Peridotite

Peridotite is a type of ultramafic igneous rock that is composed primarily of the mineral olivine, along with smaller amounts of other minerals such as pyroxenes and amphiboles. It is typically dark green in color and has a coarse-grained texture.

Peridotite is an important rock in the Earth’s mantle, which is the layer of the Earth that lies below the crust. It is believed to be one of the main rock types that make up the upper mantle, which extends from the base of the crust down to a depth of about 400 kilometers (250 miles) or more. Peridotite is thought to be a residue left behind after partial melting of the mantle, with the molten portion of the mantle rising to form basaltic crust, leaving behind the denser peridotite.

Peridotite is named after the mineral peridot, which is a gem-quality variety of olivine that is often found in peridotite rocks. Peridot is known for its distinctive green color, which is due to the presence of iron in its crystal structure. Peridotite is also an important rock in the study of plate tectonics, as it is believed to be the source of the material that makes up oceanic lithosphere, which is the rigid outer layer of the Earth’s surface that forms the oceanic crust and the uppermost part of the mantle. When peridotite is brought to the Earth’s surface through processes such as uplift and erosion, it can provide valuable insights into the composition and behavior of the Earth’s mantle.

Group: Plutonic.
Colour: Generally dark greenish-grey.
Texture: Phaneritic (coarse grained).
Mineral content: Generally olivine with lesser pyroxene ( augite) (dunite is dominantly olivine), always contains some metallic minerals, e.g. chromite, magnetite. Silica (SiO ₂) content – < 45%.

Definition and composition of peridotite

Peridotite is a type of ultramafic igneous rock that is primarily composed of the mineral olivine, along with smaller amounts of other minerals such as pyroxenes and amphiboles. It is one of the main rock types found in the Earth’s mantle, which is the layer of the Earth that lies below the crust.

The composition of peridotite typically consists of the following minerals:

1. Olivine: Olivine is the dominant mineral in peridotite and can make up more than 90% of its composition. Olivine is a silicate mineral with a chemical formula of (Mg,Fe)_2SiO_4, where Mg represents magnesium and Fe represents iron. Olivine is typically green in color and has a glassy or granular texture.
2. Pyroxene: Pyroxenes are another important group of minerals in peridotite. They are silicate minerals that can have a range of chemical compositions, but in peridotite, they are typically rich in iron and/or magnesium. Common pyroxenes found in peridotite include orthopyroxene (Mg,Fe)_2Si_2O_6 and clinopyroxene (Ca,Mg,Fe)(Si,Al)_2O_6.
3. Amphibole: Amphiboles are another group of silicate minerals that can be found in peridotite, although they are typically present in smaller amounts compared to olivine and pyroxenes. Amphiboles are complex minerals with varying chemical compositions, but they often contain calcium, magnesium, and iron. Common amphiboles found in peridotite include tremolite Ca_2Mg_5Si_8O_22(OH)_2 and actinolite Ca_2(Mg,Fe)_5Si_8O_22(OH)_2.

In addition to these primary minerals, peridotite can also contain minor amounts of other minerals such as spinel (MgAl_2O_4), garnet (a group of silicate minerals with varying compositions), and chromite (FeCr_2O_4), among others, depending on the specific composition and conditions of formation. Peridotite is typically coarse-grained, meaning that its individual mineral crystals are visible to the naked eye, and it can have a variety of textures ranging from granular to massive.

Occurrence and distribution of peridotite in the Earth’s mantle

Peridotite is one of the main rock types that make up the Earth’s mantle, which is the solid layer of the Earth that lies below the crust and extends to a depth of about 2,900 kilometers (1,800 miles). The occurrence and distribution of peridotite in the Earth’s mantle are fundamental to our understanding of the Earth’s interior and its geodynamic processes.

Peridotite is believed to be a residue left behind after partial melting of the mantle, with the molten portion of the mantle rising to form basaltic crust, leaving behind the denser peridotite. This process is known as partial melting or partial melting differentiation. The peridotite that remains in the mantle is then subjected to various geodynamic processes, such as convection, which is the movement of material within the mantle due to heat transfer, and upwelling or downwelling of mantle material due to mantle plumes or subduction.

Peridotite is found in various parts of the Earth’s mantle, and its occurrence and distribution are complex and dynamic. Some of the main occurrences of peridotite in the Earth’s mantle include:

1. Upper Mantle: Peridotite is believed to make up a significant portion of the upper mantle, which extends from the base of the crust down to a depth of about 400 kilometers (250 miles) or more. This is the region where most of the mantle melting is thought to occur, leading to the formation of basaltic crust and leaving behind peridotite residue.
2. Transition Zone: The transition zone is a region in the mantle that lies between the upper and lower mantle, typically between depths of about 400 to 660 kilometers (250 to 410 miles). Peridotite is also thought to occur in this region, although its composition and properties may differ from those in the upper mantle due to changes in pressure and temperature.
3. Lower Mantle: The lower mantle is the region of the mantle that extends from the bottom of the transition zone to the core-mantle boundary, which is about 2,900 kilometers (1,800 miles) below the Earth’s surface. The composition and properties of peridotite in the lower mantle are not well known due to the extreme conditions at these depths, but it is believed to be more enriched in iron and other elements compared to peridotite in the upper mantle.
4. Mantle Plumes: Mantle plumes are believed to be hot upwellings of material from the deep mantle that can rise to the Earth’s surface and create hotspots, such as the Hawaiian Islands and Iceland. Peridotite is thought to be a major component of mantle plumes, and the melting of peridotite in these regions is believed to be responsible for the formation of large volumes of basaltic magma.

The distribution and composition of peridotite in the Earth’s mantle are still topics of ongoing research and study, and scientists use various techniques, such as seismic studies, geochemical analyses, and experimental petrology, to gain insights into the nature and behavior of peridotite in the Earth’s interior.

Importance of peridotite in geology and geophysics

Peridotite plays a significant role in geology and geophysics due to its importance in understanding the Earth’s interior, geodynamic processes, and the formation of igneous rocks. Some of the key importance of peridotite in these fields includes:

1. Mantle Composition: Peridotite is a major component of the Earth’s mantle, which constitutes a significant portion of the Earth’s volume. Studying the composition, structure, and properties of peridotite provides valuable insights into the overall composition and behavior of the Earth’s mantle, including its mineralogy, melting processes, and geothermal properties.
2. Mantle Melting: Peridotite is a residue left behind after partial melting of the mantle, and the melting of peridotite is believed to be a fundamental process in the formation of basaltic crust and the generation of magma. Understanding the melting behavior of peridotite, including its melting temperatures, melt compositions, and melt generation processes, is crucial for understanding the formation of igneous rocks, such as basalts and other volcanic rocks, and the origin of magmas in different tectonic settings.
3. Geodynamic Processes: Peridotite is involved in various geodynamic processes, such as mantle convection, which is the process of material movement within the mantle due to heat transfer. The properties of peridotite, such as its density, viscosity, and rheology, influence the behavior of mantle convection, and studying peridotite helps us understand the dynamics of mantle convection and its role in plate tectonics, volcanism, and other geological phenomena.
4. Geophysical Studies: Peridotite has unique physical properties that can be studied using geophysical techniques, such as seismic studies, electromagnetic surveys, and gravity measurements. These studies provide important information about the composition, structure, and dynamics of the Earth’s mantle and can help us better understand the subsurface geology, seismicity, and geophysical anomalies associated with peridotite-rich regions, such as mantle plumes, subduction zones, and mid-ocean ridges.
5. Economic Importance: Peridotite can also have economic importance as a source of valuable minerals, such as chromite, which is used in the production of stainless steel, and platinum-group elements, which are used in various industrial applications. Peridotite-hosted mineral deposits can be studied to understand their formation processes and economic potential, and peridotite can also serve as a target for mineral exploration.

In summary, peridotite is a key rock type in geology and geophysics, providing valuable insights into the composition, structure, properties, and dynamics of the Earth’s mantle, as well as the formation of igneous rocks and the economic potential of mineral deposits. Studies of peridotite contribute to our understanding of the Earth’s interior and its geodynamic processes, and have broad implications in various fields of geoscience.

Petrology of Peridotite

The petrology of peridotite involves the study of its mineralogy, texture, and composition, as well as its formation and evolution processes. Peridotite is an ultramafic rock composed predominantly of the minerals olivine and pyroxene, with minor amounts of other minerals such as spinel, garnet, and plagioclase.

Mineralogy: Peridotite is typically composed of the mineral olivine (Mg2SiO4-Fe2SiO4), which makes up the majority of the rock. Pyroxenes, such as clinopyroxene (Ca-Mg-Fe silicate) and orthopyroxene (Mg-Fe silicate), are also common minerals in peridotite. Other minor minerals may include spinel, garnet, and plagioclase, depending on the composition and conditions of formation of the peridotite.

Texture: Peridotite can have a variety of textures, depending on its formation and subsequent processes. It can have a granular texture (known as equigranular or poikilitic texture) where olivine and pyroxene grains are roughly equal in size and well-mixed. Alternatively, it can have a layered texture (known as cumulate texture) where different mineral layers are formed due to crystal settling during solidification. Peridotite can also show foliation, which is a preferred orientation of mineral grains resulting from deformation and recrystallization processes.

Composition: Peridotite typically has a high magnesium (Mg) and iron (Fe) content, and low silica (SiO2) content, making it an ultramafic rock. The specific composition of peridotite can vary depending on its origin, and may have different trace element and isotopic signatures. Peridotite can also contain small amounts of water in the form of hydrous minerals, such as serpentine, which can affect its properties and behavior.

Formation and Evolution: Peridotite forms through various processes, including partial melting of the mantle, crystal fractionation, and metasomatism. Partial melting of the mantle can generate basaltic magmas, leaving behind peridotite residues that can be exposed at the Earth’s surface through tectonic uplift and erosion. Peridotite can also form through crystal fractionation, where minerals crystallize and settle out from a melt, leading to the formation of layered intrusions or cumulate rocks. Metasomatism, which involves the alteration of rock compositions by fluids or melts, can also lead to the formation of peridotite through chemical reactions.

The petrology of peridotite provides important information about the origin, evolution, and properties of this rock type, and helps us understand the processes that shape the Earth’s mantle, the formation of igneous rocks, and the behavior of ultramafic rocks in different geologic settings. Studying the mineralogy, texture, composition, and formation processes of peridotite contributes to our understanding of the Earth’s geology, geodynamics, and petrological processes.

Types of peridotite

There are several types of peridotite based on their mineralogy, texture, and composition. Some of the commonly recognized types of peridotite include:

1. Harzburgite: Harzburgite is a type of peridotite that is composed predominantly of olivine and orthopyroxene, with minor amounts of clinopyroxene and/or spinel. It is a coarse-grained rock with a granular texture and is often found in the Earth’s mantle.
2. Dunite: Dunite is a type of peridotite that is composed almost entirely of olivine, with little or no pyroxene or other minerals. It is an ultramafic rock with a high olivine content, and it often occurs as lenses or pockets within other peridotite rocks. Dunite is typically light green in color due to its high olivine content.
3. Wehrlite: Wehrlite is a type of peridotite that contains both olivine and clinopyroxene, typically with olivine being more abundant than pyroxene. It is a coarse-grained rock with a granular texture and may also contain minor amounts of other minerals such as spinel or plagioclase.
4. Lherzolite: Lherzolite is a type of peridotite that contains both olivine and pyroxene, with clinopyroxene being more abundant than orthopyroxene. It has a characteristic spotted appearance due to the presence of rounded or elongated pyroxene grains within the olivine matrix.
5. Pyroxenite: Pyroxenite is a type of peridotite that is composed predominantly of pyroxene minerals, such as clinopyroxene or orthopyroxene, with minor amounts of other minerals. It is typically dark-colored and can occur as intrusive rocks, xenoliths in other rocks, or as part of mantle rock assemblages.

These are some of the main types of peridotite, and their characteristics can vary depending on their mineralogy, texture, and composition. The types of peridotite can provide important information about the conditions and processes of their formation, as well as their geologic significance in various tectonic settings.

Geochemistry of Peridotite

The geochemistry of peridotite is an important aspect of studying this rock type, as it provides insights into its composition, origin, and evolution. Peridotite is an ultramafic rock that typically has a high content of magnesium (Mg) and iron (Fe), and low silica (SiO2) content. The geochemistry of peridotite involves the study of its major element, trace element, and isotopic compositions, which can reveal information about its source, melting processes, and alteration history.

Major element composition: The major element composition of peridotite is dominated by the abundance of olivine and pyroxene minerals. Olivine is a magnesium-rich silicate mineral (Mg2SiO4-Fe2SiO4), and its abundance in peridotite can influence the overall composition of the rock. Pyroxenes, such as clinopyroxene and orthopyroxene, are also important minerals in peridotite, and their composition can vary depending on the conditions of formation. The major element composition of peridotite can be determined using techniques such as X-ray fluorescence (XRF) or electron probe microanalysis (EPMA).

Trace element composition: The trace element composition of peridotite can provide important information about the source and melting processes that have affected the rock. For example, the abundance of trace elements such as chromium (Cr), nickel (Ni), and platinum-group elements (PGEs) in peridotite can provide insights into the processes of partial melting and melt extraction in the mantle. The trace element composition of peridotite can be analyzed using techniques such as inductively coupled plasma mass spectrometry (ICP-MS) or laser ablation ICP-MS (LA-ICP-MS).

Isotopic composition: The isotopic composition of peridotite can provide clues about its origin and evolution. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons, and their ratios can be used to track the sources and processes that have affected the rock. For example, isotopes of elements such as oxygen (O), strontium (Sr), neodymium (Nd), and osmium (Os) can provide insights into the sources and ages of peridotite rocks. Isotopic analysis of peridotite can be done using techniques such as radiogenic isotope analysis or stable isotope analysis.

Alteration and weathering: Peridotite can undergo various types of alteration and weathering processes, which can affect its geochemical composition. For example, peridotite can be altered by hydrothermal fluids, leading to the formation of serpentine minerals, such as antigorite or lizardite. This alteration can result in changes in the major and trace element compositions of peridotite. Weathering processes at the Earth’s surface, such as chemical weathering or leaching by water, can also affect the geochemical composition of peridotite.

The geochemistry of peridotite is an important tool for understanding its origin, evolution, and behavior in different geologic settings. It provides insights into the processes that shape the Earth’s mantle, the formation of igneous rocks, and the alteration of ultramafic rocks. Geochemical studies of peridotite contribute to our understanding of the Earth’s geology, geodynamics, and petrological processes.

Petrogenesis of Peridotite

The petrogenesis of peridotite involves the processes of its formation, evolution, and modification in the Earth’s mantle. Peridotite is believed to originate from the upper mantle, specifically the asthenosphere, which is a partially molten and highly viscous region beneath the Earth’s lithosphere. The exact petrogenesis of peridotite is complex and can involve multiple processes, including partial melting, melt-rock interaction, metasomatism, and recrystallization.

Partial melting: Partial melting is one of the key processes in the petrogenesis of peridotite. Under high temperatures and pressures in the mantle, peridotite can undergo partial melting, resulting in the formation of melt pockets or channels. The composition of the melt can vary depending on the source peridotite, the degree of melting, and other factors. The residual peridotite that does not melt becomes more enriched in minerals such as olivine and pyroxene.

Melt-rock interaction: Melt-rock interaction can occur when the partial melts generated from peridotite interact with the surrounding peridotite rocks. The melts can migrate through the peridotite, reacting with the solid minerals and exchanging chemical components. This process can result in the formation of different types of peridotite with varying mineralogical and geochemical compositions.

Metasomatism: Metasomatism is the process by which peridotite is altered by the introduction of new chemical components from an external source. This can occur through the infiltration of fluids, such as water, carbon dioxide, or melts, into the peridotite. Metasomatic processes can lead to the formation of different types of peridotite, such as serpentinite, which is peridotite altered by the addition of water, resulting in the formation of serpentine minerals.

Recrystallization: Recrystallization is the process by which peridotite undergoes mineralogical changes due to changes in temperature, pressure, or other conditions. This process can result in the formation of new minerals or the transformation of existing minerals in the peridotite. For example, olivine in peridotite can recrystallize to form spinel or pyroxene minerals under certain conditions.

Other processes: Other processes such as deformation, melting and solidification, and chemical reactions can also play a role in the petrogenesis of peridotite. Deformation can lead to the formation of different types of peridotite, such as harzburgite, which is a type of peridotite that has undergone plastic deformation. Melting and solidification can result in the formation of igneous rocks, such as basalt or gabbro, which can have peridotite as their source material. Chemical reactions, such as redox reactions or phase transformations, can also influence the petrogenesis of peridotite.

The petrogenesis of peridotite is a complex and dynamic process that involves various geologic and geophysical factors. Studying the petrogenesis of peridotite provides insights into the origin, evolution, and behavior of this important rock type in the Earth’s mantle, and contributes to our understanding of the geology and geophysics of the Earth’s interior.

Economic Importance of Peridotite

Peridotite is not generally considered to have significant economic importance in its natural state, as it is a relatively rare rock type and lacks economically valuable minerals. However, there are some specific contexts where peridotite can be of economic interest due to its unique properties and occurrences.

1. Gemstone industry: Peridotite is the primary source of the gemstone peridot, which is a green gemstone that is used in jewelry. Peridot is a variety of olivine, a mineral commonly found in peridotite rocks. Peridot gemstones are highly valued for their unique color and are used in various types of jewelry, including rings, earrings, necklaces, and bracelets.
2. Industrial applications: Peridotite has high melting points and is highly refractory, meaning it can withstand high temperatures and is resistant to heat and chemical corrosion. As such, peridotite has been investigated for potential industrial applications, such as in the production of refractory materials used in furnaces, kilns, and other high-temperature processes.
3. Carbon capture and storage (CCS): Peridotite has been studied as a potential rock type for carbon capture and storage (CCS), which is a technology aimed at reducing greenhouse gas emissions from power plants and other industrial processes. Peridotite has the ability to react with carbon dioxide (CO2) and form stable minerals through a process called mineral carbonation, which can potentially store CO2 in a solid, stable form for long-term sequestration.
4. Geothermal energy: Peridotite rocks can be associated with geothermal energy resources. Geothermal energy is harnessed by tapping into the heat stored in the Earth’s crust, and peridotite-rich areas can be associated with high-temperature geothermal systems. In these areas, peridotite can act as a potential heat source for generating electricity through geothermal power plants.
5. Exploration indicator: Peridotite can also serve as an indicator rock in mineral exploration. In some cases, the presence of peridotite at the Earth’s surface or in the subsurface can indicate the potential for valuable mineral deposits associated with the rock, such as nickel, chromium, or platinum group elements (PGEs). Peridotite can serve as a guide for exploration efforts to locate economically viable mineral deposits.

While peridotite itself may not be economically valuable in most cases, it can have indirect economic importance through its association with other valuable minerals or its potential use in industrial applications, carbon capture and storage, geothermal energy, and as an exploration indicator. Further research and exploration may uncover additional economic uses for peridotite in the future.

Summary of key points of Peridotite

Peridotite is a type of ultramafic rock that is composed predominantly of the minerals olivine and pyroxene, and it is an important rock type in geology and geophysics due to its unique properties and occurrences. Here are the key points about peridotite:

1. Definition and composition: Peridotite is a coarse-grained rock composed mainly of olivine and pyroxene minerals, and it typically has a greenish color due to the high iron content of olivine. It is classified as an ultramafic rock because it contains very low levels of silica, making it chemically distinct from other common rock types.
2. Occurrence and distribution: Peridotite is abundant in the Earth’s mantle, where it is believed to be a major constituent of the upper mantle. It is also found in smaller quantities at the Earth’s surface, primarily in ophiolite complexes, which are sections of oceanic crust that have been uplifted and exposed on land through tectonic processes.
3. Petrology: Peridotite can be further classified into different types based on its mineralogy, texture, and geochemical characteristics. Common types of peridotite include harzburgite, dunite, and lherzolite, which differ in their mineral assemblages and textures.
4. Geochemistry: Peridotite has a unique geochemical composition with low silica (SiO2) content, high levels of iron (Fe) and magnesium (Mg), and relatively low levels of other elements. Peridotite is an important source rock for mantle-derived magmas, such as basaltic magma, and it is believed to play a key role in the composition and evolution of the Earth’s crust and mantle.
5. Petrogenesis: The formation of peridotite is complex and can occur through various processes, including partial melting of the mantle, mantle metasomatism, and solid-state transformation of other rock types. Peridotite is believed to be a key rock type in the formation of oceanic crust, and it is also associated with the formation of kimberlite pipes, which are the primary source of diamonds.
6. Economic importance: While peridotite itself is not typically considered economically valuable, it can have indirect economic importance. Peridotite is the primary source of the gemstone peridot and can also be associated with valuable mineral deposits, such as nickel, chromium, and platinum group elements (PGEs). Peridotite has also been investigated for potential industrial applications, carbon capture and storage, and geothermal energy.

In summary, peridotite is an important rock type in geology and geophysics due to its unique properties, occurrences, and petrogenesis. It is abundant in the Earth’s mantle, has a distinct geochemical composition, and can have economic importance through its association with gemstones, valuable minerals, and potential industrial applications.

Peridotite FAQ

Q: What is peridotite?

A: Peridotite is a type of ultramafic rock composed mainly of the minerals olivine and pyroxene. It is characterized by its low silica content, high iron and magnesium content, and greenish color.

Q: Where is peridotite found?

A: Peridotite is abundant in the Earth’s mantle, where it is believed to be a major constituent of the upper mantle. It is also found in smaller quantities at the Earth’s surface, primarily in ophiolite complexes, which are sections of oceanic crust that have been uplifted and exposed on land.

Q: What are the different types of peridotite?

A: Common types of peridotite include harzburgite, dunite, and lherzolite, which differ in their mineral assemblages and textures. Harzburgite is composed mostly of olivine and pyroxene, dunite is almost entirely made of olivine, and lherzolite is a mix of olivine, pyroxene, and other minerals.

Q: What is the geochemistry of peridotite?

A: Peridotite has a unique geochemical composition with low silica (SiO2) content, high levels of iron (Fe) and magnesium (Mg), and relatively low levels of other elements. It is an important source rock for mantle-derived magmas, and its geochemistry plays a key role in the composition and evolution of the Earth’s crust and mantle.

Q: How is peridotite formed?

A: Peridotite can be formed through various processes, including partial melting of the mantle, mantle metasomatism (chemical alteration), and solid-state transformation of other rock types. It is believed to be a key rock type in the formation of oceanic crust and is also associated with the formation of kimberlite pipes, which are the primary source of diamonds.

Q: What is the economic importance of peridotite?

A: While peridotite itself is not typically considered economically valuable, it can have indirect economic importance. Peridotite is the primary source of the gemstone peridot and can also be associated with valuable mineral deposits, such as nickel, chromium, and platinum group elements (PGEs). Peridotite has also been investigated for potential industrial applications, carbon capture and storage, and geothermal energy.

Q: What are some uses of peridotite?

A: Peridotite has various uses, including as a gemstone (peridot), a potential source of valuable minerals (nickel, chromium, PGEs), and in potential industrial applications, such as in the production of iron and steel. It has also been studied for its potential in carbon capture and storage, as well as geothermal energy production.