Carbon and nitrogen isotope, and mineral inclusion

Contributions to Mineralogy and Petrology (2018) 173:72 https://doi.org/10.1007/s00410-018-1499-5

ORIGINAL PAPER

Carbon and nitrogen isotope, and mineral inclusion studies on the diamonds from the Pozanti–Karsanti chromitite, Turkey Dongyang Lian1,2,3   · Jingsui Yang2 · Michael Wiedenbeck3 · Yildirim Dilek4 · Alexander Rocholl3 · Weiwei Wu5 Received: 28 April 2018 / Accepted: 31 July 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The Pozanti–Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro-diamonds were recovered from the podiform chromitites, and these diamonds were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed-habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ13C range of the PKO diamonds varies between − 18.8 and − 28.4‰, with a principle δ13C mode at − 25‰. Nitrogen contents of the diamonds range from 7 to 541 ppm with a mean value of 171 ppm, and the δ15N values range from − 19.1 to 16.6‰, with a δ15N mode of − 9‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of ­(Ca0.81Mn0.19)SiO3, NiMnCo-alloy and nanosized, quenched fluid phases were observed as inclusions in the PKO diamonds. We believe that the 13C-depleted carbon signature of the PKO diamonds derived from previously subducted crustal matter. These diamonds may have crystallized from C-saturated fluids in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts. Keywords  Ophiolite · Diamonds · Carbon isotope · Nitrogen isotope · Inclusion

Introduction

Communicated by Jochen Hoefs. Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s0041​0-018-1499-5) contains supplementary material, which is available to authorized users. * Dongyang Lian [email protected] 1



School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

2



CARMA, Key Laboratory of Deep‑Earth Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

3

Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany

4

Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056, USA

5

Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China



Diamonds on the Earth mainly occur in volcanic rocks such as kimberlites and lamproites (Stachel and Luth 2015), but can also be found in ultrahigh-pressure metamorphic rocks (Yang et al. 2003; Ogasawara 2005), in meteorites (Huss 2005) and in sedimentary placer deposits (Tappert 2006; Shatsky et al. 2014). Diamonds of different origins generally differ in morphology, grain size and chemical composition (Cartigny 2005; Shirey et al. 2013; Stachel and Luth 2015), where diamonds in kimberlites and lamproites are considered to be mantle-derived xenocrysts which form via redox reactions occurring in sub-cratonic environments at depths exceeding 150 km and temperatures over 950 °C (Cartigny et al. 2014; Stachel and Luth 2015). Such mantle-derived diamonds are commonly viewed as metasomatic minerals, which crystalized from a C–O–H–N–S fluid through the reduction of oxidized carbon or oxidation of reduced carbon phases (Stachel and Harris 2009; Cartigny et al. 2014; Stachel and Luth 2015; Stagno et al. 2015). In recent years, diamonds have been recovered from chromitites and peridotites in different ophiolites (Bai

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et al. 1993; Yang et al. 2014, 2015a; Lian et al. 2017a). The discovery of “ophiolitic” diamond has drawn research interest to both the origin of this new class of diamond source and to the evolution of their hosting peridotites and chromitites (Ruskov et al. 2010; Yang et al. 2014; Zhou et al. 2014; Satsukawa et al. 2015; Griffin et al. 2016; Zhang et al. 2017). Ophiolitic diamond had been initially dismissed as the result of natural or artificial contamination. However, more and more evidence either directly or indirectly confirmed that these diamonds are of natural origin (Yang et al. 2007; Yamamoto et al. 2009; Howell et al. 2015a; Satsukawa et al. 2015; McGowan et al. 2015; Zhang et al. 2017). Diamonds recovered from the Luobusa, Dangqiong, Purang and Dongbo ophiolite in Tibet, China (Bai et al. 1993; Yang et al. 2007; Xu et al. 2009; Howell et al. 2015a; Xiong et al. 2016), Sartohay ophiolite in Xinjiang Province, China (Tian et al. 2015), Hegenshan ophiolite in Inner Mongolia Province, China (Huang et al. 2015), Ray-Iz ophiolite of the Polar Urals, Russia (Yang et al. 2015a), Nidar ophiolite (Das et al. 2017); and Myitkyina ophiolite, Myanmar (Chen et al. 2018), imply common presence of diamonds in ophiolitic peridotites and chromitites. Diamond is an important source of information about deep mantle compositions and processes (Walter et al. 2011; Pearson et al. 2014; Kaminsky et al. 2015; Stachel and Luth 2015). The morphology, internal structure, mineral inclusions, and isotopic compositions can provide crucial clues about the formation environment and material input which lead to diamond crystallization (Cartigny 2005; Cartigny et al. 2014; Stachel and Luth 2015). Yang et al. (2014) reported the carbon isotopic compositions (δ13C = − 18 to − 28‰) of diamonds from ophiolites in China, Russia and Myanmar. Howell et al. (2015a) studied diamonds from the Luobusa ophiolite in detail, and concluded that the Luobusa diamonds are of natural origin and they crystallized in a disequilibrium high-T environment with a short mantle residence time (Howell et al. 2015a; Griffin et al. 2016). Despite steady flow of reports of new discoveries of diamonds from different ophiolites, the origin of ophiolitic diamonds remains enigmatic; the limited geochemical database has hampered our understanding of the origin of ophiolitic diamond. Lian et al. (2017a) reported diamonds from podiform chromitites in the Pozanti–Karsanti ophiolite, Turkey. These diamonds are accompanied by SiC, rutile, zircons and other minerals. Here, we report new data on morphology, internal structures, mineral inclusions, N contents, and C and N isotopic compositions of these diamonds. These data, combined with previous data from Howell et al. (2015a) and Yang et al. (2015a), provide important new clues to the origin of ophiolitic diamonds.

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Geological setting A series of ophiolitic massifs (e.g. the Lycian nappe, Antalya, Mersin, Pozanti–Karsanti and Divirigi) are distributed along the E–W trending Tauride belt of southern Turkey (Fig. 1) (Parlak and Delaloye 1999; Dilek et al. 1999; Çelik and Michel 2003; Çelik and Chiaradia 2008; Parlak et al. 2009). These ophiolitic massifs are relicts of oceanic basins that once existed off the southern continental margin of what is today the Anatolian peninsula (Robertson et al. 2012; Parlak 2016). The Tauride belt consists of Paleozoic and Early Mesozoic platform carbonates, Paleozoic and early Mesozoic volcano-sedimentary and epiclastic rocks, Cretaceous ophiolitic complexes and late Cretaceous and younger post-collisional sedimentary and volcanic rocks (Lytwyn and Casey 1995; Dilek et al. 1999; Parlak 2016; Lian et al. 2017b). The Pozanti–Karsanti ophiolite (PKO, also referred as Aladag ophiolite), one of the largest fragments of Late Cretaceous oceanic lithosphere in the eastern Tauride belt, covers an area of approximately 1300 km2 (25 km in width and 80 km in length) (Lytwyn and Casey 1995; Polat and Casey 1995; Parlak et al. 2002). This ophiolite is offset from the Mersin ophiolite by the left-lateral Ecemis Strikeslip Fault (Fig. 1). The PKO is bounded by Oligocene and Neogene deposits within the left-lateral Ecemis Strike-slip Fault and Tertiary andesite in the west, by Neogene sediments of the Adana basin in the south, and by the Paleozoic Tauride carbonate rocks in the north and east (Polat and Casey 1995; Saka et al. 2014). The PKO contains a relatively intact ophiolitic sequence, including from bottom to top, mantle peridotites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and basaltic pillow lavas (Fig. 2) (Parlak et al. 2000; Saka et al. 2014). The mantle peridotites are dominated by harzburgites, with subordinate dunites occurring as lenses or patches in the harzburgites (Fig. 2b) (Lian et al. 2017a, b). The ultramafic cumulate rocks are composed of dunite, wehrlite, olivine clinopyroxenite and olivine websterite (Parlak et al. 2002), while the mafic cumulate rocks mainly consist of gabbro and gabbronorite (Parlak et al. 2000; Saka et al. 2014). These ultramafic and mafic cumulate rocks were suggested to have formed in a suprasubduction environment under variable pressures (Parlak et al. 2000, 2002). Chromitites in the PKO occur in two horizons including the mantle horizon and the mantle–crust transition horizon (Fig. 2) (Avcı et al. 2016; Lian et al. 2018). Chromitites in the mantle–crust transition zone have clear cumulate textures and are mainly disseminated to massive ores. Chromitites in the mantle zone have nodular, disseminated, or massive textures, which are typical for podiform

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Fig. 1  Simplified tectonic map of Turkey, modified after (Advokaat et al. 2014; Hinsbergen et al. 2016)

chromitite (Fig.  2c). Dunite envelopes are commonly observed between the chromitites and their hosting harzburgites. Diamonds were recovered from the chromitites hosted by mantle harzburgites (Lian et al. 2017a).

Samples description and analytical methods Scanning electron microscope (SEM) imaging methods A subset of 36 diamond grains, in the size range of 50–250 µm, was embedded in a total of three epoxy mounts. The diamond grains were polished using grinding paste and then twice cleaned for 5 min in a high-purity ethanol ultrasonic bath. Cathodoluminescence (CL) images were collected using the FEI Nova NanoSEM 450 scanning electron microscope (SEM) equipped with a GATAN MonoCL4 cathodoluminescence system and an Oxford INCA X-Max50 energy-dispersive spectrometer at the Institute of Geology, Chinese Academy of Geological Sciences.

Secondary ion mass spectrometer (SIMS) analyses Carbon isotope ratio (13C/12C), nitrogen abundance and nitrogen isotope ratio (15N/14N) were determined using the CAMECA 1280-HR large geometry Secondary Ion Mass

Spectrometer (SIMS) at the GeoForschungsZentrum (GFZ), Potsdam. Prior to analyses, diamond mounts were again polished by grinding paste for 10 min and twice cleaned for 5 min using high-purity ethanol (purity > 99.8%) in an ultrasonic bath. As the diamond for polishing was very finegrained (0.25 µm), thus the abrasive diamond could not have become contaminant mineral. Diamond mounts were then argon sputter coated with a 35-nm-thick, high-purity gold film to assure electric conductivity. The sample was placed in a low magnetic susceptibility SIMS sample holder, being held in place with tension springs. The sample was then placed in the airlock of the CAMECA 1280-HR SIMS instrument. Carbon isotope measurements were performed using ~ 2 nA 133Cs+ primary beam with a total impact energy of 20 keV, which was focused to a ~ 10 µm diameter Gaussian spot on the polished sample surface. The target area on the sample was pre-sputtered for 90 s using a 20 µm square raster to remove the gold coat and to suppress any surface contamination. A 10 µm raster (in conjunction with an activated dynamic transfer circuit) was applied during data collection to assure a flat-bottom sputter crater. Low energy electron flooding (diameter ~ 100 µm; 1.5 µA current flow from the sample holder) was used to suppress charge building up on the sample surface at the point of primary beam impact; the electron cloud was carefully centered under the secondary ion extraction optics at the beginning of each session.

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Fig. 2  a Simplified geological column of the Pozanti-Karsanti ophiolite, modified after Parlak et al. (2000); b Peridotite in the Pozanti–Karsanti ophiolite; c Podiform chromitite hosted by haruburgite

To determine the carbon isotope ratios of diamond, negative secondary ions were extracted using a − 10 kV potential applied to the sample holder, and these ions were injected into the mass spectrometer. The mass spectrometer was operated in multi-collection mode, using a mass resolution of M/ΔM ≈ 4000. The 12C− and 13C− were collected using Faraday cups at the L’2 and FC2 positions using 10E+10 and 10E+11 Ω resistors, respectively. Using these settings, the 12C− count rate on diamond was typically 1.4 billon ions per second. A single analysis consisted of 20 cycles of 4 s each. To assure maximum stability, the 1280-HR magnetic field was monitored throughout using a nuclear magnetic resonance-controlled feedback loop. All data were filtered for analytical outliers at the 3 standard deviation (SD) level. A single carbon isotope analysis, including the pre-sputtering and centering routines, lasted around 190 s. N contents and isotope ratios were measured adjacent to the locations of the carbon isotope analyses. Nitrogen isotope measurements were acquired following the similar procedure to that used for carbon isotope ratio determinations, but with some minor differences. Nitrogen isotope measurements used a 10 pA 133Cs+ primary beam current

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and the sample was pre-sputtered for 200 s using a 20 µm square raster. The mass spectrometer was operated in monocollection mode, with a mass resolution of M/ΔM ≈ 8880. The 12C14N− and 12C15N− were collected using an ETP 133H electron multiplier, to which a synthetic 46.2 ns deadtime was applied to the preamplifier circuit. Each nitrogen isotope analysis consisted of 25 cycles of 12C12C− (1 s integration), 12 14 − C N (4 s), and 12C15N− (15 s). A single nitrogen isotope analysis, including the pre-sputtering and centering routines lasted around 12 min. All analytical data are summarized in supplementary 1 and 2. In total, 13C/12C ratios were acquired for all 35 diamonds, whereas 15N/14N ratios and nitrogen contents could only be obtained for 29 diamonds. 13C/12C ratios are expressed in terms of δ 13C = ( 13C/ 12C sample/ 13C/ 12 C RM  −  1) × 1000, which are the relative differences in ‰ (part per thousand) compared to the Pee Dee Belemnite (PDB, 13C/ 12C  =  0.011237) (Craig 1957). 15N/ 14N ratios are expressed in terms of δ 15N = (15N/14Nsample/15 N/14NRM − 1) × 1000, which are the relative differences in ‰ to the AIR where 14N/15N = 272 (Coplen et al. 1992). Three reference materials used for C isotopic analyses

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were diamonds 1–105, 1–32 and 5–103 from Juina (Brazil) (Palot et  al. 2012), while the reference diamonds used for N contents and isotopes were in-house RMs of Vad150 (δ15N = − 5.0, N content = 215 ppm) and Vad140 (δ15N = 0.0, N content = 460 ppm). We have conducted 121 carbon isotope analyses, consisting of 60 analyses of our three diamond reference materials and 61 analyses for the PKO diamonds; our three RMs 1–102, 1–35 and 5–103 have assigned δ13C values of − 4.4, − 5.5 and − 5.1, respectively (Palot et al. 2012). The analytical repeatability of these three RMs analyzed by SIMS instrument is ~ 0.15‰ (1 SD). RM 5–103 was used to calibrate the carbon isotopic compositions of the PKO diamonds, whereas RM 1–102 and RM 1–35 were used as data quality control materials. An inter-comparison of our three RMs demonstrates that our SIMS data were consistent with the gas source data within analytical uncertainty (Palot et al. 2012). We have conducted 73 nitrogen analyses, including 32 analyses of our two RMs and 41 analyses of the PKO diamonds. The SIMS-defined repeatability of the two RMs was around ± 1.7‰ (1SD). RM Vad-140 was used to calibrate the nitrogen contents and isotopic compositions of the PKO diamonds. RM Vad-150 acts as a quality control material. The deviation in absolute nitrogen contents ΔN (= (­ Nmeasured − Ntrue)/Ntrue) of Vad150 is around 10%. The deviation in the measured mean δ15N values from the assigned δ15N values was around ± 1.5‰.

Transmission electron microscopy (TEM) analyses Seven electron transparent foils of 15 × 10 × 0.15 µm in size were cut normal to the surface of seven polished diamond grains with a Focus Ion Bean device (FEI FIB 200) (Wirth 2004) at the GFZ. These diamond foils were studied using the Potsdam TEM (FEI TECNAI G2 F20 X-Twin) operated at 200 kV with a Schottky field emitter gun as the electron source, coupled with Energy-dispersive X-ray (EDX) and Electron Energy Loss Spectroscopy (EELS) detectors. TEM images were acquired as filtered images by applying a 20 eV window to the zero loss peak of the energy-loss spectrum. EDX analyses were performed by scanning a pre-selected area, thus avoiding mass loss. The sample was tilted 20° towards the detector during data acquisition and the counting time was 60–120 s. The analytical data were processed using the TIATM software package. The NiMnCo alloys were calculated in at% with uncertainty estimates of ± 3%. Electron diffraction data from nano-crystals were acquired as selected area diffraction (SAED) images or derived from high-resolution lattice fringe images.

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Results Internal structure Cathodoluminescence images of several samples from the epoxy mounts are shown in Fig. 3. CL images of diamonds reveal sectors with differing brightness (Fig. 3). Bright sectors in some grains show layered-growth characteristics (Fig. 3d), while the growth stratigraphy of the dull sectors is invisible due to the lack of contrast for such sector. The bright sector was suggested to represent an octahedral {111} growth sector, while the dull sector was suggested to be a cuboid {100} growth sector, via a comparison with diamonds recovered from the Luobusa ophiolite in Tibet (Howell et al. 2015a).

Carbon isotopic composition The determined δ13C values for the PKO diamonds vary between − 18.8 and − 28.4‰ (Fig. 4a), with a mean value of − 25.3 ± 1.9‰ (n = 61, 1SD). These diamonds show a principal δ13C mode of − 25‰, and minor mode of − 20‰, both of which clearly deviate from the value of normal mantle carbon reservoir (δ13C = − 5 ± 1‰) (Javoy et al. 1986; Cartigny 2005; Shirey et al. 2013). The bright growth sectors have δ13C of − 18.8 to − 28.4‰ (Fig. 4b), and the dull growth sectors have δ13C of − 19.8 to − 27.8‰ (Fig. 4c). Thus, carbon isotopic compositions show little or no differences between the bright {111} and dull {100} growth sectors.

Nitrogen composition The nitrogen contents for the PKO diamonds range from 7 to 541 ppm with a median value of 171 ppm (n = 41) (Fig. 4d). There are distinct differences in nitrogen concentrations between bright growth sectors and dull growth sectors. The bright growth sectors have N concentrations ranging over 181–541 ppm (Fig. 4e), with an average of 313 ppm (n = 18), whereas the dull growth sectors have N concentrations ranging over 7–78 ppm (Fig. 4f), with an average of 49 ppm (n = 21). As the dull sectors generally have low N contents ( ~ 12 GPa, β-Ca2SiO4 + CaSi2O5 forms C ­ aSiO3-perovskite. The (Ca, Mn)SiO3 inclusion in the PKO diamond has crystal structures consistent with wollastonite. However, as wollastonite is only stable at pressure  5 GPa). Therefore, the (Ca,Mn)SiO3-wollastonite represents the retrograde phase at least transformed from ­C aSiO 3-walstromite, which has happened during the exhumation of diamonds. C ­ aSiO3-walstromite is also commonly interpreted to form by successively retrograde of ­CaSiO3-perovskite, which is an important phase in the mantle transition zone and lower mantle (Shim et al. 2000; Stachel et al. 2000; Kaminsky 2012; Nestola et al. 2018). Thus, it is commonly suggested that diamonds containing ­CaSiO3-walstromite/perovskite are derived from the transition zone or lower mantle (Joswig et al. 1999; Stachel et al. 2000). Different from the C ­ aSiO3-silicate in the Kankan diamonds (Guinea), the (Ca, Mn)SiO3 in the PKO diamond has higher Mn concentrations. Due to the ionic radius of Mn being between those of Mg and Ca (Shannon 1976) while also at the same time having the same charges with these elements, it is easy for Mn to substitute for Ca in a crystal structure. At ambient conditions, M ­ nSiO3 is stable as rhodonite with a pyroxenoid structure (Narita et al. 1977). According to high-temperature and high-pressure experiments, ­MnSiO3-perovskite appears at pressure of 23 GPa and temperature of 1200 K and can be stable at pressure up to 85 GPa (Fujino et al. 2008). However, unfortunately, the single (Ca,Mn)SiO3-wollastonite inclusion in the PKO diamond cannot provide precise pressure information. Although there is little evidence that (Ca, Mn)SiO3-wollastonite in the PKO diamond transforms from (Ca, Mn)SiO3-perovskite, such possibility cannot be excluded, because some highpressure minerals such as pseudomorphic stishovite, qinsongite, octahedral silicate, etc have been reported from the peridotites and chromitites in diamond-hosted ophiolites (Yang et al. 2007; Dobrzhinetskaya et al. 2009; Griffin et al. 2016; Das et al. 2017). Another important inclusion found in the PKO diamonds is NiMnCo alloy. Similar micrometer-sized NiMnCo alloy with bulbous shapes have also been observed in the Luobusa diamonds (Howell et al. 2015a). However, differences exist between the alloy inclusions in the PKO diamonds and those of the Luobusa diamonds. Howell et al. (2015a) suggested that the micrometer-sized anhedral alloy represents melt

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phase from which the Luobusa diamonds crystallized. In contrast, the NiMnCo alloy inclusions found in the PKO diamonds are nanometer-sized and idiomorphic in shape. The idiomorphic shape may suggest the alloy inclusions were trapped in the diamond as a solid-state material. Metal alloy (mainly Fe, and Ni) inclusions have been occasionally found in carbonado (De et al. 1998) and also in sublithospheric diamonds (Hayman et al. 2005; Bulanova et al. 2010; Smith and Kopylova 2014). According to theoretical calculations and experiments, the Earth’s mantle at > ~ 250 km depth is metal-saturated (Rohrbach et al. 2007, 2011; Frost and McCammon 2008). The existence of metal alloys in the PKO diamonds implies that these diamonds may have crystallized at depths over 250 km. As manganese nodules containing complex mixtures of metallic elements (e.g. Co, Ni, Fe) are widely distributed across oceanic floor at the global scale (Bender et al. 1966; Hein et al. 2015), it is likely that the metal alloys in the PKO diamond may be the result of the processing of manganese nodules within the subducting crust (personal communication with Paul Robinson, 2017). Considering the composition of the silicate inclusion and redox condition indicated by the metal alloy, we conclude that the PKO diamonds likely formed at least in the asthenospheric mantle at depth of over 250 km. Pore space composed of quenched phases were observed at the edges of metal alloy and (Ca, Mn)SiO3 inclusions in the PKO diamonds (Fig. 6b, c). Such pore space in the PKO diamonds indicates the existence of fluids during the formation of the PKO diamonds.

Origin of the Pozanti–Karsanti diamonds Diamonds occurring in chromitite were first reported in the Luobusa ophiolite by the diamond group of Chinese Academy of Geological Sciences in 1980s (Bai et al. 1993). Since then multiple research groups have reported that ophiolitic chromitites and peridotites of different ages and from different orogenic belts contain micro-diamonds (Robinson et al. 2004; Yang et al. 2015a, b; Tian et al. 2015; Huang et al. 2015; Xiong et al. 2016; Das et al. 2017; Wu et al. 2017). Due to the rarity of diamonds (~ 1 grain per kg) in peridotites and chromitites, it is exceptionally difficult to find in situ diamonds in thin-sections (Yang et al. 2015a). With enormous work on the Luobusa and Ray-Iz ophiolites, Jingsui Yang and his coworkers found 6 such in situ grains in chromites after polishing and checking 9.6 m2 of samples (Yang et al. 2014, 2015a). In addition, other minerals such as SiC, coesite, nitrides, retrograde wadsleyite, and native elements have also been found as in situ minerals or from mineral concentrates in different ophiolites (Yang et al. 2007; Dobrzhinetskaya et al. 2009; Satsukawa et al. 2015; Xu et al. 2015; Zhang et al. 2016).

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The origin of ophiolitic diamonds remains a topic of debate. Different models have been proposed for the formation of ophiolitic diamonds. Zhou et al. (2014) suggests that the Luobusa diamonds may have formed in the subducting slab below about 150 km, proposing a model whereby chromitite-forming magma assimilated metamorphic diamonds and other crustal minerals when passing through the slab window created by slab break-off. However, metamorphic diamonds occurring in ultrahigh-pressure metamorphic rocks generally are very fine grained ( 400 km) followed by rapid exhumation in less than 10 Ma. However, the rock units (e.g. pillow lava, isotropic gabbro and cumulate rocks) above the mantle residues in the Pozanti–Karsanti ophiolite show no evidence of UHP metamorphism (Dilek et al. 1999; Saka et al. 2014; Lian et al. 2017a, b). Thus, this deep-subduction and rapid-exhumation scenario also seems to be incompatible with the formation processes of the PKO chromitite. Recently, lightning-strike formationmechanism has been proposed for the origin of ultra-high pressure (UHP) and ultra-reduced (UR) minerals in ophiolites (Ballhaus et al. 2017, 2018), however, considering (1) the common occurrence of these UHP and UR minerals in different ophiolites (Yang et al. 2015a; Das et al. 2017; Wu et al. 2017; Lian et al. 2017a); (2) the occurrence of in situ diamond associated with amorphous carbon (Yang et al. 2015a); (3) the mineral inclusions such as olivine, garnet, chromite in diamonds (Yang et al. 2015a; Huang et al. 2015); and (4) the isotopic compositions of these diamonds (Howell et al. 2015a; Yang et al. 2015a), this lightning-strike formation mechanism can hardly be accepted (Yang et al. 2018, in preparation). Experimental studies have demonstrated that magnesiochromite is stable up to 14 GPa (~ 400 km) (Wu et al. 2016). Combining with the occurrence of stishovite/ coesite, ultra-highly reduced phases (e.g. SiC, native elements and metallic alloys), and the exsolution lamellae of diopside and coesite, Yang et al. (2015a) and Robinson et al. (2015) speculated that ophiolitic diamond may have formed

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first and then been encapsulated by chromite above the transition zone. Consequently, these minerals were suggested to be widespread in the convecting asthenospheric mantle. However, the nitrogen aggregation states of the Luobusa diamonds imply that these diamonds could only have spent a very short time at the high temperature conditions present in the upper mantle (Howell et al. 2015a). Subduction initiation plays an important role in the generation of suprasubduction zone (SSZ) ophiolites (Whattam and Stern 2011; Stern et al. 2012; Reagan et al. 2017). Geochemical studies demonstrate that the Pozanti–Karsanti ophiolite is of SSZ characteristics following the “subduction initiation rule” (Dilek et al. 1999; Parlak et al. 2000; Saka et al. 2014; Lian et al. 2017b). Slab rollback will result in the upwelling of the asthenosphere mantle, which forms the mantle part of the Pozanti–Karsanti ophiolite (Lian et al. 2017b). Despite the lack of nitrogen aggregation information for the PKO diamonds, the nitrogen aggregation state of the Luobusa and Ray-Iz diamond suggests a short mantle residence of “ophiolitic” diamonds in the upper mantle (Howell et al. 2015a; Griffin et al. 2016; Xu et al. 2017). Thus, ophiolitic diamonds are not likely to have been in the upper mantle long before the upwelling of asthenospheric mantle. Based on the isotopic compositions and inclusions, it appears that the PKO diamonds may have crystallized under sublithospheric conditions. Exsolutions of needle-shape clinopyroxene in chromite have also been reported in the PKO chromitites (Lian et al. 2018), which is similar to those observed in the Luobusa chromitite (Yamamoto et al. 2009). Hence, some chromites in the PKO chromite may also have crystalized at depth > 250 km allowing them to encapsulate UHP minerals from the asthenospheric mantle. Some diamonds may be captured by chromites and carried upward rapidly by the asthenospheric melts. When passing through the slab window created by subducting slab break-off, these melts apparently assimilated some crustal minerals (e.g. quartz, zircons, rutile) (Zhou et al. 2014; Robinson et al. 2015). Boninitic melts were generated due to the melting of the already depleted mantle peridotites. The upward moving melts may have then interacted with boninitic melts, leading to the formation of podiform chromitites (Lian et al. 2017a, b). The asthenospheric melts also have metasomatized the mantle wedge above the subduction zone while introducing diamonds into the mantle peridotites.

Conclusions Micro-diamonds were recovered from the Pozanti–Karsanti chromitite. CL images of these diamonds reveal sectors with different brightness, reflecting that the Pozanti–Karsanti diamonds are mainly mixed-habit diamonds. This 13C-depleted isotopic characteristic of the PKO diamonds suggests that

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organic material was the source for the carbon. The nitrogen contents and nitrogen isotopic compositions of the PKO diamonds are consistent with those of peridotitic and eclogitic diamonds. The growth sectors in PKO diamonds have no detectable differences in carbon isotopic composition, but big contrasts in both their N contents and δ15N values. Such big shifts in nitrogen contents and isotopic compositions between different sectors can be attributed to the crystallographic effect of the area active at the time of crystallization. The octahedral growth sectors of the PKO diamonds have higher N contents and δ15N values, which differs from the pattern seen in synthetic, mixed-habit diamonds. Combinations of ­(Ca0.81Mn0.19)SiO3, NiMnCo-alloy and quenched fluid phases were observed in the PKO diamonds, where the (Ca, Mn)SiO3 inclusions are believed to represent the retrograde transformation from C ­ aSiO3-walstromite. Metal alloy inclusions in the PKO diamonds imply a formation depth of > ~ 250 km. We conclude that some chromites have crystallized in the asthenosphere and encapsulated diamonds and other UHP minerals. These minerals were then carried rapidly upward by asthenospheric melts. These asthenospheric melts would have then metasomatized the mantle wedge and mixed with boninitic melts, allowing for the introduction of diamonds into the mantle peridotites and chromitites. Acknowledgements  We thank Fahui Xiong, Wenda Zhou and Prof. Ibrahim Uysal for assistance in the field work, Bin Shi for the assistance in CL imaging. Frédéric Couffignal conducted the SIMS analyses, Anja Schreiber cut the TEM foils of the diamonds, and Richard Wirth conducted TEM analyses. We appreciate their help very much. We would also like to thank Pengfei Zhang, Fei Liu, Paul T. Robinson and Vadim N. Reutsky for their valuable suggestions. We thank the editor and three anonymous reviewers for their thorough and valuable comments that improved this manuscript. This research was supported by the funded by Fundamental Research Funds for the Central Universities (020614380069, 020614380072), the Ministry of Science and Technology of China (2014DFR21270, 201511022, J1618), the National Science Foundation of China (Grants 41672063, 41773029, 41373029,), the China Geological Survey (DD20160023-01, DD20160022-01), and the IGCP-649 project. Y Dilek acknowledges the financial support for this project provided to him by a Lishiguang Scholarship through the Geological Survey of China and the Chinese Academy of Geological Sciences.

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