RAMAN SPECTRA OF ZIRCON INCLUSIONS IN SAPPHIRE

Raman spectroscopy of zircon inclusions in sapphire can be used to determine the “fossilized pressure” acting on the inclusion. Ideally, the ν₃(SiO₄) Raman band of zircon is linearly upshifted to higher wavenumbers with increasing pressure, allowing estimation of the entrapment pressure and depth. However, changes in volume of host sapphire, orientation and shape of zircon, and radiation damage can affect the accuracy of pressures.

Over 60 zircon inclusions in 21 sapphires from 9 different localities were investigated. Most samples are detrital, basalt-associated sapphires. Multiple confocal Raman analyses were collected on each zircon to observe spectroscopic changes along the length of the 10 to 200 μm crystals and were corrected for radiation-induced Raman downshift.

Multiple analyses on single zircons are homogeneous for some samples (e.g., Dry Cottonwood Creek, Montana). However, at others (e.g., Pailin, Cambodia) pressure differences are greater because of larger changes in the FWHM (~2-3 cm^-1/20 μm) and spectral position. Also, within a single sapphire, zircons can yield different pressures. Some lower pressures correlate to cracks, other inclusions nearby, and too shallow depths within the sapphire. But, other zircons are free of these defects yet the Raman spectra change along their length.

Some zircons were later exposed for geochemical characterization and cathodoluminescence imaging. CL images show complex textures that correlate to changes in the Raman spectrum along the zircon length. Brighter CL zones have narrower FWHM, indicating less radiation damage. Since chemistry and crystallinity affect the Raman spectrum, the presence of separate growth domains contributes to pressure variability within single zircons.

Changes in the Raman spectrum along the zircon length complicate estimates of entrapment pressure, but also provide information about the internal features. This preliminary work suggests that some zircons have separate growth domains and perhaps inherited cores. Measured pressures differ systematically among localities and range from 2 to 0.6 GPa (Avg. = 1.1 GPa), suggesting formation in the sub-continental lithosphere. The methodical differences of zircon inclusion Raman spectra are a promising, non-destructive tool for determining the provenance of gemstones.