Genesis Of Diamond Research Articles

Physicogeochemical Mechanisms of the Genesis of Matryoshka-Type Diamonds on the Basis of the Mantle-Carbonatite Theory

A physicochemical analysis of the genesis of a unique Matryoshka-type diamond from the Nyurbinskaya kimberlite pipe of the Nakyn kimberlite field, Yakutia, Russia, was performed. The specimen consists of a host diamond with a cavity containing a loose diamond inclusion; two through holes with a width of 0.1–0.4 mm emerge from the cavity. The analysis is based on the mantle-carbonatite theory of the genesis of diamonds and associated phases developed on the basis of consistent results of a physicochemical experiment and an analytical study of paragenetic inclusions in natural diamonds from kimberlite deposits. The published data of crystal morphological and physical studies of the Matryoshka diamond were used as well. As a result, the physicogeochemical mechanisms of nucleation and crystallization of the host diamond with a cavity and diamond inclusion under the conditions of mass genesis of diamonds in completely miscible carbonate–eclogite–carbon melts of the upper-mantle diamond-forming chamber are substantiated. The initially closed cavity was filled with a diamond-forming carbonate–silicate melt with dissolved carbon. In addition, an analysis of the conditions of etching and dissolution of the host diamond and diamond inclusion during the kimberlite transport of diamond-bearing material from the mantle chamber to the depths of the Earth’s crust was performed. The reasons for the explosive formation of cone-shaped through holes in the main diamond, which was accompanied by the ejection of the diamond-forming medium and the filling of the cavity with (С–О–Н)-fluid-containing kimberlite melt, are considered. The final episodes of partial dissolution of diamonds from the Matryoshka sample by kimberlite and assimilated melts continued during the formation of a cumulative chamber in the Earth’s crust and its solidification with the release of a highly compressed C–O–H fluid. “Fluid drilling” of the top of the cumulative chamber stimulated the explosive formation of the Nyurbinskaya pipe and its filling with kimberlite and assimilated diamondiferous matter. With prolonged compaction of this substance, the Matryoshka diamond was subjected to atmospheric and hydrothermal factors, as is evidenced by fine-grained sedimentary barite, a mineral of barium sulphate, found in the through holes of the host diamond.

Genesis of kimberlites and diamond

Genesis of kimberlites and diamond

Color Genesis of Brown Diamond from the Mengyin Kimberlite, China

The Mengyin diamondiferous kimberlite cluster in Shandong province is one of the three major sources of natural diamond in China, where many brown diamonds are mined, but the genesis of their color is still controversial. In this paper, studies including microscopic examination, optical properties of orthogonal polarization, low temperature photoluminescence spectra, infrared spectra, Raman spectra, ultraviolet-visible absorption spectra, luminescence of cathodoluminescence, and transmission electron microscopy have been carried out on the uncut brown diamonds and their slice samples to constrain on the color genesis of brown diamond from the Mengyin deposit. The results show that the brown color is dominantly caused by plastic deformation, and some samples are also caused by non-deformation-related defects and inclusions.

Genesis of Diamond in Metal–Carbon and Metal–Sulfur–Carbon Melts: Evidence from Experimental Data

The experimental data on diamond growth in the Fe–Ni–S–C and Fe–S–C systems with a sulfur content of 5–14 wt % at 5.5 GPa and 1300–1350°C are reported. Colorless and light yellow diamond crystals with a weight of 0.1–0.8 ct were synthesized. It is shown in the Fe–S–C system that at 5.5. GPa diamond may crystallize in a very narrow temperature range, from 1300 to 1370°C. Based on comparative analysis of the experimental data and the results of the study of native iron inclusions in natural diamonds from kimberlite pipes, it is suggested that diamond genesis may be partly controlled by the pre-eutectic (by the concentration of sulfur in relation to metal) metal–sulfide melt.

57Fe Mössbauer spectroscopy used to develop understanding of a diamond preservation index model

57Fe Mossbauer spectroscopy has provided precise and accurate iron redox ratios Fe2+/Fe3+ in ilmenite, FeTiO3, found within kimberlite samples from the Catoca and Camatxia kimberlite pipes from N.E. Angola. Ilmenite is one of the key indicator minerals for diamond survival and it is also one of the iron-bearing minerals with iron naturally occurring in one or both of the oxidation states Fe3+ and Fe2+. For this reason it is a good indicator for studying oxygen fugacities (fO2) in mineral samples, which can then be related to iron redox ratios, Fe2+/Fe3+. In this paper we demonstrate that the oxidation state of the ilmenite mineral inclusion from sampled kimberlite rock is a key indicator of the oxidation state of the host kimberlite assemblage, which in turn determines the genesis of diamond, grade variation and diamond quality. Ilmenite samples from the two different diamondiferous kimberlite localities (Catoca and Camatxia) in the Lucapa graben, N.E. Angola, were studied using Mossbauer spectroscopy and X-Ray Diffractometry, in order to infer the oxidation state of their source regions in the mantle, oxygen partial pressure and diamond preservation conditions. The iron redox ratios, obtained using Mossbauer spectroscopy, show that the Catoca diamond kimberlite is more oxidised than kimberlite found in the Camatxia pipe, which is associated within the same geological tectonic structure. Here we demonstrate that57Fe Mossbauer spectroscopy can assist geologists and mining engineers to effectively evaluate and determine whether kimberlite deposits are economically feasible for diamond mining.

Diamond crystallization and growth in N–H enriched environment under HPHT conditions

Advances in understanding the state and transformation of impurity-related defects in natural diamond have been achieved by means of high-pressure and high-temperature annealing experiments. However, the existing literature featuring complex behaviors of aggregated nitrogen and hydrogen-related defect (3107 cm−1 center) has remained controversial. Their formation mechanism during diamond crystallization is still unclear, thus further investigation is required. In this work, we have successfully synthesized a variety of high-quality diamonds containing nitrogen impurities with IaA (nitrogen pairs), IaB (group of four nitrogen atoms around a vacancy) and Ib (single-substitutional nitrogen) characteristics ranging from <1 ppm to 3380 ppm at pressures ranging from 5.0 GPa to 6.3 GPa and temperatures of 1300–1650 °C. Our results provide new experimental evidence for the aggregation of nitrogen impurities (A- and B-centers) during diamond growth. We notice that the hydrogen is easily trapped by nitrogen atoms to form nitrogen–hydrogen complexes (–NH, –NH2, –NH3) when the nitrogen tends to form C-centers in hydrogen-enriched environments. Additionally, we observed that the high reaction temperature and formation of A-centers during diamond growth play important roles in the formation of the 3107 cm−1 center. We believe the current results could be helpful for further understanding and constructing a clear model of impurity-related defects, and thus provide us deeper insights into the genesis of natural diamond.

Diamond crystallization with a carbonyl nickel catalyst under high pressure and temperature

For decades, the origin of aggregated nitrogen in diamond has been studied and has led to an important stage in solving the problems of natural diamond genesis. However, clarifying how B-centers (groups of four nitrogen atoms around a vacancy) form in diamond is still one of the most challenging issues in material science. In this work, we apply carbonyl nickel powders as a catalyst to grow diamond over a wide range of pressures, 5.0–6.2 GPa, and temperatures, 1300 to 1800 °C, and we report the first synthesis of high-quality diamonds containing nitrogen impurities with IaB characteristics. Both spontaneous nucleation of diamond and growth of diamond on seeds have been achieved by film growth and temperature gradient growth methods. By Fourier-transform infrared spectrometry, we reveal a novel formation mechanism for aggregated nitrogen impurities in B-centers, that is, B-centers can be readily aggregated by four C-centers (single-substitutional nitrogen atoms) during a diamond growth process under high pressure and temperature. High reaction temperatures can effectively contribute to the formation of nitrogen-vacancy defects in diamond and play a key role in the formation of B-centers during diamond growth. Our current findings will be helpful for further understanding and constructing a clear model of aggregated nitrogen, and thus provide us with deeper insights into the genesis of natural diamond.

Crystal growth and characterization of hydrogen-doped single diamond with Fe(C5H5)2 additive

In this paper, a series of high-quality hydrogen-doped diamonds is successfully synthesized in Ni70Mn25Co5-C system by using Fe(C5H5)2 as hydrogen source at pressures ranging from 5.5 GPa to 6.0 GPa and temperatures of 1280-1400 ℃. We find that both pressure and temperature conditions strengthen with adding the Fe(C5H5)2. Scanning electron microscope micrographs show that the obtained diamonds at low levels of Fe(C5H5)2 additive have smooth surfaces. However, many defects are found and some pores appear on the diamond surface with increasing the Fe(C5H5)2 additive in the system. From the obtained Fourier transform infrared (IR) spectrum, we notice that there is no significant change of nitrogen concentration in the synthesized diamond with the Fe(C5H5)2 additive lower than 0.3 wt%, while the nitrogen concentration gradually decreases with the further increase of Fe(C5H5)2 additive. In the system with 0.5 wt% Fe(C5H5)2 additive, the nitrogen concentration in synthesized diamond is only half that of system without Fe(C5H5)2 additive. Meanwhile, the hydrogen associated IR peaks of 2850 cm-1 and 2920 cm-1 are gradually enhanced with the increase of Fe(C5H5)2 additive in the system, indicating that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp3-CH2-symmetric (2850 cm-1) and sp3 CH2-antisymmetric (2920 cm-1) vibrations. From the obtained Raman spectrum, we find the incorporation of hydrogen impurity leads to a significant shift of the Raman peak towards higher frequencies from 1333.90 cm-1 to 1334.42 cm-1 with increasing the concentration of Fe(C5H5)2 additive from 0.1 wt% to 0.5 wt%, thereby giving rise to some compressive stress in the diamond crystal lattice. This is the first time that the gem-grade hydrogen-doped diamond single crystal, with size up to 3.5 mm has been successfully synthesized by using new hydrogen source Fe(C5H5)2 additive. We believe that our work can provide a new method to study the influence of hydrogen impurity on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.

Effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond under high temperature and high pressure

As is well known, most natural diamonds usually contain not only aggregated nitrogen up to thousands of ppm but also hydrogen. Therefore, the studies of nitrogen and hydrogen impurities in a diamond are of interest for improving the physical properties of a diamond and solving the problems about natural diamond genesis. From this point of view, in this paper, we choose C3N6H6 powders as a nitrogen and hydrogen source and select high-quality seed crystals with {100} facets as the growth facets. The effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond in the NiMnCo-C system at pressures ranging from 5.5 GPa to 6.2 GPa and temperatures of 1280-1450 ℃ are investigated. Experimental results show that both pressure and temperature, which are the synthesis conditions, increase with the increases of nitrogen and hydrogen content in diamond-growth environment, and the V-shape region of diamond-forming moves up. From the obtained Fourier transform infrared spectra, we notice that there is a significant change of the nitrogen concentration in the synthesized diamond with increasing the nitrogen and hydrogen content in the diamond-growth environment. We calculate the nitrogen concentrations in those diamonds and the results indicate that the highest concentration of nitrogen is up to 2000 ppm. Meanwhile, we notice that the hydrogen associated infrared peaks of 2850 and 2920 cm-1 are gradually enhanced, which shows that both nitrogen and hydrogen are successfully co-doped into the diamond. Scanning electron microscope micrographs show that the {111} face is elongated and has triangulated textures appearing on the surface with nitrogen and hydrogen co-doped into the diamond. This result indicates that the synergistic doping of nitrogen and hydrogen has a great influence on the morphology of {100}-oriented single diamond. From the obtained Raman spectra, we find a shift towards higher frequency of the Raman peak from 1330.23 cm-1 to 1330.40 cm-1 and the full width at half maximum increases from 3.12 cm-1 to 4.66 cm-1 with increasing the concentrations of nitrogen and hydrogen in diamond-growth environment. This is the first report about nitrogen and hydrogen co-doped on 100-oriented single diamond by far. This work can provide a new method to study the influences of nitrogen and hydrogen impurities on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.

Radiation damage to Kokchetav UHPM diamonds in zircon: Variations in Raman, photoluminescence, and cathodoluminescence spectra

We conducted detailed in-situ Raman, photoluminescence (PL) and cathodoluminescence (CL) studies on microdiamonds in a tourmaline-rich quartzofeldspathic rock from the Kokchetav Massif, Kazakhstan. The microdiamonds occur as inclusions in the cores of K-tourmaline and in zoned zircons with varying U contents. The results of 2D Raman mapping of zircon showed that the U-rich parts were more metamictized than the U-poor parts. All the diamonds showed a strong Raman band at approximately 1332 cm − 1 , however, the features of the Raman bands were distinctly different depending on the host minerals. On the one hand, diamonds in tourmaline had a sharp Raman band that is similar to that of kimberlite diamonds [full width at half maximum (FWHM): 2–3 cm − 1 ]. On the other hand, diamonds in zircon had a broad and downshifted band compared to those in tourmaline. In particular, diamonds in U-rich cores of zircons (up to 0.15 wt.% UO 2 ) showed broader and more downshifted Raman bands (FWHMs and peak positions varied up to 9.3 cm − 1 and 1328 cm − 1 , respectively), with additional small bands at approximately 1490 cm − 1 and 1630 cm − 1 . A negative correlation was observed between the peak position and the FWHM of the principal Raman band of microdiamonds. Furthermore, the PL and CL spectra showed systematic variations. Diamonds in zircons with low to moderate U-concentrations had very strong PL and CL compared to diamonds in U-rich zircons and in tourmalines. Several characteristic peaks appeared in the PL and CL spectra, indicating the presence of irradiation and/or nitrogen-related point defects in the diamonds. PL and CL of microdiamonds in high-U zircon were weak, but still showed irradiation-related peaks. The relationship between the occurrence of microdiamonds (i.e., corresponding to the estimated total α-dose since crystallization) and the Raman, PL, and CL spectral characteristics of microdiamonds strongly suggests that radiation damage predominantly caused by α-particle emission from radioactive decay of actinides (mainly U) in zircon is a principal factor for the spectral variations. Radiation damage (metamictization) is probably a common phenomenon among microdiamonds in zircons in various diamond-bearing rocks from the Kokchetav Massif. To discuss the genesis of diamonds based on their Raman, PL, or CL spectra, it is highly recommended that microdiamonds included in actinide-bearing minerals (e.g., zircon) should be treated with the most careful attention, because post-crystallization radiation damage significantly influences the spectra. • We compared Raman, PL, and CL spectra of microdiamonds in tourmaline and zircon. • The Raman peak of diamonds in zircon is downshifted and broad. • Diamonds in zircon show PL and CL that are characteristic of irradiated diamonds. • Radiation damage is important when genesis of diamond is discussed by spectroscopy.

Diamond crystallization in the Fe-Co-S-C and Fe-Ni-S-C systems and the role of sulfide-metal melts in the genesis of diamond

Diamond crystals 0.1–0.8 carats were synthesized in experiments conducted in a BARS split-sphere multianvil high-pressure apparatus in the systems Fe-Co-S-C and Fe-Ni-S-C at a pressure of 5.5 GPa and temperature of 1300°C. The microtextures of the samples and the phases accompanying diamond (carbides, graphite, monoslufide solid solution, pentlandite, and taenite) are examined in much detail, the properties of metal-sulfide-carbon alloys are discussed, and issues related to the genesis of sulfide inclusions in diamonds and graphite crystallization in the diamond stability field are considered. The experiments demonstrate that diamonds can be synthesized and grow in pre-eutectic metal-sulfide melts with up to 14 wt % sulfur at relatively low P-T parameters, which correspond to the probable temperatures and pressures of natural diamond-forming processes at depths of approximately 150 km in the Earth’s upper mantle.

Nano-inclusion suite and high resolution micro-computed-tomography of polycrystalline diamond (framesite) from Orapa, Botswana

A single polycrystalline diamond aggregate from the Orapa kimberlite (Botswana) contains a syngenetic micro- and nano-inclusion suite of magnetite, pyrrhotite, omphacite, garnet, rutile and C–O–H fluid in order of abundance. This suite of inclusions is distinctly different from those in fibrous diamonds, although the presence of sub-micrometer fluid inclusions provides evidence for a similarly important role of fluids in the genesis of polycrystalline diamond. It is the first study of polycrystalline diamond by High resolution μ-CT (Computed Tomography) reaching a resolution of 1.3 μm using polychromatic X-rays. Combined with Focused Ion Beam assisted Transmission Electron Microscopy the method reveals epigenetic replacement coatings of hematite and late stage sheet silicates around the magnetites showing that magnetites are often (but not always) interstitial to the diamond and, thus, were open to late stage overprinting. It is proposed that the polycrystalline diamond formed by a redox reaction between a small-scale carbonatitic melt and a sulfide-bearing eclogite. The water released from the melt during diamond precipitation fluxed local melting of the surrounding eclogite, and oxidation of sulfide phases to magnetite, which mingled with the carbonatitic melt and (re-)precipitated locally.

Parental media for diamonds and primary inclusions by evidence of physicochemical experiment

The mantle-carbonatite conception of genesis of diamond (Litvin, 2007, 2009) is briefly outlined. A generalized of compositions of the multi-component heterogeneous parental medium for diamond and primary inclusions therein is first developed. Boundary compositions of the parental medium are represented by mineral components of the peridotitic and eclogitic parageneses, mantle carbonatites and C-O-H-fluids, phases of accessory type both soluble in the carbonate-silicate melts (chlorides, phosphates, etc.) and insoluble in them (sulfides), as well as carbon. Phase relations of the multi-component eclogite-carbonatite-sulfide-diamond are studied at 7 GPa. Syngenesis diagram for diamond, paragenetic and xenogenetic minerals (including the solubility curves of diamond in carbonate-silicate and sulfide melts) is constructed. Physicochemical mechanism of formation of diamond and paragenetic minerals is revealed. Genetic classification of primary inclusions in natural diamonds is worked out. Analysis of physicochemical history of diamond formation in the mantle chamber of carbonatite magma is accomplished. Introduction .The mantle-carbonatite conception of genesis of diamond of kimberlite deposits (Litvin, 2007 a, b , 2009) is justified by the complex of mineralogical, experimental and theoretical data. Completely miscible carbonate-silicate growth melts with dissolved elemental carbon (carbon source for diamond) form a basis for the parental media for the dominating mass of natural diamonds and paragenetic minerals. The parental carbonate-silicate-carbon melts are multi-component heterogeneous media with strongly changeable compositions. So, silicate constituents are presented by minerals and components both peridotite and eclogite parageneses (Sobolev, 1974). It is established in the high- pressure high-temperature experiments that both peridotite-carbonatite-carbon and eclogite-carbonatite- carbon melts are represented as highly efficient diamond-forming media. An example of diamond crystallization in melt oversaturated in dissolved carbon of the eclogite-carbonatite system is demonstrated in the fig. 1. Composition of parental medium for diamond and primary inclusions. A generalized of compositions of the multi-component heterogeneous parental medium for diamond and primary inclusions therein is developed (Fig. 2).The major composition tetrahedron of the system peridotite-eclogite-carbonatite-soluble admixture componens is its basic element. Belonging of the mineral assemblage of syngenetic inclusions in natural diamonds to two major parageneses, - peridotitic and eclogitic, - is took into consideration. The boundary compositions of the parental medium are determined with mineral components of the auxiliary tetrahedrons - peridotitic (Ol-Opx-Cpx-Grt), eclogitic (Cpx-Grt-Crn-Coes), mantle carbonatite (Mg, Fe, Ca, K, Na, etc. carbonates), phases and components of admixtured accessory sort - soluble in carbonate-silicate melts (chlorides, phosphates, etc.) including the components of C-O-H-volatile compounds, and carbon too. All the components of the boundary tetrahedrons mentioned above are responsible for formation of paragenetic mineral phases together with diamond and, correspondingly, for capture of the paragenetic inclusions by natural diamonds. At the same time due to physicochemical experiment it turned out that xenogenetic mineral phases are presented within the assemblage of so called syngenetic inclusions. Sulfide minerals which are not soluble in diamond-forming carbonate-silicate melts as well as sulfide melts which are completely immiscible with these are among the xenogenetic phases. In the fig. 2, the auxiliary boundary tetrahedron for sulfide components is positioned out of the major tetrahedron and is

Experimental characterization of diamond crystallization in melts of mantle silicate-carbonate-carbon systems at 7.0–8.5 GPa

Diamond crystallization from carbon solutions in compositionally variable melts of model eclogite with dolomite [CaMg(CO3)2], potassium carbonate (K2CO3), and multicomponent K-Na-Ca-Mg-Fe carbonates was studied at 7.0–8.5 GPa. Concentration barriers for the nucleation of the diamond were determined at a standard pressure of 8.5 GPa for variable proportions of silicate and carbonate components in the growth solutions. They correspond to 35, 65, and 40 wt % of silicate components for systems with dolomite, K2CO3, and carbonatites, respectively. At higher contents of silicates in silicate-carbonate melts, the nucleation of diamond phase ceases, but diamond crystallization on seed crystals continues and is accompanied by the spontaneous crystallization of thermodynamically unstable graphite. In melts of the albite (NaAlSi3O8)-K2CO3-C compositions, the concentration barrier of diamond nucleation at 8.5 GPa is up to 90–92 wt % of the albite component, and diamond growth on seeds was observed in albite-carbon melts. Using mineralogical and experimental data, we developed a model of mantle carbonate-silicate (carbonatite) melts as the main parental media for natural diamonds; it was shown that the composition of the silicate constituent of such parental melts is variable and corresponds to the mantle ultrabasic-basic series. With respect to concentration contributions and dominant role in the genesis of diamond in the Earth’s mantle, major (carbonate and silicate) and minor or admixture components were distinguished. The latter include both soluble in carbonate-silicate melts (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble components (sulfides, metals, and carbides). Both major and minor components may affect the position of the concentration barriers of diamond nucleation in natural parent media.

Carbon, part 2: diamond

In Part 1 (Minerals explained 43, Geology Today 2006, v.22, no.2, pp.71–77) graphite was examined, the polymorph of carbon that is stable over a wide temperature range, but only at relatively low pressures. The other principal polymorph of carbon, diamond, is dealt with here in Part 2. Diamond has a very large stability range over both temperatures and pressures, although it is created at similar depths in the Earth's crust, probably in the mantle (Fig. 1). It would probably have remained there unsuspected, had it not been brought to the Earth's surface by volcanic mechanisms. This will be looked at in detail in the section on the genesis of diamond below, as will the apparently anomalous stability of diamond at NTP.Pressure/temperature phase diagram for diamond.image

RECENT DEVELOPMENT IN DIAMOND SYNTHESIS

The high pressure-high temperature (HPHT) method led to wide applications of artificial diamonds in industry. Low temperature coating of diamonds has been studied for some time, and chemical vapor deposition has been developed. In this contribution, we will focus on the most recent developments in diamond synthesis, and especially the important progress upon hydrothermal synthesis and the reduction of carbide (HSRC), which has attracted more and more attention due to its great commercial potential. Especially, we will discuss the synthesis of colorless, large diamond crystals from the reduction of carbon dioxide or carbonates. In addition, some recent progress on the theoretical studies of the metastable nucleation and growth of diamonds will also be reviewed. Finally, theories about the genesis of natural diamond were briefly reviewed, and a new theory based on our experimental results was proposed, which suggests that diamond within the Earth could be produced from carbon dioxide wherever carbon dioxide existed and the conditions (e.g., temperature, pressure, and reducing environment) are satisfied.

High-pressure synthesis of diamond from graphite and Fe&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;

A mixture of graphite and Fe2SiO4 in a ratio 2:3 by weight has been studied under pressures from 6 to 10GPa and temperatures between 1000 and 1960°C using an octahedral-anvil apparatus. At 8GPa, diamond with a size -10μm across was found to be formed at temperatures higher than 1900°C. This threshold temperature is by 150°C higher than the melting temperature of Fe2SiO4 at this pressure. The result offers a clue to the genesis of diamond in the deep interior of the earth.

High-Pressure Synthesis of Diamond from Organic Compounds.

Diamond has been synthesized from cyclic hydrocarbons under high pressures between 6 to 9GPa and high temperatures ranging from 710 to 1290°C. The result offers a clue to the genesis of diamond in the deep interior of the earth.

Measurement of reduced peridotite-C-O-H solidus and implications for redox melting of the mantle

Methane-bearing C-O-H fluids may be important carriers of carbon and hydrogen in the Earth's deeper mantle1,2. It has been proposed that such fluids have a key function in the genesis of kimberlitic magmas and diamond in the subcontinental mantle2–6. Because the melting behaviour of mantle peridotite in the presence of C-O-H fluids has not been accurately located as a function of pressure, temperature and oxygen fugacity1,4, it has not been possible to assess the validity of reduced fluid 'infiltration' models of this kind. We report here advances in high-pressure fluid buffering techniques that have allowed the C-O-H fluid-saturated solidus of a fertile peridotite to be determined to 35 kbar. The results show that initial melting of peridotite saturated with H2O-CH4 dominated fluids occurs at temperatures well above stable continental geotherms, even under quite H2O-rich conditions. This implies that C-O-H fluid-induced melting beneath continents will be restricted to regions where CH4 can be largely eliminated from the fluid by oxidation (redox melting) or where the geotherm is abnormally high (hot-spot melting), or where there is some combination of these processes.

Chromium and the genesis of diamond

Several of the mineral inclusions that occur in natural diamond are major chromian phases, or contain unusually high chromium concentrations. Forsterite olivine inclusions contain ten times as much chromium as ordinary forsteritic olivine. Diopsides are commonly Cr-rich and some contain up to 12 wt.% Cr 2O 3. Chrome-pyrope garnets occur characteristically as inclusions in diamond and rarely in kimberlite. Chromite inclusions are the most magnesian and chromian of terrestrial chromites. Without further experimental studies of Cr-bearing systems at high pressure the significance of these observations remain somewhat enigmatic.