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

Abstract

Alluvial gem sapphires from the Denchai area, northern Thailand, have O-isotope compositions ranging from + 4.7 per mil to + 6.1 per mil(15 samples), with one outlier, + 8.4 per mil. 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 delta 18 O= + 4.9 per mil to + 5.1 per mil and are the presumed sources of these gem sapphires. Homogeneous delta 18 O 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 delta 18 O values higher than + 5.5 per mil. 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.