Oxygen isotope composition of the Denchai sapphire, Thailand: a clue to its enigmatic origin

Alluvial gem sapphires from the Denchai area, northern Thailand, have O-isotope compositions ranging from +4.7‰ to +6.1‰ (15 samples), with one outlier, +8.4‰. It is suggested that these sapphires are not in O-isotope equilibrium with and must be xenocrysts from the nearby alkali basalts, which contain olivine phenocrysts with δ18O=+4.9‰ to +5.1‰ and are the presumed sources of these gem sapphires. Homogeneous δ18O values within individual sapphire crystal indicate limited interactions with the carrier magmas during transport to the surface. The O-isotope compositions of these sapphires suggest that crustal rocks might have been involved in the genesis of those sapphires with δ18O values higher than +5.5‰. A hybrid origin model for the Denchai sapphires is, therefore, postulated, which involves different degrees of interactions between a fractionated partial melt from (metasomatized) mantle and a lower/mid-crust Al-rich rock.