Zircon from Ratanakiri Province, northeastern Cambodia, is well known in the gem trade for its vivid blue colour that results from heat treatment. The untreated brown material turns blue under reducing conditions at ~900-1,000°C. Ratanakiri zircon is characterised by remarkably low contents of trace elements. In particular, the actinides have low concentrations (e.g. approximately 120 ppm U and 95 ppm Th). Together with the very young age of the zircon (<1 million years [Ma]), this results in an extremely low self-irradiation dose, which in turn is in agreement with its non-radiation-damaged, nearly perfectly crystalline state. The heat treatment, therefore, does not result in detectable changes in the zircon's structural state. The cause of the blue colour, presumably related to a valence change upon heating in the reducing environment, is still under debate. The absorption of the treated Ratanakiri zircon is decidedly different from that of blue U 4+-doped and blue V 4+-doped synthetic ZrSiO 4. Absorption spectra show a strongly pleochroic band at 18,200-13,000 cm-1 (corresponding to ~550-770 nm wavelength) that is clearly responsible for the treated blue colour; however, its assignment remains unresolved.

* This article is dedicated to George Bosshart (1943-2012), who since the late 1980's had devoted most of his research activities to the study of processes causing natural and artificial green diamond colouration. His contributions to diamond research have stimulated also the investigation presented here.

The Mogok area in Myanmar (Burma) is known since historic times as a source for some of the finest rubies and spinels in the world. In this study, we focus on in-situ U–Pb geochronological analyses of zircon and zirconolite, either present as inclusions in gem-quality ruby and spinel or as accessory minerals in ruby- and spinel-bearing marble and adjacent granulite facies gneisses. The age determination was carried out using both laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS) and sector-field mass spectrometry (LA-ICP-SF-MS). In addition, we present multi-element data (REE) of zircon and zirconolite collected with LA-ICP-TOF-MS to further characterize these inclusions. Most of the studied zircon grains display growth zoning (core/rim) regardless if as inclusion in gemstones, or as accessory mineral in host rock samples. U–Pb dating was conducted on both core and rim of zircon grains and revealed most ages ranging from ~200 Ma in the core to ~17 Ma in the rim. The youngest U–Pb ages determined from the rim of zircon inclusions in gem-quality ruby and spinel are 22.26 ± 0.36 Ma and 22.88 ± 0.72 Ma, respectively. This agreement in U–Pb ages is interpreted to indicate a simultaneous formation of ruby and spinel in the Mogok area. In ruby- and spinel-bearing marble from Bawlongyi, the youngest zircon age was determined as 17.11 ± 0.22 Ma. Furthermore, U–Pb age measured on the rim of zircon grains in a biotite-garnet gneiss reveals a Late Oligocene age (26.13 ± 1.24 Ma), however older ages up to Precambrian age were also recorded in the cores of zircon as accessory minerals from this gneiss. These old ages point to a detrital origin of the analysed zircon cores. Although non-matrix matched standard was applied, zirconolite U–Pb age results are narrower in distribution from ~35 Ma to ~17 Ma, falling within the range of zircon ages. Based on results which are well in accordance with previous geochronological data from the Mogok Metamorphic Belt (MMB), we deduce that gem-quality ruby and spinel from Mogok probably formed during a granulite-facies regional metamorphic event in Oligocene to Early Miocene, related to post collision tectonics of the Eurasian and Indian plates. Our data not only provide key information to understand the formation of gem-quality ruby and spinel in the so-called Mogok Stone Tract, but also provide assisting evidence when determining the country of origin of gemstones in gemmological laboratories.

No abstract

No abstract