Diamond Formation Research Articles

Diamond-forming media through time – Trace element and noble gas systematics of diamonds formed over 3 billion years of Earth’s history

Ten individual gem-quality monocrystalline diamonds of known peridotite/eclogite paragenesis from Southern Africa (Koffiefontein, Letlhakane, Orapa) were studied for trace element concentrations and He and Ar abundances and isotopic compositions. In addition, two samples, consisting of pooled fragments of gem-quality peridotitic diamonds from Finsch and DeBeers Pool respectively, were analysed for noble gases. Previous studies (Richardson et al., 1984; Pearson et al., 1998; Gress et al., 2017; Timmerman et al., 2017) provided age constraints of 0.09, 1.0–1.1, 1.7, 2.3, and 3.2–3.4 Ga on mineral inclusions in the studied diamonds, allowing us to study trace elements and noble gases over 3 Gyr of geological time. Concentrations of trace elements in the diamonds are very low – a few hundred ppt to several tens of ppbs – and are likely dependent on the amount of sub-micron inclusions present. Trace element patterns and trace element/3He ratios of the studied monocrystalline diamonds are similar to those in fibrous diamonds, suggesting that trace elements and stable noble gas isotopes reside within the same locations in diamond and track the same processes that are reflected in the trace element patterns. We cannot discern any temporal differences in these geochemical tracers, suggesting that the processes generating them have been occurring over at least the past 2.3 Ga. 3He/4He ratios decrease and 4He and 40Ar* contents increase with increasing age of peridotitic and some eclogitic diamonds, showing the importance of in-situ radiogenic 4He and 40Ar ingrowth by the decay of U-Th-Sm and K respectively. For most gem-quality monocrystalline diamonds, uncertainties in the 3He/4He evolution of the continental lithospheric mantle combined with large analytical uncertainties and possible spatial variability in U-Th-Sm concentrations limit our ability to provide estimates of diamond formation ages using 4He ingrowth. However, the limited observed 4He ingrowth (low U + Th/3He) together with a R/Ra value of 5.3 for peridotitic diamond K306 is comparable to the present-day sub-continental lithospheric mantle value and supports the young diamond formation age found by Re-Os dating of sulphides in the same diamond by Pearson et al. (1998). After correction for in-situ radiogenic 4He produced since diamond formation a large variation in 3He/4He remains in ∼1 Ga old eclogitic diamonds that is suggested to result from the variable influence of subducted altered oceanic crust that has low 3He/4He. Hence, the 3He/4He isotope tracer supports an origin of the diamond-forming fluids from recycled oceanic crust for eclogitic diamonds, as indicated by other geochemical proxies.

Synthesis of diamond nanostructures from carbon nanotube and formation of diamond-CNT hybrid structures

We report direct conversion of multiwall carbon nanotubes (CNTs), synthesized by chemical vapor deposition, into diamond by nanosecond pulsed laser melting process at ambient temperature and pressure in air without any catalysts. The Raman spectroscopy of the CNTs after the laser irradiation showed the characteristic diamond peak at around 1324-1325 cm−1. The downshift of this peak from its theoretical position (at 1332 cm−1) is explained by phonon confinement in nanostructured diamond. The SEM and TEM images show the formation of diamond mostly at the tip and bends of the CNTs. The grain size distribution and the shape of the converted nanodiamonds suggest that the transformation takes place by melting of the CNTs in a super undercooled state by nanosecond laser pulses, and subsequent rapid quenching to convert it into phase-pure diamond. The EBSD analysis illustrates the phase-pure single crystal diamond formation at the tips and bends of the CNTs. The high-resolution electron energy-loss spectrum in the STEM contains characteristic σ* peak at 292 eV for sp3 bonding of diamond. This study on the laser-induced direct conversion of CNTs to diamond marks a major breakthrough in the formation of diamond nanostructures and diamond-CNT hybrid for a variety of potential applications.

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

Methane and other hydrocarbons are major components of the mantle regions of icy planets. Several recent computational studies have investigated the high-pressure behaviour of specific hydrocarbons. To develop a global picture of hydrocarbon stability, to identify relevant decomposition reactions, and probe eventual formation of diamond, a complete study of all hydrocarbons is needed. Using density functional theory calculations we survey here all known C-H crystal structures augmented by targeted crystal structure searches to build hydrocarbon phase diagrams in the ground state and at elevated temperatures. We find that an updated pressure-temperature phase diagram for methane is dominated at intermediate pressures by CH 4 :H 2 van der Waals inclusion compounds. We discuss the P-T phase diagram for CH and CH 2 (i.e., polystyrene and polyethylene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH 4 (H 2 ) 2 van der Waals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa.

FTIR Spectroscopy Data and Carbon Isotope Characteristics of the Ophiolite‐hosted Diamonds

AbstractWe report new δ13C ‐values data and N‐content and N‐aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray‐Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 up to 589 ± 20% atomic ppm) with a consistently low aggregation state, show identical IR spectra dominated by strong absorption between 1130 cm−1 and 1344 cm−1, and hence characterize Type Ib diamond. Microdiamonds from the Luobusa peridotites have δ13C ‐PDB‐values ranging from ‐28.7‰ to ‐16.9‰, and N‐contents from 151 to 589 atomic ppm. The δ13C and N‐content values for diamonds from the Luobusa chromitites are ‐29‰ to ‐15.5‰ and 152 to 428 atomic ppm, respectively. Microdiamonds from the Ray‐Iz chromitites show values varying from ‐27.6 ‰ to ‐21.6 ‰ in δ13C and from 108 to 499 atomic ppm in N. The carbon isotopes values bear similar features with previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N‐contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds; we also analyzed showing enhanced δ13C ‐variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N‐aggregation state and N‐content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e. <950°C), and in a short residence time (i.e. within several million years) at high temperatures in the deep mantle.

Carbon isotopic signatures of diamond formation mediated by iron redox chemistry

Carbon isotopic signatures of diamond formation mediated by iron redox chemistry

Diamond-Bearing Root Beneath the Northern East European Platform (Arkhangelsk Region, Russia): Evidence from Cr-Pyrope Trace-Element Geochemistry

In this study, we reconstruct the composition and metasomatic evolution of the lithospheric mantle beneath the poorly-studied southern Arkhangelsk region, based on the geochemistry of 145 Cr-pyrope grains recovered from samples of modern rivers and stream sediments, to evaluate the diamond exploration potential of these territories. Based on the concentrations of Cr2O3, CaO, TiO2, and rare earth elements (REEs), the garnets are divided into four groups: (1) low-chromium lherzolitic pyropes with fractionated heavy REE patterns; (2) low- to medium-chromium pyropes of lherzolitic and megacryst associations with flat heavy REE patterns; (3) high-chromium lherzolitic pyropes with “humped” REE patterns; and (4) high-chromium and low-chromium lherzolitic and harzburgitic pyropes with sinusoidal REE patterns. The pyrope geochemistry suggests a multi-stage model for the evolution of the lithospheric mantle, including partial melting to different degrees and further metasomatic overprints by silicate and carbonatite melts. The results confirm that the lithospheric mantle beneath the study area is suitable for the formation and preservation of diamonds. The significant percentage of diamond-associated pyropes (15%) emphasizes the likelihood of high diamond contents in kimberlites to be discovered within the study area.

Evaluating mechanisms for eclogitic diamond growth: An example from Zimmi Neoproterozoic diamonds (West African craton)

Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ13C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ13C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ13C values) and carbonates (high δ13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ13C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.

From Kenorland to modern continents: tectonics and metallogeny

There are three stages in tectonic evolution of the Earth: (1) nucleation — from origin of protocratons to their assembly into Supercontinent Kenorland (2.7–2.5 Ga); (2) cratonization — from breakup of Kenorland (2.45 Ga) to the assembly of Columbia (1.85 Ga) and its reorganization into Rodinia (1.0–0.72 Ga); (3) modern plate tectonics — from breakup of Rodinia at 720 Ma until present. Analysis of time-space reorganizations of Archean granulite-gneiss terranes, which correspond to continental lithospheric keels, reveals five groups of protocratons (Nena, Ur, Congo-Sahara, NAsia and Atlantica) that remained almost intact during long time intervals.
 After the breakup of Kenorland, the continental crust rotated counter-clockwise. NAsia and Atlantica the least rotated and drifted relative to Nena, however the latter was rotated by 180°. Congo-Sahara, Ur and Kalahari were the most rotated. The assembly and breakup of the supercontinents clearly correlates with secular changes in dominant types of base, precious and ferrous metal deposits, as well as formation and emplacement of diamonds.

Extended Deep Earth Water Model for predicting major element mantle metasomatism

Abstract Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H 2 CO 3 0 . Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.

U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages

The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, α-implantation, α-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.

The study of the ontogenesis of crystals of the Ural type diamonds by the method of IR-spectrometry

The collection of the Ural, Anabar and Brazilian diamonds conducted research by infrared spectrometry method. It was found that a diamond with a low degree of aggregation of nitrogen defects composing the crystals of the later generation from mantle eclogite xenoliths and outer zones of the greater part of the crystals from deposits of kimberlite-lamproite type is missing or is poorly developed in the crystals of the Ural type. Conclusions about the thermal conditions of the Ural diamonds formation and their possible affinity to the specification and type of original sources.

An effect of reduced S-rich fluids on diamond formation under mantle-slab interaction

Experimental study, dedicated to understanding the effect of S-rich reduced fluids on the diamond-forming processes under subduction settings, was performed using a multi-anvil high-pressure split-sphere apparatus in Fe3C-(Mg,Ca)CO3-S and Fe0-(Mg,Ca)CO3-S systems at the pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–60 h. At the temperatures of 900 and 1000 °C in the carbide-carbonate-sulfur system, extraction of carbon from cohenite through the interaction with S-rich reduced fluid, as well as C0-producing redox reactions of carbonate with carbide were realized. As a result, graphite formation in assemblage with magnesiowüstite, cohenite and pyrrhotite (±aragonite) was established. At higher temperatures (≥1100 °C) formation of assemblage of Fe3+-magnesiowüstite and graphite was accompanied by generation of fO2-contrasting melts - metal-sulfide with dissolved carbon (Fe-S-C) and sulfide-oxide (Fe-S-O). In the temperature range of 1400–1600 °C spontaneous diamond nucleation was found to occur via redox interactions of carbide or iron with carbonate. It was established, that interactions of Fe-S-C and Fe-S-O melts as well as of Fe-S-C melt and magnesiowüstite, were С0-forming processes, accompanied by disproportionation of Fe. These resulted in the crystallization of Fe3+-magnesiowüstite+graphite assemblage and growth of diamond. We show that a participation of sulfur in subduction-related elemental carbon-forming processes results in sharp decrease of partial melting temperatures (~300 °C), reducting the reactivity of the Fe-S-C melt relatively to FeC melt with respect to graphite and diamond crystallization and decrease of diamond growth rate.

From Kenorland to Modern Continents: Tectonics and Metallogeny

There are three stages in tectonic evolution of the Earth: (1) nucleation, from the origin of protocratons to their assembly into the Kenorland supercontinent (2.7–2.5 Ga); (2) cratonization, from the breakup of Kenorland (2.45 Ga) to the assembly of Columbia (1.85 Ga) and its reorganization into Rodinia (1.0–0.72 Ga); and (3) modern plate tectonics, from the breakup of Rodinia 720 Ma until the present. Analysis of the time-space reorganizations of Archean granulite–gneiss terranes, which correspond to continental lithospheric keels, reveals five groups of protocratons (Nena, Ur, Congo–Sahara, NAsia and Atlantica) that remained almost intact during long time intervals. After the breakup of Kenorland, the continental crust rotated counterclockwise. NAsia and Atlantica rotated the least and drifted relative to Nena; however, the latter rotated by 180○. Congo‒Sahara, Ur, and Kalahari rotated the most. The assembly and breakup of the supercontinents clearly correlates with secular changes in dominant types of base, precious, and ferrous metal deposits, as well as the formation and emplacement of diamonds.

Corrigendum to “Plume-lithosphere interaction, and the formation of fibrous diamonds” by Broadley et al., 2018

Corrigendum to “Plume-lithosphere interaction, and the formation of fibrous diamonds” by Broadley et al., 2018

Theoretical Investigation of Phase Transitions of Graphite and Cubic 3C Diamond Into Hexagonal 2H Diamond Under High Pressures

Possible techniques for experimentally obtaining hexagonal diamond are studied in the scope of the density functional theory method. It has been found that hexagonal diamond may be created as a result of structural transition at 61–68 GPa uniaxial compression from orthorhombic AB graphite and at 57–66 GPa from hexagonal AA graphite. Also the formation of hexagonal diamond is shown to take place in case of very strong (300–380 GPa) compression of cubic diamond. X‐ray and electron‐microscopic data on nanodiamonds from meteorite craters are analyzed for the presence of hexagonal diamond. The analysis shows that impact‐origin carbon materials do not contain pure cubic and hexagonal diamonds, and the layers of nascent crystals of diamond polytypes are randomly packed.

Development of novel TiO2-Cu2(OH)PO4 heterojunction as nanophotocatalyst for improved Cr (VI) reduction

TiO2-Cu2(OH)PO4 heterojunction nanomaterials with 2–10% of Cu2(OH)PO4 were synthesized by hydrothermal method. The physiochemical properties of synthesized nanomaterials were studied using XRD, FESEM, HRTEM, N2 physisorption, UV–vis (DRS) and XPS. XRD indicates the formation of anatase phase. HRTEM conforms the proper heterojunction formation of diamond shaped crystalline nanoparticles with average size 20–30 nm. Addition of 10% Cu2(OH)PO4 with titania extend the light absorption to visible-NIR region and decrease the overall band gap to 2.6 eV thus enhancing the visible light absorption. The in-situ generated Ti3+ and oxygen vacancies in the TiO2 lattice along with the metallic copper at the interface results in extended visible light absorption up to NIR range and improved charge separation. Photocatalytic reduction of Cr (VI) under visible light was studied to evaluate the effect of Cu2(OH)PO4 addition. TiO2 heterojunction with 10% of Cu2(OH)PO4 can completely reduce Cr (VI) into Cr (III) within 90 min of visible light irradiation whereas in same interval TiO2 and Cu2(OH)PO4 shows negligible and 10% reduction, respectively. Improved photocatalytic activity of TiO2-Cu2(OH)PO4 nanophotocatalyst under visible radiation is attributed to the synergistic effect of high surface area, improved visible-NIR light absorption and formation of proper heterojunction.

Depth of diamond formation obtained from single periclase inclusions

Depth of diamond formation obtained from single periclase inclusions

Формирование структуры гексагонального алмаза: ab initio расчеты

Modeling of the structure formation of hexagonal diamond from graphite and cubic diamond was performed using the density functional theory method. The transformation of orthorhombic AB graphite into hexagonal diamond can occur as a result of uniaxial compression at pressures above 61 GPa. For the formation of hexagonal diamond from AA graphite, the pressure must exceed 57 GPa. Another possible way to obtain hexagonal diamond is uniaxial compression of cubic diamond at pressures from 300 to 380 GPa. The data of theoretical calculations were used to interpret the diffraction data obtained as a result of X-ray diffraction and electron microscopic analysis of natural and synthetic diamond-like materials. It was found that crystals with the ideal cubic and hexagonal structure are not contained in diamond-like carbon materials, and their structure is characterized by random packing of molecular layers.

Large diamond education mechanism

The article shows the conditions for the formation and existence of both polymorphic modifications of carbon in natural conditions The formation of diamond and carbon can be realized in the presence of carbon containing rocks in near surface and surface reducing conditions with a low partial pressure of oxygen The formation of explosion tubes is carried out due to a chain reaction between deep hydrogen and oxygen in the air with the release of a huge amount of heat increasing the temperature to deg C during a typical exothermic reaction

Graphite and Diamond Formation in the Carbide–Oxide–Carbonate Interactions (Experimental Modeling under Mantle P,T-Conditions)

Experimental modeling of the formation of graphite and diamond as a result of carbide–fluid interactions was performed in the Fe3C–SiO2–Al2O3–(Mg,Ca)CO3 systems at 6.3 and 7.5 GPa and 1100–1650 °C. In the experiments with ƒO2-gradient (7.5 GPa, 1250–1350 °C), graphite + magnesiowüstite + garnet ± cohenite assemblage was formed. Graphite was produced through the redox interactions of carbide with carbonate or CO2 (reducing conditions), and redox reactions of magnesiowüstite and CO2 (oxidizing conditions). At 1450–1650 °C, crystallization of graphite, garnet, magnesiowüstite and ferrospinel, as well as generation of Fe2+,3+-rich carbonate–silicate melt occurred. This melt, saturated with carbon, acted as a medium of graphite crystallization and diamond growth on seeds. In the experiments without ƒO2-gradient (6.3 GPa), decarbonation reactions with the formation of CO2-fluid and Fe,Mg,Ca-silicates, as well as C0-producing redox reactions of CO2-fluid with cohenite were simultaneously realized. As a result, graphite (± diamond growth) was formed in assemblage with Fe2+,Fe3+,Mg-silicates and magnetite (1100–1200 °C), or with Fe3+-rich garnet and orthopyroxene (1300–1500 °C). It has been established that a potential mechanism for the crystallization of graphite or diamond growth is the oxidation of cohenite by CO2-fluid to FeO and Fe3O4, accompanied by the extraction of carbon from Fe3C and the corresponding reduction of CO2 to C0.