Diamond inclusions are the non-diamond materials that get encapsulated inside diamond during its formation process in the mantle. The trapped materials can be other minerals or fluids like water. Since diamonds have high strength and low reactivity with either the inclusion or the volcanic host rocks which carry the diamond to the Earth's surface, the diamond serves as a container that preserves the included material intact under the changing conditions from the mantle to the surface. Although diamonds can only place a lower bound on the pressure of their formation, many inclusions provide additional constraints on the pressure, temperature and even age of formation.
Types and materials of diamond inclusions (Summary) | |
---|---|
Types | Main materials |
Mineral (solid) | Silicates (e.g. garnet, silicate perovskites), oxides, sulfides |
Fluid | Fluids (containing carbonates, silicates, sulfides, halides, hydroxyl groups, etc.), water, brines |
Multiphase | Fluid inclusions coexisting with mineral inclusions in the same diamond |
Mineral inclusions, especially the silicate inclusions in lithospheric diamonds, can be classified into two dominant types depending on the mantle parental rocks of the host diamond: eclogite (E-type) and peridotite (P-type). These are the two main parental rocks for the diamond formation which mostly lead to silicate inclusions. [1] [4] P-type and E-type inclusions can be distinguished based on the content of specific materials in the trapped mineral. For instance, in garnet inclusions, the content ratio of chromium(III) oxide (Cr2O3) and calcium oxide (CaO) can be the basis for the classification. [5] E-type garnet inclusion contains less Cr2O3 while P-type contains less CaO. Trace elements such as rare earth elements (REE) can also characterize P-type and E-type garnet inclusions. [6] Similarly, nitrogen inclusions can be classified into P-type and E-type inclusions by analyzing their stable isotopes. [7] For sulfide inclusions, osmium contents from rhenium-osmium dating can differentiate P-type and E-type inclusions. [8]
In the craton ic crust of the Kaapvaal-Zimbabwe craton, Southern Africa, seismic velocity at 150-km depth correlates with the nature of diamond inclusions, whether peridotitic or eclogitic. This suggests that lithospheric P-wave speeds can be used, perhaps elsewhere as well as in souther Africa, to map the distribution of different diamond source regions. [9]
Sub-lithospheric mineral inclusions such as majorite and silicate perovskites (e.g. bridgmanite, davemaoite) can be also classified into ultramafic type (peridotitic) and basaltic type (eclogitic) inclusions. [11] However, these additional classifications are harder than the lithospheric inclusions due to the rarity of samples, small grain size, and difficulties in recognizing the original mineral assemblages under deep-mantle conditions. [1]
The timing of mineral crystallization can be used to categorize diamond inclusions into three types: protogenetic, syngenetic, and epigenetic inclusions. [14] Minerals in the protogenetic inclusions were crystallized earlier than the diamond formation. The host diamond encapsulated pre-existing minerals during its crystallization. Therefore, protogenetic inclusions provide information on the conditions that existed before diamond formation. This can explain isotopically different mineral inclusions found from the same generation of diamonds. [15] For syngenetic mineral inclusions, the crystallization of the trapped mineral and the diamond occur simultaneously. [1] In this case, the environmental records from included minerals match that of the host diamond. Syngenetic inclusions can be evidenced by the imposition of host diamond morphology on the trapped mineral. [16] Epigenetic inclusions are formed from minerals that crystallized after the diamond formation. The after-formed minerals can crystallize along diamond fractures or the pre-existing protogenetic/syngenetic inclusions may have been altered into new material. [1]
Mineral inclusions can preserve materials formed under the extreme environments in Earth's mantle back to surface conditions. [1] This enables the discovery of the natural form of minerals which have previously been only synthesized in the laboratory. [17] For instance, the natural calcium silicate perovskite (CaSiO3), was recently given the mineral name davemaoite, when it was discovered as a mineral inclusion in a diamond in 2021. [18] The discovery was surprising due to the extreme conditions necessary to synthesize davemaoite which made it seem unlikely that it could be preserved at the Earth's surface. [17]
Classification of mineral inclusions (Summary) | ||
---|---|---|
Classification | Types | Etc. |
Location of the inclusion | - Lithospheric - Sub-lithospheric | |
Parental rocks of the host diamond | - P-type (peridotitic) - E-type (eclogitic) |
|
Timing of crystallization of the included mineral | - Protogenetic - Syngenetic - Epigenetic |
Fluid inclusions trap fluids containing materials like silicates, carbonates and hydroxyl groups, water and brine. [19] Such fluid inclusions can be found in coated diamonds (monocrystalline diamonds coated by polycrystalline diamonds with fluid inclusions) and fibrous diamonds (diamonds coated by rods or blades of diamonds with fibrous structures). [1] Fluid microinclusions mostly contain carbonates with the silicate or halides forming the silicate-carbonate or halide-carbonate assemblages. [20] Similarly, subduction-derived saline fluids with a high concentration of K and Cl can be found from microinclusions in the cloudy diamonds (fluid-rich central fibrous diamonds transforming into fluid-poor outward diamonds). [21] Saline and silicic fluid inclusions do not co-exist, implying the immiscibility of the two fluids during the diamond formation. [22] [23] The presence of volatile materials originating from subduction zones such as sulfide inclusions can suggest the viability of subduction-related crustal recycling during the diamond formation in specific continents where the diamond was created. [24]
In 2018, the high-pressure form of water known as ice-VII was found in the diamond inclusion. This discovery suggests the presence of water-rich fluids in the transition zone. [25]
In the diamond-forming conditions of high pressures and temperatures, hydrous silicate melt and the aqueous fluid make a single-phase supercritical mixture. This mixture forms fibrous, cloudy, or polycrystalline diamonds with multiphase inclusions. [26] Multiphase inclusions host fluids (mainly containing carbonates and silicates, high density aqueous fluids, and brines) and the mineral inclusions in the same diamond. [27]
High-resolution techniques like Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) imaging, and Electron Microprobe (EPMA) are commonly used to analyze the composition and phase of the trapped material in the diamond. [1] Non-destructive elastic methods such as micro-Raman spectroscopy, strain birefringence analysis, and single-crystal X-ray diffraction are used to estimate the pressure-temperature conditions of the material inside the diamond while minimizing the sample damage. [1]
Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.
Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.
A xenolith is a rock fragment that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock entrained during magma ascent, emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing body of lava on the Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g. rhyolite-dominated lava of Niijima volcano (Japan) contains two types of gabbroic xenoliths which are of different origin - they were formed in different temperature and pressure conditions.
Coesite is a form (polymorph) of silicon dioxide (SiO2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953.
Eclogite is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum and, rarely, diamond. The chemistry of primary and accessory minerals is used to classify three types of eclogite. The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.
Omphacite is a member of the clinopyroxene group of silicate minerals with formula: (Ca, Na)(Mg, Fe2+, Al)Si2O6. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing diopside and jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history. It exhibits the typical near 90° pyroxene cleavage. It is brittle with specific gravity of 3.29 to 3.39 and a Mohs hardness of 5 to 6.
Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce the Mid-Ocean Ridge Basalt by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names PYR-oxene and OL-ivine. However, whether pyrolite is representative of the Earth's mantle remains debated.
A melt inclusion is a small parcel or "blobs" of melt(s) that is entrapped by crystals growing in magma and eventually forming igneous rocks. In many respects it is analogous to a fluid inclusion within magmatic hydrothermal systems. Melt inclusions tend to be microscopic in size and can be analyzed for volatile contents that are used to interpret trapping pressures of the melt at depth.
Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorphic rocks, as evidenced by a multitude of geochemical, geophysical and petrological studies.
In geochemistry, the primitive mantle is the chemical composition of the Earth's mantle during the developmental stage between core-mantle differentiation and the formation of early continental crust. The chemical composition of the primitive mantle contains characteristics of both the crust and the mantle.
In geology, the slab is a significant constituent of subduction zones.
Silicate perovskite is either (Mg,Fe)SiO3 or CaSiO3 when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and mainly exist in the lower part of Earth's mantle, between about 670 and 2,700 km depth. They are thought to form the main mineral phases, together with ferropericlase.
Elephant Moraine 79001, also known as EETA 79001, is a Martian meteorite. It was found in Elephant Moraine, in the Antarctic during the 1979–1980 collecting season.
Robert Norman Clayton was a Canadian-American chemist and academic. He was the Enrico Fermi Distinguished Service Professor Emeritus of Chemistry at the University of Chicago. Clayton studied cosmochemistry and held a joint appointment in the university's geophysical sciences department. He was a member of the National Academy of Sciences and was named a fellow of several academic societies, including the Royal Society.
The geochemistry of carbon is the study of the transformations involving the element carbon within the systems of the Earth. To a large extent this study is organic geochemistry, but it also includes the very important carbon dioxide. Carbon is transformed by life, and moves between the major phases of the Earth, including the water bodies, atmosphere, and the rocky parts. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respirated back into the atmosphere. However an important part of the carbon cycle involves the trapping of living matter into sediments. The carbon then becomes part of a sedimentary rock when lithification happens. Human technology or natural processes such as weathering, or underground life or water can return the carbon from sedimentary rocks to the atmosphere. From that point it can be transformed in the rock cycle into metamorphic rocks, or melted into igneous rocks. Carbon can return to the surface of the Earth by volcanoes or via uplift in tectonic processes. Carbon is returned to the atmosphere via volcanic gases. Carbon undergoes transformation in the mantle under pressure to diamond and other minerals, and also exists in the Earth's outer core in solution with iron, and may also be present in the inner core.
Toshiko K. Mayeda was a Japanese American chemist who worked at the Enrico Fermi Institute in the University of Chicago. She worked on climate science and meteorites from 1958 to 2004.
Mantle oxidation state (redox state) applies the concept of oxidation state in chemistry to the study of the Earth's mantle. The chemical concept of oxidation state mainly refers to the valence state of one element, while mantle oxidation state provides the degree of decreasing of increasing valence states of all polyvalent elements in mantle materials confined in a closed system. The mantle oxidation state is controlled by oxygen fugacity and can be benchmarked by specific groups of redox buffers.
The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States, in the central part of the Colorado Plateau. The volcanic field consists of over 80 volcanoes and associated intrusions of unusual potassium-rich compositions, with an age range of 26.2 to 24.7 million years (Ma).
Davemaoite is a high-pressure calcium silicate perovskite (CaSiO3) mineral with a distinctive cubic crystal structure. It is named after geophysicist Ho-kwang (Dave) Mao, who pioneered in many discoveries in high-pressure geochemistry and geophysics.
Marta E. Torres is a marine geologist known for her work on the geochemistry of cold seeps and methane hydrates. She is a professor at Oregon State University, and an elected fellow of the Geochemical Society and the Geological Society of America.