Yuri N. Palyanov

The role of mantle ultrapotassic fluids in diamond formation.

Analysis of data on micro- and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400–1,800°C, with different compositions of melts and fluids in the KCl/K2CO3/H2O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes.

Mantle-slab interaction and redox mechanism of diamond formation.

Significance The primary question that we address in this study is what happens when a carbonate-bearing crust is subducted to depths where the Earth’s mantle is metal saturated. Subduction plays an important role in the evolution of the Earth’s interiors, but the mechanism of the interaction between the oxidized slab and reduced mantle remains unclear. Here we report the results of high-pressure redox-gradient experiments on the interaction between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary, and present mechanisms of diamond formation ahead of and behind the redox front. We demonstrate that the redox mechanism revealed in this study can explain the contrasting heterogeneity of natural diamonds on the composition of inclusions, carbon isotopic composition, and nitrogen impurity content. Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle–slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.

The system K2CO3-MgCO3 at 6 GPa and 900–1450 °C

Abstract Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1450 °C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 °C. We propose that K2Mg(CO3)2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K2Mg(CO3)2+MgCO3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900-1000 °C. Thus, the K2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K2Mg(CO3)2 compound was studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K2Mg(CO3)2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)2 exhibited a trigonal R3̅m structure previously established at ambient conditions.

Germanium: a new catalyst for diamond synthesis and a new optically active impurity in diamond.

Diamond attracts considerable attention as a versatile and technologically useful material. For many demanding applications, such as recently emerged quantum optics and sensing, it is important to develop new routes for fabrication of diamond containing defects with specific optical, electronic and magnetic properties. Here we report on successful synthesis of diamond from a germanium-carbon system at conditions of 7 GPa and 1,500–1,800 °C. Both spontaneously nucleated diamond crystals and diamond growth layers on seeds were produced in experiments with reaction time up to 60 h. We found that diamonds synthesized in the Ge-C system contain a new optical centre with a ZPL system at 2.059 eV, which is assigned to germanium impurities. Photoluminescence from this centre is dominated by zero-phonon optical transitions even at room temperature. Our results have widened the family of non-metallic elemental catalysts for diamond synthesis and demonstrated the creation of germanium-related optical centres in diamond.

Monitoring diamond crystal growth, a combined experimental and SIMS study

Detailed ion microprobe measurements have been made on two synthetic diamond crystals in order to investigate how variations in chemical and isotopic compositions are related to growth sectors and overall growth history. The crystals were grown by the metal catalyst technique under identical T - P conditions of 1450 °C and 5.5 GPa, but with different source nitrogen abundances. Measurements for carbon and nitrogen isotope compositions and nitrogen abundance were made in traverses across the crystal sectors, which included cubic sectors and octahedral sectors of both relatively rapid and relatively slow growth. In both crystals an early phase of growth dominated by falling δ 13 C and rising of N ppm is followed by an extensive phase of growth with moderately constant δ 13 C and gradually descending N ppm . The change from falling to stable δ 13 C ratios has been numerically modelled on the basis of the carbon isotope fractionation between the carbon solution in metal melt and the growing diamond in closed system; the stabilization of the diamond carbon isotope composition is achieved once a steady state is attained and diamond grows with the same carbon isotope composition as the graphite source. The decreasing N ppm values appear to be a product of Rayleigh fractionation. Carbon isotope compositions show little difference between different growth sectors, and δ 15 N values in any given sector show no change with time of growth. However, the nitrogen isotope compositions show major differences of ca. 30 ‰ between octahedral and cubic sectors. These differences are not related to growth rate or time of growth and appear to represent a consistent difference in N isotope adhesion between the two faces.

Melting and subsolidus phase relations in the system Na2CO3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle

Abstract Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1400 °C temperatures. Sub-solidus assemblages are represented by Na2CO3+Na2Mg(CO3)2 and Na2Mg(CO3)2+MgCO3, with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na2CO3 occurs between 1350 and 1400 °C. We propose that Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2Mg(CO3)2 compounds have been studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X‑ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2Mg(CO3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under “dry” conditions or at 900-1100 °C under hydrous conditions. Thus, the Na2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples-sheared peridotite xenoliths (190-230 km depth).

Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900–1700 °C and its relation to the system CaCO3-FeCO3-MgCO3

Abstract The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multianvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900-1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R3c, while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn, to calcite, R3c, structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. At X(Ca) = 0-30 mol%, 6 GPa and 1500-1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X(Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO2 in (Fe,Ca)CO3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO3- MgCO3-FeCO3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.

Crystal Growth of Diamond

Abstract In this chapter we have presented a review of the state-of-the-art of diamond crystal growth using high pressure high temperature (HPHT) and chemical vapor deposition (CVD) approaches. We considered the main methods, techniques, and equipment used for single crystal diamond growth. Special attention is given to discussing the effects of growth conditions on the growth and properties of diamond. Current achievements in HPHT and CVD growth of single crystal diamond and its applications are considered.

Phase relationships in the system K2CO3-CaCO3 at 6 GPa and 900-1450 °C

Abstract Phase relations in the system K2CO3-CaCO3 have been studied in the compositional range, X(K2CO3), from 100 to 10 mol%, at 6.0 GPa and 900-1450 °C. At 900-950 °C, the system has three intermediate compounds: K6Ca2(CO3)5, K2Ca(CO3)2, and K2Ca3(CO3)4. The K2Ca(CO3)2 compound decomposes to the K6Ca2(CO3)5 + K2Ca3(CO3)4 assembly above 950 °C. The K6Ca2(CO3)5 and K2Ca3(CO3)4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 64 and 44 mol% near 1200 °C and at 23 mol% near 1300 °C. K2CO3 remains as a liquidus phase at 1300 °C and 75 mol% and melts at 1425 ± 20 °C. Aragonite remains as a liquidus phase at 1300 °C and 20 mol% and at 1400 °C and 10 mol%. CaCO3 solubility in K2CO3 and K2CO3 solubility in aragonite are below the detection limit (<0.5 mol%). Infiltration of subduction-derived K-rich Ca-Mg-Fe-carbonatite into the Fe0-saturated mantle causes the extraction of (Mg,Fe)CO3 components from the melt, which shifts its composition toward K-Ca-carbonatite. According to our data this melt can be stable at the P-T conditions of subcratonic lithosphere with geothermal gradient of 40 mW/m2 corresponding to temperature of 1200 °C at 6 GPa.

Phase relations on the K2CO3-CaCO3-MgCO3 join at 6 GPa and 900–1400 °C: Implications for incipient melting in carbonated mantle domains

Abstract To constrain the ternary K2CO3-CaCO3-MgCO3 T-X diagram at 6 GPa and to expand upon the known K-Mg, K-Ca, and Ca-Mg binary systems we have carried out multi-anvil experiments along the K2CO3-Ca0.5Mg0.5CO3 join. The diagram has primary phase fields for K2CO3, K2Mg(CO3)2, K2Ca0.1–0.5 Mg0.9–0.5(CO3)2, K4CaMg(CO3)4, Ca-magnesite, and dolomite. The system has two liquidus minima near 1000 °C. At one minimum, a liquid with the composition of 36 K2CO3·64(Ca0.65Mg0.35)CO3 is in equilibrium with three phases: Ca-magnesite, K2Ca0.1–0.5Mg0.9–0.5(CO3)2, and K6Ca2(CO3)5. The other minimum, a liquid with the composition of 62 K2CO3·38Ca0.72Mg0.28CO3 is in equilibrium with K2CO3, K4CaMg(CO3)4, and K6Ca2(CO3)5. At 900 °C, the ternary diagram contains two- and three-phase regions with Ca-magnesite, aragonite, K2Ca3(CO3)4, K2Ca(CO3)2, K6Ca2(CO3)5, K2CO3, K2Ca0.1–0.5Mg0.9–0.5(CO3)2 solid solution, K2Mg0.9Ca0.1(CO3)2, and K4CaMg(CO3)4. We also expect an existence of primary phase fields for K6Ca2(CO3)5, K2Ca3(CO3)4 and aragonite. We suggest that extraction of K from silicate to carbonate components should decrease the minimum melting temperature of dry carbonated mantle rocks up to 1000 °C at 6 GPa and yield ultrapotassic Ca-rich dolomite melt containing more than 10 mol% K2CO3. As temperature increases above 1200 °C the melt evolves toward an alkali-poor, dolomitic liquid if the bulk molar CaO/MgO ratio >1, or toward K-Mg-rich carbonatite if bulk CaO/MgO <1. The majority of compositions of carbonatite inclusions in diamonds from around the world fall within the magnesite primary field between the 1300 and 1400 °C isotherms. These melts could be formed by partial melting of magnesite-bearing peridotite or eclogite with bulk Ca/Mg <1 at temperatures ≤1400 °C. A few compositions revealed in the Ebelyakh and Udachnaya diamonds (Yakutia) fall within the dolomite primary field close to the 1200 °C isotherm. These melts could be formed by partial melting of dolomite-bearing rocks, such as carbonated pelite or eclogite with bulk Ca/Mg <1 at temperatures ≤1200 °C.