Zircon inclusions in two selected Kashmir sapphire reference samples and one faceted gemstone were studied by HR SIMS and LA-ICP-MS for U-Pb age determination. The two independent analytical methods result in the same conclusion that the zircons have crystallization ages of 26-25 Ma. The provided result is a good reference for U-Pb age of Kashmir sapphire that can be used for origin determination in gemmological laboratories with LA-ICP-MS routine method. Additionally,cathodoluminescence imaging showed that the zircon inclusions in Kashmir sapphires have regular oscillatory zoning. High U concentrations of up to 20 000 ppm have been detected in the zircon inclusions.
Sapphire from the Kashmir Mountains is one of the most sought-after gems on the market. Its deposit was first discovered in 1879 in the Paddar region of Kashmir, now in India. However, the major source has already become exhausted in the 1930s, and only sporadic production entered the market again since the 1980s. Because of its rarity and the distinctive beauty of its cornflower blue colour and a slightly milky (sleepy) quality which is sometimes compared to "blue velvet", auction records for large faceted gemstones recently reached as much as $243.703 per carat (Sotheby's Oct. 2015 in Hong Kong).
The Paddar sapphire mine complex is located approximately 4 600 m above sea level on the southern slopes of the Zanskar range, northwest of the Indian Great Himalayan range. The series comprises a succession of marble, amphibolite and garnet-graphite-biotite and amphibole-bearing gneiss which have been intruded by pegmatite (Atkinson & Kathavala, 1983). The sapphires are located in pockets or lenses that are associated with metamorphic rocks formed of olivine, talc and spinel. The deposit belongs to the metamorphic types and might be a consequence of hydrothermal metasomatism occurring in the contact to intrusive aluminosilicate plutonic rocks (Giuliani et al., 2014). But because of the remote location of the source region and the lack of reference samples, the geological formation and age of Kashmir sapphire deposits have not been determined as intensively as many other corundum occurrences.
This study focuses on U-Pb age determination of zircon inclusions in Kashmir sapphire, using HR SIMS and LA-ICP-MS and its application for gemmological laboratories. U-Pb dating methods and results on zircon inclusions in faceted gemstones have first been described for sapphires from Madagascar (Link, 2015), Sri Lankan (Elmaleh et al., 2019), and Myanmar (Link, 2016). The age of zircons in Kashmir sapphires fills a further gap and makes this method an even more powerful tool for origin determination.
MATERIALS AND METHODS
The samples (Fig. 1) used for this study were selected from the Gübelin Reference Collection. This collection contains approximately two hundred rough sapphire samples from the Kashmir region, most of them were collected by Dr. E. Gübelin in the last century. These samples have been gemmologically, spectroscopically and chemically studied and described as typical materials from the Sumjam mine. These studies not only published the microscopic characteristics of the samples in the Photoatlas(Gübelin & Koivula, 1986), but also described the inclusions in the samples in subsequent publications and compared them with inclusions from other sources(Schwieger, 1990; Hänni, 1990). The two most suitable samples containing zircon inclusions (SK1 and SK2) were prepared for this study. The weight of sample SK1 is 2.9 ct and the weight for sample SK2 is 1.1 ct. These samples were polished to expose as many zircon inclusions as possible on the surface for testing. HR SIMS and CL were carried out on the 14 zircon grains in sample SK1, whereas LA-ICP-MS was applied on sample SK2, which carried only one zircon crystal for U-Pb age dating. Raman spectra were collected from the zircon inclusions in the two samples. The data is complemented by the zircon age of a faceted gem quality sapphire SKG1 that originates from Kashmir which weights 2.6 ct. Despite the limited number of applied samples, the consistent results add significant knowledge to the gemmological community.
Figure
1.
Kashmir sapphire sample SK1 (a-d), sample SK2 (e-f) and faceted blue sapphire sample SKG1 from Kashmir (g-h)
a. Sample SK1, 10×; b. Two corroded zircon crystals in prismatic shape, 60×; c. Euhedral needle-like zircon crystals randomly oriented, 60×; d. Crossed cluster of euhedral zircon crystals, 60×; e. Sample SK2, 15×; f. A strongly corroded prismatic zircon crystal, 60×; g. Sample SKG1, 10×; h. A whitish zircon crystal in prismatic shape cut on surface, 60×
The zircon inclusions in the sapphire sample were first identified by Raman spectrometer. Then the selected sapphire samples were mounted in epoxy resin and polished until the zircon grains were exposed.
Cathodoluminescence (CL)
The prepared samples were coated with a very thin layer of gold in order to examine the internal structure of the zircon grains using a NovaNano SEM 450 scanning microscope equipped with a Gatan Mono CL4 cathodoluminescence (CL) spectrometer at the scanning electron microscopy (SEM) laboratory, IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing). 45 seconds of scanning time at 10 kV and 120 nA were applied as optimum conditions.
High-Resolution Secondary Ion Mass Spectroscopy(HR SIMS)
Measurements of U, Th and Pb isotopes were conducted using a Cameca IMS-1280 HR SIMS at IGGCAS. The primary O2- ion beam spot was accelerated at 13 kV and focused at about 10 μm ×15 μm in size. Positive secondary ions were extracted with a 10 kV potential. In the secondary ion beam optics, a 60 eV energy window was used, together with a mass resolution of circa 5 400 at 10% peak height to separate Pb+ peaks from isobaric interferences. A single electron multiplier was used in ion-counting mode to measure secondary ion beam intensities by peak jumping mode. Each measurement consists of 7 cycles and takes about 12 min. Pb/U calibration was performed relative to the zircon standard Plesovice (206Pb/238U age = 337 Ma; Sláma et al., 2008); U and Th concentrations were calibrated against the zircon standard 91500 (Th = 29 ppm, and U = 81 ppm; Wiedenbeck et al., 1995). A long-term uncertainty of 1.5% (1s RSD) for 206Pb/238U measurements of the standard zircons was propagated to the unknowns (Li et al., 2010), despite that the measured 206Pb/238U error in a specific session is generally ≤1% (1s RSD). Measured compositions were corrected for common Pb using non-radiogenic 204Pb. Corrections are sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey & Kramers, 1975) is used for the common Pb assuming that the common Pb is largely surface contamination introduced during sample preparation. Data reduction was carried out using the Isoplot/Ex v.2.49 program (Ludwig, 2001). Uncertainties on individual analyses in data tables are reported at 1σ level; Concordia U-Pb ages are quoted with 95% confidence interval, except where noted otherwise. In order to monitor the external uncertainties of HR SIMS U-Pb zircon dating calibrated against Plesovice standard, an in-house zircon standard Qinghu was alternately analyzed as an unknown together with other unknown zircons. Twenty-two measurements on Qinghu yield a Concordia age of 160 ± 1 Ma, which is identical within error with the recommended value of 159.5 ± 0.2 Ma (Li et al., 2013).
Laser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS)
The LA-ICP-MS system consists of an ESI NWR193UCArF excimer-based UV laser ablation system with a large-format sample chamber and a flexible cup that collects the ablated material, which is coupled with a PerkinElmer ELAN DRC-e quadruple ICP mass spectrometer. As reference material GJ-zircons (Jackson et al., 2004) and zircon standard 91500(Wiedenbeck et al., 1995) were used for quality control whereas a Plesovice-zircon (Sláma et al., 2008) was used as primary reference for standardization. Common lead was monitored using the non-radiogenetic lead isotope 204Pb whereas potential interferences with 204Hg were controlled via 204Hg. Common lead corrections were not required as its potential effect would have been covered by the calculated age uncertainties.
The samples were mounted in sample holder with zircon inclusion exposed and polished on surface. The faceted sapphire was fixed using customized sample holders (Blu-Tack). 20 μm to 50 μm diameters were applied for laser spots size. The applied method is analogue to Link (2015).
RESULTS
Microscopic Observation
Sample SK1 (Fig. 1a) is a slab oriented perpendicular to the c-axis, with a slightly rounded hexagonal crystal outline, and intensive blue colour blocks distributed at the outer crystal edges and tip. It shows medium saturated blue colour with oriented whitish milky bands made of minute particles. Typical Kashmir sapphire inclusions such as whitish TiO2 dust-like particles, colourless rounded feldspar crystals, and healing fissures can also be found in sample SK1. Sample SK2 (Fig. 1e) has a spindle-shaped hexagonal pyramidal crystal form. The center part of the stone shows transparent milky white colour and intensive blue spots distributed near the crystal surface between whitish feldspar crystal clusters.
Zircon inclusions observed in the Kashmir sapphire sample SK1 and SK2 possess a dipyramidal prismatic shape, partially forming well-developed long needle-like crystals (Fig. 1c and Fig. 1d), partially with etching surfaces (Fig. 1b and Fig. 1f). The long needle-like zircon crystals in Fig. 1c are oriented randomly in the host sapphire with a few reaching the surface, and in Fig. 1d, a pair of long needle-like crystals are intersected. In Fig. 1b the narrow prismatic crystal on the right side was much stronger corroded than the one on its left side. Fig. 1f shows an isolated elongated zircon crystal with a strongly corroded surface. Based on microscopic observation, crystal faces [100]> [110] and [101] can be recognized. Zircon crystals with and without corrosion have been found in sample SK1, while in sample SK2 only one corroded zircon inclusion was observed.
In the faceted gemstone SKG1 (Fig. 1g), inclusions of fine whitish particles in straight bands are observed as a few reflective dust particles in tracks, and one whitish crystal in prismatic shape, which is identified as zircon (Fig. 1h). The zircon is exposed on a surface at the pavilion, its section is big enough to measure two spots with LA-ICP-MS.
Cathodoluminescence Image
Sample SK1 includes numerous prismatic zircon crystals and 13 of them were exposed on the surface after sample preparation and were named SK1-z1 to SK1-z13. The crossed crystal cluster shown in Fig. 1d was cut parallel to its c-axis. The high-resolution images of internal zoning patterns of the zircons were obtained by CL (cathodoluminescence) (Fig. 2). Crystals SK1-z1 to SK1-z13 (Fig. 2a-Fig. 2h) were scanned on cross sections vertical or oblique to the c-axis. In situ U-Pb dating was performed on those zircon inclusions that showed the best euhedral morphology and oscillatory zoning with irregular patch zones which are the overgrowth of the youngest generation. SK1-z14 (Fig. 3), which was cut parallel to the c-axis, displays a relatively homogeneous internal microstructure with a slight contrast parallel to the c-axis.
Figure
2.
Cathodoluminescence images of zircon inclusions in the sample SK1
a. HR SIMS measuring spots 1 and 2; b. Measuring spots 3 and 4; c. Measuring spots 5 and 6; d. Measuring spots 7 and 8; e. Measuring spots 9 and 10; f. Measuring spot 11; g. Measuring spot 12; h. Measuring spot 13
HR SIMS was carried out on the zircon inclusions of sample SK1 illustrated in Fig. 2. Based on CL-scanning, the sampling regions were selected for measuring (red circles). 207Pb/235U and 206Pb/238U ratios, U, Th and Pb concentration, and their U-Pb decay ages are summarized in Table 1 and Table 2. The 18 measurements show relatively consistent results, indicating that the U-Pb ages of the zircon inclusions in Kashmir sapphire sample SK1 are 24.97 Ma on average with standard deviation of 0.22 Ma (Table 1, Table 2 and Fig. 4).
Table
1.
U-Pb dating results of zircon inclusions in Kashmir sapphire sample SK1 according to HR SIMS results
Figure
4.
Conventional U-Pb Concordia plot showing SIMS analytical data of zircon inclusions in Kashmir sapphire sample SK1. The red ellipsoids each represent one measurement from a single zircon, and the blue ellipsoid gives the resulting weighted mean concordant age and error from the zircons within the sapphire sample SK1
One measurement was carried out on the surface reaching zircon crystal in sample SK2 by LA-ICP-MS, the results are displayed in Table 2 and Fig. 5. LA-ICP-MS concluded a U-Pb age of 26.3±3.8 Ma. Due to much bigger standard deviation of LA-ICP-MS, the result still coincides well with the Concordia age concluded by HR SIMS.
Figure
5.
U-Pb Concordia plot of LA-ICP-MS analytical data of zircon inclusions in Kashmir sapphire sample SK2. One measuring spot was car-ried out.
U-Pb age dating of a zircon that was exposed at the surface of the faceted sapphire sample SKG1 was also carried out by LA-ICP-MS(Fig. 6). It was possible to place two sampling pits with 20 μm diameter on the zircon inclusion without compromising the quality and appearance of the gemstone. The method and measuring conditions are following Link (2015). The resulting Concordia age averaged at 23 Ma. The U-Pb ages gained from the reference material are comparable to this sapphire when considering the errors too.
Figure
6.
U-Pb Concordia plot of the LA-ICP-MS analytical data of the zircon exposed in the 2.6 ct faceted sapphire SKG1
Geological studies from the central Zanskar area (Dèzes, 1999) (Fig. 7), located approximately 50 km southeast from Sumjam, show that leucogranites are widespread and exposed within the upper structural levels of the whole area. These leucogranites are generally described to have an equigranular texture and range between aplites and pegmatites. They are essentially formed of quartz, plagioclase, K-feldspar and muscovite with variable amounts of biotite, garnet and greenish tourmaline, whereas plagioclase and greenish tourmaline are the typical associated minerals of sapphires from the Paddar mine (Hänni, 1990; Gübelin et al., 1986). The leucogranites of the Zanskar area yield a monazite U-Pb age between 19 Ma to 22 Ma (Dèzes, 1999) while leucogranites in other Himalayan areas yield around 25 Ma to 20 Ma (Schärer, 1984; Harrison et al., 1995), which matches well to the U-Pb age of the zircons in the sapphires from Paddar mine in this study. Furthermore, the monazite in leucogranite contains relatively high U concentrations, similar to the zircon inclusions tested in this study, with values between 5 000 ppm to 20 000 ppm (Dèzes, 1999). A comparison of associated minerals, formation age, as well as chemical fingerprinting suggests that the leucogranitic magma or their related fluids are presumably related to the zircon growth and thus potentially presumably related to the sapphire crystallization. It is quite possible that these melts intruded into the ultramafics and amphibolites and their reaction zones formed the "plumasite" deposits analogue to Peretti (1990).
Figure
7.
Geological overview of the central Himalayan in Zanskar area (Kashmir, India) and location of the Kashmir sapphire deposit Sumjam in Paddar area (Mod-ified after Dèzes 1999 and Atkinson et al., 1983)
In the interpretation of geochronological data, the distinction between magmatic and metamorphic zircon is mainly based on morphology, internal zoning or Th/U ratio (Moller et al., 2003). The typical morphology of zircon observed in Kashmir sapphires is prismatic or long elongated and forms needles. The CL images show internally regular oscillatory zoning parallel to c-axis. This kind of regular oscillatory zoning is usually considered as magmatic derived zircons (e.g. Pidgeon, 1992). Th/U ratio of zircon with Th/U > 0.2—0.4 are regarded as magmatic, while Th/U ratios < 0.1 are attributed to metamorphic origins (Williams & Claesson, 1987; Rubatto & Gebauer, 2000). In zircon crystals in the Kashmir sapphire sample SK1, the Th/U ratio is far lower, around 0.02 to 0.06, mainly due to the unusually high concentrations of U potentially pointing to a metamorphic formation. However, an interpretation only based on Th/U ratio could lead to mistakes as Moller (2003) highlights. It may be possible that the apparently conflicting observations are the result of a reaction of magmatically derived (pegmatite) melt with the mafic metamorphic host rock, thus supporting the assumed unusual metasomatic which often in gemmology referred to "plumasitic" (Lawson, 1903), formation models of the zircon hosting sapphires (Peretti et al., 1990). Additionally, recrystallization rim is presented in zircon crystals in Kashmir sapphire samples SK1 and SK2 in general. Microscopically small crystals often display corroded surfaces (Fig. 1b and Fig. 1f), and all the zircon grains show a thin CL-bright rims cutting primary oscillatory zoning.
Our newly presented study conclude that zircon formation ages in Kashmir sapphires match to the tectonic environment described in the geological studies of the central Zanskar area. This allows the conclusion that the formation age of 25-26 Ma of zircon inclusion in Kashmir sapphire is correct, and it can be used as reference age for origin determination of the gemstone. Zircon inclusions in other metamorphic sapphire origins have also been determined, e.g. Myanmar sapphire enclosed zircon inclusions have U-Pb ages of 31-32 Ma, Sri Lankan Sapphire zircon inclusions have been determined to have an age of 549 Ma (Elmaleh et al., 2019), and Madagascar sapphire zircon inclusions U-Pb age range between 560-650 Ma (Kröner et al., 1999; Link, 2015). Kashmir sapphires can therefore be easily separated from Sri Lanka or Madagascar sapphires through determining the formation ages because of their clear age differences. The tested sample SKG1 clearly shows that its age is different from zircon inclusion ages found in zircons from Sri Lanka or Madagascar sapphires, and a consistent U-Pb age is repetitively found in Kashmir sapphires' zircon inclusions, in low quality sample as well as in gem quality material. This does not apply to Myanmar sapphires as they have formation ages which are comparable to the Kashmir ones (Link, 2016). Nevertheless, according to our long-term lab experience, it is noteworthy that in most cases, sapphires originating from Myanmar can simply be separated by optical features.
CONCLUSION
In this study, U-Pb dating applying HR SIMS and LA-ICP-MS on zircon inclusions in blue sapphire from the Sumjam region in Kashmir was successfully carried out on reference samples, and its age is concluded 25-26 Ma. This can be used as a reference U-Pb age of zircon inclusions for origin determination of Kashmir sapphire in gemmological laboratories by applying LA-ICP-MS on faceted gemstones where the zircon is exposed to the surface. In addition, cathodoluminescence imaging showed that the zircon grains in Kashmir sapphires have regular oscillatory zoning. Evidence seems to be their high U concentrations of up to 20 000 ppm which is related to the leucogranitic magma of the Zanskar area. As a side effect the unique combination of two independent analytical methods, the very advanced HR SIMS and the gemmologically established LA-ICP-MS permits evaluate the LA-ICP-MS method presented previously (Link, 2015) and strengthens its validity as a routine method in gemmological laboratories. The examined 2.6 ct sapphire in gem quality underlines and demonstrates the power of age determination to securely exclude Sri Lanka or Madagascar as potential origins and together with other observations support the conclusion of Kashmir as origin.
ACKNOWLEDGMENTS
The authors would like to thank Dr.Tobias Häger Mineralogical Institute, University of Mainz, Germany for helping and providing equipment of sample preparation.
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