GPa) at 1400 °C. Natural stishovite has only been observed in meteorite impact structures. Terrane : a crustal block with a distinct stratigraphy, structure, and geologic history. Exotic terranes are fault-bounded blocks that have a foreign origin compared to the surrounding rocks. Most UHP terranes are of known provenance and belong to a craton, although exotic UHP terranes that originated as continental arcs are also known. A terrain can be thought of as any physical landmass, and the term sometimes refers to topography, as in “rough terrain.” Ultrahigh-pressure (UHP) metamorphism : metamorphism of crustal rocks that occurs at pressures in the coesite stability field. UHP metamorphism is identified by the presence of coesite or diamond, or by the equivalent pressure–temperature conditions calculated from thermodynamic models of mineral compositions.