Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine,RSA

Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF₃. ¹⁸O/¹⁶O ratios are determined on oxygen converted to CO₂ over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O₂) enhanced CO production in the graphite reactor causes a systematic shift in both δ¹³C and δ¹⁸O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ¹³C (CO₂) depletion relative to δ¹³C (graphite), is used to obtain corrected δ¹⁸O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ¹³C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ¹⁸O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ¹⁸O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ¹⁸O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.     

Formation of metastable graphite inclusions during diamond crystallization in model systems

Metastable graphite inclusions have been studied in diamond, forsterite, and orthopyroxene synthesized in silicate-carbonate-fluid and aqueous chloride systems at 6.3–7.5 GPa and 1400–1600°C. The graphite inclusions were studied using optic microscopy and Raman spectroscopy. It has been established that graphite in diamond and liquidus silicate minerals is represented by a highly ordered variety. Depending on parameters of runs, the graphite inclusions are hexagonal, irregular polygonal, or rounded in shape. The morphology of graphite inclusions involving metastable graphite in run products is compared with previously established crystallization sequence of carbon phases. It has been revealed that the protogenetic graphite inclusions in diamond are rounded, and this shape was caused by dissolution of the newly formed graphite. Polygonal graphite inclusions are syngenetic and represented by metastable graphite that crystallized contemporaneously with diamond.     

Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle

The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ¹³C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.     

Raman spectra of polycrystalline microdiamond inclusions in zircons,and ultrahigh-pressure metamorphism of a quartzofeldspathic rock from the Erzgebirge terrane,Germany

ABSTRACT

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm^?1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm^?1. These data combined with previously reported diamond band data (1331–1337 cm^?1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disordered sp³-carbon, but show downshifting of the Raman band from the ideal value (1332 cm^?1), and have a broader diamond band (FWHM >3 cm^?1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm^?1. Our study together with previously reported C-isotopic compositions (δ¹³C, ?17 to ?27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of the P–T time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.     

Syngenetic inclusions of yimengite in diamond from Sese kimberlite (Zimbabwe) — evidence for metasomatic conditions of growth

Syngenetic inclusions of yimengite K (Cr, Ti, Mg, Fe, Al)₁₂O₁₉, a potassium member of the magnetoplumbite mineral group, have been recorded in an octahedral macrodiamond from the Sese kimberlite (50 km south of Masvingo, Zimbabwe). One yimengite inclusion carries lamellae of chromite suggesting peridotitic diamond paragenesis. The diamond and inclusions were studied in situ in a plate polished parallel to (011). Cathodoluminescence (CL) imaging has shown blue colour and octahedral zonation of the diamond, lack of cracks and the location of five yimengites in different growth zones. Nitrogen (N) contents (at. ppm) in the diamond determined by Fourier transform infrared spectroscopy (FTIR) steadily decrease from 576 (core) to 146 (rim). N aggregation (%1aB) is correspondingly 40% in the core and 30% in the rim. Hydrogen (H) content is high in the core, moderate in the intermediate and very high in the rim zones. Four yimengites were dated using the laser ⁴⁰Ar/³⁹Ar method. Three inclusions yielded total gas ages that agree with, or are younger than, or within error of, the Sese kimberlite eruption age (538±11 Ma) but may be compromised by gas loss. One inclusion, with the highest tapped interface gas yield, gave a total gas age of 892±21 Ma that is a likely minimum yimengite age. Time–T °C constraints from N aggregation systematics give a range of possible ages from kimberlite eruption date back to Archean and do not resolve the variable results of the ⁴⁰Ar/³⁹Ar dating. Compared with the published chemistry of yimengite from kimberlites, inclusions from the Sese diamond contain higher Al, Mg, and Sr and have lower concentration of Fe³⁺. The chondrite-normalised REE pattern of the yimengite shows enrichment in LREE and depletion in HREE, but LREE/HREE fractionations are lower than for lindsleyite–mathiasite series mantle titanates and rather similar to the REE concentrations in kimberlite and lamproite rocks. It is suggested that Sese yimengite formed in the lithospheric mantle from metasomatism of chrome spinel by a fluid rich in Ti, K, Ba and LREE.     

Djerfisherite in xenoliths of sheared peridotite in the Udachnaya-East pipe (Yakutia): origin and relationship with kimberlitic magmatism

Djerfisherite, a Cl-bearing potassium sulfide (K₆Na(Fe,Ni,Cu)₂₄S₂₆Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe–Ni–Cu sulfides when xenoliths interact with a K-and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths—sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite+ pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the melt inclusions and xenolith interstices crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe–Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt.     

Diamonds and Their Mineral Inclusions,and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K₂O than the Cpx grains in the host.

The δ¹³C compositions are relatively constant at ?5% both within and between diamonds, whereas δ¹⁵N values vary from ?2.8% to ?15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond.

This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.     

Olivine with diamond-imposed morphology included in diamonds. Syngenesis or protogenesis?

The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of the possible chemical re-equilibration during diamond-forming processes, at least some of the olivines may have existed prior to the diamond (i.e. they are protogenetic). Our results imply that a diamond-imposed morphology alone cannot be considered as unequivocal proof of syngenicity of mineral inclusions in diamonds.     

Formation of epigenetic graphite inclusions in diamond crystals: experimental data

To elucidate the conditions of formation of epigenetic graphite inclusions in natural diamond, we carried out experiments on high-temperature treatment of natural and synthetic diamond crystals containing microinclusions. The crystal annealing was performed in the CO–CO₂ atmosphere at 700–1100 °C and ambient pressure for 15 min to 4 h. The starting and annealed diamond crystals were examined by optical microscopy and Raman spectroscopy. It has been established that the microinclusions begin to change at 900 °C. A temperature increase to 1000 °C induces microcracks around the microinclusions and strong stress in the diamond matrix. The microinclusions turn black and opaque as a result of the formation of amorphous carbon at the diamond–inclusion interface. At 1100 °C, ordered graphite in the form of hexagonal and rounded plates is produced in the microcracks. A hypothesis is put forward that graphitization in natural diamond proceeds by the catalytic mechanism, whereas in synthetic diamond it is the result of pyrolysis of microinclusion hydrocarbons. The obtained data on the genesis of graphite microinclusions in diamond are used to evaluate the temperature of kimberlitic melt at the final stage of formation of diamond deposits.     

Diamond in metasedimentary crustal rocks from Pohorje,Eastern Alps: a window to deep continental subduction

We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp³‐bonded carbon, in most cases centred between 1332 and 1330 cm^?1, with a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered graphitic (sp²‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO₂ and CH₄ in polyphase inclusions_. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.     

A Find of Coesite in Diamond-Bearing Kyanite Eclogite from the Udachnaya Kimberlite Pipe,Siberian Craton

Doklady Earth Sciences - A find of coesite in a kyanite graphite–diamond-bearing eclogite xenolith from the Udachnaya–Vostochnaya kimberlite pipe is described in this paper. The coesite...     

Mineralogy of the upper mantle: A review of the minerals in mantle xenoliths from kimberlite

The importance of ultramafic and eclogitic xenoliths in kimberlite as representing the rocks and minerals of the upper mantle has been widely perceived during the last decade. Studies of the petrology and mineral chemistry of these mantle fragments as well as of inclusions in diamond, have led to significant progress in our understanding of the mineralogy and chemistry of the upper mantle. For example, it is now known that textural differences in the ultramafic xenoliths (lherzolite, harzburgite, pyroxenite and websterite) are partially reflected in chemical differences. Thus xenoliths that display a ‘fluidal’ texture, indicative of intense deformation are less depleted in Ca, Al, Na, Fe and Ti than those xenoliths in which granular textures are predominant. It is believed this relative depletion may indicate the sheared (fluidal texture) xenoliths are representative of primary, undifferentiated mantle. This material on partial melting would produce ‘basaltic-type’ material, and leave a residuum whose chemistry and mineralogy is reflected by the granular xenoliths.Also present in kimberlite are large single phase xenoliths that may be either one single crystal (xenocryst, megacryst) or an aggregate of several crystals of the same mineral (discrete xenolith, or discrete nodule). These large single phase samples consist of similar minerals to those occurring in the ultramafic xenoliths but chemically they are distinct in being generally more Fe-rich. The relation between these xenocrysts to their counterparts in the ultramafic xenoliths is unknown. Also unknown, at the present time, is the exact relation between diamond and kimberlite. Evidence obtained from study of the mineral inclusions in diamond suggests that diamond forms in at least two chemically distinct environments in the mantle; one eclogitic, the other, ultramafic. Interestingly, this suggestion is true for diamonds from worldwide localities.The mineral-chemical results of studies on xenoliths and inclusions in diamond have been convincingly interpreted in the light of experimental studies. It is now possible based on several different geothermometers and barometers to determine relatively reasonable physical conditions for the final genesis of many of these mantle rocks. For the most part the final equilibration temperatures range between 1000 and 1400°C and pressure in the region 100–200 km. These conditions are consistent with an upper mantle origin. Future studies will undoubtedly attempt to more concisely, and accurately, define these conditions, as well as understand better the chemical and spatial relationship of the rock-types in the mantle.     

Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.     

Metasomatic parageneses in deep-seated xenoliths from pipes Udachnaya and Komsomol'skaya-Magnitnaya as indicators of fluid transfer through the mantle lithosphere of the Siberian craton

The petrography, major element, and trace element (TE) compositions of minerals from two types of modal metasomatites (metasomatized peridotites and pyroxenites) from kimberlite pipes Udachnaya and Komsomol'skaya-Magnitnaya, Yakutia, have been studied. It is shown that texturally and chemically equilibrated metasomatites A consist of a set of superimposed minerals: phlogopite + diopside ± ilmenite ± apatite ± sulfides ± graphite. Their major and trace element compositions have specific features. The contents of TEs in garnet and clinopyroxene from these metasomatites are close to those in garnet and clinopyroxene from low-temperature coarse-grained peridotites richest in TEs. The distribution of a significant portion of TEs between garnet and clinopyroxene from A-type metasomatites and from coarse-grained lherzolites rich in TEs is close to experimental values reported for minerals coexisting with carbonatitic and basaltic fluids. We assume that this metasomatic process was nearly synchronous with the global metamorphism and cratonization of the mantle lithosphere and that high-density silicate–carbonate fluidmelts were metasomatizing agents.Another large mantle metasomatism process in the lithosphere of the Siberian craton was associated with the Middle Paleozoic kimberlite magmatic event, induced by the Yakutian thermochemical plume. Metasomatic minerals (Mg phlogopite + Cr diopside + chromite ± sulfides ± graphite) intensely replaced the minerals of the primary paragenesis, particularly, garnet. These reaction metasomatites show a sine-shaped REE pattern in garnet and disequilibrium between garnet and clinopyroxene. It is supposed that the reaction metasomatism in the mantle lithosphere of the Siberian craton was associated with ingress of reduced asthenospheric fluids at early stages of the kimberlite formation cycle. Metasomatic graphite formed in metasomatites of both types, and this fact evidences for two diamond formation epochs in the history of the mantle lithosphere of the Siberian craton.     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

Discovery of diamond in the Tromsø Nappe,Scandinavian Caledonides (N. Norway)

We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm^?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm^?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp²‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.     

Ar/Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by ⁴⁰Ar/³⁹Ar techniques. Previous ⁴⁰Ar/³⁹Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, ⁴⁰Ar/³⁹Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana.Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated ⁴⁰Ar/³⁹Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages.The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve.It is argued that the majority of older ⁴⁰Ar/³⁹Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all ⁴⁰Ar/³⁹Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the ⁴⁰Ar/³⁹Ar dating technique. However, if the first model is valid, then the older ⁴⁰Ar/³⁹Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.     

Shrilankite Inclusions in Garnets from Kimberlite Bodies and Diamondiferous Volcanic–Sedimentary Rocks of the Yakutian Kimberlite Province,Russia

Pyrope–almandine garnets (Mg# = 28.3–44.9, Ca# = 15.5–21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano–sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti₂ZrO₆), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet–pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.     

Mg-ilmenite megacrysts from the Arkhangelsk kimberlites, Russia: Genesis and interaction with kimberlite melt and postkimberlite fluid

In the present work we studied Mg-ilmenite megacrysts from the Arkhangelsk kimberlites (the Kepino kimberlite field and mantle xenoliths from the Grib pipe). On the basis of isotopic (Rb/Sr, Sm/Nd, δ¹⁸O) and trace-element data we argue that studied Mg-ilmenite megacrysts have a genetic relation to the “protokimberlitic” magma, which was parental to the host kimberlites. Rb-Sr ages measured on phlogopite from ilmenite-clinopyroxenite xenoliths and the host Grib kimberlite overlap within the error (384 Ma and 372 ± 8 Ma, respectively; Shevchenko et al., 2004) with our estimation of the Kotuga kimberlite emplacement (378 ± 25 Ma). Sr and Nd isotopic compositions of megacrysts are close to the isotopic composition of host kimberlites (Mg-ilmenites from kimberlites have ⁸⁷Sr/⁸⁶Sr_{(t = 384)} = 0.7050–0.7063, ?Nd_{(t = 384)} = + 1.7, +1.8, ilmenite from ilmenite-garnet clinopyroxenite xenolith has ⁸⁷Sr/⁸⁶St_{(t = 384)} = 0.7049, ?Nd_{(t = 384)} = +3.5). Oxygen isotopic composition of ilmenites (δ¹⁸O = +3.8–+4.5‰) is relatively “light” in comparison with the values for mantle minerals (δ¹⁸O = +5–+6‰). Taking into account ilmenite-melt isotope fractionation, these values of δ¹⁸O indicate that ilmenites could crystallize from the “protokimberlitic” melt. Temperatures and redox conditions during the formation of ilmenite reaction rims were estimated using ilmenite-rutile and titanomagnetite-ilmenite thermo-oxybarometers. New minerals within the rims crystallized at increasing oxygen fugacity and decreasing temperature. Spinels precipitated during the interaction of ilmenite with kimberlitic melt at T = 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [QFM] ≈ 1. Rims comprised with rutile and titanomagnetite crystallized at T ≈ 1100°C, $\Delta \log f_{O_2 }$ [NNO] ≈ 4 and T = 600–613°C, $\Delta \log f_{O_2 }$ [QFM] ≈ 3.7, respectively. Rutile lamellae within ilmenite grains from clinopyroxenitic xenolith were formed T ≥ 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [NNO] = ?3.7. Since the pressure of clinopyroxene formation from this xenolith was estimated to be 45–53 kbar, redox conditions at 135–212 km depths could be close to $\Delta \log f_{O_2 }$ [NNO] = ?3.7.     

Peculiarities of Diamonds in Eclogitic Xenoliths from the Komsomolskaya Kimberlite Pipe,Yakutia

The first studies of diamonds in eclogitic xenoliths from the Komsomolskaya kimberlite pipe are described. Among round and oval-shaped xenoliths with diamond ingrowths, samples with a garnet content of 40–90% of the xenolith volume dominate. Two eclogite samples contain grains of accessory rutile; a kyanite sample is also revealed. Certain samples contain two or more crystals of diamonds. Diamonds with an octahedral habit and crystals with transitional habits, which belong to an octahedral-rhombic dodecahedral row, dominate in eclogites; there are many variety VIII aggregates. A high concentration of structural nitrogen, commonly in the A form, was registered in most of the crystals. Diamonds with a small content of nitrogen impurities, 40–67% in the B1 form, are present in a number of xenoliths. The calculated temperatures of the formation of eclogitic xenoliths is 1100–1300°C. Diversity in the impurity compositions of diamonds in the same xenolith shows that these diamonds were formed at various times and in different settings. The diamond position in xenoliths, the various level of nitrogen aggregation in the diamonds, and a number of other factors point to the later formation of the diamonds, as compared to minerals of eclogites, from fluid or fluid-melts in the process of metasomatosis.