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Characteristics of HPHT diamond grown at sub-lithosphere conditions (10–20 GPa)
Diamond & Related Materials 20 (2011) 11–17
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Characteristics of HPHT diamond grown at sub-lithosphere conditions (10–20 GPa) Emma L. Tomlinson a,⁎, Daniel Howell b,1, Adrian P. Jones b, Daniel J. Frost c a b c
Department of Geology, Royal Holloway University of London, Egham Hill, Egham, Surrey, TW20 0EX, UK Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK Bayerisches Geoinstitut, Universität Bayreuth, 95440, Bayreuth, Germany
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Article history: Received 23 December 2009 Received in revised form 7 September 2010 Accepted 12 October 2010 Available online 27 October 2010 Keywords: Diamond Transition zone Superdeep Synthesis HPHT
a b s t r a c t We have conducted high pressure–high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10–20 GPa), equivalent to the transition zone, using MgCO3 carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite–diamond transformation were observed in these short duration experiments (20 min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite–diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤ 10 GPa in MgCO3 and ≤ 15 GPa in FeNi) and, cubo-octahedral at higher pressure. All grown diamonds show a high degree of twinning. The diamonds lack planar deformation features such as laminations or slip planes, which are commonly associated with natural superdeep diamonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Natural diamonds typically come from a ‘diamond window’ in the Earth's upper mantle, bounded by the graphite–diamond stability curve at ~ 140 km and the maximum thickness of the lithosphere at ~ 220 km. However, rare diamonds containing various, often isolated inclusions of majorite garnet, ferroperricalse, stishovite, tetragonal almandine-pyrope phase (TAPP garnet), MgSi perovskite and CaSi perovskite, indicate much deeper derivation from the transition zone (410–660 km) or even the lower mantle (N660 km). Primary inclusion phase structures are not seen as a result of retrograde phase transitions caused by high temperature plastic deformation of the diamond host, so their occurrence is inferred on the basis of chemistry. Superdeep diamonds are known from Monastery and Jagersfontein, South Africa [20,38]; Kankan, Guinea [33]; Juina, Brazil [10,14]; Guaniamo, Venezuela [14]; Yubileynaya, Siberia [30]. Although recognised for more than a decade [9,28] as a new category of diamond, there is still relatively little data for superdeep diamonds, yet they represent the deepest samples of the Earth and are important for understanding large-scale geophysical mineralogical and geodynamic processes. Here, we present results from high–pressure high–temperature (HPHT) diamond growth experiments at ‘superdeep’ diamond conditions of 10 to 20 GPa and temperatures along a model terrestrial ⁎ Corresponding author. E-mail address: [email protected] (E.L. Tomlinson). 1 Now at GEMOC, Department of Earth & Planetary Science, Macquarie University, NSW 2109, Australia. 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.10.002
mantle geotherm (T range 1900–2100 °C). These pressures are much higher than used in conventional diamond synthesis. The purpose of this study is to investigate the morphology of synthetic diamonds grown under sublithospheric conditions using two very different compositions, one oxidising and one reducing. Two different carbonsaturated mixtures were used: C–MgCO3 and C–Fe90Ni10. Previous synthesis experiments using carbonates (including MgCO3) as solvent catalysts have shown that diamonds grown at 5.7–10 GPa are octahedral [21,22,26,31]. However, subordinate growth on the {100} face does occur in the carbonate systems with the highest reaction rates e.g. Li2CO3 and Na2CO3, leading to cubooctahedral morphologies [21]. In addition to the use of carbonate as a solvent catalyst, Bayarjargal et al. [2] have shown that diamonds can be crystallised directly by the decomposition of CaCO3. Diamond growth using metallic solvent catalysts (group VIII metals such as Fe, Ni, Co and their alloys) is generally conducted at 5–8 GPa and grown diamonds are typically cubo-octahedral at these conditions [32,36]. Diamond morphology is thought to change from cube to cubooctahedral to octahedral with increasing temperature and in the opposite direction with increasing pressure [4]. 2. Methods 2.1. HPHT experiments Starting materials were: 1) carbon: high purity graphite powder (UCP1-100: by Ultra Carbon Group); 2) solvent catalyst: MgCO3 (natural magnesite) and a 9:1 mix of Fe and Ni powders. A 50:50 mix of graphite and solvent catalysts were weighed and mixed by lightly
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Fig. 1. Raman spectra of experimental run products. Spectra have been baseline-corrected and displaced vertically.
grinding together in an agate mortar. The sample powder was packed into a 1.6 mm diameter Re-foil capsule. A graphite capsule liner was used for the Fe9Ni1 solvent catalyst to prevent alloying between the catalyst and capsule. Multi-anvil press experiments were carried out at 10–20 GPa and 1900–2100 °C at the Bayerisches Geoinstitut, Germany. The highpressure cells were MgO–Cr2O3 octahedra with pyrophyllite fin gaskets. Different cell assemblies were used for the 10–15 GPa experiments (8 mm anvil truncation edge length and 5.5 mm diameter sample hole) and for the 20 GPa experiments (5 mm anvil truncation edge length and 4.5 mm diameter sample hole). Pressure has been calibrated at room temperature using the phase transitions: BiI–II (2.5 GPa) and BiIII–V (7.7 GPa) and at high temperature from the phase transitions: α–β Mg2SiO4 (13.7 GPa, 1300 °C and 15 GPa, 1540 °C) and coesite–stishovite (9 GPa, 1500 °C). Heating was achieved with an electrical resistive LaCrO3 furnace. A stepped heater was used for the 10–15 GPa experiments, to reduce longitudinal thermal gradients. Temperature was measured using a single W97%Re3%–W75Re25% thermocouple mounted axially through the furnace. The holding time for all experiments was 15 min. At the end of each experiment, thermal quenching was achieved by shutting off the electrical power and no compensation was made for an instantaneous coupled reduction in pressure caused by thermal contraction.
spectra of the diamond run products were frequently complicated due to fluorescence. All spectra have been background corrected. The peak for cubic diamond (1332 cm− 1) is present in the Raman spectra of all diamond synthesis runs (Fig. 1). 3. Results 3.1. Carbonate MgCO3 Carbonate is segregated into melt pockets in all three of the experimental run products from the MgCO3–C system. This indicates that the catalyst was a melt during the experiments and may have
2.2. Analytical methods The foil capsules and their contents were split transversely to reveal a circular cross section, splitting was facilitated by hoop-like fractures probably formed as a result of rapid quenching and decompression, which weakened the specimens. Selected run products were treated by washing in a mixture of H2SO4 and HCl acids (3:1) to remove the FeNi and carbonate solvent catalysts. The samples were not polished, but mechanically broken. Backscatter electron (BSE) images were collected using a Hitachi S-3000 scanning electron microscope operating at an accelerating voltage of 20 KeV and a working distance of 9.9 mm. Raman spectra were collected to confirm the presence of diamond. Raman spectra were collected with a Renishaw inVia Raman system fitted to a Leica DM/LM microscope using primarily a laser of wavelength of 514.5 nm. As is common for diamond, the Raman
Fig. 2. P–T graph showing the conditions of diamond growth in this study and the published data for HPHT diamond synthesis experiments using pure MgCO 3 [1,22,26,37]. Open symbols represent no spontaneous diamond growth, closed symbols represent successful spontaneous nucleation. Note the symbol for Pal'yanov et al. [22] which represents no diamond growth at these conditions during their 2 h experiment, but successful growth during their 18 h experiment. Magnesite solidus* is taken from [15], and the diamond–graphite stability field** is taken from [16].
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been quenched as a glass. Peaks for MgCO3 are observed at 329, 735 and 1095 cm− 1 (v1, v4 and v1) in the Raman spectra for the runs at 10 and 15 GPa (Fig. 1), carbonate peaks are not seen in the spectrum of the 20 GPa run product because the analysis was taken at a point away from any carbonate pockets. The carbonate peaks are quite broad (v1 peak FWHM = 15 cm− 1), which would be consistent with amorphous carbonate. However, we will not discuss the state of the carbonate in detail because it may have undergone partial recrystallisation during storage at ambient conditions following the diamond growth experiments. The runs at 10 and 15 GPa show graphite Raman peaks (centred at 1580 cm− 1) in addition to diamond. At 10 GPa, the graphite peak is broad (FWHM ~ 120 cm− 1) suggesting distorted graphite and consistent with high-pressure quenched graphite [27]. BSE images of quenched products show the presence of graphite flakes 10–20 μm in size. At 15 GPa the graphite peak is resolved into two broad peaks at 1500 and 1600 cm− 1, suggesting the occurrence of distorted crystalline graphite and also possibly amorphous carbon. 3.1.1. S3635 (10 GPa, 1900 °C) This run represents the lowest HPHT conditions in this study (Fig. 2), although it is still a little higher than either commercial methods for diamond synthesis, or inferred conditions for long-term residence of typical natural diamonds in the Earth's upper mantle (~5–7 GPa). The run product consists dominantly of well-crystallised graphite (10–20 μm), with a weak preferred orientation in a texture
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where numerous individual graphite plates tend to lie within relatively small angles of offset from similar planes. This is coarser and texturally different to the starting graphite. Some areas show extremely small (≪1 μm) granular crystals amidst the graphite, thought to be coexisting diamonds; the diamond is confirmed in the Raman spectra of this run product (Fig. 1). However the grown diamond is too small and sparse to assess morphology, so the products of this experiment will not be discussed further. 3.1.2. S3633 (15 GPa, 2000 °C) The product is characterised by a relatively smooth agglomeratelike mass of polycrystalline diamond crystallised from the carbonate solvent-catalyst. Overall the diamond crystal orientation appears to be random (Fig. 3A) with a wide range of grain sizes from 5 to 10 μm down to some very fine-grained crystals b0.1 μm (Fig. 3B). Part of the diamond matrix appears to consist of sub-grains outlined by polygonized diamond networks. In detail, many of the diamond crystals have poor crystal shapes, but with some visible inter-grown twins of dominantly cubo-octahedral forms (Fig. 3B). The Raman spectrum indicates that graphite-to-diamond conversion was not completed during this run. 3.1.3. H2383 (20 GPa, 2100 °C) This run represents the highest HPHT conditions in this study. The product shows an agglomerated mass of randomly oriented diamond with few distinct crystals (Fig. 3C). These crystals show extensive
Fig. 3. BSE images of MgCO3 run products: (A) texture of diamond in S3633 (15 GPa, 2000 °C), individual diamond crystallites can be seen where pockets of carbonate have been removed. Note the brittle fracture seen in this image; (B) small individual diamond crystals in S3633 (15 GPa, 2000 °C), with poor crystal shapes; (C) areas of H2383 (20 GPa, 2000 °C) consist of more agglomerated masses of randomly oriented diamond with fewer distinct crystals; and (D) larger, well-formed diamond crystals in H2383 (20 GPa, 2000 °C); note the extensive twinning in images (B) and (D).
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twinning, with crystal morphological evidence for abundant intergrown twins of dominantly cubo-octahedral forms (Fig. 3D) and elsewhere agglomerated masses of diamond with grains outlined by polygonized diamond networks. The range of crystal sizes observed is in the range of 10–20 μm (Fig. 3D), with better formed diamond crystals bordering former pockets of carbonate, now represented by faceted hollows from where carbonate has been dissolved by acid treatment. There is no preferred orientation and the crystals have random orientation (Fig. 3C,D). The Raman spectrum and BSE images indicate that 100% of graphite was transformed to diamond. 3.2. Metallic Fe90Ni10 Two runs (S3634 at 15 GPa, and S3636 at 20 GPa, Fig. 4) showed similar textural relationships between the diamond products and metal phase and produced extensive crystalline diamonds, forming a solid ~1 mm sized slug. These pressures are 2–3 times greater than the metallic melt catalyst methods for commercial diamond synthesis, and represent similarly reduced oxygen conditions. Raman spectra show the characteristic sharp single diamond peak, centred close to 1332 cm− 1 (Fig. 1). Examination of the run products shows that individual diamond crystallites are rare, and most of the capsule contains an agglomerated diamond mass with a “brain-like” texture, formed of grains outlined by polygonized diamond networks, interspersed with pockets of FeNi metal (Fig. 5A,B,E). Where individual crystals are present, they are larger than at equivalent conditions in the carbonate system (Table 1). Individual diamond crystallites occur in cavities and irregularities previously occupied by metal. These crystallites are uniformly octahedral (S3634, 15 GPa) and cubo-octahedra (S3636, 20 GPa) and show preferred growth orientation, with families of adjacent crystals closely aligned in 3 dimensions (Fig. 5D) with some quite coarse-grained crystals of cubooctahedra (Fig. 5F). Parallel stepped growth layers are developed on some diamond faces. This may be due to carbon supersaturation, where a new layer starts to form before the old one is complete, with nucleation perhaps associated with dislocations (Fig. 5C). Curvilinear
Fig. 4. P–T graph showing the published data for HPHT diamond synthesis experiments using pure Fe–Ni. The bulk of the previously published data is focused upon the synthesis of the diamond under the lowest P and T conditions possible. * The lower bound of this field (c. 1280 °C) is taken from [36]. ** The diamond–graphite stability field is taken from [16]. *** The solidus for pure Fe is taken from [11] and references therein.
ridges on diamond surfaces (Fig. 5B,D,E) were formed as a result of mechanical breaking of the grown diamond sample, and are similar to those observed in carbonado diamond where cleavage places are not activated [23]. The metal mixture is interpreted to have been a liquid under run conditions [18]. Inclusions of the metal catalyst in the diamond are common, and occur as small isolated droplets, sometimes forming trails of similar size. These melt remnants were probably trapped by rapidly transgressing diamond grain boundaries. Larger irregular to cuspate areas of metal are also common, into which protrude good crystals of diamond, with some signs of melt “necking”, consistent with low melt viscosity of the liquid FeNi metal at these PT conditions. 4. Discussion We have shown that under HPHT conditions (15–20 GPa, equivalent to the transition zone at 450–600 km depth) diamond crystallisation might occur at very high growth rates (~mm per day) using both MgCO3 carbonate (oxidising) and FeNi (reducing) solvent catalysts. Polycrystalline growth and twinning is common. A high degree of graphite to diamond transformation was achieved in both the MgCO3–C and FeNi–C systems at 15 and 20 GPa. The degree of graphite–diamond transformation is higher in the FeNi–C system than in the MgCO3–C system at 15 GPa, suggesting that FeNi has a higher catalytic activity. The fact that the degree of transformation is dependent on the solvent catalyst indicates that diamond is not growing by direct transformation from graphite in these experiments. We now compare these synthesis results with natural conditions in the Earth. 4.1. Morphology The run products from these experiments are characterised by an agglomerated, cryptocrystalline mass of randomly oriented diamond with a few distinct crystals in cavities. The polycrystalline diamond matrix appears to consist of sub-grains outlined by polygonized diamond networks, reminiscent of textures seen in carbonado [8] a natural porous microcrystalline diamond aggregate, and in sintered diamond. The production of cryptocrystalline diamond masses in these experiments is due to: 1) a high density of nucleation sites as a result of pressure–temperature conditions far above the graphite– diamond equilibrium line. Significant pressure and/or temperature overstepping has been suggested as a mechanism for the formation of carbonado [23], this study supports this method for carbonado growth. 2) high degrees of carbon supersaturation. The majority of work using FeNi as a solvent catalyst for diamond synthesis has focused upon the production of large, well faceted diamonds, grown in the shortest period of time. A key to this is not having excessive nucleation of interfering crystals, something which is minimized when between 10 and 60% of the carbon is dissolved [35]. At the conditions of the experiments of this study, it is likely that the entire carbon source was dissolved, resulting in high degrees of carbon supersaturation and the production of interfering crystal nucleation. Euhedral and twinned micrometer-sized diamond crystals grown in cavities were formed later than the polycrystalline aggregate i.e. in the latter stages of graphite–diamond transformation under conditions of decreasing carbon supersaturation. The diamonds grown from the MgCO3 solvent catalyst are cubooctahedral, this is in contrast to diamonds grown at lower pressures (7–10 GPa) using MgCO3, which are typically octahedral [21,22,26,31] (Fig. 6). Therefore, higher pressure appears to favor more rapid growth on the {111} face. The diamonds grown from the FeNi catalyst are octahedral at 15 GPa and cubo-octahedral at 20 GPa. Therefore, as in the MgCO3 system, higher pressure appears to favor more rapid growth on the {111} face; the characteristic field of cubo-octahedral
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Fig. 5. BSE images of FeNi run products (A) S3636 (20 GPa, 2100 °C) capsule slice with solvent catalyst is removed to show ‘brain-like’ texture of amalgamated diamond mass present in both FeNi runs; (B) S3634 (15 Gpa, 2000 °C) showing two different configurations of FeNi, either dismembered semi-continuous filaments or isolated spherule-like blebs; C) parallel growth layers on the surface of the growing diamond in S3634 (15 Gpa, 2000 °C); (D) S3634 (15 Gpa, 2000 °C) showing octahedral growth morphology of individual crystallites in amalgamated diamond mass; (E) S3636 (20 GPa, 2100 °C) showing brittle fracture features; and (F) texture of S3636 (20 GPa, 2100 °C) showing a couple of distinct cubo-octahedral crystals in the amalgamated diamond mass.
growth is shifted to higher pressure in the FeNi system relative to the MgCO3 system (Fig. 6). The MgCO3 grown diamonds also have a high degree of crystal twinning. Twinning generally forms under applied shear stress e.g. when diamonds are deformed under conditions that inhibit brittle fracture. The occurrence of twinning has been proposed as the most likely product of plastic deformation during the actual growth of diamond [39], and the product of the experiments of this study would strongly support that. Abundant twinning on {111} may have formed
by destroying the stacking sequence of a close packed layer, which minimises the energy of misfit when close packing. This suggests that there may also be features such as stacking faults, parallel dislocation lines and dislocation networks, which may be visible by transmission electron microscopy (TEM). Both intergrown and star twins are present in the SEM images of the carbonate run products (e.g. Fig. 3D). Star twins are characterised by a 7° lattice mismatch [6,17] and occur under conditions of rapid diamond formation [17] and the ratio between single and twinned
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Table 1 Experimental results of diamond growth during 15 min at sublithospheric conditions. Abbreviations:% trans. — percentage transformation of graphite to diamond; I1332/ I1580 = intensity of Raman diamond peak at 1332 cm− 1/intensity of Raman graphite peak at 1580 cm− to give an estimate of the proportion of diamond to graphite; V(100)/V(111) — diamond shape factor (low — cubic, high — octahedral) based on relative growth rates of b100N and b 111N faces. Run
Starting materials
P (GPa)
T (°C)
% trans.
Size (μm)
Morphology (V(100)/V(111))
S3635 S3633 H2383 S3634 S3636
C–MgCO3 C–MgCO3 C–MgCO3 C–Fe90Ni10 C–Fe90Ni10
10 15 20 15 20
1900 2000 2100 2000 2100
b100% (I1332/I1580 = 0.6) b100% (I1332/I1580 = 2.8) 100% 100% 100%
b0.1 5 20 20 40
– Twinned cubo-octahedron (1.30) Twinned cubo-octahedron (1.30) Octahedron (1.65) Cubo-octahedron (0.70)
crystals depends on the transformation rate [6], therefore the star twins in this study may represent formation conditions intermediate between single crystals and the cryptocrystalline diamond mass. Twinning is less evident in the FeNi run products due to the paucity of single crystals amongst the cryptocrystalline diamond at even higher degrees of graphite–diamond transformation. The degrees of transformation required to form star twins are unlikely to be common during the formation of natural diamonds (superdeep or cratonic), with the possible exception of carbonado in which star twins may occur. The most common feature of plastic deformation seen in natural diamond is slip planes/lamination lines occurring in the (111) orientation. These are not seen in the experimental run products reported here. Slip planes in natural diamonds are thought to be postgrowth features formed during the fracture propagation of kimberlitic melts, which is responsible for the subsequent transportation of the diamonds [12]. 4.2. Pressure, temperature and fO2 conditions The PT conditions used in this study would correspond to geothermal gradients of 50 mWm− 2 (10 GPa) to 45 mWm− 2 (15– 20 GPa) (calculated after [24]). These conditions are at the high temperature end of the range of PT conditions determined from geothermobarometry of mantle xenoliths, which range from 35 to 45 mWm− 2. The high temperatures are responsible for the high graphite–diamond transformation rates in our experimental systems. The high pressure is thought to be responsible for the high density of nucleation sites, which resulted in the aggregated morphology of the grown diamond. Natural superdeep diamonds are not typically characterised by polycrystalline and cryptocrystalline morphologies,
this may reflect lower degrees of carbon supersaturation in the highpressure growth environment of superdeep diamonds. Our two model systems cover a range of oxygen fugacity from metallic iron to relatively oxidised carbonate, both buffered by carbon saturation. To a first approximation, these two systems span the oxygen fugacity of the mantle from the continental crust (−2 to + 5 log units relative to the log fayalite–magnetite–quartz reference: FMQ −2 to FMQ+5) to the lower mantle (bFMQ−5) [19] and core (−2 log units below iron–wustite equilibrium, IW−2 [25]). In the transition zone, which is equivalent to the pressures used in our experiments, oxygen fugacity is approximately FMQ−4 [19]. The experiments in this study indicate that diamond could be formed in the mantle transition zone in the presence of either carbonate or metallic iron. Inclusions in diamonds from Brazil and Tanzania originating from the lower part of the transition zone below indicate that least locally carbonates exist at this depth [3,34]. Experimental work also suggests that carbonatites can originate from the transition zone [5], supporting the presence or carbonate melts at this depth. Inclusions of Fe–metal and FexC carbide have also been documented [13]. Metallic Fe and carbonate inclusions may well record ambient mantle conditions [7] or they may represent transient redox perturbations linked to local conditions during diamond growth [13]. In the Fe–C system, diamond is stable at higher temperatures than iron carbide (Fe3C breaks down to 3Feliq + Cliq): iron carbide is stable below 2000 °C at 15 GPa and below 2030 °C at 20 GPa [29]. Our experiments were run within error of these temperatures, however iron carbides have not specifically been recognised in the FeNi experimental run products. It is possible that small carbides were formed during the experiments, however diamond is the dominant phase. This may be due to the additional presence of Ni, or perhaps to the rapid quench-rate of the carbon-saturated FeNi melt in our experiments.
5. Conclusions
Fig. 6. P–T plot showing the morphologies of diamond grown in this study at 15 and 20 GPa (the MgCO3 run at 10 GPa is not shown because grown diamonds are too small to distinguish morphology). Carbonate-grown diamonds (grey) are compared to lower pressure diamonds grown in MgCO3 [1,22,26,37]. Diamonds grown in FeNi are shown in white.
Diamond has been grown experimentally at ‘superdeep’ conditions (15–20 GPa, equivalent to the transition zone at 450–600 km depth) using both oxidising (MgCO3) and reducing (FeNi) materials as a solvent catalyst for graphite. We infer that a diamond can be formed in the mantle transition zone in the presence of metallic iron or carbonate. Experimental conditions were close to the solidus in both carbonate and metallic runs at 15 and 20 GPa. Graphite-to-diamond transformation rates were high, and the transformation rate was higher in the FeNi system. Grown diamonds formed a polycrystalline mass of agglomerated sub-grains outlined by polygonized diamond networks as a result of high degrees of carbon supersaturation and formation pressures significantly above the graphite–diamond stability line. Where individual crystals occur, these show extensive twinning, including star twinning. Higher pressure appears to favour the growth of cubo-octahedral diamond, whereas octahedral diamond is grown at lower pressure conditions (≤10 GPa in MgCO3 and ≤15 GPa in FeNi). The run products lack the slip planes and lamination lines commonly associated with superdeep
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diamonds, we suggest that these deformation features are due to transport, rather than growth conditions. Acknowledgements The high-pressure experiments were performed courtesy of the Bayerisches Geoinstitut under the EU Research Infrastructures: Transnational Access Program (Contract No. 505320 (RITA) — High Pressure). Daniel Howell was funded by an EPSRC industrial CASE studentship and acknowledges the sponsorship and advice provided by the Diamond Trading Company (DeBeers). Four anonymous reviewers are thanked for constructive comments on this manuscript. References [1] M. Akaishi, H. Kanda, S. Yamaoka, Journal of Crystal Growth 104 (1990) 578. [2] L. Bayarjargal, T.G. Shumilova, A. Friedrich, B. Winkler, European Journal of Mineralogy 22 (1) (2010) 29. [3] F.E. Brenker, et al., Earth and Planetary Science Letters 260 (1–2) (2007) 1. [4] R.E. Clausing, in: M.A. Prelas, A. Popovici, L.K. Bigelow (Eds.), Handbook of Industrial Diamonds and Diamond Films, 1997, p. 19. [5] C. Dalou, K.T. Koga, T. Hammouda, F. Poitrasson, Geochimica et Cosmochimica Acta 73 (1) (2009) 239. [6] T.L. Daulton, D.D. Eisenhour, T.J. Bernatowicz, R.S. Lewis, P.R. Buseck, Geochimica et Cosmochimica Acta 60 (23) (1996) 4853. [7] D.J. Frost, et al., Nature 428 (6981) (2004) 409. [8] J. Garai, S.E. Haggerty, S. Rekhi, M. Chance, Astrophysical Journal 653 (2) (2006) L153. [9] B. Harte, J.W. Harris, Mineralogical Magazine 58A (1994) 384. [10] P.C. Hayman, M.G. Kopylova, F.V. Kaminsky, Contributions to Mineralogy and Petrology 149 (4) (2005) 430. [11] R.J. Hemley, H.K. Mao, International Geology Review 43 (2001) 1. [12] Howell D., 2009. Quantifying stress and strain in diamond. PhD Thesis, University College London, London. [13] D. Jacob, A. Kronz, K. Viljoen, Contributions to Mineralogy and Petrology 146 (2004) 566.
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Characteristics of HPHT diamond grown at sub-lithosphere conditions (10–20 GPa) Emma L. Tomlinson a,⁎, Daniel Howell b,1, Adrian P. Jones b, Daniel J. Frost c a b c
Department of Geology, Royal Holloway University of London, Egham Hill, Egham, Surrey, TW20 0EX, UK Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK Bayerisches Geoinstitut, Universität Bayreuth, 95440, Bayreuth, Germany
a r t i c l e
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Article history: Received 23 December 2009 Received in revised form 7 September 2010 Accepted 12 October 2010 Available online 27 October 2010 Keywords: Diamond Transition zone Superdeep Synthesis HPHT
a b s t r a c t We have conducted high pressure–high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10–20 GPa), equivalent to the transition zone, using MgCO3 carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite–diamond transformation were observed in these short duration experiments (20 min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite–diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤ 10 GPa in MgCO3 and ≤ 15 GPa in FeNi) and, cubo-octahedral at higher pressure. All grown diamonds show a high degree of twinning. The diamonds lack planar deformation features such as laminations or slip planes, which are commonly associated with natural superdeep diamonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Natural diamonds typically come from a ‘diamond window’ in the Earth's upper mantle, bounded by the graphite–diamond stability curve at ~ 140 km and the maximum thickness of the lithosphere at ~ 220 km. However, rare diamonds containing various, often isolated inclusions of majorite garnet, ferroperricalse, stishovite, tetragonal almandine-pyrope phase (TAPP garnet), MgSi perovskite and CaSi perovskite, indicate much deeper derivation from the transition zone (410–660 km) or even the lower mantle (N660 km). Primary inclusion phase structures are not seen as a result of retrograde phase transitions caused by high temperature plastic deformation of the diamond host, so their occurrence is inferred on the basis of chemistry. Superdeep diamonds are known from Monastery and Jagersfontein, South Africa [20,38]; Kankan, Guinea [33]; Juina, Brazil [10,14]; Guaniamo, Venezuela [14]; Yubileynaya, Siberia [30]. Although recognised for more than a decade [9,28] as a new category of diamond, there is still relatively little data for superdeep diamonds, yet they represent the deepest samples of the Earth and are important for understanding large-scale geophysical mineralogical and geodynamic processes. Here, we present results from high–pressure high–temperature (HPHT) diamond growth experiments at ‘superdeep’ diamond conditions of 10 to 20 GPa and temperatures along a model terrestrial ⁎ Corresponding author. E-mail address: [email protected] (E.L. Tomlinson). 1 Now at GEMOC, Department of Earth & Planetary Science, Macquarie University, NSW 2109, Australia. 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.10.002
mantle geotherm (T range 1900–2100 °C). These pressures are much higher than used in conventional diamond synthesis. The purpose of this study is to investigate the morphology of synthetic diamonds grown under sublithospheric conditions using two very different compositions, one oxidising and one reducing. Two different carbonsaturated mixtures were used: C–MgCO3 and C–Fe90Ni10. Previous synthesis experiments using carbonates (including MgCO3) as solvent catalysts have shown that diamonds grown at 5.7–10 GPa are octahedral [21,22,26,31]. However, subordinate growth on the {100} face does occur in the carbonate systems with the highest reaction rates e.g. Li2CO3 and Na2CO3, leading to cubooctahedral morphologies [21]. In addition to the use of carbonate as a solvent catalyst, Bayarjargal et al. [2] have shown that diamonds can be crystallised directly by the decomposition of CaCO3. Diamond growth using metallic solvent catalysts (group VIII metals such as Fe, Ni, Co and their alloys) is generally conducted at 5–8 GPa and grown diamonds are typically cubo-octahedral at these conditions [32,36]. Diamond morphology is thought to change from cube to cubooctahedral to octahedral with increasing temperature and in the opposite direction with increasing pressure [4]. 2. Methods 2.1. HPHT experiments Starting materials were: 1) carbon: high purity graphite powder (UCP1-100: by Ultra Carbon Group); 2) solvent catalyst: MgCO3 (natural magnesite) and a 9:1 mix of Fe and Ni powders. A 50:50 mix of graphite and solvent catalysts were weighed and mixed by lightly
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Fig. 1. Raman spectra of experimental run products. Spectra have been baseline-corrected and displaced vertically.
grinding together in an agate mortar. The sample powder was packed into a 1.6 mm diameter Re-foil capsule. A graphite capsule liner was used for the Fe9Ni1 solvent catalyst to prevent alloying between the catalyst and capsule. Multi-anvil press experiments were carried out at 10–20 GPa and 1900–2100 °C at the Bayerisches Geoinstitut, Germany. The highpressure cells were MgO–Cr2O3 octahedra with pyrophyllite fin gaskets. Different cell assemblies were used for the 10–15 GPa experiments (8 mm anvil truncation edge length and 5.5 mm diameter sample hole) and for the 20 GPa experiments (5 mm anvil truncation edge length and 4.5 mm diameter sample hole). Pressure has been calibrated at room temperature using the phase transitions: BiI–II (2.5 GPa) and BiIII–V (7.7 GPa) and at high temperature from the phase transitions: α–β Mg2SiO4 (13.7 GPa, 1300 °C and 15 GPa, 1540 °C) and coesite–stishovite (9 GPa, 1500 °C). Heating was achieved with an electrical resistive LaCrO3 furnace. A stepped heater was used for the 10–15 GPa experiments, to reduce longitudinal thermal gradients. Temperature was measured using a single W97%Re3%–W75Re25% thermocouple mounted axially through the furnace. The holding time for all experiments was 15 min. At the end of each experiment, thermal quenching was achieved by shutting off the electrical power and no compensation was made for an instantaneous coupled reduction in pressure caused by thermal contraction.
spectra of the diamond run products were frequently complicated due to fluorescence. All spectra have been background corrected. The peak for cubic diamond (1332 cm− 1) is present in the Raman spectra of all diamond synthesis runs (Fig. 1). 3. Results 3.1. Carbonate MgCO3 Carbonate is segregated into melt pockets in all three of the experimental run products from the MgCO3–C system. This indicates that the catalyst was a melt during the experiments and may have
2.2. Analytical methods The foil capsules and their contents were split transversely to reveal a circular cross section, splitting was facilitated by hoop-like fractures probably formed as a result of rapid quenching and decompression, which weakened the specimens. Selected run products were treated by washing in a mixture of H2SO4 and HCl acids (3:1) to remove the FeNi and carbonate solvent catalysts. The samples were not polished, but mechanically broken. Backscatter electron (BSE) images were collected using a Hitachi S-3000 scanning electron microscope operating at an accelerating voltage of 20 KeV and a working distance of 9.9 mm. Raman spectra were collected to confirm the presence of diamond. Raman spectra were collected with a Renishaw inVia Raman system fitted to a Leica DM/LM microscope using primarily a laser of wavelength of 514.5 nm. As is common for diamond, the Raman
Fig. 2. P–T graph showing the conditions of diamond growth in this study and the published data for HPHT diamond synthesis experiments using pure MgCO 3 [1,22,26,37]. Open symbols represent no spontaneous diamond growth, closed symbols represent successful spontaneous nucleation. Note the symbol for Pal'yanov et al. [22] which represents no diamond growth at these conditions during their 2 h experiment, but successful growth during their 18 h experiment. Magnesite solidus* is taken from [15], and the diamond–graphite stability field** is taken from [16].
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been quenched as a glass. Peaks for MgCO3 are observed at 329, 735 and 1095 cm− 1 (v1, v4 and v1) in the Raman spectra for the runs at 10 and 15 GPa (Fig. 1), carbonate peaks are not seen in the spectrum of the 20 GPa run product because the analysis was taken at a point away from any carbonate pockets. The carbonate peaks are quite broad (v1 peak FWHM = 15 cm− 1), which would be consistent with amorphous carbonate. However, we will not discuss the state of the carbonate in detail because it may have undergone partial recrystallisation during storage at ambient conditions following the diamond growth experiments. The runs at 10 and 15 GPa show graphite Raman peaks (centred at 1580 cm− 1) in addition to diamond. At 10 GPa, the graphite peak is broad (FWHM ~ 120 cm− 1) suggesting distorted graphite and consistent with high-pressure quenched graphite [27]. BSE images of quenched products show the presence of graphite flakes 10–20 μm in size. At 15 GPa the graphite peak is resolved into two broad peaks at 1500 and 1600 cm− 1, suggesting the occurrence of distorted crystalline graphite and also possibly amorphous carbon. 3.1.1. S3635 (10 GPa, 1900 °C) This run represents the lowest HPHT conditions in this study (Fig. 2), although it is still a little higher than either commercial methods for diamond synthesis, or inferred conditions for long-term residence of typical natural diamonds in the Earth's upper mantle (~5–7 GPa). The run product consists dominantly of well-crystallised graphite (10–20 μm), with a weak preferred orientation in a texture
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where numerous individual graphite plates tend to lie within relatively small angles of offset from similar planes. This is coarser and texturally different to the starting graphite. Some areas show extremely small (≪1 μm) granular crystals amidst the graphite, thought to be coexisting diamonds; the diamond is confirmed in the Raman spectra of this run product (Fig. 1). However the grown diamond is too small and sparse to assess morphology, so the products of this experiment will not be discussed further. 3.1.2. S3633 (15 GPa, 2000 °C) The product is characterised by a relatively smooth agglomeratelike mass of polycrystalline diamond crystallised from the carbonate solvent-catalyst. Overall the diamond crystal orientation appears to be random (Fig. 3A) with a wide range of grain sizes from 5 to 10 μm down to some very fine-grained crystals b0.1 μm (Fig. 3B). Part of the diamond matrix appears to consist of sub-grains outlined by polygonized diamond networks. In detail, many of the diamond crystals have poor crystal shapes, but with some visible inter-grown twins of dominantly cubo-octahedral forms (Fig. 3B). The Raman spectrum indicates that graphite-to-diamond conversion was not completed during this run. 3.1.3. H2383 (20 GPa, 2100 °C) This run represents the highest HPHT conditions in this study. The product shows an agglomerated mass of randomly oriented diamond with few distinct crystals (Fig. 3C). These crystals show extensive
Fig. 3. BSE images of MgCO3 run products: (A) texture of diamond in S3633 (15 GPa, 2000 °C), individual diamond crystallites can be seen where pockets of carbonate have been removed. Note the brittle fracture seen in this image; (B) small individual diamond crystals in S3633 (15 GPa, 2000 °C), with poor crystal shapes; (C) areas of H2383 (20 GPa, 2000 °C) consist of more agglomerated masses of randomly oriented diamond with fewer distinct crystals; and (D) larger, well-formed diamond crystals in H2383 (20 GPa, 2000 °C); note the extensive twinning in images (B) and (D).
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twinning, with crystal morphological evidence for abundant intergrown twins of dominantly cubo-octahedral forms (Fig. 3D) and elsewhere agglomerated masses of diamond with grains outlined by polygonized diamond networks. The range of crystal sizes observed is in the range of 10–20 μm (Fig. 3D), with better formed diamond crystals bordering former pockets of carbonate, now represented by faceted hollows from where carbonate has been dissolved by acid treatment. There is no preferred orientation and the crystals have random orientation (Fig. 3C,D). The Raman spectrum and BSE images indicate that 100% of graphite was transformed to diamond. 3.2. Metallic Fe90Ni10 Two runs (S3634 at 15 GPa, and S3636 at 20 GPa, Fig. 4) showed similar textural relationships between the diamond products and metal phase and produced extensive crystalline diamonds, forming a solid ~1 mm sized slug. These pressures are 2–3 times greater than the metallic melt catalyst methods for commercial diamond synthesis, and represent similarly reduced oxygen conditions. Raman spectra show the characteristic sharp single diamond peak, centred close to 1332 cm− 1 (Fig. 1). Examination of the run products shows that individual diamond crystallites are rare, and most of the capsule contains an agglomerated diamond mass with a “brain-like” texture, formed of grains outlined by polygonized diamond networks, interspersed with pockets of FeNi metal (Fig. 5A,B,E). Where individual crystals are present, they are larger than at equivalent conditions in the carbonate system (Table 1). Individual diamond crystallites occur in cavities and irregularities previously occupied by metal. These crystallites are uniformly octahedral (S3634, 15 GPa) and cubo-octahedra (S3636, 20 GPa) and show preferred growth orientation, with families of adjacent crystals closely aligned in 3 dimensions (Fig. 5D) with some quite coarse-grained crystals of cubooctahedra (Fig. 5F). Parallel stepped growth layers are developed on some diamond faces. This may be due to carbon supersaturation, where a new layer starts to form before the old one is complete, with nucleation perhaps associated with dislocations (Fig. 5C). Curvilinear
Fig. 4. P–T graph showing the published data for HPHT diamond synthesis experiments using pure Fe–Ni. The bulk of the previously published data is focused upon the synthesis of the diamond under the lowest P and T conditions possible. * The lower bound of this field (c. 1280 °C) is taken from [36]. ** The diamond–graphite stability field is taken from [16]. *** The solidus for pure Fe is taken from [11] and references therein.
ridges on diamond surfaces (Fig. 5B,D,E) were formed as a result of mechanical breaking of the grown diamond sample, and are similar to those observed in carbonado diamond where cleavage places are not activated [23]. The metal mixture is interpreted to have been a liquid under run conditions [18]. Inclusions of the metal catalyst in the diamond are common, and occur as small isolated droplets, sometimes forming trails of similar size. These melt remnants were probably trapped by rapidly transgressing diamond grain boundaries. Larger irregular to cuspate areas of metal are also common, into which protrude good crystals of diamond, with some signs of melt “necking”, consistent with low melt viscosity of the liquid FeNi metal at these PT conditions. 4. Discussion We have shown that under HPHT conditions (15–20 GPa, equivalent to the transition zone at 450–600 km depth) diamond crystallisation might occur at very high growth rates (~mm per day) using both MgCO3 carbonate (oxidising) and FeNi (reducing) solvent catalysts. Polycrystalline growth and twinning is common. A high degree of graphite to diamond transformation was achieved in both the MgCO3–C and FeNi–C systems at 15 and 20 GPa. The degree of graphite–diamond transformation is higher in the FeNi–C system than in the MgCO3–C system at 15 GPa, suggesting that FeNi has a higher catalytic activity. The fact that the degree of transformation is dependent on the solvent catalyst indicates that diamond is not growing by direct transformation from graphite in these experiments. We now compare these synthesis results with natural conditions in the Earth. 4.1. Morphology The run products from these experiments are characterised by an agglomerated, cryptocrystalline mass of randomly oriented diamond with a few distinct crystals in cavities. The polycrystalline diamond matrix appears to consist of sub-grains outlined by polygonized diamond networks, reminiscent of textures seen in carbonado [8] a natural porous microcrystalline diamond aggregate, and in sintered diamond. The production of cryptocrystalline diamond masses in these experiments is due to: 1) a high density of nucleation sites as a result of pressure–temperature conditions far above the graphite– diamond equilibrium line. Significant pressure and/or temperature overstepping has been suggested as a mechanism for the formation of carbonado [23], this study supports this method for carbonado growth. 2) high degrees of carbon supersaturation. The majority of work using FeNi as a solvent catalyst for diamond synthesis has focused upon the production of large, well faceted diamonds, grown in the shortest period of time. A key to this is not having excessive nucleation of interfering crystals, something which is minimized when between 10 and 60% of the carbon is dissolved [35]. At the conditions of the experiments of this study, it is likely that the entire carbon source was dissolved, resulting in high degrees of carbon supersaturation and the production of interfering crystal nucleation. Euhedral and twinned micrometer-sized diamond crystals grown in cavities were formed later than the polycrystalline aggregate i.e. in the latter stages of graphite–diamond transformation under conditions of decreasing carbon supersaturation. The diamonds grown from the MgCO3 solvent catalyst are cubooctahedral, this is in contrast to diamonds grown at lower pressures (7–10 GPa) using MgCO3, which are typically octahedral [21,22,26,31] (Fig. 6). Therefore, higher pressure appears to favor more rapid growth on the {111} face. The diamonds grown from the FeNi catalyst are octahedral at 15 GPa and cubo-octahedral at 20 GPa. Therefore, as in the MgCO3 system, higher pressure appears to favor more rapid growth on the {111} face; the characteristic field of cubo-octahedral
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Fig. 5. BSE images of FeNi run products (A) S3636 (20 GPa, 2100 °C) capsule slice with solvent catalyst is removed to show ‘brain-like’ texture of amalgamated diamond mass present in both FeNi runs; (B) S3634 (15 Gpa, 2000 °C) showing two different configurations of FeNi, either dismembered semi-continuous filaments or isolated spherule-like blebs; C) parallel growth layers on the surface of the growing diamond in S3634 (15 Gpa, 2000 °C); (D) S3634 (15 Gpa, 2000 °C) showing octahedral growth morphology of individual crystallites in amalgamated diamond mass; (E) S3636 (20 GPa, 2100 °C) showing brittle fracture features; and (F) texture of S3636 (20 GPa, 2100 °C) showing a couple of distinct cubo-octahedral crystals in the amalgamated diamond mass.
growth is shifted to higher pressure in the FeNi system relative to the MgCO3 system (Fig. 6). The MgCO3 grown diamonds also have a high degree of crystal twinning. Twinning generally forms under applied shear stress e.g. when diamonds are deformed under conditions that inhibit brittle fracture. The occurrence of twinning has been proposed as the most likely product of plastic deformation during the actual growth of diamond [39], and the product of the experiments of this study would strongly support that. Abundant twinning on {111} may have formed
by destroying the stacking sequence of a close packed layer, which minimises the energy of misfit when close packing. This suggests that there may also be features such as stacking faults, parallel dislocation lines and dislocation networks, which may be visible by transmission electron microscopy (TEM). Both intergrown and star twins are present in the SEM images of the carbonate run products (e.g. Fig. 3D). Star twins are characterised by a 7° lattice mismatch [6,17] and occur under conditions of rapid diamond formation [17] and the ratio between single and twinned
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Table 1 Experimental results of diamond growth during 15 min at sublithospheric conditions. Abbreviations:% trans. — percentage transformation of graphite to diamond; I1332/ I1580 = intensity of Raman diamond peak at 1332 cm− 1/intensity of Raman graphite peak at 1580 cm− to give an estimate of the proportion of diamond to graphite; V(100)/V(111) — diamond shape factor (low — cubic, high — octahedral) based on relative growth rates of b100N and b 111N faces. Run
Starting materials
P (GPa)
T (°C)
% trans.
Size (μm)
Morphology (V(100)/V(111))
S3635 S3633 H2383 S3634 S3636
C–MgCO3 C–MgCO3 C–MgCO3 C–Fe90Ni10 C–Fe90Ni10
10 15 20 15 20
1900 2000 2100 2000 2100
b100% (I1332/I1580 = 0.6) b100% (I1332/I1580 = 2.8) 100% 100% 100%
b0.1 5 20 20 40
– Twinned cubo-octahedron (1.30) Twinned cubo-octahedron (1.30) Octahedron (1.65) Cubo-octahedron (0.70)
crystals depends on the transformation rate [6], therefore the star twins in this study may represent formation conditions intermediate between single crystals and the cryptocrystalline diamond mass. Twinning is less evident in the FeNi run products due to the paucity of single crystals amongst the cryptocrystalline diamond at even higher degrees of graphite–diamond transformation. The degrees of transformation required to form star twins are unlikely to be common during the formation of natural diamonds (superdeep or cratonic), with the possible exception of carbonado in which star twins may occur. The most common feature of plastic deformation seen in natural diamond is slip planes/lamination lines occurring in the (111) orientation. These are not seen in the experimental run products reported here. Slip planes in natural diamonds are thought to be postgrowth features formed during the fracture propagation of kimberlitic melts, which is responsible for the subsequent transportation of the diamonds [12]. 4.2. Pressure, temperature and fO2 conditions The PT conditions used in this study would correspond to geothermal gradients of 50 mWm− 2 (10 GPa) to 45 mWm− 2 (15– 20 GPa) (calculated after [24]). These conditions are at the high temperature end of the range of PT conditions determined from geothermobarometry of mantle xenoliths, which range from 35 to 45 mWm− 2. The high temperatures are responsible for the high graphite–diamond transformation rates in our experimental systems. The high pressure is thought to be responsible for the high density of nucleation sites, which resulted in the aggregated morphology of the grown diamond. Natural superdeep diamonds are not typically characterised by polycrystalline and cryptocrystalline morphologies,
this may reflect lower degrees of carbon supersaturation in the highpressure growth environment of superdeep diamonds. Our two model systems cover a range of oxygen fugacity from metallic iron to relatively oxidised carbonate, both buffered by carbon saturation. To a first approximation, these two systems span the oxygen fugacity of the mantle from the continental crust (−2 to + 5 log units relative to the log fayalite–magnetite–quartz reference: FMQ −2 to FMQ+5) to the lower mantle (bFMQ−5) [19] and core (−2 log units below iron–wustite equilibrium, IW−2 [25]). In the transition zone, which is equivalent to the pressures used in our experiments, oxygen fugacity is approximately FMQ−4 [19]. The experiments in this study indicate that diamond could be formed in the mantle transition zone in the presence of either carbonate or metallic iron. Inclusions in diamonds from Brazil and Tanzania originating from the lower part of the transition zone below indicate that least locally carbonates exist at this depth [3,34]. Experimental work also suggests that carbonatites can originate from the transition zone [5], supporting the presence or carbonate melts at this depth. Inclusions of Fe–metal and FexC carbide have also been documented [13]. Metallic Fe and carbonate inclusions may well record ambient mantle conditions [7] or they may represent transient redox perturbations linked to local conditions during diamond growth [13]. In the Fe–C system, diamond is stable at higher temperatures than iron carbide (Fe3C breaks down to 3Feliq + Cliq): iron carbide is stable below 2000 °C at 15 GPa and below 2030 °C at 20 GPa [29]. Our experiments were run within error of these temperatures, however iron carbides have not specifically been recognised in the FeNi experimental run products. It is possible that small carbides were formed during the experiments, however diamond is the dominant phase. This may be due to the additional presence of Ni, or perhaps to the rapid quench-rate of the carbon-saturated FeNi melt in our experiments.
5. Conclusions
Fig. 6. P–T plot showing the morphologies of diamond grown in this study at 15 and 20 GPa (the MgCO3 run at 10 GPa is not shown because grown diamonds are too small to distinguish morphology). Carbonate-grown diamonds (grey) are compared to lower pressure diamonds grown in MgCO3 [1,22,26,37]. Diamonds grown in FeNi are shown in white.
Diamond has been grown experimentally at ‘superdeep’ conditions (15–20 GPa, equivalent to the transition zone at 450–600 km depth) using both oxidising (MgCO3) and reducing (FeNi) materials as a solvent catalyst for graphite. We infer that a diamond can be formed in the mantle transition zone in the presence of metallic iron or carbonate. Experimental conditions were close to the solidus in both carbonate and metallic runs at 15 and 20 GPa. Graphite-to-diamond transformation rates were high, and the transformation rate was higher in the FeNi system. Grown diamonds formed a polycrystalline mass of agglomerated sub-grains outlined by polygonized diamond networks as a result of high degrees of carbon supersaturation and formation pressures significantly above the graphite–diamond stability line. Where individual crystals occur, these show extensive twinning, including star twinning. Higher pressure appears to favour the growth of cubo-octahedral diamond, whereas octahedral diamond is grown at lower pressure conditions (≤10 GPa in MgCO3 and ≤15 GPa in FeNi). The run products lack the slip planes and lamination lines commonly associated with superdeep
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diamonds, we suggest that these deformation features are due to transport, rather than growth conditions. Acknowledgements The high-pressure experiments were performed courtesy of the Bayerisches Geoinstitut under the EU Research Infrastructures: Transnational Access Program (Contract No. 505320 (RITA) — High Pressure). Daniel Howell was funded by an EPSRC industrial CASE studentship and acknowledges the sponsorship and advice provided by the Diamond Trading Company (DeBeers). Four anonymous reviewers are thanked for constructive comments on this manuscript. References [1] M. Akaishi, H. Kanda, S. Yamaoka, Journal of Crystal Growth 104 (1990) 578. [2] L. Bayarjargal, T.G. Shumilova, A. Friedrich, B. Winkler, European Journal of Mineralogy 22 (1) (2010) 29. [3] F.E. Brenker, et al., Earth and Planetary Science Letters 260 (1–2) (2007) 1. [4] R.E. Clausing, in: M.A. Prelas, A. Popovici, L.K. Bigelow (Eds.), Handbook of Industrial Diamonds and Diamond Films, 1997, p. 19. [5] C. Dalou, K.T. Koga, T. Hammouda, F. Poitrasson, Geochimica et Cosmochimica Acta 73 (1) (2009) 239. [6] T.L. Daulton, D.D. Eisenhour, T.J. Bernatowicz, R.S. Lewis, P.R. Buseck, Geochimica et Cosmochimica Acta 60 (23) (1996) 4853. [7] D.J. Frost, et al., Nature 428 (6981) (2004) 409. [8] J. Garai, S.E. Haggerty, S. Rekhi, M. Chance, Astrophysical Journal 653 (2) (2006) L153. [9] B. Harte, J.W. Harris, Mineralogical Magazine 58A (1994) 384. [10] P.C. Hayman, M.G. Kopylova, F.V. Kaminsky, Contributions to Mineralogy and Petrology 149 (4) (2005) 430. [11] R.J. Hemley, H.K. Mao, International Geology Review 43 (2001) 1. [12] Howell D., 2009. Quantifying stress and strain in diamond. PhD Thesis, University College London, London. [13] D. Jacob, A. Kronz, K. Viljoen, Contributions to Mineralogy and Petrology 146 (2004) 566.
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