Peridotite

Olivine-Dunit

Peridotite is a dense, coarse-grained igneous rock consisting primarily of the minerals olivine and pyroxene. Peridotite is ultramafic, as it contains less than 45% silica (SiO2). It is high in magnesium (MgO) and iron (FeO), giving it a dark color. This rock type is significant in the Earth's mantle, particularly in the upper mantle, and is the primary source of lava in mid-ocean ridges and some hotspot volcanoes.

Formation and Composition

Peridotite forms deep within the Earth's mantle at high temperatures and pressures. It can be brought to the surface by tectonic processes, such as continental collisions, or by volcanic activity, where it is often found in ophiolite sequences, which are slices of the oceanic crust and upper mantle obducted onto the continental crust.

The primary mineral in peridotite is olivine, often accompanied by various types of pyroxenes, and less commonly, amphibole and phlogopite. Some peridotites also contain chromite, garnet, and plagioclase. The specific mineral composition can vary, leading to different types of peridotite, such as harzburgite (rich in olivine and low in clinopyroxene), lherzolite (which has more clinopyroxene and is considered to represent the most common type of mantle peridotite), and dunite (almost entirely composed of olivine).

Role in the Earth's Mantle

Peridotite plays a crucial role in the Earth's mantle, particularly in the process of mantle convection and plate tectonics. Mantle convection involves the slow creeping motion of Earth's solid silicate mantle caused by convection currents carrying heat from the interior of the Earth to the surface. Peridotite undergoes partial melting to produce magma, which can lead to volcanic eruptions when it reaches the surface. The composition of the magma is often altered during its ascent, becoming more enriched in silica and other elements, but its source is typically peridotitic.

Economic Importance

Peridotite is not only significant geologically but also economically. It is a major source of chromium, nickel, and platinum group elements (PGEs), which are extracted for industrial and commercial use. These elements are concentrated in sulfide minerals that can be found in association with peridotite.

Environmental Aspects

Recently, peridotite has gained attention for its potential in carbon sequestration. The natural weathering process of peridotite involves the reaction of the rock with carbon dioxide (CO2) from the atmosphere to form solid carbonate minerals, effectively removing CO2 from the atmosphere. This process, known as in situ carbonation, is being studied as a potential method for mitigating climate change.

Types of Peridotite

• Harzburgite: Characterized by a high proportion of olivine and low clinopyroxene content.
• Lherzolite: Contains more clinopyroxene and is indicative of a more fertile mantle source.
• Dunite: Composed almost entirely of olivine, often found in mantle plumes and as cumulates in layered intrusions.

See Also

• Olivine
• Pyroxene
• Mantle (geology)
• Igneous rock
• Ophiolite