CC BY
25GEOLOGICAL AND MINERALOGICAL SCIENCES
PETROGENETIC EVIDENCE AND FTIR CONSTRAINTS ON THE ORIGIN OF DIAMONDS IN XENOLITHS FROM YUBILEYNAYA AND KOMSOMOLSKAYA PIPES, YAKUTIA
Spetsius Z.
Dr.Sci., Chief Researcher,
Diamond and Precious Metal Geology Institute Siberian Branch Russian Academy of Sciences, Yakutsk,
Russia Griffin W. Dr.Sci., Professor
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS/GEMOC), Macquarie University, NSW
2109, Australia O'Reilly S. Dr.Sci., Professor
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS/GEMOC), Macquarie University, NSW
2109, Australia Bogush I.
Candidate of Geological and Mineralogical Sciences, Senior Researcher Geo-Scientific Investigation Enterprise, ALROSA PJSC, Mirny, Russia
Ivanov A.
Candidate of Geological and Mineralogical Sciences, Senior Researcher Saint Petersburg Mine University, Saint Petersburg, Russia
Abstract
35 diamondiferous xenoliths from the Komsomolskaya and Yubileynaya pipes of Yakutia comprise bimin-eralic eclogites, predominantly of Group B, rare kyanite eclogites and some garnet megacrysts. Most xenoliths contain two or more diamond crystals (0.5-8.0 mm), predominantly with octahedral or transitional forms. Finegrained interstitial metasomatic mineral assemblages and partial-melting phases occur in most specimens.
Clinopyroxene-garnet thermobarometry suggests equilibration at 1000-1225 °C, at pressures of 4-5 GPa and integrated residence temperatures of ~1160-1200 °C for diamond growth. Garnets in eclogites from the Komsomolskaya pipe generally have Mg# >60 and convex-upward REE profiles. Most clinopyroxenes and some garnets in xenoliths from both pipes display LREE and MREE enrichments consistent with minor amounts of cryptic metasomatism. FTIR-data display wide variation in total nitrogen content and aggregation state in the diamonds of xenoliths from both pipes. About 80% of the crystals from Yubileynaya xenoliths have high total contents of nitrogen (600-1500 at. ppm) but low aggregation state (20-30%), while 20% of the crystals have total nitrogen contents < 500 at. ppm with high aggregation state (30-80%). Crystals in Komsomolskaya xenoliths have moderate nitrogen contents (200-900 at. ppm) and predominantly 20-50% aggregation. Diamonds with different nitrogen aggregation states occur within individual xenoliths from both pipes. The distribution of diamonds in the xenoliths, and the presence of diamonds with different nitrogen aggregation characteristics in the same xenolith, suggest multistage growth of diamond from metasomatic fluids. These data are consistent with observations on diamon-diferous eclogites from other Yakutian kimberlites. Correlations between diamond grade and the relative abundance of metasomatised eclogite xenoliths and garnets indicate that the overall grade of these kimberlites is strongly controlled by the abundance or absence of eclogitic horizons in the deeper parts of the lithospheric mantle.
Keywords: Kimberlites, Xenoliths, Diamonds, Trace elements, FTIR-data.
Introduction
Eclogites are an important component in mantle xenolith suites transported to the surface by kimberlite magmas. Although peridotitic xenoliths dominate the xenolith suite in most kimberlites, and peridotitic (P-type) diamonds are globally more abundant than eclo-gitic (E-type) diamonds, diamondiferous eclogite xen-oliths are significantly more abundant world-wide than are diamondiferous peridotites and are present in all of the industrial kimberlite deposits of the Yakutian province (Jacob et al., 1994; Kostrovizky et al., 2015; Snyder et al., 1995; Spetsius et al., 2008). Despite the relative rarity of eclogitic xenoliths in most kimberlites, they offer key constraints on the formation of the Ar-chean cratonic lithosphere, as well as the extent of modification resulting from interaction with melt or fluids
in mantle environments and during kimberlite transport (e.g., Pearson et al., 1995, 2003; Snyder et al., 1997; Spetsius and Taylor, 2002). Studies of eclogites are important for refining models of global crust-mantle evolution and provide constraints on the origin of diamonds, as eclogites represent significant diamond reservoirs in some portions of the subcontinental lithospheric mantle (SCLM) of the Siberian Craton (e.g., Jacob, 2004; Spetsius et al., 2008).
The late Devonian Komsomolskaya and Yubi-leynaya pipes are located in the center of the Yakutian province, within the Alakit-Marhinsky kimberlitic field. The diamond grade of the Yubileynaya kimberlites (0.90 ct/t) is higher than in Komsomolskaya (0.36 ct/t) but the per-carat value of diamonds from Komsomolskaya is twice as high. The aim of this study is to
characterize petrologically the Yubileynaya and Komsomolskaya eclogite xenoliths and their diamonds in order to understand the differences in the origin of diamonds within the SCLM beneath these pipes. Examination of these diamondiferous eclogites can also have important implications for understanding diamond genesis and lithosphere evolution within the Siberian Platform, in general, and can provide criteria for the prioritizing of exploration targets. Preliminary data on diamonds and mineralogy of xenoliths from these pipes are given in a papers (Spetsius and Bogush, 2018; Spetsius et al., 2018). Here, we present major- and trace-element data on garnet and clinopyroxene along with FTIR data for diamonds from this new set of eclo-gites.
Samples and methods
Our suite of 35 diamondiferous xenoliths from the Komsomolskaya and Yubileynaya pipes of Yakutia comprises bimineralic eclogites, rare kyanite eclogites and some garnet megacrysts. One sample from the Komsomolskaya pipe (K-7) contains about 20% of ky-anite by volume and xenolith K-6 essentially a garnet-ite, with 90% of garnet. Five samples from the Yubileynaya pipe are garnet megacrysts (Yb-1, 5, 7, 10 and 20), but one garnetite xenolith (Yb-3) contains <10 % clinopyroxene. Xenolith Yb-21 from the Yubileynaya pipe is presented by 5 megacrystic grains of purple garnet that correspond to the harzburgite-dunite paragenesis. Most xenoliths in both pipes contain two or more diamond crystals (0.5-8.0 mm) with predominantly octahedral or transitional forms (Fig. 1). Coated diamonds were found in two xenoliths from the Yubileynaya pipe. In sample K-15 that corresponds to magnesium eclogite, the diamond is a colorless crystal of octahedral habit with a distinct central host crystal and the coat. The distribution of crystals in the xenoliths is irregular and is not related to the specimen surfaces. Mineral inclusions in the diamonds are rare and represented by sulfides, garnet, clinopyroxene and very rare rutile. Fine-grained interstitial metasomatic mineral assemblages (phlogopite and amphibole) and partial-melting phases (fine-grained secondary clinopyroxene, spinel, and glass and/or plagioclase feldspar) are a characteristic feature of most of the specimens, and a specific feature of diamondiferous eclogites (Spetsius, Taylor, 2002). Samples from Yubileynaya are more intensively metasomatised and altered.
Major-element compositions of garnets and clino-pyroxenes in the xenoliths were determined with a Superprobe JXA-8800R electron microprobe at the "ALROSA" PJS Company (Mirny, Yakutia). Natural minerals and synthetic materials were used as standards. Analytical conditions included an accelerating voltage of 15 keV, a beam current of 20 nA, beam size of 5 ^m, and 20 seconds counting time for all elements. All analyses underwent a full ZAF correction.
The trace elements have been measured by laser Ablation ICP-MS (LAM) in the Geochemical Analysis Unit at Macquarie University, with the NIST 610 glass as external standard and Ca as internal standard; pit diameters were 40 -50 mm. Some samples were analyzed at Virginia Polytechnic and State University, and details are given by Pernet-Fisher et al. (2014).
The morphology of about 300 diamond crystals from eclogite xenoliths of both pipes was studied, and selected diamonds from Komsomolskaya (102) and Yubileynaya (167) were analyzed by micro-Fourier transform infrared (FTIR) spectroscopy to determine both nitrogen content (N FTIR) and nitrogen aggregation state. IR spectra were obtained over the range of 370-4200 cm-1 with the use of Vertex 70 FTIR spectrometer and Hyperion 2000 microscope. The spectrum resolution was 2 cm-1. Errors in nitrogen content (N FTIR) and nitrogen aggregation state are estimated to be better than 20% and 5% respectively. The self-absorption of the diamond was taken as an internal standard. Other details are given in (Spetsius and Bogush, 2018). All determinations were done at Geo-Scientific Investigation Enterprise, ALROSA PJSC, Mirny.
Carbon-isotope compositions were determined in diamonds from 8 eclogitic xenoliths from the Yubi-leynaya pipe, predominantly from eclogites. Diamond crystals were converted to CO2 via combustion (900 °C) in vacuum in a quartz reactor in the presence of CuO and Pt catalyzers. Carbon-isotope determinations were performed using a Finnegan-MAT Delta mass spectrometer in Analytical Centre Institute of Geology and Mineralogy (Novosibirsk). Values of 513C were calibrated relative to graphite standard USGS-21 and normalized to the standard Pee Dee Belemnite (PDB). The accuracy of C-isotope compositions is within ± 0.5%o based on multiple analyses of a standard diamond powder (Fitzsimons et al., 2000).
a b
___ I « > Л* I *
6 mm
4 mm
Figure 1. Examples of diamondiferous xenoliths from Komsomolskaya (a, b) and Yubileynaya (c, d) pipes
Analytical results Mineralogy of xenoliths
Investigations of 35 xenoliths with diamonds from Yubileynaya and Komsomolskaya pipes confirmed the petrology of these unique rocks and provides new results on the properties of diamonds. Data on major- and trace elements in the minerals of 35 diamondiferous xenoliths from the Komsomolskaya and Yubileynaya kimberlites are summarized. Garnet compositions of xenoliths from both pipes are given respectively in Tables 1, 2. For eclogite classification we use plot of Taylor and Neal (1989), based on the proportions of py-rope, almandine and grossular; this classification has been widely used for Siberian eclogites (Fig. 2). Nearly all garnets from xenoliths of both pipes in this study fall
within group B in this classification scheme. Based on the almandine components, this suite can be further divided into two sub-groups: group BI with a high alman-dine component (>20 mol.%) and group BII with a low almandine component (<20 mol.%); this subdivision was established for eclogites from the Komsomolskaya (Pernet-Fisher et al., 2014) and also can be applied to the Yubileynaya xenoliths. As indicated above, one sample from the Yubileynaya pipe is a megacrysts of chromian harzburgite-dunite garnet (&2O3 =9.85 wt.%) that contains inclusions of octahedral diamonds. Two samples from the Komsomolskaya pipe correspond to Group C eclogite' one of these contains minor amounts of kyanite.
Table 1
Representative garnet compositions (wt.%) in diamondiferous xenoliths from the Komsomolskaya pipe
Sample
SiO9
AUa TiO
Cr9O, FeO
MnO
MgO CaO
Na,O Total
K-1
K-2
K-3
K-4
K-5
K-6
K-7
K-8
K-9
K-10
K-11
K-12
K-13
K-14
K-15
40.71 41.12
40.98 40.37 40.47 40.43 39.93 40.76 40.56
40.27 40.90 41.25 40.64
39.99
40.28
21.34 22.33 22.82 22.54 22.00 22.70 22.43 22.75 21.57 22.21 22.94 23.11 22.88 22.04 21.54
0.25 0.12 0.12 0.12 0.24 0.12 0.32 0.09 0.30 0.24 0.10 0.08 0.08 0.39 0.26
0.94 0.15 0.06 0.15 0.04 0.05 0.00 0.14 0.95 0.47 0.11 0.23 0.13 0.03 1.20
17.08
10.63 8.90
11.60 11.85 9.75 10.10 10.25
17.64 17.42 10.08 11.03 10.47 18.95 17.05
0.32 0.22 0.17 0.26 0.22 0.21 0.17 0.20 0.30 0.30 0.21 0.21 0.22 0.38 0.30
15.80 13.24 12.04 11.58 10.07 10.63 9.94 13.11 15.37 16.50 13.13 16.27 14.26 11.82 15.80
3.54 12.15 15.01 13.68 15.22
16.27 16.91 12.88
3.54 2.79 12.76 8.16
11.28 7.00 4.06
0.12 0.02 0.07 0.01 0.05 0.11 0.11 0.07 0.17 0.07 0.05 0.05 0.10 0.17 0.10
100.10 99.98 100.18 100.32 100.16 100.28 99.91 100.26 100.39 100.25 100.28 100.39 100.05 100.77 100.60
Komsomolskaya eclogitic garnets have variable Cr2Os, low TiO2 (0.10-0.39 wt.%), and a range of FeO (9.0-17 wt.%) and CaO (3.6-17.2 wt.%) contents. Group B garnets generally have &2O3 extending up to
~1.2 wt.% at a near-constant CaO of ~3 wt% and FeO contents >17 wt%; in contrast to the garnet of sample K14 (Group C), that has high CaO (6.7 wt.%) and lower FeO (16.4 wt.%) than other Group B eclogites.
Table 2
Representative garnet compositions (wt.%) in diamondiferous xenoliths from theYubileynaya pipe
Sample SiO2 M2O3 TiO2 &2O3 FeO MnO MgO CaO Na2O Total
Yb-2 40.69 21.98 0.16 0.10 16.58 0.36 13.15 6.92 0.06 100.00
Yb-1* 40.88 21.63 0.50 0.07 13.20 0.29 13.61 9.66 0.15 99.99
Yb-3 42.15 22.32 0.18 0.09 6.28 0.15 11.29 17.49 0.05 100.00
Yb-4 40.77 21.91 0.18 0.02 16.87 0.37 12.97 6.82 0.08 100.00
Yb-6 39.96 21.88 0.39 0.10 15.70 0.29 12.81 8.71 0.15 99.98
Yb-8 41.03 21.85 0.38 0.05 14.76 0.39 14.59 6.80 0.14 99.99
Yb-5* 40.82 22.09 0.22 0.13 11.92 0.28 12.52 11.93 0.09 99.99
Yb-10* 40.83 21.94 0.18 0.08 16.82 0.35 11.97 7.71 0.13 100.01
Yb-9 40.52 21.99 0.24 0.01 16.29 0.32 12.77 7.78 0.10 100.00
Yb-11 39.51 21.89 0.27 0.06 16.42 0.35 12.54 8.85 0.07 99.97
Yb-12 40.16 21.93 0.17 0.05 17.64 0.41 11.64 7.88 0.11 100.00
Yb-13 40.54 21.66 0.22 0.05 16.50 0.34 11.97 8.61 0.10 100.00
Yb-14 41.66 22.75 0.02 0.02 4.52 0.05 13.14 17.72 0.04 99.92
Yb-18 40.95 22.27 0.24 0.08 16.23 0.37 13.49 6.28 0.10 99.99
Yb-17 40.53 22.15 0.24 0.06 14.94 0.31 12.36 9.31 0.09 99.98
Yb-16 40.24 21.82 0.20 0.03 18.12 0.39 12.47 6.62 0.10 99.99
Yb-15 40.39 21.93 0.26 0.05 15.58 0.32 12.36 9.00 0.11 100.00
Yb-19 40.26 21.80 0.22 0.08 17.10 0.39 12.15 7.90 0.10 100.00
Yb-20* 41.51 21.84 0.21 0.05 11.40 0.22 12.59 12.11 0.05 100.00
Notes: *- means that xenoliths were presented by megacrysts of garnet.
Garnets in Yubileynaya eclogites generally have near-constant MgO of 11.2-13.4 wt.%; in contrast CaO low &2O3 <0.2 wt.% and ТЮ2 (0.15-0.45 wt.%) at a and FeO contents show large variations, respectively
(7.2-17.7 wt.%) and (4.4-16.4 wt.%) defining two different groups (Fig. 2).
Figure 2. Garnets compositions in diamondiferous xenoliths from Yubileynaya and Komsomolskaya pipes
Clinopyroxenes from xenoliths of both pipes contain between 25-50 mol.% of the jadeite component, classifying them as omphacite. The 'A B C' classification (Taylor and Neal, 1989) can also be applied to clinopyroxenes based on their MgO and Na2O concentrations, and the majority of xenolith samples from both pipes fall largely within Group B. However, two clino-pyroxenes from the Komsomolskaya plot significantly outside the Group B field, within the fields of Groups C and A.
Minerals in eclogites from this study display no zonation in their chemistry; therefore, the major-element compositions can be used to estimate the equilibration temperatures of these samples. Using the thermometer of Krogh (1988) equilibration temperatures for the Komsomolskaya eclogites range from 985-1190 oC, corresponding to pressures of 4.4-5.5 GPa when projected to the xenolith-based geotherm for the Udachnaya pipe. According to thermometer of Ellis and Green (1979), the results suggest equilibration at 1000-1225 °C at 4 GPa for xenoliths of both pipes.
Trace elements distribution
Garnets of Group B and C eclogites from Komsomolskaya pipe generally have Mg# >60 and convex-
upward REE profiles. All Komsomolskaya eclogitic garnets are clearly divided into two obvious groups with differences in HREE (Fig. 3). One group has high HREE and low LREE and the other has moderate HREE but enriched in LREE. All garnets of the latter group show positive Eu-anomalies and generally have sub-chondritic MREE- HREE. Garnets of both groups have positive Yb-anomalies. Systematic REE and trace element differences are also observed in the clinopy-roxenes, corresponding to these sub-divisions (Pernet-Fisher et al., 2014). About 40% of clinopyroxenes have higher REE concentrations and show enrichments in LREE-MREE, displaying a 'humped' REE profile and clinopyroxenes of this group have strong positive Yb-anomalies (Fig. 4). Most clinopyroxenes generally have sub-chondritic HREE, but display enrichments in LREE-MREE and depleted in HREE. Some clinopy-roxenes have 'humped' REE patterns, whereas other show LREE-enriched patterns. Most clinopyroxenes are characterized by strong positive Sr-anomalies and negative Nb-anomalies. 30% of clinopyroxenes display positive Eu-anomalies.
1000
0,01 -1-1-1-1-1-1-1-1-1-I-1-1-1-
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Figure 3. Chondrite-normalized REE abundances of Komsomolskaya eclogite xenolith garnets. All normalization factors from McDonough and Sun (1995).
Figure 4. Chondrite-normalized REE abundances of Komsomolskaya eclogite xenolith clinopyroxenes. All normalization factors from McDonough and Sun (1995)
Yubileynaya eclogitic garnets also define two ob- MREE- HREE. The second group of garnets with low viously different groups with low and high LREE (Fig. LREE is characterised by positive Eu-anomalies. 5). Garnets enriched in LREE show sub-chondritic
Figure 5. Chondrite-normalized REE abundances of Yubileynaya xenolith garnets. All normalization factors
from McDonough and Sun (1995)
The most salient features of the trace-element analyses can be summarized as follows:
- chondrite-normalized REE pattern of garnets predominantly are convex-upwards and slightly to strongly enriched in LREE (Figs. 3, 5) whereas clino-pyroxenes are characteristically strongly depleted in HREE.
- on the basis of the garnet REE distribution two different rock suites can be distinguished with garnets: (1) a "normal" group showing LREE depletion, flat MREE to HREE and variable Eu anomalies, (2) a "HREE depleted" group with (Gd/Yb)n > 1 and small positive Eu anomalies.
- wide variations in REE and Sr are observed for clinopyroxenes; the most suitable explanation for this enrichment is partial melting connected with metasomatism (Spetsius, 2004).
- some the whole rocks eclogitic xenoliths have small positive Eu-anomalies and low HFSE abundances (Pernet-Fisher et al., 2014) that may be evidence for crustal protoliths (Jacob and Foley, 1999).
Diamond isotopic compositions and FTIR data Carbon-isotope compositions were determined in diamonds from 8 eclogite xenoliths from the Yubileynaya pipe. The range of 513C is from -5.3 to -8.9%o, and more than 80% of the 513C values are obviously outside of the mantle range (e.g., Cartigny, 2005; Deines, 2002) with 513C from(-) 7.2 to 8.9% (Table 3). Among these are diamonds from eclogite xenolith Yb-6, which contains both cubic crystals and combined rombododecahedrons, as well as from eclogite xenolith Yb-19 with a high content (43%) of coated diamonds (IV variety of Orlov, 1987).
Table 3
Isotopic compositions of diamonds in the Yubileynaya eclogitic xenoliths
Xenolith # 513C, %o (PDB)
Yb-2 -6.8
Yb-3 -5.3
Yb-6 -8.4
Yb-12 -5.7
Yb-15 -8.5
Yb-17 -8.9
Yb-18 -7.2
Yb-19 -8.4
The FTIR data display differences in total nitrogen content and aggregation state in the diamonds of xeno-liths from these two pipes (Fig. 6). About 80% of the crystals from xenoliths of the Yubileynaya pipe have high total contents of nitrogen (600-1500 at. ppm) but low aggregation state (20-30%), while 20% of crystals have total nitrogen contents < 500 at. ppm with high aggregation states (30-80%). Crystals in xenoliths from
Komsomolskaya pipe have moderate nitrogen contents (200-900 at. ppm) and predominantly 20-50% aggregation. In Figure 5, the diamonds in eclogites from the Yubileynaya pipe define two definite trends that most likely answer two generation of diamonds formation. These two trends are present also, but less obvious, in the eclogitic diamonds population of the Komsomolskaya pipe. According the annealing theory on the
formation of nitrogen defects (Evans, Qi, 1982), a high level of nitrogen aggregation in the diamonds or their zones shows that the diamonds were in the mantle conditions over a long period of time in comparison with the crystals (or their zones) with a small level of impurity aggregation in the same xenolith. Figure 6 illustrates the isochronous lines calculated in the order of reaction 2 (Evans, Qi, 1982) for a temperature of 1150°C. The average temperature of the diamond annealing was determined for the eclogites based on the
g 1500
Oh
Cl
\ 1250
o -*—>
Z
Fe-Mg distribution between garnet and pyroxene (Ellis and Green, 1979), assuming that the pressure was 50 kbar and the diamonds grew and were annealed (while in the mantle) at the same temperature (Spetsius and Bogush, 2018). Integrated residence temperatures of ~1150-1200 °C was calculated for diamonds growth. These determinations are conditional; however, they allow us to assume a time gap of more than a billion years between the diamond generations (Fig. 6).
1000 750 500 250
%B=100*Nb/(Na+Nb)
♦ p. Komsomolskaya £ p. Yubileynaya
Figure 6. Distribution of total nitrogen and aggregation state in diamonds from Yubileynaya and
Komsomolskaya eclogitic xenoliths
Discussion
Origin(s) of the eclogites
Eclogitic xenoliths occur in all kimberlite provinces worldwide; the petrology of these rocks has been properly characterised but their origin still is discussed. Two contrasting petrogenic hypotheses exist: either mantle eclogites represent (1) high-pressure magmatic cumulates which occur as magma chambers or dykes within the upper mantle (e.g., Dawson et al., 1990; Snyder et al., 1995) or (2) recycled and metamorphosed Archaean oceanic crust (Ireland et al., 1994; Jacob et al., 1994; Barth et al., 2002). The first idea is based on the presence of subsolidus exsolution in eclogitic pyroxene and by sets of highly aluminous xenoliths, ranging from kyanite eclogites to alkremites, which show linear differentiation trends (Exley et al., 1989; Spetsius, 1995). The second alternative is based on the
oxygen isotope evidence and the presence of coesite-bearing eclogites in kimberlites of Yakutia, as well as in other kimberlite provinces (e.g., Barth et al., 2001; Jacob, 2004; Spetsius, 2004). There thus is mineralogi-cal and isotopic evidence that mantle eclogites may have multiple origins and both types may occur even in one kimberlite pipe (e.g., Snyder et al., 1997). This diversity of their origin has been further enhanced by the following evolution of eclogites through processes of partial melting and mantle metasomatism(e.g., Spetsius, Taylor, 2002; Greau et al., 2011; Huang et al., 2012).
Metasomatism is generally described as either 'modal' or 'cryptic' (Dawson, 2002 and references therein). The former is used to describe changes in modal proportions due to the addition of clearly meta-
somatic phases, whereas the latter is identified by compositional changes (e.g. LREE-MREE enrichment) of primary rock-forming minerals, without the occurrence of metasomatic phases. In diamondiferous eclogites, there typically is evidence for significant modal metasomatism, introducing mica, amphibole and rutile. Fine-grained interstitial metasomatic mineral assemblages have been identified in most diamondiferous ec-logites, and ascribed to late-stage percolating fluids (e.g., Spetsius, Taylor, 2002). Stealth metasomatism, which changes the abundances and compositions of the major „primary" phases (O'Reilly and Griffin, 2012) also may have affected the eclogites, but is difficult to confirm because few cases of mineral zoning have been observed except garnets in diamondiferous xenoliths from the Nyurbinskaya pipe (Spetsius et al, 2008).
Partial-melting products sometimes with the presence of phlogopite could be seen in all diamondiferous eclogite xenoliths from the Udachnaya pipe (Spetsius and Taylor, 2002) and from other Yakutian kimberlite pipes. Partly altered phlogopite is particularly concentrated around diamonds in the overwhelming majority of xenoliths from the Nyurbinskaya pipe (Spetsius et al., 2008). Many clinopyroxenes and most eclogitic garnets display LREE and MREE enrichments, consistent with influence of cryptic metasomatism. The lack of zoning (except the garnet from xenoliths of the Nyurbinskaya pipe) suggests that the cryptic metasomatism occurred over relatively large time scales and predating entrainment of xenoliths in the host kimber-lite. We can conclude that modal and cryptic metasomatism have significantly changed the mineralogy and composition of diamondiferous eclogites from Yaku-tian kimberlites, as observed in other mantle eclogites worldwide (e.g., Chinn et al., 2017; Jacob et al., 2009; Greau et al., 2011; Huang et al., 2012, Kopylova et al., 2012).
The rare earth and other trace elements of the garnets and clinopyroxenes from most of the samples from Komsomolskaya and Yubileynaya pipes provide further evidence on these processes (see Figs. 3-5). Most clinopyroxenes and some garnets in investigated xeno-liths display LREE and MREE enrichments consistent with cryptic metasomatism (Menzies et al., 1987). About 40% of the analyzed garnets and clinopyroxenes show positive or rarely negative Eu-anomalies that suggest plagioclase fractionation, and hence the involvement of recycled crust in the formation of eclogitic xen-oliths. Komsomolskaya and Yubileynaya eclogites are characterized predominantly by highly magnesian garnets but some eclogitic garnets enriched in almandine component exhibiting strong Eu-anomalies, and high Sr/Lu values, which are consistent with plagioclase-rich crustal cumulates (Pernet-Fisher et al., 2014).
Most garnets from the Komsomolskaya diamon-diferous eclogites have mantle-like 518O values (Pernet-Fisher et al., 2014), implying that the host rocks were derived from deep within the mantle and most probably have cumulate origins. However, a crustal li-thology also is suggested by some 518O values higher than the mantle range, interpreted to reflect low-temperature (<350oC) interaction with seawater. Overall, this suite of xenoliths adds to the large body of evidence
that suggests that eclogites from the Siberian SCLM are derived partly from high-pressure mantle cumulates and partly from recycled crustal material crust (e.g., Barth et al, 2001, 2002; Dawson, Carswell, 1990; Jerde et al., 1993; Snyder et al., 1997; Spetsius et al, 2008).
Origin(s) of the eclogitic diamonds
A steadily growing body of evidence (Spetsius, 1999; Spetsius and Taylor, 2008; Shatsky et al., 2008; Spetsius et al., 2009; Thomassot et al., 2009; Tomlin-son et al., 2009) indicates that metasomatic growth of diamonds is more favored in eclogitic substrates. There are several lines of evidence pointing to the metaso-matic origin of diamonds in eclogites (Spetsius, 1999): (1) sharp boundaries between zones with different nitrogen contents and aggregation states; (2) large variations in carbon and nitrogen isotopic composition between the inner and outer parts of the same crystal (Hauri et al., 2002; Spetsius et al., 2016); (3) abundance of sulfide inclusions and the heterogeneity of sulfur isotopes (Deines and Harris, 1995); (4) inclusions of eclo-gitic and peridotitic paragenesis in a single crystal (Prinz et al, 1975); (5) large variation in the Pb-isotope compositions of sulfides within a single diamond (Rud-nick et al., 1993).
Maps of the distribution of total nitrogen and hydrogen impurities in ca 100 plates cut from diamonds of eclogite xenoliths from Udachnaya and Nyurbin-skaya pipes show that more than 50% of eclogitic suite diamonds show obvious zonation, with many cases of multistage and/or interrupted growth (Spetsius et al., 2008, 2012). FTIR analyses show nitrogen abundance and aggregation state can from the center to the periphery of a crystal (e.g. from 1400 to 370 at. ppm and from 45 to 28% IaB respectively), and the 3107 cm-1 hydrogen absorption line may change in a stepwise fashion from 6.0 cm-1 to 0.1 cm-1. In our experience <50% of all diamonds in these eclogites grew in a single-stage process. These observations do not appear to be consistent with any magmatic process, but could be realized through migrating metasomatic fluids that initiated and caused the growth of diamonds through the precipitation of transported carbon.
Petrographic observations of diamonds in the ec-logites reveal not only multi-stage growth, but features indicative of the late formation of diamonds, related to partial melting and mantle metasomatism shortly before kimberlite eruption (Spetsius and Taylor, 2002). Such evidence includes: (a) correlations between the abundance of diamonds and the intensity of features interpreted as caused by partial melting; (b) correlations of diamonds with deformation zones in the host eclo-gite; (c) the distribution of diamonds around large grains of garnet and in metasomatised clinopyroxene (Spetsius and Taylor, 2002, Fig. 6). Macro- and microdiamonds commonly differ in their color, morphology and physical properties and belong to different generations (Spetsius, 1999; Spetsius and Taylor, 2008). For example, two different morphologies of diamond crystals (smooth-faced octahedra and coated diamonds) occur in a kyanite eclogite from Udachnaya (Pokhilenko et al., 1992) and two generations of diamonds in one eclogite xenolith (Ug31) from this pipe differ in morphology, nitrogen content (1st generation 478-685, 2nd
generation 681-1418 ppm) and N-aggregation state (1st generation 36.7-61.5%, 2nd generation 8.4-17.7% (Spetsius et al., 2012).
In both pipes studied here, diamonds with different morphologies, photoluminescence characteristics and nitrogen contents can be found within single eclo-gite xenoliths. The diversity in morphological types and the impurity composition of diamonds in the same xen-olith and the presence of zonal crystals in eclogites of Komsomolskaya (Spetsius and Bogush, 2018, Fig. 1b) assumes several stages of diamond formation. Two obvious trends are evident on the plot of total nitrogen vs aggregation state, confirming the presence of two diamond populations in xenoliths from the Yubileynaya and Komsomolskaya pipe (see Fig. 6).
The two trends shown in Figure 5 suggest at least two periods of diamond growth in the eclogitic xeno-liths. Diamonds with different nitrogen aggregation states occur in individual xenoliths from both pipes. According to the annealing theory of nitrogen defect formation in diamonds (Evans, 1992), a higher state of aggregation at a given nitrogen content suggests a significant mantle residence time for the diamonds, in contrast to crystals that have lower aggregation states. The nitrogen-aggregation data thus suggest that the diamond population in the xenoliths contains subpopulations with different time-temperature histories. Such differences in the morphological varieties of crystals and impurities of diamonds in a given xenolith strongly suggests the genesis of diamonds in different episodes, related to different pulses of metasomatic fluids.
Metasomatic mineral assemblages (phlogopite and others) are found in eclogites from the Komsomolskaya pipe where partial melting phases also are present, distributed between the rock-forming minerals or cutting garnet and clinopyroxene grains. This evidence, combined with the ubiquitous presence of diamonds with multiple growth features, is consistent with the conclusion that the eclogitic diamonds in Komsomolskaya and Yubileynaya eclogite xenoliths are connected with the processes of mantle metasomatism and partial melting, as in eclogites from the Udachnaya pipe (Spetsius, 1999).
Diamonds with low 513C (< 8 %o) are found in many kimberlites from different cratons (e.g., Cartigny, 2005). In the Nyurbinskaya eclogites, the majority of the diamonds with isotopic compositions lighter or heavier than the mantle range belong to the eclogitic paragenesis (Spetsius et al., 2016; Table 3). The carbon-isotope data presented here for diamonds of 8 ec-logite xenoliths from the Yubileynaya pipe includes mostly low 513C values and all diamonds from these xenoliths have 513C-values falling outside the so-called 'main mantle range' of -5±2%o but diamonds data of two samples (Yb-3 and Yb-12) are close to the ambient mantle value.
Conclusions
Our results on trace-element distribution in minerals from Komsomolskaya and Yubileynaya eclogites, combined with the carbon isotopes of diamonds and 518O values of garnets, imply that some of the diamon-diferous eclogitic xenolith suite formed from a subducted crustal protolith but most of these eclogites
probably were derived from high-pressure mantle cumulates. The carbon-isotope compositions of diamonds from xenoliths vary significantly from -5.3 to -8.9%o and about 50% of the diamonds are markedly enriched in light carbon isotopes, with 513C (%) between (-) 8.4 - 8.9, but the mode of the data is close to typical mantle values, suggesting that either recycled components mix effectively to produce a carbon isotope composition close to -5 % or that mantle-derived carbon is volumet-rically dominant through the SCLM. Approximately 50% of the diamondiferous eclogite xenoliths in these pipes record evidence of metasomatic enrichment that was recent enough to retain trace element zonation. Combined trace-element and non-mantle oxygen isotope values indicate a low-pressure origin for the pro-toliths of eclogites from the Komsomolskaya pipe and for the most of Yubileynaya eclogites.
Metasomatic fluids play an important role in the modification of primary mantle eclogites and can be linked to the diamond formation. The irregular distribution of diamonds in the Komsomolskaya and Yubi-leynaya xenoliths, the presence of diamonds with different morphology and various nitrogen aggregation states in the same xenolith, and other petrographic evidence suggest multistage growth of diamond from met-asomatic fluids. Metasomatized eclogites represent a major reservoir of diamonds in this segment of the Ya-kutian lithospheric mantle.
Acknowledgements We are grateful to Marina Shalkina from NIGP "ALROSA" PJSC for the help with the recovering and description of xenoliths. Ale-ksey Aminov from NIGP "ALROSA" Co is thanked for the support with the preparing some figures. Thanks are given to Alla Logvinova from Institute Geology and Mineralogy (Novosibirsk) for the assistance with the carbon isotope analysis of diamonds. This study used instrumentation funded by ARC LIEF and DEST Systemic Infrastructure Grants, Macquarie University, NCRIS AuScope and Industry. This is contribution XXX from the ARC National Key Centre for Geo-chemical Evolution and Metallogeny of Continents (www.GEMOC.mq.edu.au) and paper XXX from the ARC Centre of Excellence for Core to Crust Fluid Systems.
References
1. Barth MG, Rudnick RL, Horn I, McDonough WF, Spicuzza MJ, Valley JW, Haggerty SE (2001) Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archaean crust formation. Geochimica et Cosmo-chimica Acta 65: 1499-1527.
2. Barth MG, Rudnick RL, Horn I, McDonough WF, Spicuzza MJ, Valley JW, Haggerty SE (2002) Geochemistry of xenolithic eclogites from West Africa, part 2: Origin of the high MgO eclogites. Geochim. Cosmochim. Acta 66: 4325-4345.
3. Cartigny P (2005) Stable isotopes and origin of diamond. Elements 1, 79-84.
4. Chinn IL, Perritt SH, Stiefenhofer J, Stern RA (2017) Stable isotope data and FTIR analyses of diamonds from Orapa Mine: a clear subduction signature. 11thIKC Long Abstracts. N 4450.
5. Dawson JB, Carswell DA (1990) High temperature and ultrahigh pressure eclogites. In: Carswell DA (Ed.), Eclogite-facies rock. Chapmen Hall, New York, pp. 316-319.
6. Deines P (2002) The carbon isotope geochemistry of mantle xenoliths. Earth Sci. Rev 58: 247278.
7. Ellis DJ, Green DH (1979) An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria. Contrib Mineral Petrol 71: 1322
8. Exley RA, Smith JF, Dawson JB (1983) Alkremite, garnetite and eclogite xenoliths from Bells-bank and Jagersfontein, South Africa. Amer Mineral. 68: 512-516.
9. Evans T, Qi Z (1982) The kinetics of aggregation of nitrogen atoms in diamonds. Proc. Roy. Soc. London, A381, p. 238-242.
10. Fitzsimons ICW, Harte B, Clark RM (2000) SIMS stable isotope measurement: counting statistics and analytical precision. Mineralogical Magazine 64: 59-83.
11. Greau Y, Huang J-X, Griffin WL, Renac C, Alard O, O'Reilly S Y (2011) Type I eclogites from Roberts Victor kimberlites: Products of extensive mantle metasomatism: Geochim Cosmochim Acta 75 (22): 6927-6954.
12. Jacob DE (2004) Nature and origin of eclo-gite xenoliths from kimberlites. Lithos 77: 295- 316.
13. Jacob DE, Foley S F (1999) Evidence for Ar-chean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths. Li-thos 48: 317-336.
14. Jacob DE, Jagoutz E, Lowry D, Mattey D, Kudijavtseva GP (1994) Diamondiferous eclogites from Siberia: Remnants of Archean oceanic crust. Geochim Cosmochim Acta 58: 5191-5207.
15. Jerde EA, Taylor LA, Crozaz G, Sobolev NV and Sobolev VN. Diamondiferous eclogites from Yakutia, Siberia: evidence for a diversity of protoliths, Contrib. Mineral. Petrol. 114, 189-202, 1993.
16. Ireland TR., Rudnick RL, Spetsius ZV (1994) Trace elements in diamond inclusions from eclogites reveal link to Archean granites, Earth and Planet. Sci. Lett. 128:199-213.
17. Kostrovizky SI, Spetsius ZV, Yakovlev DA, Fon-der-Flaas GS, Suvorova LF, Bogush IN (2015) Atlas of primary diamond deposits of Yakutian kimberlite province. Mirny. OOO MGT. 480 p. (in Russian).
18. Krogh EJ (1988) The garnet-clinopyroxene Fe\Mg geothermometer—a reinterpretation of existing experimental data, Contrib. Mineral. Petrol 99: 44-48.
19. McDonough WF, and Sun S.-S (1995) The composition of the Earth, Chem. Geol. 120: 223-253.
20. Menzies M A, Rogers N, Tindle A. & Hawkesworth CJ (1987) Metasomatic and enrichment processes in lithospheric peridotites, an effect of as-thenosphere-lithosphere interaction. In: Menzies, M. A. & Hawkesworth, C.J. (eds) Mantle Metasomatism. London: Academic Press, pp. 313-361.
21. Orlov Yu (1977) The Mineralogy of Diamond. John Wiley and Sons, New York, 235 p.
22. McDonough WF, Sun S.-S (1995) The composition of the Earth. Chem. Geol. 120: 223-253.
23. Pearson D. G., Snyder G. A., Shirey S. B., Taylor L. A., Carlson R.W. and Sobolev N. V. (1995) Archaean Re-Os age for Siberian eclogites and constraints on Archean tectonics. Nature 374 (6524): 711713.
24. Pearson DG, Canil D, Shirey SB (2003) Mantle samples included in volcanic rocks: xenoliths and diamonds. Treatise Geochem 2: 171-275.
25. Pernet-Fisher JF, Howarth GH, Liu Y, Barry PH, Carmody L, Valley JW, Bodnar RJ, Spetsius ZV, Taylor LA (2014) Komsomolskaya diamondiferous ec-logites: Evidence for oceanic crustal protoliths. Contrib Mineral Petrol 167(3): 1-17.
26. Prinz M.D.V., Manson P.F., Hlava & Keil K. (1975) Inclusions in diamonds: Garnet lherzolite and eclogite assemblages. Physics & Chemistry of the Earth 9: 797-815.
27. Rudnick RY, Eldridge CS & Bulanova GP (1993) Diamond growth history from in situ measurement of Pb and S isotopic compositions of sulfide inclusions. Ceology 21: 13-16.
28. Snyder GA, Taylor LA, Jerde EA, Deines P, Clayton RN, Mayeda T, Rossman GR, Sobolev NV (1995) Archean mantle heterogeneity and the origin of diamondiferous eclogites, Siberia: Evidence from stable isotopes and hydroxyl in garnets. Amer Mineral. 80: 799-809.
29. Snyder GA, Taylor LA, Crozaz G, Halliday AN, Beard B, Sobolev VN, Sobolev NV (1997) The origin of Yakutian eclogite xenoliths. Journ. of Petrology 38: 85-113.
30. Spetsius ZV (1995) Occurrence of diamond in the mantle: a case study from the Siberian Platform. Journ. of Geochem Exploration 53: 25-39.
31. Spetsius ZV (1999) Two generations of diamonds in the eclogite xenoliths. In: Gurney, J.J., Gur-ney, J.L., Pascoe, M.D. and Richardson, S.H., eds. Proceedings of 7th International Kimberlite Conference, v. 2. Cape Town: Red Roof Design, p. 823-828.
32. Spetsius ZV (2004) Petrology of highly aluminous xenoliths from kimberlites of Yakutia. Lithos 77: 525-538.
33. Spetsius ZV and Bogush IN (2018) Peculiarities of Diamonds in Eclogitic Xenoliths from the Komsomolskaya Kimberlite Pipe, Yakutia. Doklady Akademii Nauk, 2018, 480 (3): 339-343. (in Russian).
34. Spetsius ZV, Boguch IN, Ivanov AS (2018) Xenoliths of Eclogites with Diamonds from the Yu-bileinaya Kimberlite Pipe, Yakutia. Doklady Akademii Nauk, 478 (2): 217-220. (in Russian).
35. Spetsius ZV, Cliff J, Griffin WL, O'Reilly SY (2016) Carbon isotopes of eclogite-hosted diamonds from the Nyurbinskaya kimberlite pipe, Yakutia: The metasomatic origin of diamonds. Chem. Geol. 455: 131-147.
36. Spetsius ZV, Taylor LA (2002) Partial melting in mantle eclogite xenoliths: connection with diamond genesis. International Geology Review 44: 973987.
37. Spetsius ZV, Taylor LA (2008) Diamonds of Siberia: Photographic evidence for their origin. Tran-quility Base Press, Lenoir City, Tennessee, USA, 278p
38. Spetsius ZV, Taylor LA, Valley JW, De An-gelis MT, Spicuzza M, Ivanov AS, Banzeruk VI (2008) Diamondiferous xenoliths from crustal subduction: garnet oxygen isotopes from the Nyurbinskaya pipe, Yakutia. Eur. Jour. Mineral. 20: 375-385.
39. Taylor LA, Neal CR (1989) Eclogites with oceanic crustal and mantle signatures from the Bells-bank kimberlite, South Africa, Part I: Mineralogy, petrography, and whole-rock chemistry. J. Geol. 97: 551567.
40. Thomassot E., Cartigni P, Harris JW, Lorand JP, Rollion-Bard C, Chaussidon M (2009) Metasomatic diamond growth: a multi-isotope study 13C, 15N, 33S, 34S of sulphide inclusions and their host diamonds from Jwaneng (Botswana). Earth Planet. Sci. Lett.282:79-90.
41. Tomlinson E.L., Müller W., EIMF (2009) A snapshot of mantle metasomatism: Trace element analysis of coexisting fluid (LA-ICP-MS) and silicate (SIMS) inclusions in fibrous diamonds. Earth and Planet. Sci. Lett. 279: 362-372
