A study of eclogitic diamonds and their inclusions from the Finsch kimberlite pipe, South Africa

Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K₂O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

Trace element abundance patterns of garnet inclusions in peridotite-suite diamonds

Individual, sub-calcic, chrome-pyrope crystals from Finsch and Kimberley diamonds, Finsch and Bultfontein kimberlite heavy mineral concentrate, and from diamondiferous harzburgite-dunite xenoliths from the Udachnaya kimberlite pipe were analyzed for rare earth elements (REE), Sc, Ti, and Zr with the ion microprobe. The abundances and abundance ratios of these trace elements including LREE enrichment and low Ti, together with high and variable Cr contents, are inconsistent with a simple equilibrium relationship between peridotite-suite garnet and silicate and carbonate liquids. It is suggested that the trace element abundance patterns represent a signature of ancient mantle metasomatism which preceded the formation of peridotite-suite garnet and diamond.     

Experimental investigation of the origin of majoritic garnet inclusions in diamonds

 Multianvil experiments were carried out at 10–15 GPa and 1600–1700 °C to match the compositions of majoritic garnet inclusions from diamonds, and to determine the compositions of other phases potentially coexisting with these inclusions in the source. Most experiments produced coexisting majoritic garnet, diopsidic clinopyroxene, one or more (Mg,Fe)₂SiO₄ polymorphs, and quenched carbonatic melt. The experimental garnets had relatively high Ca and Fe contents similar to the observed Ca and Fe contents of the inclusions. The resulting Si contents confirmed that the depth of origin of the inclusion with the highest Si content did not exceed 410 km, thus none of the majoritic garnet inclusions found so far originated in the transition zone (410–660 km). The evidence from inclusions and experiments is consistent with the presence of an eclogite layer occurring globally between 200 and 410 km. Compositional variations observed among more than 100 majoritic garnet inclusions with their Si content, which is a measure of pressure and depth, are consistent with the origin of the eclogite layer by crystal fractionation in a magma ocean. The compositions of olivine coexisting with majoritic garnet in the experimental products had the average Fe/(Fe + Mg) ratios between 0.16 and 0.28. Inclusions with such high Fe contents have not been found; the Fe/(Fe + Mg) ratio of the olivine inclusions in diamonds usually varies between 0.05 and 0.09. Hence, the mantle between 200 and 410 km may not contain olivine. In the absence of olivine, the discontinuity at 410 km is most likely a chemical boundary between the 200-km-thick eclogite layer and a more mafic transition zone. Received: 15 March 2001 / Accepted: 14 September 2001     

Ultrahigh pressure macro diamonds from Copeton (New South Wales, Australia), based on Raman spectroscopy of inclusions

Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (± 100 °C and ± 4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous ⁴⁰Ar/³⁹Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth's surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin.     

Trace elements in sulfide inclusions from Yakutian diamonds

 Sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides, and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere. Trace-element abundances were measured by proton microprobe in >50 sulfide inclusions (SDI) from Yakutian diamonds; about half of these were measured in situ in polished plates of diamonds, providing information on the spatial distribution of compositional variations. Many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions. Known peridotitic diamonds contain SDIs with Ni contents of 22–36%, consistent with equilibration between olivine, monosulfide solid solution (MSS) and sulfide melt, whereas SDIs in eclogitic diamonds contain 0–12% Ni. A group of diamonds without silicate or oxide inclusions has SDIs with 11–18% Ni, and may be derived from pyroxenitic parageneses. Eclogitic SDIs have lower Ni, Cu and Te than peridotitic SDIs; the ranges of the two parageneses overlap for Se, As and Mo. The Mo and Se contents range up to 700 and 300 ppm, respectively; the highest levels are found in peridotitic diamonds. Among the in-situ SDIs, significant Zn and Pb levels are found in those connected by cracks to diamond surfaces, and these elements reflect interaction with kimberlitic melt. Significant levels of Ru (30–1300 ppm) and Rh (10–170 ppm) are found in many peridotitic SDIs; SDIs in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum-group elements (PGEs) as well, and high chondrite-normalized Ir/Pd. Comparison with experimental data on element partitioning between crystals of monosulfide solid solution (MSS) and sulfide melts suggests that most of the inclusions in both parageneses were trapped as MSS, while some high-Cu SDIs with high Pd±Rh may represent fractionated sulfide melts. Spatial variations of SDI composition within single diamonds are consistent with growth histories shown by cathodoluminescence images, in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation. Received: 5 July 1995 / Accepted: 21 February 1996     

Granulitic and eclogitic inclusions from basic pipes at Delegate,Australia

Basic breccia-nephelinite pipes at Delegate (N.S.W., Australia) contain abundant two-pyroxene granulite, garnet granulite and fassaite eclogite inclusions and rare spinel pyroxenite, peridotite and charnockite inclusions.Petrographic, mineralogical and chemical data on the inclusions and their co-existing phases are consistent with the hypothesis that the fassaite eclogite, garnet granulite and spinel pyroxenite inclusions all crystallized or recrystallized in about the same temperature-pressure region, within the range 7–15 kb and 700–1200° C. This means that these particular inclusions were formed within the uppermost part of the mantle and/or the lowermost part of the crust.The two-pyroxene granulites may also have crystallized in the same region but there are some data which are indicative of crystallization at lower pressures within the crust. The charnockite inclusion is also considered to be of crustal origin.     

Coesite inclusions in diamonds of Yakutia

The results of the study of diamonds with inclusions of high-pressure modification of SiO₂ (coesite) by Raman spectroscopy are reported. It is established that the octahedral crystal from the Zapolyarnaya pipe is characterized by the highest residual pressure (2.7 ± 0.07 GPa). An intermediate value of this parameter (2.1 ± 0.07 GPa) was obtained for a crystal of transitional habit from the Maiskaya pipe. The minimal Raman shift was registered for coesite in diamond from the Komsomol’skaya–Magnitnaya pipe and provided a calculated residual pressure of 1.8 ± 0.03 GPa. The residual pressures for crystals from the placer deposits of the Kuoika and Bol’shaya Kuonamka rivers are 2.7 ± 0.07 and 3.1 ± 0.1 GPa, respectively. Octahedral crystals were formed in the mantle at a higher pressure than rhombododecahedral diamonds.     

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.     

Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds

Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations. Received: 27 April 1999 / Accepted: 1 March 2000     

A high-temperature and high-pressure Raman spectroscopic study of CaGeO3 garnet

 High-pressure and high-temperature Raman spectra of CaGeO₃ tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO₃ majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO₃ garnet. The observed mode shifts of CaGeO₃ garnet were used to determine the isothermal and isobaric mode Grüneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a _i , for CaGeO₃ garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10⁻⁵ K⁻¹ to −1.3 × 10⁻⁵ K⁻¹, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO₃ majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO₃ tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour. Received: 21 April 1999 / Revised, accepted: 11 September 1999     

Mineral inclusions in diamonds from the River Ranch kimberlite, Zimbabwe

More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo_92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich. Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f _O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m², similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines. This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient, chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well buffered and homogeneous with respect to f _O2 at the time of peridotitic diamond crystallization. Received: 11 January 1995 / Accepted: 10 June 1997     

Rare and unusual mineral inclusions in diamonds from Mwadui, Tanzania

Syngenetic diamond inclusions from the Mwadui kimberlite reveal that an unusually fertile section of lithospheric mantle beneath the Central African Craton was sampled. This is shown by a very high ratio of lherzolitic to harzburgitic garnet inclusions (1:2) and low Mg/Fe-ratios in olivine and orthopyroxene. Geothermometry applied to the peridotitic inclusions indicates disequilibrium between non-touching inclusion pairs to be common. Disequilibrium between garnet-olivine and garnet-orthopyroxene pairs suggests successive iron enrichment during diamond formation, e.g. leading to the presence of harzburgitic garnet and lherzolitic olivine in the same diamond. Apart from the dominant peridotitic inclusion suite (88%), rare eclogitic inclusions occur (2%) and a number of uncertain paragenesis. Two diamonds, one with eclogitic garnets with moderate pyroxene solid solution and the other with a single ferro-periclase inclusion, suggest the contribution of a small sub-lithospheric component. The finding of the association Fe-FeO-Fe₃O₄ in one single diamond indicates diamond formation over a large range of f _O2 conditions, possibly along redox fronts. Steep compositional gradients may also be reflected by the joint occurrence of harzburgitic garnet and a SiO₂-phase in the same diamond. Alternatively the formation of the SiO₂-phase may be due to extreme carbonation of the peridotitic source. Further unusual findings include the exsolution of a silicate phase from magnetite inclusions, (i.e. primary solution of γ-olivine) and an ilmenite inclusion with an eskolaite (Cr₂O₃) component of 14.5 mol%, the latter together with harzburgitic paragenesis silicate inclusions. Received: 23 August 1997 / Accepted: 7 January 1998     

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.     

Polyphase hydrocarbon inclusions in garnet from the Mir diamondiferous pipe

The garnet nodules from the enrichment concentrate of the Mir kimberlite pipe, which have different saturation levels of regularly oriented mineral and primary polyphase fluid inclusions, are investigated. The garnets that contain inclusions are classified in the grossular-almandine-pyrope series and are usually characterized by an increased content of Si, Mg, Al, and the absence of Fe³⁺. The polyphase fluid inclusions are studied using IR-Fourier- and Raman spectroscopy. The presence of polyaromatic hydrocarbons, H₂O and CO₂ is established in their structure, which indicates the hydrocarbon specific character of fluids that participate in the processes of deep mineral formation.     

Melt inclusions in eclogite garnet: experimental study of natural processes

Experiment with poikilitic garnet at 3 GPa and 800 °C showed dehydration melting of its mineral inclusions, which is accompanied by the growth of (sub)euhedral garnet crystals inside the inclusion and/or xenomorphic garnets replacing the host mineral. The newly formed and host garnets differ drastically in composition. The inclusion surface is complicated by specific wedge-like protrusions or thin branches composed of melt or its crystallization products. The above features have been discovered in polymineral inclusions in garnet from low-temperature (650 °C) eclogite from the Yukon-Tanana terrane, Canada. The inclusions are interpreted as the crystallization products of in situ formed melt.     

The birth of the continents: new clues from inclusions in diamonds

Tiny inclusions found in diamonds may hold the clue to the origin of the Earth's continental crust. This is the new idea proposed by a group of Earth scientists from Australia and Russia studying a diamond pipe in Siberia.     

The formation of peridotitic suite inclusions in diamonds

Olivine, orthopyroxene and garnet grains belonging to the peridotitic suite of mineral inclusions in natural diamonds typically show compositions poorer in Ca and Al and richer in Mg and Cr than the same minerals in peridotite nodules in kimberlite. Other features suggest the crystallisation of diamonds from magmas of kimberlitic affinities, and it is suggested that the genesis of peridotitic suite diamonds is linked with that of a CO₂-bearing magma. It is shown that the generation of kimberlitic magma from common garnet-peridotite (with 5 wt.% clinopyroxene) in the presence of CO₂ may rapidly remove by melting all Ca-rich solid phases (clinopyroxene and/or carbonate). Further melting may form liquids in equilibrium with olivine, orthopyroxene, and garnet with the distinctive compositions of the diamond inclusions. The amount of melting and CO₂ necessary for the loss of clinopyroxene (and/or carbonate) are estimated at approximately 5.0 wt.% and 0.5 wt.% respectively.     

The mineralogy of Ca-rich inclusions in sublithospheric diamonds

This paper discusses mineralogy of Ca-rich inclusions in ultra-deep (sublithospheric) diamonds. It was shown that most of the Ca-rich majoritic garnets are of metabasic (eclogitic) affinity. The observed variation in major and trace element composition is consistent with variations in the composition of the protolith and the degree of enrichment or depletion during interaction with melts. Major and trace element compositions of the inclusions of Ca minerals in ultra-deep diamonds indicate that they crystallized from Ca-carbonatite melts that were derived from partial melting of eclogite bodies in deeply subducted oceanic crust in the transition zone or even the lower mantle. The occurrence of merwinite or CAS inclusions in ultra-deep diamonds can serve as mineralogical indicators of the interaction of metaperidotitic and metabasic mantle lithologies with alkaline carbonatite melts. The discovery of the inclusions of carbonates in association with ultra-deep Ca minerals can not only provide additional support for their role in the diamond formation process but also help to define additional mantle reservoirs involved in global carbon cycle.