gems, corundum, Geology.11, Thailand.12, Phrae, Den Chai, basalt sapphire

Alluvial gem sapphires from the Denchai area in northern Thailand exhibit oxygen isotope compositions ranging from +4.7‰ to +6.1‰ (based on 15 samples), with one outlier at +8.4‰. These values suggest that the sapphires are not in oxygen isotope equilibrium with the host alkali basalts and are instead xenocrysts derived from them. The associated basalts contain olivine phenocrysts with δ¹⁸O values of +4.9‰ to +5.1‰ and are considered the likely sources of the sapphires. The homogeneous δ¹⁸O values within individual sapphire crystals imply minimal interaction with the carrier magmas during their ascent. The higher δ¹⁸O values (above +5.5‰) observed in some sapphires point to the possible involvement of crustal materials in their formation. A hybrid origin model is thus proposed for the Denchai sapphires, involving varying degrees of interaction between a fractionated partial melt from a metasomatized mantle source and an Al-rich rock of lower or mid-crustal origin.