Diamond Resorption Research Articles

Heterogeneous water distribution in between peridotite xenoliths from Kaapvaal Craton kimberlites: Constraints on diamond barren nature of kimberlites

Nominally anhydrous mantle minerals (olivine, pyroxenes, garnets, etc.) in 11 peridotite xenoliths from four different uneconomic and economic Kaapvaal Craton kimberlite pipes (Matsoku, Thaba Putsoa, Pipe 200 and Bultfontein) have been investigated using Fourier transform infrared spectroscopy (FTIR). All xenoliths contain accessories of garnet, diopside, chromite, and phlogopite. High orthopyroxene content (>30 mol vol.%) in most xenoliths from all kimberlites and its interconnected channel-like nature hint towards hydrous siliceous fluid metasomatism. Peridotite xenoliths from uneconomic kimberlites show development of phlogopite and clinopyroxene (± chromite) forming veins and in garnet rims suggesting metasomatism by alkaline silico-carbonatite (possibly kimberlite-related) melt. The xenoliths contain significant H2O in olivine (17–62 ppm), orthopyroxene (21–230 ppm), and clinopyroxene (87–833 ppm), whereas garnets are dry and only show IR absorbance bands at > 3,670 cm−1 for contamination of hydrous minerals. Compared to the economic kimberlites in the Kaapvaal Craton, the uneconomic kimberlite xenoliths from this study have lower orthopyroxene and olivine H2O content. In the xenoliths affected by garnet breakdown metasomatism, the H2O content of orthopyroxene and olivine is higher and lower, respectively. The structural hydroxyl distribution profile across olivine and higher inter-mineral water partition coefficient, suggest diffusion of hydrogen and possible re-equilibration. Statistical analysis of the olivine spectra suggests that hydrogen bands at 3540, 3624, 3638, and 3672 cm−1 are a good discriminant of economic and uneconomic kimberlites and in literature, they are associated with metasomatism, weathering-associated processes, high water activity, and oxygen fugacity. The lower water concentration in xenoliths from uneconomic kimberlite from the margin of the craton than the economic kimberlites from the interior of the Kaapvaal Craton and identified metasomatism hints towards dehydration of xenoliths by water-poor and CO2-rich melts in tectonized cross-lithospheric zones causing diamond resorption and may be responsible for the diamond-poor nature of uneconomic kimberlites in northern Lesotho.

Experimental Modeling of Diamond Resorption during Mantle Metasomatism

The morphology of resorbed diamond crystals is a valuable source of information on the composition and ascent rate of kimberlite magmas, as well as on possible redox conditions in protolith. Previously, diamond resorption was thoroughly investigated at P–T–fO2 parameters of the kimberlite magma ascent. In this study, we investigated diamond resorption using unaltered group I kimberlite and model carbonatite at P–T–fO2 parameters that are typical of the peridotite source of kimberlite magmas in the subcontinental lithospheric mantle. An analysis of previous studies made it possible to determine the rate of diamond octahedron transformation into a spherical tetrahexahedron depending on the composition of the carbonate–silicate melt. It was shown that the rate of diamond resorption at 6.3 GPa increases in all the investigated systems as fO2 and temperature rise. There is a steady decrease in the diamond resorption rate as pressure increases from 1 GPa to 6.3 GPa. The morphology comparison of the experimentally produced samples with natural diamonds is indicative of the significant contribution of metasomatic alteration of protolith by the oxidized agent and at the initial stages of kimberlite magma ascent to the resorption of natural diamonds.

The effect of oxygen fugacity on diamond resorption in ascending kimberlite melt

When transported by magmas to the Earth's surface, diamond crystals underwent resorption, the intensity of which significantly differed in various kimberlite pipes. We experimentally simulated diamond resorption at different oxygen fugacities (fO2) in ascending kimberlite magma enriched in CO2 and H2O. The experiments were carried out using specially prepared unaltered Group I kimberlite from the Udachnaya East pipe (Yakutia) and model carbonatite at 3.0 GPa, 1200–1400 °C, and fO2 in a range of NNO–2 to NNO + 3.2 log units (where NNO is Ni-NiO buffer). Over the investigated range of conditions, resorption of octahedral diamond crystals is found to occur according to a single scenario. Negative trigons and shield-shaped laminae develop on the {111} faces and crystal edges are truncated by the surfaces of tetrahexahedroids. The rate of diamond resorption increases in all studied systems as fO2 and temperature are raised. In this case, water-enriched melts are the most aggressive media in the investigated T–fO2 interval. Among the most oxidized high-temperature melts, it is carbonatite melts depleted in SiO2 that provide the maximum rate of diamond resorption. Furthermore, the rates of diamond resorption we obtained are an order of magnitude higher than those previously measured in silicate melts containing CO2 and H2O, at fO2 values from the NNO buffer to NNO-2. Therefore, high oxygen fugacity, a temperature of ~1400 °C, and essentially carbonate composition of water-containing magma could provide a high intensity of diamond resorption at the mantle stage of magma ascent to the surface. Apparently, this process primarily influenced the formation of the appearance and preservation of natural diamond crystals in kimberlite pipes.

Diamond resorption and immiscibility of C-O-H fluid in kimberlites: Evidence from experiments in H2O – CO2 – SiO2 – MgO – CaO system at 1–3 GPa

Kimberlites are the deepest sourced magmas emplaced at the Earth's surface. They provide a “window” into the processes at the base of the subcratonic mantle. A better understanding of the origin, composition, and emplacement mechanisms of kimberlites is hampered by uncertainties in the contents of the two main volatiles, H2O and CO2. Diamond dissolution in H2O and in CO2 fluids produces distinct resorption features offering an opportunity to determine the composition of the magmatic fluid in kimberlites. Here we examined the relationship between H2O:CO2 ratio of the fluid and the style of diamond resorption by conducting experiments in C-O-H fluid saturated with silicates with variable H2O:CO2 ratios at the conditions of kimberlite ascent of 1–3 GPa and 1150–1350 °C. Our results showed that the geometry of etch pits on diamond and the resorption style evolve consistently as bulk CO2 content of the fluid changes from 0 to 50 to 50–90 and 90–100 mol%. The fluid composition at the run conditions was monitored by entrapment of synthetic fluid inclusions in olivine and quartz. The inclusions demonstrated the existence of a fluid miscibility gap at 1–3 GPa and 1250 °C with two fluid endmembers, an aqueous and a carbonic phase, which H2O:CO2 ratio at 1 GPa determined with confocal Raman microscopy is (H2O)0.62(CO2)0.38 and (H2O)0.12(CO2)0.88 respectively. Hence, diamond resorption morphology depends on the proportions of the end-member aqueous and carbonic fluids, which vary with the bulk composition of the fluid. The different density and ability of aqueous and carbonic fluids to dissolve silicates (olivine) would promote their separation in the rising magma column. Concentration of the lower density aqueous fluid towards the tip of the propagating dyke would facilitate more efficient fracturing of the country rocks and faster ascent of the kimberlite magma causing explosive eruption. We propose that preferential attachment of aqueous fluid bubbles would help to increase the buoyancy of olivine xenocrysts and possibly diamond in the kimberlite magma offering a mechanism for transporting the heavy mantle cargo.

Microrelief of Rounded Diamond Crystals as an Indicator of the Redox Conditions of Their Resorption in a Kimberlite Melt

We conducted a detailed study of the morphology of diamond crystals partially dissolved in a water-bearing kimberlite melt at pressure of 6.3 GPa, temperature of 1400 °C, and two oxygen fugacities (fO2) corresponding to the Re-ReO2 buffer and near the magnetite–hematite (MH) buffer. The triangular etch pits on the {111} faces, which formed during experimental diamond dissolution, were found to completely correspond to negative trigons on natural diamond crystals in the shape and sidewalls inclination angle. Furthermore, two experimental fO2 values were associated with two relief types of the rounded tetrahexahedroid surfaces typical of natural rounded diamonds. Therefore, the surface microrelief on rounded natural diamond crystals was concluded to be an indicator of the redox conditions of natural diamond resorption.

Influence of ions on diamond resorption

The paper presents a summary of extensive experiments on diamond resorption rates in presence of various ions performed at Prof. Rudenko lab at Moscow State University. For the first time all experimental data are shown together allowing direct comparison. Surface features of the samples etched in different conditions were studied using optical, scanning electron and atomic force microscopy. It is shown that catalytic theory of diamond resorption, a variant of topochemical adsorption theory of crystal etching, explains dramatic differences between activities of ions from different groups of periodic table on diamond resorption rate at least on qualitative level. Strong variations in surface features on diamonds etched in presence of ions with various catalytic activities are observed.

Geochemistry of Magmatic and Xenocrystic Spinel in the No.30 Kimberlite Pipe (Liaoning Province, North China Craton): Constraints on Diamond Potential

There are two genetic types of spinel (magmatic spinel crystallizing directly from kimberlite magma and xenocrystic spinel derived from mantle xenoliths) in the No.30 kimberlite pipe (Liaoning Province, North China Craton). Their geochemistry is investigated to reveal processes of diamond capture and resorption during kimberlite magmatism to constrain the diamond potential. Magmatic spinels are mostly euhedral to subhedral, 20 to 60 µm in size, and have compositional zones: the cores are classified as chromite with high Cr and Mg contents, and the rims are classified as magnetite with low Cr and high ferric Fe. The compositional trends suggest that magmatic spinel and olivine phenocrysts are crystallized contemporaneously during the early stages of kimberlite crystallization. During this period, temperature (T) and oxygen fugacity (fO2) values calculated at an assumed pressure of 1 GPa are in the range of 994–1274 °C and 1.6–2.4 log fO2 units below the nickel-nickel oxide (NNO) buffer, respectively. The high values of fO2 suggest heavy diamond resorption during kimberlite magmatism. Estimated temperatures of xenocrystic spinel range from 828 to 1394 °C, and their distributions indicate that only a small proportion of xenocrystic spinels are derived from the diamond stabilization field, which suggests a low potential of diamond capture. The low diamond capture and heavy diamond resorption during kimberlite magmatism contributed to the low diamond grade of the No.30 kimberlite.

Diamond destruction and growth during mantle metasomatism: An experimental study of diamond resorption features

Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1–3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500°C in synthetic CaO–MgO–SiO2–CO2–H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1–3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate–carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.

Three styles of diamond resorption in a single kimberlite: Effects of volcanic degassing and assimilation

Three styles of diamond resorption in a single kimberlite: Effects of volcanic degassing and assimilation

Diamond resorption features as a new method for examining conditions of kimberlite emplacement

The study develops a new approach utilizing parameters of trigonal etch pits on diamond crystals to infer the conditions of diamond residence in kimberlite magma. Diamond crystals from dissolution experiments conducted at 1 GPa and 1150–1350 °C in the presence of H2O-rich or CO2-rich fluid were studied with atomic force microscopy (AFM). The AFM data of resorbed diamond surfaces show that much deeper surface relief was produced in CO2 fluid. It also clearly distinguishes the profiles of the trigonal etch pits forming regular flat-bottomed trigons in H2O fluid, and round- or pointed-bottomed trigons in CO2 fluid. The relationship between the diameter and the depth of the trigonal pits is found to be another important indicator of the fluid composition. Dissolution in H2O fluid develops trigons with constant diameter and variable depth where the diameter increases with temperature. Trigons developed in CO2 fluid have a large range of diameters showing a strong positive correlation with the depth. The developed criteria applied to the natural diamond crystals from three Ekati Mine kimberlites indicate significant variation in CO2–H2O ratio and temperature of their magmatic fluid. This conclusion based on diamond resorption agrees with the mineralogy of microphenocrysts and groundmass of the studied kimberlites offering new method to study crystallization conditions of kimberlite magma.

Evolution of diamond resorption in a silicic aqueous fluid at 1–3 GPa: Application to kimberlite emplacement and mantle metasomatism

Natural diamonds grow and partially dissolve during mantle metasomatism and undergo further resorption during the ascent to the Earth's surface in kimberlite magmas. This study uses atomic force microscopy (AFM) for quantitative characterization of diamond resorption morphology in order to provide robust constraints of the composition of kimberlitic and mantle metasomatic fluids. We performed experiments in a piston–cylinder apparatus at pressures (P) of 1–3GPa and temperatures (T) of 1150–1400°C to examine the impact of P, T, and silica content of an aqueous fluid on diamond dissolution. Petrographic observation and microthermometry of synthetic fluid inclusions trapped in olivine at the run conditions provide constraints on the composition and density of the fluid reacting with the diamond. Our results confirm an inverse relationship between P and T on diamond dissolution kinetics. A P increase of 1GPa suppresses diamond oxidation rates by the same value as a T decrease by ~50°C, while the transformation rate of diamond crystal morphology from octahedron to tetrahexahedron increases with both P and T. All dissolved diamonds develop glossy surfaces, ditrigonal {111} faces, sheaf striations, and negative trigons, while circular pits only occur in aqueous fluids with low silica content (≤4.2mol/kg) at 1GPa. We identify five distinct morphological groups of trigons: two types of point-bottomed (p/b) (trumpet- and V-shaped) and three types of flat-bottomed (f/b) (trumpet-shaped, trapezoid-shaped and rounded). AFM measurements of trigons from two successive runs showed three stages of their evolution. Etch pits nucleate at defects as trumpet p/b trigons with the vertical dissolution rate (Vd) faster than the dissolution rates at the surface free of defects; they further develop by growth of the bottoms in (111) plane to create trumpet-shaped f/b trigons accompanied by decrease in Vd; and finally form trapezoid-shaped f/b trigon with constant wall angles. The diameter of f/b trigons developed in the aqueous fluids depends on the diamond weight loss and dissolution kinetics, and does not correlate with their depth. Integration of our AFM data with the theoretical model for trigon formation suggests that the change from point- to flat-bottomed trigons depends on the defect sizes and dissolution conditions. Application of our results to the diamonds from Ekati diamond Mine, Canada, suggests that variations in diamond rounding in different pipes implies variable depth of fluid exsolution; presence of circular pits on diamonds indicates predominantly aqueous fluid during the latest stages of kimberlite emplacement; and comparison to the mantle-derived morphologies on Ekati diamonds implies the importance of CO2-rich fluids and/or carbonate melts during mantle metasomatism. The constrained effect of P on diamond dissolution kinetics indicates that appreciable diamond weight loss can only happen at P<1GPa and therefore the conditions at the latest stages of kimberlite emplacement are very important for assessments of diamond preservation in a kimberlite pipe.

The origin of high hydrogen content in kimberlitic olivine: Evidence from hydroxyl zonation in olivine from kimberlites and mantle xenoliths

Abstract Olivine macrocrysts in kimberlites are the most H-rich natural olivines known. Their xenocrystic provenance makes unclear whether H-enrichment occurred in the mantle prior to entrainment in kimberlite magma or during ascent. We present a Fourier transform infrared (FTIR) spectroscopy-based study of H zonation in kimberlite-derived olivine macrocrysts and/or olivine in xenoliths from the kimberlites Jericho (Nunavut, Canada), Beartooth (Northwest Territories, Canada), and Pipe 200 and Matsoku (Lesotho). The objective of the study was to determine whether changes in the concentration or speciation of H defects in proximity to the host kimberlite could yield insights into the origin of H-enrichment. Within the Group 1 region of the FTIR spectrum (~ 3420–3700 cm − 1 ), we find that olivines within xenoliths and macrocrysts show weak zonation or significant H depletions in rims. Peaks in the Group 2 region (~ 3260–3420 cm − 1 ) in Beartooth olivines show marked decrease in the crystal margins in comparison to Group 1 peaks. We find no preserved evidence for kimberlite-related H-enrichment in our dataset. We ascribe H depletion to diffusive H loss to infiltrating kimberlitic media. Diffusion models using coefficients previously applied to dehydrogenation of olivine and assuming a dry environment produce extremely short ascent durations incompatible with the results of other geospeedometers. This mismatch suggests that the poorly developed zonation in Jericho and Beartooth olivines indicates water-enrichment and/or faster ascent of these melts (in comparison to Pipe 200 and Matsoku), resulting in greater retention of mantle-derived H. A further significant factor in the mismatch is the differing speciation of H in most natural mantle-derived olivines in comparison to those used in diffusion experiments. We argue that the presence of presumably mantle-derived Group 2 H in olivine macrocrysts indicates rapid ascent of magma exsolving water-rich fluid, permitting preservation of these faster-diffusing defects only in particularly rapidly ascending kimberlite magmas. This interpretation explains empirical correlations between the presence of Group 2 bands, water-derived diamond resorption features, and volcaniclastic kimberlite facies.

H 2O and CO 2 in kimberlitic fluid as recorded by diamonds and olivines in several Ekati Diamond Mine kimberlites, Northwest Territories, Canada

Surface dissolution features on diamonds and Fourier Transform Infra Red spectroscopy (FTIR) of phenocrystal and xenocrystal olivines from kimberlites contain a record of magmatic fluid in kimberlite magmas. We investigated composition and behavior of kimberlitic fluid and the effect of volatiles on the eruption style and geology of kimberlites using microdiamonds and olivine concentrates from six kimberlite pipes with different lithologies and the character of diamond resorption (Ekati Diamond Mine, Northwest Territories, Canada). The study showed a clear correlation between the resorption style of diamond population of the kimberlites and the type of infrared (IR) spectra of their olivines. Four kimberlites have high quality diamonds with smooth regular surface features and high H 2O content of the olivines indicating the presence of H 2O-rich fluid during the emplacement. Fast ascent rates of fluid-rich magma can explain explosive eruption and filling the pipes with volcaniclastic kimberlite facies. Conversely, Grizzly and Leslie kimberlites have diamonds with complex sharp features diminishing diamond quality and indicating loss of the fluid. The slower ascent rates and less explosive eruption of the fluid-free magmas produced kimberlite pipes filled with magmatic facies kimberlite. Distinctive peaks in olivine IR spectra at 3356 and 3327 cm − 1 were found to correlate with the presence of hydrous magmatic fluid. Character of diamond morphology suggests that during the whole ascent of all six kimberlites, the magmatic fluid when present had a high H 2O:CO 2 ratio.

Evaluation of kimberlite diamond potential using FTIR spectroscopy of xenocrystic olivine

Xenocrysts or phenocrysts of olivine in kimberlite show a diversity of hydroxyl-related infrared absorption bands that reflect compositional variation of their mantle source and conditions of last equilibration. Interpretation of infrared spectra of olivine in terms of equilibration/crystallization conditions may help to isolate spectral features associated with good diamond carrying and diamond preservation potential. As an exploratory study we examine by FTIR the spectroscopic features of olivine inclusions in diamond, olivine coexisting with diamond in peridotite xenoliths and olivine from diamond-free peridotite xenoliths. The three environments are respectively characterized by spectroscopically anhydrous olivine, olivine with low water concentrations (H 2O < 60 wt. ppm) and olivine that may be fairly rich in water (H 2O up to 260 ppm). Previous studies have shown that the majority of olivine macrocrysts in kimberlite contain > 100 ppm H 2O, with a significant proportion containing > 250 ppm H 2O. Our new results suggest that hydrogenation of olivine occurs in the mantle during equilibration with kimberlite-related hydrous fluids and/or during equilibration with, or crystallization from, kimberlite magmas during ascent. The association of gem diamonds with water-poor mantle olivine suggests that such samples experienced limited interaction with hydrous fluids/melts over a short time span, thus preventing diffusional equilibration. Future FTIR studies of olivine from kimberlites with widely variable diamond grades and reconciliation with diamond resorption features may permit development of new FTIR based procedures to assist in the evaluation of new kimberlite discoveries.

Mechanisms of diamond oxidation and their bearing on the fluid composition in kimberlite magmas

Diamond oxidation experiments were undertaken in a piston-cylinder apparatus at 1150 to 1500 °C and 1 GPa to understand the mechanism of diamond oxidation in kimberlite melts and to determine the main rate-controlling parameters for this process. Only surface graphitization, and no diamond resorption, occurs in melts that are fluid undersaturated (synthetic kimberlite, carbonate melt, alkaline basalt, CaO-MgO-SiO 2 -H 2 O-CO 2 melts). In contrast, fluid oversaturated conditions (as evidenced by the presence of bubbles) produce resorption features commonly seen in natural diamonds recovered from kimberlites. The diamond oxidation rate is the same in the melts with a free fluid phase, in a pure H 2 O or CO 2 fluid, suggesting that the process of diamond oxidation is its reaction with the fluid and not with the melt. Both CO 2 and H 2 O oxidize diamonds at a similar rate, but produce very different surface features. Therefore, the surface features of natural kimberlite-hosted diamonds may provide information on the relative proportion of H 2 0 and CO 2 in the kimberlitic fluid. The common diamond morphologies imply significant amount of H 2 O. The absence of diamonds with surface graphitization and the abundance of resorbed diamonds in kimberlites suggest the presence of a free fluid phase in kimberlite magmas for hours or days. We found no correlation between the rate and character of diamond oxidation and the physical properties of diamonds (nitrogen content, color).

8th International Kimberlite Conference: Selected Papers, Volume I: The C. Roger Clement Volume, Volume II: The J. Barry Hawthorne Volume.: R. H. MITCHELL, H. S. GRUTTER, L. M. HEAMAN, B. H. SCOTT-SMITH, AND T. STACHEL, EDITORS. Volume I: Pp. 593; Volume II: Pp. 949. Hardcover. Elsevier. 2006. reprinted from Lithos, v. 76-77 (1-4). ISBN 0070931097. Price US$235, two-volume set.

The 85 papers contained in this two-volume set and reprinted from the journal Lithos were presented at the 8th International Kimberlite Conference in Victoria, British Columbia, Canada, in June 2003. They represent the most current published compendium of knowledge regarding the wide range of topics that the study of diamonds includes. Those economic geologists wishing to learn about the current state of knowledge in the diamond industry and in exploration for kimberlites will be compelled to read these papers, which cover subjects related to diamond growth, grade assessment, exploration methods, kimberlite petrogenesis, emplacement mechanisms, geochronology, mantle geochemistry, tectonics, and geometallurgy. Examples given are from Australia, Botswana, Brazil, Canada, China, Finland, India, Namibia, Russia, South Africa, Tanzania, USA, Venezuela, West Africa, and Zimbabwe, and from the deep mantle to the crust and the laboratory. These volumes address the interests of the mineral deposit and exploration community by providing the details of diamond formation, emplacement, exploration, and recovery, and then some. In exploration, target selection begins with analysis of craton architecture, regional geological terrain characteristics, prospectivity for kimberlite emplacement, and the potential for geologic diamond-producing or diamond-destructive factors at depth or at the surface. Techniques for indicator mineral analysis and diamond grade assessment, stone color, shape, and resorption characteristics, and resource estimation and geometallurgical factors all influence the viability of a project. Economic geologists will be interested to learn that the diamond business encompasses geology from the earth’s core (ultradeep diamonds) …

Dissolution forms in Lac de Gras diamonds and their relationship to the temperature and redox state of kimberlite magma

The degree and character of diamond dissolution were compared to crystallization temperatures (T) and oxygen fugacities (fO2) estimated from chromite inclusions in olivine phenocrysts in several kimberlites from Lac de Gras, Northwest Territories, Canada. The T and fO2 values calculated at an assumed pressure of 1 GPa are in the range of 970–1,140±50°C and 2.8–4.4 log fO2units below the nickel–nickel oxide (NNO) buffer. The T and fO2 vary between kimberlites from northwest and southeastern clusters within 150°C and 1 log unit, respectively. A detailed description of morphological forms and surface dissolution features for diamond parcels from the Panda, Beartooth, Grizzly, Misery and Jay kimberlites (>7000 stones) show that an increase in diamond resorption in the kimberlites corresponds to increase in T. The development of various surface dissolution pits and structures correlates with higher fO2 of kimberlites and therefore mainly happens in the magma. The two processes of diamond dissolution, volume resorption and surface etching, do not show a strong correlation with each other, since some of the resorption occurs in the mantle. We suggest that the fO2 of the kimberlite magma plays an important role in both the processes. The proportion of plastically deformed brown diamonds does not correlate with the degree of volume resorption, but does correlate with the development of surface forms. The diamond grade is higher in kimberlites with lower fO2, confirming that conditions of kimberlite crystallization can have notable effect on diamond dissolution.

On the dilation of diamond by nitrogen impurity aggregated in A defects

Lattice parameter measurements by modern methods of photography of transmitted divergent–beam X–ray diffraction patterns are reported. A small (less than 10 μm diameter) source of divergent CuKα X–rays is produced by an electron beam impinging on a thin copper film evaporated on to one surface of the diamond, which is placed in a scanning electron microscope. Patterns of high dispersion and high resolution are obtained by use of a 700mm camera length and an Ilford L4 nuclear emulsion as the recording medium. A synthetic diamond volume containing only about one atomic part per 10 6 nitrogen impurity served as a pure–diamond standard. This was compared with a natural diamond of nearly pure IaA spectral type, unusually rich and uniform in content of nitrogen impurity aggregated in A–defect form (N concentration circa 1135 atomic parts per 10 6 ). In both specimens, volumes free from dislocations and bounded by well–polished damage–free surfaces were probed. Advances in pattern–measuring methods included a film–stacking technique to improve signal–to–noise ratio, digitized microdensitometry and the measurement of pattern shifts relative to invariant datum points in the pattern provided by angularly sharp features arising from coherent multiple diffraction effects. Procedures were developed to correct for diffraction–line–profile asymmetry (which depends upon lattice perfection as well as other diffraction parameters). An absolute lattice parameter determination on the pure diamond yielded , a 0 = 0.356710(4)nm, considered to be a good value. After subtracting the calculated dilatation contribution (24%) due to a small population of {001} platelet defects in the nitrogen–rich diamond, the dilatation due to A defects derived is Δa 0 /a 0 = (0.6 ± 0.25) x 10 –6 [μ A(1282cm–1) /cm –1 ], where μ A(1282cm–1) is the infrared absorption at 1282cm –1 due to A defects, in units of cm –1 . In terms of c N(A) , the fractional atomic concentration in the diamond of nitrogen impurity present in A defects, and subject to more uncertainty, Δa 0 /a 0 = 0.036c N(A) , which indicates that a nitrogen atom in an A defect occupies circa 1.11 times the volume of the normal carbon atom it replaces. This dilatation is compared with those expected from computed structures of the A defects.

Origin and Significance of Ilmenite Megacrysts and Macrocrysts from Kimberlite

Ilmenite populations (megacrysts and macrocrysts) from 26 kimberlites in North America have been characterized by electron microprobe analysis to assist in the understanding of the origin and significance of ilmenite in kimberlites worldwide. Most belong to the Cr-poor megacryst suite. Geochemical trends in Cr-poor-suite ilmenites are consistent with a mantle fractional crystallization origin, with ilmenite forming only a minor proportion of the crystallizing assemblage. Coprecipitating magnesite is inferred to be an important host for Mg, with its crystallization causing Mg depletion in coexisting ilmenite. Decrepitation of magnesite megacrysts during kimberlite ascent may have enriched kimberlite hosts in Mg, contributing to the Mg increase characteristic of ilmenite rims. Ilmenite rims commonly have lower hematite contents than do cores, suggesting that the oxidation state of the kimberlite, and thus its potential for diamond resorption, can be overestimated if core compositions alone are considered. N...