What is left after the basaltic component is removed is called a depleted mantle. These types of peridotite yield basaltic magma when they melt partially. Lherzolite and wehrlite are sometimes referred to as a fertile mantle. This is the mechanism how different types of peridotite came to be. Rest of the rock that did not melt have also different composition from the source rock, it is enriched in minerals with a higher melting temperature. Some minerals start melting earlier and form basaltic magma which migrates upward because of lower density. Basalt and peridotite differ in composition because rocks are mixtures of minerals, but each mineral has its own melting temperature. Basaltic magma forms when peridotite is partially melted. The main minerals are diopside (green), pyrope (purple) and olivine (yellowish green). Wehrlite (clinopyroxene peridotite) from Norway. Massive samples are often brought to the surface as xenoliths inside basalt. The Troodos ophiolite in Cyprus is a well-known locality where such rocks can be seen at the surface. Layered ultramafic typically occur at the base of gabbroic intrusions. Layered peridotite is a cumulate rock formed by crystal settling from magma. Peridotite is often massive (homogeneous) just like other plutonic rocks, but it can be also layered. It is an ultramafic rock (mafic minerals make up more than 90% of the rocks composition). Other notable minerals that are often present are chromite, garnet, and plagioclase. Peridotite is a plutonic rock, it is mostly composed of visible mineral grains. A rock type with less olivine is named pyroxenite (or hornblendite if there are more amphiboles than pyroxenes). Olivine, the principal component of peridotite, makes up more than 40% of the pair olivine + pyroxene (or amphibole). This rock is relatively rare at the surface and is often altered by hydrothermal metamorphism or weathering because its constituent minerals are unstable at the weathering environment. Lherzolite is the least pretentious of them - it is a mixture of two pyroxenes and olivine 1. Harzburgite is rich in olivine and orthopyroxene but there can be only up to 5% clinopyroxene. Wehrlite contains almost no orthopyroxene but is rich in clinopyroxene and olivine. Dunite is a type of peridotite that is almost monomineralic (more than 90% of it is olivine). Ol, Opx, and Cpx represent olivine, orthopyroxene, and clinopyroxene respectively. How to read this diagram? It is a typical ternary plot which are often used in geology. There are four subtypes: dunite, harzburgite, lherzolite, and wehrlite. Note that neither peridotite nor pyroxenite are single rock types. It is an important rock type because the Earth’s mantle is predominantly composed of it.Ī classification diagram of ultramafic rocks. Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene.
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