STUDY OF ZIRCON INCLUSIONS IN SAPPHIRE: U/PB AGE DATING OF KASHMIR SAPPHIRE AND APPLICATION OF RAMAN SPECTROSCOPY ON ORIGIN CHARACTERIZATION

This study focuses on zircon inclusions in Kashmir sapphire and other 3 important commercial metamorphic sapphire deposits Myanmar, Sri Lanka, and Madagascar. Zircon inclusions in selected sapphire samples from Kashmir origin were studied by HR SIMS and Laser-Ablation-ICPMS for U-Pb age determination. Raman spectra were collected in the zircon inclusions in total 115 sapphires, all unheated and originating from Myanmar, India (Kashmir), Sri Lanka, and Madagascar, aiming to find relationship between their origin and Raman spectroscopic features.

U/Pb age dating of Kashmir sapphire: 18 measurements of HR SIMS show relatively consistent results, indicating that the U-Pb ages of the zircon in Kashmir sapphire are 24.97 Ma on average with standard deviation of 0.22 Ma. One LA-ICPMS measurement concluded a U-Pb age of 26.3±3.8 Ma. An age of 25-26 Ma fits well within the age range of Alpine-Himalayan orogeny.

Application of Raman spectroscopy in origin characterization: By comparing Raman features of zircon inclusions in sapphires from Myanmar, India(Kashmir), Sri Lanka and Madagascar, it could be established that those of younger age (sapphires from Myanmar and India (Kashmir)) contain zircon inclusion which exhibit relatively low ʋ1 and ʋ3 band positions and also smaller FWHM (ʋ3) than those in the older sapphires from Sri Lanka and Madagascar. Binary plotting of ʋ1 versus ʋ3 frequencies (Figure 3), and ʋ3 wavenumber versus FWHM (Figure 4 and 5) of the studied zircon inclusions provide a method to distinguish young sapphires formed during Alpine-Himalayan orogeny (Kashmir, Myanmar) about 50-5 Ma years from those related to the Pan-African orogeny (Sri Lanka, Madagascar) about 600-500 Ma ago.

Raman spectroscopic features of zircon inclusions in sapphire combine influences of their trace element content, ambient pressure, metamictization degree and formation age. Summary effect of these four factors could be considered as a kind of geological “fingerprint” which gives indication of geological origin of the sapphire.

The Raman spectroscopically study on zircon inclusions shows the potential using the approach to characterize sapphire`s origin as a service to the commercial gem trade in a non-destructive way.