Mineral Inclusions In Diamonds Research Articles

Shape Change of Mineral Inclusions in Diamond—The Result of Diffusion Processes

The paper considers the possibility of changing the morphology of inclusions in diamonds based on the study of these inclusions and the inclusion–diamond boundary. Raman spectroscopy and transmission electron microscopy methods were used. According to the literature data, it is known that the octahedral form of mineral inclusions in diamond is induced, and does not correspond to the initial conditions of joint growth of diamond and inclusion, but the mechanism of this process is not considered. Solids differ in the value of surface Gibbs energy; the harder the material, the higher its melting point and the greater the value of surface Gibbs energy In the case of the diamond–inclusion pair, the surface energy of diamond far exceeds the surface energy of the inclusion. Diamond crystals have a surface energy value for an octahedron face of 5.3 J/m2, dodecahedron—6.5 J/m2, and cube—9.2 J/m2, i.e. it is anomalously high compared to the surface tension of silicate and other minerals. Therefore, the mineral inclusion in diamond tends to the form corresponding to the minimum of free energy in the “diamond–inclusion” pair, and when the energy of diamond dominates, the final shape will be determined by it, i.e. it will be an octahedron. The authors suggest the possibility of redistribution of diamond substance around the inclusion with simultaneous change of the inclusion morphology.

A critique of using epitaxial criterion to discriminate between protogenetic and syngenetic mineral inclusions in diamond

Distinguishing syngenetic from protogenetic inclusions in natural diamonds is one of the most debated issues in diamond research. Were the minerals that now reside in inclusions in diamonds born before the diamond that hosts them (protogenesis)? Or did they grow simultaneously and by the same reaction (syngenesis)? Once previously published data on periclase [(Mg,Fe)O] and magnesiochromite (MgCr2O4) inclusions in diamond have been re-analysed, we show that the main arguments reported so far to support syngenesis between diamond and its mineral inclusions, definitely failed. Hence: (a) the epitaxial relationships between diamond and its mineral inclusion should no longer be used to support syngenesis, because only detecting an epitaxy does not tell us which was the nucleation substrate (there are evidences that in case of epitaxy, the inclusion acts as a nucleation substrate); (b) the morphology of the inclusion should no longer be used as well, as inclusions could be protogenetic regardless their shapes. Finally, we advance the hypothesis that the majority of inclusions in diamonds are protogenetic, e.g., they are constituent of rocks in which diamonds were formed and not products of reactions during diamond growth.

Raman Spectroscopy for Characterization of Peridotite Paragenesis Mineral Inclusions in Diamonds

Research subject. Spectroscopic features (Raman spectra) of mineral inclusions of peridotite paragenesis (olivine, orthopyroxene, clinopyroxene, garnet) in natural diamonds of the Yakutian diamondiferous province. Materials and methods. A series of diamonds was studied both with single mineral inclusions and with associations of inclusions of peridotite paragenesis. The chemical composition of mineral inclusions in diamonds was determined using an electron probe micro-analyzer (EPMA). The Raman spectra of inclusions were obtained on a spectrometer equipped with a Nd:YAG laser with a wavelength of 532 nm. Results. The revealed spectroscopic characteristics of mineral inclusions in natural diamonds reflect specific features of their chemical composition. Thus, the shift in the positions of the Raman peaks DB1 and DB2 in the olivine spectra reflects the forsterite - fayalite (Mg-Fe) isomorphism; changes in the positions of valence vibrational modes in the Raman spectra of clinopyroxene Si-Onbr (ν16) and Si-Obr (ν11) and orthopyroxene (ν17) reflect the isomorphism of diopside - jadeite (CaMg-NaAl) and enstatite - ferrosilite (Mg-Fe), position shifts of deformation (ν2) and valence (ν1, ν3) modes of vibrational energies of the Si-O bond in garnets reflect the Al-Cr and Ca-Mg isomorphism, respectively. Conclusions. For the identified correlations, regression lines were calculated, which can be used to determine the quantitative contents of the main chemical components of mineral inclusions (clinopyroxene and garnet) of peridotite paragenesis in situ in diamonds. The developed method for evaluating the chemical composition of garnet and clinopyroxene inclusions can be used to distinguish clinopyroxene and garnet inclusions from different mantle parageneses.

Mantle convection and diamonds

Research subject. The present evolutionary stage of geodynamic theory is associated with the idea of thermochemical convection of various levels in the Earth's mantle, where the centrifugal branches are represented by plumes, and the centripetal - by subduction zones. Aim. The study of diamonds contributes to an understanding of when, at what level in mantle, under what P-T conditions and geochemical environment particular diamonds originated, which were then transported by centrifugal convection flows to the Earth's surface, thereby permitting characterization of this flow. Materials and methods. Generalization of published materials and characterization of mineral inclusions in diamonds allow the general structure of mantle convection to be clarified in different epochs and different regions. Results. The data obtained on mineral inclusions in diamonds, along with the experimental data on the P-T conditions of their mineral parageneses and geophysical data on mantle properties, indicate that the depth of diamond formation varies from the lower lithosphere in the upper part of the upper mantle (≈150-250 km) to the bottom of the lower mantle. At the same time, the diamonds containing mineral inclusions, characteristic of the lower mantle, account for only the first percents of the general number of diamonds. Conclusions. The transport of diamonds from different depths of their origin is a reliable indication of convection processes (as a plume activity) in the mantle. This information provides evidence to the real existence of plumes, which is important in the context of ongoing discussions on the depth of their origin. However, the study of mineral inclusions in diamonds, particularly in superdeep diamonds, is a challenging task due to the retrograde changes, resorption and sometimes complete dissolution on their way to the surface. These circumstances minimize the probability of occurrence of superdeep diamonds and require consideration when making judgements about the reality of existence of superdeep diamonds.

Thermobarometry of diamond inclusions: Mantle structure and evolution beneath Archean cratons and mobile belts worldwide

Thermobarometric calculations for mineral inclusions in diamonds provide a systematic comparison of P-T-fO2 conditions for different cratons and mobile belts worldwide, using a database of 4440 mineral EPMA analyses. Beneath all cratons, the cold branch of the mantle geotherm (35-32 mWm−2) relates to PT estimates for sub-calcic garnets and rare omphacitic diamond inclusions, referring to major continental growth events in Archean. High-temperature plume-related geotherms are common in Proterozoic kimberlites such as Premier, Mesozoic-Roberts Victor etc. and are common in Slave and Siberian cratons. Eclogitic and pyroxenitic pyroxenes and garnets and Fe- and Ca-enriched pyrope diamond inclusions prevail in mobile belts like Limpopo, Magondi, Ural, Khapchan and in the marginal parts of cratons (Kimberly in Australia) They refer to diamondiferous eclogite-pyroxenite lens in the mididdle parts of lithospheric mantle. Eclogitic garnets and clinopyroxenes show trends of joint decreasing Mg# and pressures due to differentiation of rising melts. Dunitic Ca pyropes giving high-pressure estimates prevail in the central parts of cratons. The PT for the chromites reflect conditions just above the lithosphere-asthenosphere boundary formed due to interaction with the hydrous plume protokimberlite melts. Archean diamond Ca-enrich pyropes inclusions from Wawa province Canada give low-temperature conditions. Inclusions from younger kimberlites in Superior and Slave, Siberian and East European cratons show complex high-temperature geotherms due to plumes influence. Peridotite garnets beneath the Amazonian craton indicate complex layering in the lithosphere base and a pyroxene layer in the middle part of mantle lithosphere. Diamond inclusions from the Kimberley Craton of Australia show the greatest variations in the temperatures and composition. The mobile belts and marginal parts of cratons are favorable for the finding of the abundant colored diamonds. The huge plume influence for hydrated and eclogite-rich mantle like in Premier, Venetia and other kimberlites brings to the formation of giant diamonds. Diamond inclusion reveal the huge variations of PT conditions formed by different agents due to long complex stories of craton deciphered using thermobarometry and trace element and isotopic geochemistry.

Diamond formation beneath the Coromandel area, southwestern São Francisco Craton – The role of re-fertilization and subduction

The lithospheric mantle underpinning the Alto Paranaíba Igneous Province , southwestern São Francisco Craton , was investigated through a study of 82 diamonds and their mineral inclusions from the Coromandel (Douradinho River), Verde River (northern Coromandel), Abaeté, Frutal, and Romaria deposits. Mineral inclusion abundances show that lherzolite is the main diamond source-rock, followed by eclogite , harzburgite and minor websterite. The limited chemical depletion recorded in lherzolitic inclusions suggests a post-Archean origin or modification of the lithospheric mantle beneath this part of the São Francisco craton. Sinusoidal rare earth element patterns for lherzolitic garnet inclusions indicate variable but overall low degrees of metasomatism by fluids or silico‑carbonatitic (proto-kimberlitic) melts. These results contrast with previous studies on mineral inclusions in diamonds (Canastra range, southern Coromandel; Rio da Prata system, northeastern Coromandel; our new data on the Frutal area, southwestern Coromandel), where a dominance of harzburgitic inclusions documents a more depleted, likely Archean, cratonic root. Bulk rock compositions reconstructed from eclogitic and websteritic inclusions establish a clear link between subduction processes and diamond formation in the Coromandel region. Calculated bulk rare earth element patterns match typical sections of upper oceanic crust , where plagioclase accumulation is uncommon. Pyroxene inclusion-based geothermobarometry indicates that lherzolitic and websteritic diamonds formed and last resided along a ∼ 39 mW/m 2 model geotherm. Projection of eclogitic garnet-clinopyroxene pairs onto this 39 mW/m 2 geotherm places their origin near the base of the lithosphere (> 180 km depth). Low nitrogen contents and high aggregation states (up to 100 %B) indicate that eclogitic, websteritic, and part of the lherzolitic diamond populations experienced extended (2.0 Byr) mantle residence at high temperatures (> 1200 °C). The similarity in mantle residence temperatures and the shared presence of minor negative Eu anomalies for eclogitic and websteritic diamonds and their inclusions may be indicative of a common origin, following emplacement of subducted oceanic crust beneath the São Francisco Craton margin likely during a Paleoproterozoic orogeny (2.2–1.9 Ga). Based on our results, future exploration efforts in the Alto Paranaíba region should shift their focus from harzburgitic to lherzolitic and eclogitic indicator minerals to find new diamond deposits and kimberlites in this area. • Lherzolite and eclogite main diamond substrates in Alto Paranaíba Igneous Province. • Moderate olivine Mg# and rare G10 garnets indicate mild SCLM depletion. • Lherzolitic substrates affected by fluid or silico‑carbonatitic melt metasomatism. • Eclogitic and websteritic diamonds derived from base of lithosphere. • Eclogitic diamonds linked to Paleoproterozoic (2.2–1.9 Ga) subduction.

Crystallographic Methods for Non-destructive Characterization of Mineral Inclusions in Diamonds

Crystallographic Methods for Non-destructive Characterization of Mineral Inclusions in Diamonds

Electron Probe Microanalysis and Microscopy of Polishing-Exposed Solid-Phase Mineral Inclusions in Fuxian Kimberlite Diamonds

Solid-phase mineral inclusions in diamond (1–3 mm in diameter) from the No. 50 kimberlite diatreme of Liaoning Province, China, were exposed by polishing. A variety of silicate, carbonate and sulfide inclusions were recovered in the diamond. The common solid-phase inclusions are olivine, chromite, garnet and orthopyroxene; the rare phases include Ca carbonate, magnesite, dolomite, norsethite, pyrrhotite, pentlandite, troilite, a member of the linnaeite group, an unknown hydrous magnesium silicate and an Fe-rich phase. Abundance and composition of the solid-phase inclusions in diamond indicate that they belong to the peridotitic suite and are mainly harzburgitic. No eclogitic mineral inclusions were found in the diamond. The slightly lower Mg # of the olivine inclusions (peak at 93) than that of harzburgitic olivine inclusions worldwide (Mg # peak at 94), the higher Ni content (0.25–0.45 wt. %) of the olivine inclusions than those of olivine inclusions worldwide (0.30–0.40 wt. %), the higher Ti contents (up to 0.79 wt. %) in some chromite inclusions in diamond than those in chromite inclusions worldwide, the existence of carbonate inclusions in diamond, and the possible presence of hydrous silicate phases in diamond all indicate a metasomatic enrichment event in the source region of diamond beneath the North China craton, suggesting that the diamond probably formed by solid-state growth under metasomatic conditions with the presence of a fluid. Solid-state growth of diamond is also supported by abundant graphite inclusions in the diamond. Sulfide inclusions in diamond often coexist with chromite and olivine or are rich in Ni content, indicating that the sulfide inclusions belong to the peridotitic suite. From the chemical compositions, most sulfide inclusions in diamond from the No. 50 kimberlite were probably trapped as monosulfide crystals, although some may have been entrapped as melts.

Olivine in komatiite records origin and travel from the deep upper mantle

Abstract The deep upper mantle is the main source of high-temperature magmatism, but the only known naturally occurring samples of high-pressure mantle constituents are mineral inclusions in diamonds. Trace elements in olivine crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal that these crystals formed in the deep upper mantle as high-pressure phenocrysts, and some perhaps even formed in the mantle transition zone (410–600 km) where they began as wadsleyite. The crystals were entrained within ascending komatiite magma and conveyed to the surface. The olivine crystals have the highest contents of Al2O3 (0.3 wt%) recorded in any terrestrial olivine, which is indicative of formation at high pressure. The deep mantle gave rise to Archean komatiites, extraordinarily hot magmas (up to 1700 °C), which provide insight into Earth's early mantle evolution and the formation of most ancient continental and oceanic crust. In spite of extensive research since their discovery over 50 years ago, the origins of komatiites have remained contentious. Plumes—thermochemical instabilities originating at the core-mantle boundary—are the most likely source, but no direct evidence of a deep mantle origin of komatiite has yet been recognized.

Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle

AbstractMajoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.

Crystallographic Orientation and Geochemical Features of Mineral Inclusions in Diamonds

Abstract —The orientation of 76 mineral inclusions represented by olivine (25 inclusions), pyrope (13 inclusions), and magnesiochromite (38 inclusions) was measured in 16 diamond samples from the major primary diamond deposits of Yakutia: Mir, Udachnaya, Internatsionalnaya, Aikhal, and Yubileynaya kimberlite pipes. The novelty of the study is that it provides a special purposeful approach to selection of samples containing not only olivine inclusions that have been extensively studied in the most recent years after the publication of the book Carbon in Earth (2013). The present collection accounts for more than 25% of all samples studied across the world and includes the most typical mineral inclusions of the predominant peridotitic paragenesis in almost all known kimberlites. Both this experiment and similar studies conducted by foreign colleagues in 2014–2019 have found no inclusions whose orientation meets the epitaxial criterion. Only single magnesiochromite inclusions in three diamonds demonstrate an orientation close to the regular one. A significant correlation between the carbon isotope composition and the mineral composition of inclusions of peridotitic and eclogitic paragenesis diamonds as well as the lack of a correlation with other properties may be considered one of the geochemical features. However, given the numerous published and proprietary data demonstrating the complex diamond growth history and, in some cases, wide variations in the composition of mineral inclusions in different zones, along with the difference in their morphology, the authors a believe that syngenetic and protogenetic inclusions can coexist in the same diamond. This is also confirmed by the discoveries of diamondiferous peridotite and eclogite xenoliths in kimberlites where diamonds are completely enclosed in garnet or olivine. Of particular note is the constant presence of heavy hydrocarbons (rel.%), from pentane (C5H12) to hexadecane (C16H34), that are predominant in fluid inclusions in kimberlite and placer diamonds as well as in pyrope and olivine of diamondiferous peridotite xenoliths.

U–Pb ages of rare rutile inclusions in diamond indicate entrapment synchronous with kimberlite formation

The timing of diamond crystallization is generally inferred from radiometric dating of individual or multiple mineral inclusions in diamond, where ages are derived from isochrons or isotopic evolution models. Resulting ages often significantly predate the emplacement ages of host kimberlites, but age information from both approaches can be ambiguous due to long-lived mantle heterogeneities where mixing can mimic isochrons or in-situ aging. Direct dating of accessory mineral inclusions in diamonds, by contrast, has rarely been attempted because of the scarcity of such inclusions, requiring careful high grading of large amounts of diamond concentrates. Here, we report secondary ionization mass spectrometry (SIMS) U–Pb ages from a suite of rare rutile inclusions which were extracted from diamond in an eclogitic paragenesis from the Mir kimberlite pipe, Siberia. One population of rutile inclusions shows diamond crystal-shapes and was petrographically identified as completely enclosed in diamond prior to host burning for inclusion extraction. These rutile inclusions were screened for U abundances (0.24–23 μg/g U) and those with the highest U contents were dated, yielding a uniform U–Pb concordia age of 375.5 ± 7.0 Ma (error 95% confidence; mean square of weighted deviates MSWD = 0.74; number of spots n = 19). This age is nearly coeval with, but significantly (at the 95% confidence level) older than combined U–Pb rutile ages for rutile intergrown with polycrystalline fibrous diamond (362.9 ± 9.5 Ma; MSWD = 0.96; n = 7) and rutile from the eclogitic xenolith matrix (369 ± 16 Ma; MSWD = 0.30; n = 5). Rutile ages are consistent with published ages for kimberlite emplacement (ca. 360 Ma), but much younger than Re–Os sulfide isochron ages between ca. 913 Ma and 2.1 Ga for groups of individual inclusions in diamond from the same location. When assuming the oldest Re–Os sulfide age as the timing of rutile entrapment, diffusive Pb-loss from rutile inclusions over ca. 2 Ga would require diffusivities of Pb in diamond of >10−22 m2/s, as well as Pb partitioning coefficients between diamond and rutile at near or greater unity. Alternatively, if Pb is immobile in diamond, ancient rutile ages could have been reset due to processes other than volume diffusion through diamond (e.g., due to fluid migration along cracks). However, in this scenario it remains difficult to explain why all rutile inclusions dated here would have been located along cracks, whereas sulfide inclusions investigated in other studies were not. Another explanation is that rutile inclusions were entrapped at the time of diamond formation, and that this event is recorded by U–Pb rutile ages, whereas sulfides predate diamond crystallization and their Re–Os systematics were incompletely reset. Trace element heterogeneity in rutile suggests that they originated in chemically different environments, and thus rutile likely predates diamond formation. However, due to rapid diffusion of Pb in rutile, radiogenic Pb accumulated only after entrapment in diamond (for inclusions) or eruptive quenching (for intergrown or matrix rutile). Overall, rutile geochronology indicates that at least some diamond from the Mir pipe formed briefly (within the ca. 20 Ma age difference between U–Pb rutile ages for inclusions and intergrown/matrix rutile) before kimberlite eruption, supporting models that link diamond formation with carbon-rich precursors of kimberlite magmas.

Fast identification of mineral inclusions in diamondat GSECARS using synchrotron X-ray microtomography, radiography and diffraction.

Mineral inclusions in natural diamond are widely studied for the insight that they provide into the geochemistry and dynamics of the Earth's interior. A major challenge in achieving thorough yet high rates of analysis of mineral inclusions in diamond derives from the micrometre-scale of most inclusions, often requiring synchrotron radiation sources for diffraction. Centering microinclusions for diffraction with a highly focused synchrotron beam cannot be achieved optically because of the very high index of refraction of diamond. A fast, high-throughput method for identification of micromineral inclusions in diamond has been developed at the GeoSoilEnviro Center for Advanced Radiation Sources (GSECARS), Advanced Photon Source, Argonne National Laboratory, USA. Diamonds and their inclusions are imaged using synchrotron 3D computed X-ray microtomography on beamline 13-BM-D of GSECARS. The location of every inclusion is then pinpointed onto the coordinate system of the six-circle goniometer of the single-crystal diffractometer on beamline 13-BM-C. Because the bending magnet branch 13-BM is divided and delivered into 13-BM-C and 13-BM-D stations simultaneously, numerous diamonds can be examined during coordinated runs. The fast, high-throughput capability of the methodology is demonstrated by collecting 3D diffraction data on 53 diamond inclusions from Juína, Brazil, within a total of about 72 h of beam time.

Carbon and nitrogen isotopes and mineral inclusions in diamonds from chromitites of the Mirdita ophiolite (Albania) demonstrate recycling of oceanic crust into the mantle

Carbon and nitrogen isotopes and mineral inclusions in diamonds from chromitites of the Mirdita ophiolite (Albania) demonstrate recycling of oceanic crust into the mantle

Toward a Robust Elastic Geobarometry of Kyanite Inclusions in Eclogitic Diamonds

AbstractHere we report the first results from elastic geobarometry applied to a kyanite inclusion entrapped within an eclogitic diamond (from Voorspoed mine, South Africa) using micro‐Raman and Fourier transform infrared spectroscopy, electron microprobe analysis, ab initio calculations, and finite element modeling. Application of elastic geobarometry to very elastically anisotropic kyanite inclusions is challenging, as current models do not allow for elastic anisotropy. In order to minimize the effects of anisotropy, we have explored the effects of deviatoric stress on Raman modes via ab initio density functional theory. The results allowed us to select the Raman mode (at ca. 638 cm−1) that is the least sensitive to deviatoric stress. The shift of this band in the inclusion while still trapped within the diamond relative to the inclusion in air (once liberated) was used under hydrostatic approximation to determine a residual pressure on the inclusion of 0.184 ± 0.045 GPa and an entrapment pressure of 5.2 ± 0.3 GPa (~160 km depth) for an FTIR N‐aggregation residence temperature of 1119 ± 50 °C. This is the first geothermobarometric determination for a diamond from the Voorspoed kimberlite. It overlaps with P–T estimates obtained by traditional chemical geobarometry for diamonds from other kimberlites from the Kaapvaal craton, suggesting that the hydrostatic approximation does not introduce significant errors in the geobarometric evaluation. Our protocol of Raman peak selection can be used for geobarometry of further kyanite‐bearing diamonds and may provide a guide for more robust geobarometry of other types of mineral inclusions in diamonds, both eclogitic and peridotitic.

Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?

Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4–16 wt%) of garnets and the Mg# (69–92) and jadeite contents (14–48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39–40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere–transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.

Fate of water transported into the deep mantle by slab subduction

The roles of water in the mantle transition zone, lower mantle, and the core-mantle boundary are investigated. The evidence for a wet mantle transition zone has been suggested based on hydrous mineral inclusions in diamond. Seismic wave velocity and electrical conductivity profiles together with mineral physics data are consistent with existence of stagnant slabs in a wet mantle transition zone. The transition zone may contain continental crustal components in these stagnant slabs. Dense hydrous magmas may exist at the base of the upper mantle. Fluids or volatile-rich magmas may also exist at the top of the lower mantle due to the large contrast in water contents between the mineral assemblages in the mantle transition zone and the lower mantle, and the crossing of the convective descent of the cold hydrated materials. Dense magmas are not likely to be formed at the top of the lower mantle and hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported deeper into the lower mantle by gravitational collapse of the stagnant slabs, which supply water into the lower mantle, including the core-mantle boundary. Hydrous δ-H solid solution may be the most important hydrous phase in lower mantle, and existence of this phase reduces the aluminum content in coexisting bridgmanite and post-perovskite, and thus modifies the physical properties of the lower mantle. Hydrous δ-H solid solution can accumulate at the base of the lower mantle. The iron-water reaction at the core-mantle boundary can also create pyrite-type FeOOH which can be a potential candidate material for the ultralow velocity zone (ULVZ).

Mineral inclusions in diamonds from Chapada Diamantina, Bahia, Brazil: a Raman spectroscopic characterization

Abstract The Chapada Diamantina, located in the central region of the State of Bahia, is of important historical significance due to its diamond occurrences. Discovered in the nineteenth century, comprehensive research about the regional diamonds and their origins are still limited, demanding more investigation in the matter. Looking for insights about their genesis, mineral inclusions in 23 alluvial diamonds from 4 garimpos located in the Chapada Diamantina were analyzed through the use of Raman micro spectroscopy. Additionally, the characteristics of nitrogen aggregation of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIR). The diamonds from Chapada Diamantina consist mainly of well-formed crystals, with dominant dodecahedral habits, characterized by faint to very light yellow body colors, typically with green and brown radiation spots on their surface. The main surface textures observed are related to processes that took place in the late stage resorption and during the residence of the diamonds in placer environments. The diamonds are predominantly type IaAB, with a significant occurrence of poorly aggregated nitrogen (Type IaA diamond). The main mineral assemblages of the studied peridotitic inclusions refer to a harzburgitic paragenesis.

Mineral inclusions in diamonds may be synchronous but not syngenetic

It is widely assumed that mineral inclusions and their host diamonds are ‘syngenetic’ in origin, which means that they formed simultaneously and from the same chemical processes. Mineral inclusions that, instead, were formed earlier with respect to diamonds are termed protogenetic. However, minerals can have the same age as the diamonds in that they become enclosed in and isolated from any further isotopic exchange. But this is termed ‘synchronous’ not ‘syngenetic’. Here we demonstrate conclusively the protogenesis of inclusions in diamonds, based upon data from an exceptional fragment of a diamond-bearing peridotite, its clinopyroxene and a gem-quality diamond. Clinopyroxenes in the xenolith had the same chemistry and crystallographic orientation as those for inclusions in the diamond. With our results with garnets, olivines and sulfides, we can state that a major portion of the mineral inclusions in non-coated, monocrystalline-lithospheric diamonds are protogenetic. Our discovery here presented has implications for all genetic aspects of diamond growth, including their ages.

Inclusions of α-quartz, albite and olivine in a mantle diamond

Mineral inclusions in diamonds have been used to track potential information on the Earth's deep mantle. Here we report results from a detailed study on the mineral inclusions in a ca. 0.28ct diamond from the Shengli No. 1 kimberlite in Mengyin County, Shandong Province, eastern China. Our study reveals the presence of α-quartz, albite and olivine in the diamond. At an inferred depth of ca. 165km for the diamond crystallization, the inclusions of α-quartz and albite suggest the possible involvement of deep subducted crustal material, traces of which were captured during the diamond growth and magma migration.