Oxygen isotope studies on placer sapphire and ruby in the Chanthaburi-Trat alkali basaltic gemfield, Thailand

Abstract

Placer sapphire and ruby are common in the western zone and the central/ eastern zone, respectively, of the Chanthaburi-Trat (C-T) alkali basaltic gemfield in southeastern Thailand. Sapphires have O-isotope composition in the range from +5.1‰ to +6.2‰, while rubies, from +1.3‰ to +4.2‰. These data clearly show that the C-T placer sapphires and rubies should have a different primary genesis. Olivine in the nearby alkali basalts exhibits a narrow range of O-isotope composition, i.e., from +4.8‰ to +5.0‰, demonstrating that the C-T placer gem corundums cannot be phenocrysts of these basalts. The homogeneous O-isotope compositions of sapphires, combined with other available data, strongly suggest that these sapphires most probably have originally crystallized from evolved mantle magmas at lower crust/upper mantle depths. The varied O-isotope compositions of rubies are consistent with the previous suggestion that these rubies originally formed in mafic metamorphic rocks under upper mantle conditions, although the O-isotope data also require that the mafic rocks would have experienced an earlier surface (sea) water-rock interaction history. Consistent with the available data, a suggestion with garnet pyroxenite in the sub-continental mantle as the primary source for these placer gem corundums is tentatively proposed. The surface distribution of sapphire or ruby in the studied area may be a sign of composition variation of garnet pyroxenite underneath. © 2005 Elsevier B.V. All rights reserved.