Earth and Planetary Science Letters 430 (2015) 284–295
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Diamonds in ophiolites: Contamination or a new diamond growth
environment?
D. Howell a,b,c,∗, W.L. Griffin a, J. Yang d, S. Gain a, R.A. Stern e, J.-X. Huang a, D.E. Jacob a, X. Xu d, A.J. Stokes f, S.Y. O’Reilly a, N.J. Pearson a
a ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Department of Earth & Planetary Science, Macquarie University, NSW 2109, Australiab Institute of Geoscience, Goethe University, Frankfurt, Germanyc School of Earth Sciences, University of Bristol, Queens Road, Bristol, BS8 1RJ, UKd State Key Laboratory of Continental Tectonics and Dynamics (CARMA), CAGS, Beijing, Chinae Canadian Centre for Isotopic Microanalysis, University of Alberta, Edmonton, AB, T6G 2E3, Canadaf OptoFab – ANFF and MQPhotonics Research Centre, Macquarie University, NSW 2109, Australia
a r t i c l e i n f o a b s t r a c t
Article history:Received 16 April 2015Received in revised form 7 August 2015Accepted 19 August 2015Available online xxxxEditor: A. Yin
Keywords:Luobusa peridotitesreduced oxygen fugacitycarbon and nitrogen isotopestrace elementsType Ib diamonds
For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal–alloy inclusions and highly negative δ13C values are present in both sets of diamonds. The Tibetan diamonds (n = 3) show an exceptionally large range in δ15N (−5.6 to +28.7�) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with δ15N = 0� in {111} sectors and +30� in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni–Mn–Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.
* Corresponding author at: ARC Centre of Excellence for Core to Crust Fluid Sys-tems (CCFS) and GEMOC, Department of Earth & Planetary Science, Macquarie Uni-versity, NSW 2109, Australia. Tel.: +61(0)298504401; fax: +61(0)298508943.
Diamonds occur in three main geological settings: in cratonic mantle (transported to the surface via kimberlites and lamproites), in ultra high-pressure (UHP) metamorphic rocks, and in meteorite impact craters (Harlow and Davies, 2005, references therein). Dia-monds have also been reported from peridotites and chromitites in
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Fig. 1. Map showing the location of the Yarlung–Zangbo suture zone (YZSZ). The position of the Luobusa ophiolite along the YZSZ is shown in the lower image. Taken from
Xiong (2014).
Tibetan ophiolites by researchers at the Chinese Academy of Geo-logical Sciences (CAGS; Beijing) for more than 20 years (e.g. Bai et al., 1993). However, these reports have been deemed controversial by both the diamond-research community and the wider geolog-ical community, for two main reasons. One is that the ophiolitic diamonds are very similar in appearance to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents) and therefore have been written off as contami-nants. Another is that other researchers have studied these rocks without finding any diamonds.
We have characterized the diamonds reportedly found in the mantle peridotites and chromites of the Luobusa massif in Tibet, and report morphology, internal structures, trace-element patterns, N concentration, distribution and aggregation, and C and N iso-topes. We also document the recovery of several such diamonds from an independently collected hand specimen processed under clean laboratory conditions. The data are compared to the exten-sive body of literature on HPHT synthetic diamonds, with a focus on understanding the crystallographic processes that occur during growth and account for these characteristics. This work represents the first fully documented account of the Tibetan diamonds; it will allow the research community to form a more objective judgment as to whether these samples represent a previously unrecognized mantle process and environment, or a case of anthropogenic con-tamination.
1.1. Geological setting
The Luobusa ultramafic massif is the easternmost of a series of peridotites, up to 2000–3000 km3 in size, scattered along the
Yarlung–Zangbo Suture Zone (YZSZ: Fig. 1), which runs roughly E–W at ∼29◦N, for 1500 km across the southern edge of Tibet. These peridotites are commonly described as “ophiolites” though most bear only a faint similarity to the classic “Penrose model”, based on the Troodos and Semail examples (Anonymous, 1972;Nicolas et al., 1981). Similar peridotite massifs (e.g. Dongqiao, Fig. 1) occur on other sutures, subparallel to the YZSZ, further north on the Tibetan Plateau. The Luobusa and Dongqiao bod-ies each contain several podiform chromitite bodies of commercial size and grade, whereas chromitites are rare and small in most of the other bodies.
The recognition, over 30 years ago, of diamonds in mineral separates from the Luobusa chromitites led to an ongoing pro-gram of detailed mineral separation by the Chinese Academy of Geological Sciences (CAGS). It has found that the diamonds are accompanied by a remarkable assortment of minerals (coesite, stishovite, native elements, alloys, carbides, nitrides) requiring con-ditions of ultra-high pressure, high temperature and/or extremely low f O2 (Dobrzhinetskaya et al., 2009; Xu et al., 2015). The origi-nal work concentrated on the chromitites, but these super-reduced and ultra-high-pressure (SuR-UHP) phases have now been sepa-rated from the peridotitic and dunitic host rocks to the Luobusa chromitites, as well as from peridotites in other massifs (Yang et al., 2014; Fig. 1).
More than 2000 diamond grains have been recovered by the CAGS group from peridotites and chromitites in Tethyan ophiolites in southern Tibet (within two different suture zones), and the Ray-Iz ophiolite in the polar Urals, Russia (Yang et al., 2014, 2015). In all cases the sampling has involved large volumes (300–1100 kg), often of mixed lithologies (Xu et al., 2015) and usually taken from
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dumps. Samples have been processed in industrial-scale plants using crushing and sieving, followed by heavy-media and mag-netic techniques (Xu et al., 2009). Most of the separated phases thus have no clear petrological context. However, diamonds, SiC and corundum have been observed in situ in polished sections of chromitite; in each case these phases are enclosed in ovoid to ir-regular volumes of “amorphous carbon”, whose nature and origin remain undefined (Yang et al., 2014, 2015).
2. Samples
Over 400 diamonds recovered by the CAGS group from the Luobusa peridotites and chromites were examined by DH for a general population study. Diamonds from peridotites have KY sam-ple numbers, while those recovered from the chromites have KCr sample numbers. 20 diamonds were selected for separate detailed analysis. In addition, two epoxy mounts were prepared by the CAGS group with ∼120 diamonds from each of the two rock types, for laser-ablation analysis.
While there is a large body of literature on the characteristics of HPHT synthetic diamonds, there are few trace-element data. To allow comparison with the trace element data from the Tibetan diamonds, collections of industrial HPHT synthetic diamonds were purchased from one European (Element 6) and one Chinese man-ufacturer (Zhengzhou Crystal Diamond Ltd., Henan Province). The Chinese manufacturer is based in the same town, Zhengzhou, as the processing facility used by CAGS workers for mineral sepa-rations. From each supplier, packets of both highest- and lowest-quality crystals were obtained in the largest size range (commonly 650–1000 μm). The objective was to see if there was any differ-ence in the data obtained from those with more inclusions (i.e. the lower-quality crystals) and the generally inclusion-free, better-quality crystals.
3. Analytical techniques
An assessment of size, morphology and color in the KY and KCr diamond populations was performed using a Zeiss Discovery stereo-microscope, equipped with a MRc5 AxioCam for imaging (CAGS, Beijing). A data-binning approach was adopted for each of these characteristics. Crystal sizes were determined using a dig-ital graticule and placed in categories: <100 μm, 100–190 μm, 190–210 μm, 210–290 μm, 290–310 μm, 310–400 μm, and>400 μm. Crystal morphologies were determined visually and grouped as follows: purely octahedral; predominantly octahedral but with some {100} cube faces present; equal proportions of octa-hedral and cube faces; predominantly cube faces with some small octahedral faces; purely cube morphology. This classification was only possible where the crystals exhibited enough crystal faces, and their shapes could also be assessed as being euhedral, sub-hedral or anhedral. Many of the grains were fragments, and these were recorded separately. Color was assessed visually, on a scale from 0 (colorless) to 6 (strong yellow).
Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet iN10 instrument (CCFS, Macquarie University) on the ∼20 individual grains extracted from the larger collection. The spectra were deconvoluted using the DiaMap routine (see Howell et al., 2012a, 2012b). These data provide information on the ni-trogen concentration, aggregation state and other spectral features of interest (i.e. hydrogen bands, platelet characteristics) if present. Due to the small size of the crystals, they were mounted on syn-thetic fibers for analysis, but often the fragmented nature of the grains prevented the retrieval of high-quality data, and only 4 anal-yses provided fully quantitative data.
Several of these crystals were ion-beam milled using a Jeol IB-19500 cross sectional polisher (Photonics Research Centre, Mac-
quarie University), as they were too small to polish by traditional methods. This revealed the cores of the crystals, as well as ex-posing inclusions. Cathodoluminescence (CL) images were collected on these crystals using a Zeiss EVO scanning electron micro-scope (SEM; CCFS, Macquarie University). Where inclusions were exposed, energy-dispersive X-ray spectral analysis (EDX) was per-formed using an Oxford Instruments Aztec Synergy EDS/EBSD sys-tem, fitted to the same Zeiss EVO SEM.
In situ laser-ablation inductively coupled-plasma mass spec-trometry (LA-ICPMS) was performed on 30 KY diamonds and 30 KCr diamonds in the two epoxy mounts. The diamonds were ablated using a Photon Machines Excite excimer (193 nm) laser ab-lation system (Geochemical Analysis Unit, CCFS/GEMOC, Macquarie University), coupled to an Agilent 7700 ICP-MS. The methodology, as described by Howell et al. (2013a), involves the use of doped cellulose as an external calibration standard (after Rege et al., 2005). The data were reduced using the GLITTER® software (ver-sion 4.4.4; Griffin et al., 2008). All of the data presented here are above the limit of quantification (LOQ; see McNeill et al., 2009 and Howell et al., 2013a for discussions on this topic).
After LA-ICPMS analysis was carried out, both mounts were ion-beam milled using a South Bay Technology PC-2000 Plasma Cleaner (Electron Microscope Unit, University of New South Wales), which allowed a larger surface area to be removed. This exposed the cores of many of the crystals, and their CL response was im-aged as described above. After imaging, crystals (n = 19) that had not been laser-ablated were selected for analysis of carbon and nitrogen isotopic compositions and nitrogen abundance by sec-ondary ion mass spectrometry (SIMS) using a Cameca IMS 1280 multi-collector ion probe (CCIM, University of Alberta). Methods and reference materials are as detailed by Stern et al. (2014) and Howell et al. (2015), and are summarized here.
The selected diamond crystals were remounted in epoxy, trimmed to form a 4 × 5 mm block, and then pressed into an indium metal mount along with the CCIM natural diamond refer-ence material S0270 (aliquot M) with δ13CVPDB = −8.88 ± 0.10�and δ15NAIR = −0.4 ± 0.5�. The nitrogen abundance measure-ments were calibrated against a natural diamond reference mate-rial (CCIM sample S0280E) with [N] = 1670 ppm (atomic), located on a separate mount. All diamonds were pressed into the central part of the mount assembly, where there are no significant bi-ases due to stage position. Analyses were conducted using 133Cs+at 20 keV impact energy, with a spot diameter of 15 μm, and beam current of 3–4 nA. Electron charge compensation was not required. Data were collected in two analytical sessions on adja-cent spots, one for C-isotopes, and one for N-isotopes and abun-dances. For δ13C, the median uncertainty per spot for the data in this study is ±0.13� (95% confidence level). Uncertainties for δ15N range from ∼±3� (95% confidence level) for the spots with lowest [N] to ∼±1� for the highest, and N-isotope data are re-ported only for spots with [N] > ∼50 ppm. Uncertainties in [N] are ∼±10%. The uncertainties in the isotopic data do not include the 2σ absolute uncertainties in the reference materials. The isotopic data are reported as δ13CVPDB (13C/12C = 0.001118) and δ15NAIR(15N/14N = 1/272), respectively, and N-abundances are reported in atomic ppm.
4. Independent discovery
Independently of the CAGS group, workers at MQ have carried out fieldwork to collect samples of the Luobusa chromitites and peridotites. Single hand-specimens (∼1–2 kg) were collected from the rock faces or from larger blocks in dumps, using a hammer (i.e. no mechanical processing tools which might involve synthetic diamond bits). They have been processed in the clean Mineral Pro-cessing Unit (MQ), using a Selfrag high-voltage fragmentation de-
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Fig. 2. (a) Photograph of a diamond crystal recovered from a hand specimen of Lu-obusa chromite, collected by WLG and processed in the clean labs of MQ using a Selfrag mineral separator. (b) BSE image of the diamond in (a). Scale bars rep-resent 100 μm. (c) Raman spectrum showing the characteristic diamond band at 1332 cm−1. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
vice with a Teflon vessel. Disposable single-use screens were used during subsequent sieving to avoid cross-contamination. Follow-ing magnetic separation, a diamond crystal was recovered in 2014 from the <600 μm size fraction of a specimen of massive chromi-tite (13LBSIII-1, No. 3 Mining Area, Luobusa). Raman spectroscopy has confirmed the crystal as diamond (Fig. 2c). It exhibits cubo-octahedral morphology and a strong yellow color (Fig. 2a and b); it contains no visible inclusions. Six other crystals or fragments of diamonds have been recovered in 2015 and confirmed by Raman
Fig. 3. Plots showing the various characteristics of the diamond populations re-covered from the Luobusa peridotites (KY) and chromites (KCr): (a) crystal size, (b) morphology, and (c) color. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
spectroscopy (LBS14-06D, massive chromitite, Shaft 4266, Luobusa mining area; KJL14-06F, -06-2 (dunite with massive chromitite) and -07E (nodular chromitite), Orebody C-11, Kangjinla mining area). One of the euhedral crystals (LBS14-06D) is significantly smaller (<100 μm) than the other intact crystals (200–300 μm); another with several preserved faces is <50 μm. This diversity in sizes may weigh against contamination from drillbits or saws.
5. Results
5.1. Tibetan diamonds
5.1.1. Population studyThe size distributions for the KY and KCr populations are shown
in Fig. 3a. While sizes range from ∼100 to 500 μm, it is clear that most of the samples are in the 200 to 300 μm range for both sam-ple sets. The significance of this size range is unclear, as a large proportion (>50%) of samples are fragments of larger crystals. As the host rocks were crushed prior to sieving and picking, it is hard to know if the fracturing is an original feature or a result of the processing.
The morphology of the diamonds ranges from octahedral to cube-dominated cubo-octahedron (Figs. 2, 3b); no perfect cubes
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Fig. 4. Infrared spectrum obtained from one of the Luobusa diamond crystals. Shown below the obtained spectrum is an example of a pure diamond (Type II, i.e. nitrogen free) spectrum, and the inset shows the characteristic C centre absorption feature.
were found. See Xu et al. (2009) for more SEM images of the di-amonds’ varied morphologies. The most important observation is that the {100} cube faces are smooth surfaces, upon which spi-ral/dislocation growth has occurred. On one crystal, very fine den-dritic patterns were observed (but not imaged) on both {111} and {100} crystal surfaces.
The diamonds exhibit a range of colors; some are close to col-orless, whereas others have a very deep yellow color (Fig. 3c). As these data are fundamentally subjective (i.e. based on perception of color, not quantitative absorption spectroscopy) our reliance upon it for interpretation purposes will be limited.
5.1.2. FTIR spectroscopyAn example of a FTIR spectrum recorded from one of the dia-
mond grains is shown in Fig. 4. The data obtained from process-ing the four best spectra suggest nitrogen concentrations in the range of 100 to 200 ppm. All of this nitrogen is in the form of single-substitutional C centres, making the diamonds Type Ib. No evidence of aggregation to pairs of nitrogen atoms (A centres) was observed.
5.1.3. EDX analysis of exposed inclusionsTwo samples were ion-beam milled to expose inclusions at the
surface (Fig. 5). All four inclusions have bulbous shapes, suggesting that they were trapped as melt phases. While the growth stratig-raphy as revealed by CL is not very obvious in one sample, in the other it is clear that the inclusions are located within the bright {111} growth sector. EDX analysis of the inclusions reveals them to be metallic alloys, composed of Ni, Mn and Co in the ratios of ∼70:25:5.
5.1.4. Trace-element dataThe trace-element analyses of the Tibetan diamonds show con-
siderable scatter in absolute abundance, reflecting the residence of most trace elements in mineral inclusions (Fig. 6). Despite this scatter, there are subtle differences in the trace-element patterns of the diamonds from the chromitites and those from the peri-dotites. While both populations show mild enrichment of LREE relative to MREE and HREE, and strong depletion in Nb, the dia-monds from the peridotites show a more consistent depletion of Sr and enrichment in Pb. Of the redox-sensitive elements, Eu shows consistent depletion only in the peridotite diamonds, while Sm and Yb, which are only rarely quantifiable, show occasional depletion in both populations.
Many of the LA-ICPMS time-resolved analyses of the Tibetan diamonds show “inclusion bursts”, with several elements exhibit-
ing coupled behavior. Most of these inclusions, in both the KY and KCr samples, are metallic alloys containing Ni, Mn and Co. While the concentration data show that the ratios of these elements vary slightly between samples (Fig. 7), they are clearly coupled, but are decoupled from Fe. We also observed such bursts from inclusions of REE-rich phases, with La, Ce and Pr the most common elements present. One inclusion in a KY sample showed coupled signals of Y, La, Ce, Pr, Nd, Gd, Dy, Ho, Th and U.
5.1.5. Cathodoluminescence (CL) imagingCL images of several samples from the epoxy mounts are shown
in Figs. 5 and 8. They all reveal clear sector growth zoning; bright {111} sectors with layered octahedral-type growth, and dull {100} sectors. When the contrast is increased to the point that the {111} sectors are totally overexposed, some growth stratigraphy is visible in the {100} sectors (not shown).
5.1.6. Stable-isotope dataEighteen grains from the epoxy mounts were analyzed for car-
bon isotopes (6 KY and 12 KCr); 17 of these also were analyzedfor nitrogen concentrations and isotopic ratios (6 KY and 11 KCr; Appendix A). In total, 78 δ13C measurements were recorded, ex-hibiting a range in δ13C of −24.0� to −28.3� (2σ < 0.16�), with an average value of −25.8�. There is no difference in δ13C between the two populations (average δ13C; KY = −25.9�[n = 29], KCr = −25.7� [n = 49]). There are also no detectable differences between growth sectors (average δ13C; octahedral =−25.8� [n = 37], cube = −25.8� [n = 41]). The largest varia-tion within a single stone is ∼ 1.6� (−24.8 to −26.4�; KYL5_10), while the largest change in δ13C within a single growth sector from core to rim is ∼+1� (−25.8 to −24.8�; KCrL4_13).
Nitrogen concentrations were obtained for 82 points, with val-ues ranging from 9 to 430 ppm, an average of 142 ppm and a median of 95 ppm. There is a distinct difference between the ni-trogen concentrations of the octahedral growth sectors (average =214 ppm, n = 48), and the cube sectors (average = 46 ppm, n = 34). For high-quality nitrogen-isotope measurements (i.e. 2σ <
2�), concentrations of >100 ppm are required. As the cube sec-tors did not contain nitrogen above 72 ppm, it was not possible to record isotopic values with 2σ uncertainties of <±2.4� (max-imum uncertainty ±3.4�); in contrast, the data from octahedral sectors have average uncertainties of ±1.4�. As a result, cross-sector comparison of the nitrogen-isotope data is limited. δ15N in octahedral sectors ranges from −5.6� to +28.7�, with an aver-age of +8.6� and a median of +8.2� (n = 39). The peridotitic (KY) samples range from −5.6 to +14.3�, with an average value
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Fig. 5. Backscattered Electron (BSE) image (a) and a cathodoluminescence (CL) image (b) of a KCr diamond that has been ion-beam milled to expose inclusions. Secondary Electron (SE) image (c) and CL image (d) of a KY diamond, that has also been ion-beam milled. Note that the two inclusions in the KY diamond are within the octahedral (brightly luminescing) sector.
Fig. 6. Chondrite-normalized trace-element data obtained by LA-ICPMS analysis of diamonds from the two source rock types, and HPHT synthetic diamonds obtained from Chinese and European manufacturers. All data are above the limit of quantification (LOQ; see text for explanation).
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Fig. 7. Concentration data for the four transition metals, Ni, Mn, Co and Fe, obtained by LA-ICPMS. In most analyses of the Tibetan diamonds, there appears to be a consistent relationship between Ni, Mn and Co, indicating a coupled behavior. In the HPHT synthetic diamonds, this is seen for only Fe and Ni. All data are above the limit of quantification (LOQ; see text for explanation).
Table 1Average nitrogen isotope and concentration data for the three diamonds shown in Fig. 8, revealing varying degrees of isotopic fractionation between the {111} and {100} growth sectors.
of +5.8� (n = 12), while the chromitite (KCr) samples range from −1.4 to +28.7�, with an average of +9.8� and a median of +8.7� (n = 27). The largest range in the octahedral sector of a single sample is 21.8� (−0.1 to +21.7�; KCrL4_13). In the δ15Ndata obtained from the cube sectors, the range is similarly large (−7.6� to +27.1�) but with a lower average of +2.4� and a median of −2.0� (n = 21).
Data from three samples can be used to evaluate fractionation between sectors. Fig. 8a–c shows the location of analytical spots and the δ15N – [N] data recorded from samples KCrL4_2, KCrL4_23and KCrL4_24 respectively. The average values for the cube and oc-tahedral sectors of these diamonds are shown in Table 1. There does not appear to be any consistent fractionation between sec-tors across the three samples. The octahedral sectors are enriched in 15N relative to the cube sectors by 18.2 and 9.5� in two of the samples. The third sample shows 15N enrichment in the cube sectors by 3.8�, but this difference is within the analytical uncer-tainties. Thus the δ15N values of the octahedral sectors are either similar to the cube sectors, or higher.
5.2. HPHT synthetic diamonds
5.2.1. Trace element dataThe synthetic HPHT diamonds from the European manufacturer
have trace-element patterns with mild enrichment of LREE relative to MREE and HREE, strong depletion in Nb and Sr, occasional en-richment in Pb, and signs of depletion in the few data for Sm, Eu and Yb (Fig. 6). In contrast, the patterns data obtained from the Chinese HPHT diamonds show enrichment in the MREE relative to both the LREE and HREE, with relative enrichment in Sr and Nb, and the same enrichment in Pb.
As noted above, many of the analyses show short-lived peaks of certain elements in the time-resolved data, indicative of inclu-sions. For both sets of HPHT synthetic diamonds, only two ele-ments (Fe and Ni) showed coupled behavior in these bursts in the ICPMS data. However, it is important to note that both of these elements also showed base-level signals (i.e. excluding signal de-
rived from inclusions) that were well above the detection limits. Whether these elements are contained in microinclusions that are not optically visible, or are lattice-bound impurities, cannot be de-termined from these data; a combination of both possibilities is probable. The only notable difference between the low- and high-quality crystals from each manufacturer is a slight increase in the total abundances of trace elements in the former.
6. Discussion
6.1. Characteristics of HPHT synthetic diamond
Synthetic diamonds, grown from pure metal or alloy solvents under HPHT conditions, exhibit crystal morphologies that can be bounded by {111} and/or {100} faces, and thus lie in the cubo-octahedron spectrum. The competition between {111} and {100} faces is a function of temperature (Clausing, 1997); {100} faces dominate at lower temperatures. Crystal-growth theory based on the diamond structure (see Sunagawa, 2005) has shown that {111} faces are the only smooth faces upon which spiral growth (of-ten referred to as octahedral growth, in diamonds) can occur. The {100} faces correspond to a rough interface, upon which adhesive growth takes place. However, this is not the case for diamonds grown from metal solvents, where the {100} face acts as a smooth interface due to the process of surface reconstruction. This pro-cess occurs because the atoms on the surface of a crystal are not bonded in three dimensions, unlike the atoms in the interior. As a result, the surface atoms are in a higher energy state than the internal atoms, which is less than ideal. Under certain conditions, partial bonds are formed that reduce the high-energy state of the surface atoms. Surface reconstruction modifies the crystal faces, producing a different morphology than that on non-reconstructed faces. This process can only occur when diamond grows in a so-lution in which the solvent has a small ionic radius (Sunagawa, 2005), such as a metallic melt. Therefore the occurrence of smooth growth in cube sectors is commonly used to discriminate between
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Fig. 8. Cathodoluminescence (CL) images of four diamonds [(a) KCrL4_2, (b) KCrL4_23, (c) KCrL4_24, (d) KCrL4_13] from the Luobusa chromitite, showing the locations of SIMS analyses for carbon isotopes (yellow spots) and nitrogen isotopes, 2σ analytical uncertainties and concentrations (red dots). All δ13C data has 2σ analytical uncertainties of <0.15�. White scale bars represent 50 μm. Averages of the data used to investigate growth-sector fractionation are shown in Table 1. A graph of δ15N data plotted against N concentrations is shown in (e). Grey error bars are 2σ analytical uncertainties. White diamond symbols represent data from the octahedral sectors, and black squares represent data from the cube sectors. The black lines through the data from both sectors show the negative relationship, with increasing δ15N data as nitrogen concentrations decrease. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
HPHT synthetic diamonds and natural cratonic diamonds grown from C–O–N–H-rich melts.
The nitrogen characteristics of HPHT synthetic diamonds are well documented. They contain single-substitutional nitrogen(C centres); concentrations are typically around 200 ppm (Collins, 2000) but can range from 0 ppm (if N-getters are used) to at least
500 ppm. This unaggregated nitrogen (Type Ib) is thought to reflect the short time frames in which the diamonds are grown. C centres are responsible for the absorption of light that gives HPHT syn-thetic diamonds their characteristic yellow color, and contribute to their cathodoluminescence response. Where growth occurs on both {111} and {100} faces by the same spiral growth mechanism, nitro-
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gen is preferentially partitioned into the octahedral growth sectors because there are more bonding opportunities on the {111} faces than on the {100} faces.
This partitioning of nitrogen can also result in isotopic frac-tionation. Boyd et al. (1988) noted that the octahedral sectors of a large synthetic HPHT diamond had δ15N values close to atmo-spheric (0�), while the cube sectors have δ15N values of +30�; this result was reproduced by Reutsky et al. (2008). In a closed growth system, these values can change from the core of the stone to the rim, with the δ15N of octahedral sectors becoming slightly negative (−0.4 to −2.5�), while the δ15N of cube sectors in-creased to +42� (Boyd et al., 1988). It is important to point out that the absolute values are inherently related to the starting ma-terials; perhaps more importantly, both of these studies used large HPHT diamonds that were grown by the temperature-gradient method. Isotopic fractionation is not observed in the carbon data. The δ13C of synthetic diamonds is usually highly negative, reflect-ing the composition of the graphite used as a carbon source.
As synthetic diamonds are grown from metal/alloy solvents (most commonly FeNi melts), they often contain inclusions of this growth medium, as well as impurities within the lattice. The most notable lattice impurity in HPHT synthetic diamonds is nickel, which is preferentially taken up in the octahedral growth sectors (Collins et al., 1990; Babich and Feigelson, 2009); it can reach con-centrations of ∼70 ppm (Collins et al., 1998), and often forms nickel–nitrogen complexes (Collins et al., 1990; Babich and Feigel-son, 2009). These nickel defects are often revealed in the CL imag-ing of synthetic diamonds, and their presence or absence can high-light the {111} and {100} growth sectors, along with the C centres.
6.2. Comparison between Tibetan and HPHT synthetic diamonds
The most obvious similarity between the Tibetan and syn-thetic diamonds is that their morphologies both lie in the cubo-octahedron spectrum. This similarity is also manifest in the CL images of the crystal-growth stratigraphy. Cubo-octahedral mor-phology is deemed atypical of “natural” diamonds (i.e. cratonic diamonds grown from C–O–N–H fluids/melts), because diamonds grown from C–O–N–H-rich melts do not grow smooth {100} faces. Instead, they grow on the {100} faces by either the nu-cleation/birth-and-spread mechanism (cuboid growth; Moore and Lang, 1972) or the adhesive growth mechanism (fibrous growth; Lang, 1974). However, cubo-octahedral diamonds, while rare, do occur in nature (Kvasnitsa et al., 1999) and have been grown ex-perimentally from natural materials such as anhydrous alkaline carbonates (Palyanov and Sokol, 2009).
The nitrogen concentrations of the Tibetan diamonds, as deter-mined by both FTIR and SIMS, are within the ranges expected for both cratonic and HPHT synthetic diamonds. However, the N is present in the form of unaggregated single substitutional centres (Type 1b). This aggregation state is very rare in cratonic diamonds, but not unheard of Hainschwang et al. (2013); it is very common in synthetic ones. It implies that the diamonds have experienced HPHT conditions for only a relatively short period of time (in ge-ological terms) and as a result, the nitrogen atoms have not dif-fused through the lattice to aggregate with one another. Nitrogen is strongly partitioned into the octahedral growth sectors of the Tibetan diamonds. As the growth mechanisms are the same on both crystal faces, the greater availability of bonding opportunities on the {111} faces allows a faster uptake of impurities compared to the {100} faces. While this nitrogen partitioning is observed in HPHT synthetic diamonds, it is also seen in natural mixed-habit di-amonds where the two growth mechanisms are different (cuboid and octahedral growth; see Howell et al., 2013b, and references therein). N partitioning between growth sectors therefore cannot be used to discriminate between natural and synthetic diamonds.
The δ13C values of the Tibetan diamonds are highly negative (−24� to −28�), values remarkably similar to those reported from HPHT synthetic diamonds. They are outside of the range con-sidered to be representative of primitive mantle (∼−5�), but they fall within the total range observed in cratonic diamonds (Cartigny, 2005). The nitrogen isotopes of the Tibetan diamonds do differ from those of synthetic diamonds but the comparison is limited. Studies of two large HPHT diamonds grown by temperature gradi-ent methods (Boyd et al., 1988; Reutsky et al., 2008) reported δ15Nvalues from the {111} sectors of ∼0�, with ∼30� relative enrich-ment of 15N in the {100} sectors. These values depend upon the source of the nitrogen (atmospheric) and the fractionation is likely related to the growth method. Irrespective of whether the Tibetan diamonds are natural or represent anthropogenic contamination, they will not have grown by the temperature gradient method. More likely they will have formed by either redox processes (see below) or film-growth (see Palyanov et al., 2015). As they will also not have the same source of nitrogen, a direct comparison has lim-ited value. Combined with the fact that we only have data from three Tibetan diamonds to compare with two HPHT synthetics sug-gests that much more research is required into the understanding of detailed N isotope characteristics of all diamonds to fully under-stand what we are recording in the Tibetan diamonds.
The trace-element concentrations of the synthetic diamonds an-alyzed in this study are, on average, lower than those recorded in the Tibetan diamonds. The synthetic HPHT diamonds from the Eu-ropean manufacturer have trace-element patterns similar to those of the diamonds from the Tibetan peridotites, while those from the Chinese manufacturer are more distinct. The most consistent differences between the Tibetan diamonds and the synthetic HPHT diamonds are seen in the siderophile elements. The Tibetan dia-monds show a consistent, smooth decrease in abundance from Ni to Mn to Co, and typically have very low Fe contents (∼1 ppm). The HPHT diamonds from both manufacturers, in contrast, have consistently high Fe and Ni contents (10–1000 ppm), Mn contents either lower or higher than Ni contents, and a strong depletion in Co relative to Ni–Mn and Fe.
The concentrations of the transition metals suggest that the synthetic diamonds purchased for LA-ICPMS analysis were grown in Fe–Ni fluxes, while the Tibetan diamonds would appear to be have formed in contact with Ni–Mn–Co melts. This is supported by the EDX analysis of the exposed inclusions, which represent the growth medium of the diamonds. While FeNi melts are the most common solvent used for growing synthetic diamonds, other tran-sition metal alloys have also been used on a laboratory scale (see Burns et al., 1999), including NiMnCo (Zang et al., 2007).
This comparison between the Tibetan diamonds and the prod-ucts of two randomly chosen synthetic HPHT diamond manufactur-ers is instructive but ultimately limited. While samples from many different manufacturers could be analyzed, the trace-element im-purities (beyond just the metallic solvent) come from the starting materials, i.e. impurities in both the metal solvents and carbon. For any manufacturer, these starting materials could vary in impurity content over time depending on their suppliers. This means that diamonds from one manufacturer could easily change over time, and monitoring these changes would be a never-ending task.
6.3. Interpreting the characteristics of the Tibetan diamonds
It is clear from the above comparison that there are more sim-ilarities than differences between HPHT synthetic diamonds and the Tibetan diamonds recovered from ophiolitic rocks. However, it is important to understand what these characteristics are actu-ally telling us about the growth of these diamonds, and not just whether they are the same as synthetic diamonds.
D. Howell et al. / Earth and Planetary Science Letters 430 (2015) 284–295 293
The cubo-octahedral morphology of the Tibetan diamonds is the result of growth from a flux or melt that allowed the process of surface reconstruction to take place. Taking into account the in-clusions and LA-ICPMS data, it appears most likely that this growth medium was a NiMnCo alloy. The composition of the inclusions is close to the minimum-melting point on the ternary Ni–Mn–Co liq-uidus, with a temperature of ∼1150 ◦C at 1 atm (Gupta, 1999); if the pressure dependence is similar to melting curves for Fe–Ni al-loys, the melting temperature would be close to 1200 ◦C at 5 GPa. The absence of Fe is consistent with the association between the Tibetan diamonds and a range of other minerals that require f O2well below the range where all Fe would be removed from the local chemical system as native iron, Fe-carbides or other highly-reduced phases observed in the Luobusa mineral separates.
The carbon-isotope values of the Tibetan diamonds imply a source of isotopically light carbon. This could be subducted or-ganic material, which has a mean δ13C = ∼−25� (Cartigny, 2005). There is no discernable fractionation between the {111} and {100} growth sectors. When there is much variation within an individual crystal, δ13C becomes more positive, suggesting an open system and a source of carbon that is large compared to the amount of diamond crystallized.
The nitrogen isotope data, in contrast to the C-isotopes, show a very large range, extending to the most positive values reported in natural diamonds (Mikhail et al., 2014). Plots of δ15N vs N concen-tration (Fig. 8e) reveal a clear negative correlation in the octahedral sectors, and a subtler one in cube sectors. Due to the small size of the samples, only two samples have analyses near the core and rim of the same growth sector. In KCrL4_23 (Fig. 8b), a decrease in N concentration can be seen (391 ppm to 119 ppm) but differences in the δ15N values are within analytical uncertainties. In KCrL4_13(Fig. 8d), the fractionation is more pronounced; nitrogen concen-trations drop from 390 ppm to 140 ppm, while δ15N becomes significantly more positive (0� to +21�). The overall trend of the octahedral sectors is consistent with fractional crystallization of the fluid, as the �15Ndiamond–fluid appears to be negative (Petts et al., 2015). Therefore, progressively crystallized diamond will have increasingly positive δ15N. If this interpretation is valid, it suggests that the least-evolved diamond had δ15N of ∼ 0� or less, which is well within the normal range for natural diamonds (−24� to +18�; Cartigny, 2005).
The range of nitrogen concentrations and the lack of aggrega-tion can account for the range of colors observed in the Tibetan diamonds. C centres create yellow color, and the range of concen-trations of ∼10 to >400 ppm would produce a range from nearly colorless to intense yellow. The lack of nitrogen aggregation also suggests that the diamonds have experienced HPHT conditions for only a relatively short period of time, and/or mantle storage at un-usually low temperatures. The latter seems inconsistent with the presence of NiMnCo alloy as inclusions in the diamonds; tempera-tures of ∼1200 ◦C are required for this to be in liquid form to act as a solvent for diamond growth, and temperatures of ∼1350 ◦C would be needed for cubo-octahedral growth in this system.
It is hard to accurately quantify the residence times or temper-atures, as there has been no aggregation at all; therefore calcula-tions using the time–temperature relationship of nitrogen aggrega-tion (e.g. Evans and Harris, 1989) can only provide maximum con-straints. An additional complication is the possible effect of nickel, manganese, cobalt and vacancy defects on the rate of aggregation. Significantly different activation energies have been calculated for aggregation in the {111} and the {100} sectors of HPHT synthetic diamonds (Taylor et al., 1996). The higher activation energy of the {100} sector (6 eV, as opposed to 4.4 eV in the {111} sectors) has been attributed to vacancy-assisted nitrogen migration in the {111} sectors (where Ni defects were preferentially partitioned; Taylor et al., 1996). As Ni defects are likely to occur within the Tibetan
Fig. 9. Mantle residence times vs average temperatures calculated from the nitrogen aggregation relationship. The black lines represent the average nitrogen concen-tration of the {111} sectors (214 ppm), and the grey lines the average nitrogen concentration of the {100} sectors (46 ppm). The solid lines have been calculated using an activation energy of 6 eV and Ln(A) = 18.8 (Taylor et al., 1996) and an as-sumed aggregation of 10% A centres. The dashed lines use the same assumed 10% A centres, but an activation energy of 4.4 eV and Ln(A) = 12.1 (Taylor et al., 1996).
diamonds, we have performed calculations using both activation energies. Temperatures have been calculated for fixed residence times using the average nitrogen concentrations of the {111} and {100} sectors, 214 ppm and 46 ppm respectively, and an assumed 10% aggregation to A centres. This means that the estimated resi-dence times and temperatures shown in Fig. 9 represent maximum values. They show that the diamonds have spent very little time, in geological terms, at the temperatures expected for their required formation conditions.
The trace-element data and the analyses of exposed inclu-sions show that the NiMnCo inclusions most likely represent the medium from which the diamonds grew. If these diamonds are natural, this would represent a new, unique growth environment. The relative depletion in some redox-sensitive elements, such as Eu, Sm and Yb, (which are often below the LOQ) are consistent with a very low f O2, and with the occurrence of the other SuR-UHP minerals as noted above. Diamond growth in this environ-ment would require a redox process; oxidation of a highly reduced, C saturated metallic melt (Palyanov et al., 2013). The additional occurrence of REE-rich inclusions suggests that the metallic-alloy growth medium was more chemically complex than would be ex-pected for industrially-manufactured synthetic HPHT diamonds.
6.4. Models for the formation of the Tibetan ophiolite diamonds
Several models have been proposed to account for the occur-rence of diamonds in the Tibetan and other ophiolite sequences, but none of them takes into account the diamonds’ unusual char-acteristics described here. Several studies (Xiong et al., 2015;Xu et al., 2015; Yang et al., 2014, 2015) suggest that the diamonds and other UHP minerals were formed in the deep upper mantle or top of the transition zone, and have subsequently been transported into the oceanic mantle section of the ophiolite sequence by man-tle fluids or plumes. Other models (Robinson et al., 2015; Yang et al., 2007; Zhou et al., 2014) consider deep subduction to be im-portant in the formation of many of the UHP mineral phases, but suggest that the diamonds are already widespread throughout the asthenospheric mantle before this occurs. However, several obser-vations suggest that these models are not adequate to explain the diamonds:
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(1) Natural “superdeep” diamonds, which have grown in the tran-sition zone or deeper (as evidenced by inclusions of lower-mantle phases), do not share the characteristics of the Tibetan diamonds. Superdeep diamonds often have poor crystal habit due to extensive resorption, and exhibit extensive plastic de-formation (Hutchison et al., 1999).
(2) Superdeep diamonds are typically Type II (i.e. nitrogen-free), but when they do contain nitrogen, it is almost completely aggregated to Type IaB (Hayman et al., 2005), reflecting the high temperatures experienced by the diamonds in the deep mantle. The nitrogen characteristics of the Tibetan diamonds are in stark contrast to this; they are Type Ib, completely un-aggregated. If the Tibetan diamonds had formed at the high temperatures expected in the deep upper mantle or transition zone, then they should show advanced nitrogen aggregation.
(3) The Tibetan diamonds did not grow from the spectrum of C–O–N–H fluids/melts that generate cratonic diamonds (e.g. Weiss et al., 2013). The metal alloys must have been more than simply present in the growth medium; they probably constituted a large proportion of it. We do not know what proportions of metal solvent are required for surface recon-struction to take place (and therefore produce cubo-octahedral morphology), but as the majority of inclusions are of these metal alloys, it suggests that the alloy, rather than a C–O–H fluid/melt, was the dominant growth medium.
Providing a new model for the formation of these diamonds, that also takes into account the characteristics of all the other min-eral phases reported in the Luobusa rocks, is beyond the scope of this study and will be developed elsewhere. If the diamonds are accepted as natural rather than anthropogenic, then their charac-teristics place several constraints on their formation. They appar-ently formed in C-saturated metal-rich melts and the source of the carbon was most likely subducted crustal material. After formation, they spent very little time at the high temperatures of the up-per mantle. The formation of the rare dendritic features observed on a few crystal surfaces requires a rapid temperature decrease of the metal melt, akin to quenching, which seems unfeasible in a natural setting, but is in keeping with a short high-temperature history. The rate of uplift from the diamond stability field would have had to be quite high to prevent the aggregation of nitrogen, or the graphitization of the diamonds (cf. Beni Bousera; Davies et al., 1993). A recently proposed model (McGowan et al., 2015) ar-gues that the buoyant harzburgitic bodies that contain the Tibetan diamonds have been rapidly (6–10 Ma) excavated from the deep upper mantle or Transition Zone, by the channelized mantle flow associated with rollback of a slab stalled in the Transition Zone. If the diamonds formed in the peridotites during the latter stages of such ascent, this might explain their unusual N-aggregation char-acteristics.
Several questions still require answers to refine our understand-ing of the presence of diamonds in these peridotite massifs:
(1) How could a highly-reduced metal (Ni–Mn–Co) melt form within a natural geological setting? What relationship would this kind of melt have with other more common types of melt (e.g. silicic, carbonatitic)? How would the C be partitioned be-tween these melts? How would diamond growth be effected by the metal solvent interacting with a much more complex chemical environment?
(2) What natural processes can explain diamond growth in this unique chemical environment at high temperatures (∼1350 ◦C) but only for a short time, to account for the lack of N aggrega-tion, combined with the rapid T decrease of the metallic melt to create the dendritic features?
(3) What is the relationship of the diamonds to the other SuR-UHP mineral phases?
(4) What are the other, non-metallic phases that are present within these diamonds, as seen in the time-resolved LA-ICP-MS spectra?
7. Conclusions
There are many similarities between the diamonds found in the Luobusa ophiolite and those synthesized by HPHT industrial methods, but there are also important differences in the nitrogen isotope data and trace element concentrations. The nitrogen iso-tope values for the Tibetan diamonds show far more variation than has been reported from synthetic diamonds, and the systematic fractionation between growth sectors seen in large HPHT indus-trial diamonds is not apparent in the Tibetan diamonds. A second difference is the higher concentration of trace elements in the Tibetan diamonds, and the predominance of Ni–Mn–Co alloy inclu-sions and occasional REE-rich inclusions, in contrast to the Fe–Ni alloys common in synthetic HPHT diamonds.
By themselves, these two differences may be deemed insuffi-cient to confirm that these diamonds are natural. However, the discovery of diamond crystals from independently-collected Lu-obusa/Kangjinla chromitite samples, processed in clean laborato-ries by MQ workers, represents the first independent reproduction of the CAGS group’s findings. This lends weight to the view that these diamonds are natural.
This detailed report on the diamonds from the Luobusa mas-sif will allow the geology community to move past the question of whether these diamonds reflect anthropogenic contamination. A more useful question would be: how might diamonds grow in nature in an environment similar to those utilized in industrial diamond synthesis? An ongoing research program into the dia-mondiferous host rocks will shed more light on the unique growth environment of these diamonds.
Acknowledgements
Part of this work was performed at the OptoFab node of the Australian National Fabrication Facility, supported by NCRIS and NSW State Government funding. Some analytical data were ob-tained using instrumentation funded by DEST Systemic Infrastruc-ture Grants, ARC LIEF, NCRIS AuScope, industry partners and Mac-quarie University. This is contribution 653 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1024 from the GEMOC Key Centre (www.gemoc.mq.edu.au).
Appendix A. Supplementary material
Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2015.08.023.
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