An inclusion of corundum (ruby) was found in a clinopyroxene xenocryst in alkali basalt from the lateCenozoic Chanthaburi-Trat volcanics of eastern Thailand. The clinopyroxene is fairly sodic, highly aluminous and magnesian (0.12 0.14 Na, 0.31 0.33 Al IV and 0.36 0.40 Al VI per 6(O), and Mg/ (Mg+Fe 2+ ) > 0.9)) and is chemically similar to clinopyroxene inclusions in rubies from nearby alluvial gem deposits, suggesting a common origin for the two types of occurrence. Sapphirine (Mg/(Mg+Fe 2+ ) = 0.91 0.94) and garnet (py 56 67 alm 11 18 grs 18 23 ) also occur as inclusions in alluvial rubies. Thermodynamic calculation of the equilibrium 2 di + 2 crn = 2 cats + en constrains the temperatures of clinopyroxene + corundum crystallization to between 800 and 1150 +1008C. Use of other equilibria as stability limits places the pressures of crystallization between 10 and 25 kbar, implying depths of between 35 and 88 km. The most Fe-rich clinopyroxene crystallized at a pressure in the lower part of the range. The pyropic garnet inclusions in corundum crystallized at pressures of >18 kbar (i.e. at depths >~63 km).The xenocrystic clinopyroxene could have coexisted in equilibrium with garnet of similar composition to the observed inclusions at the deduced temperatures of crystallization. The rubies probably crystallized in rocks of mafic composition, i.e. garnet-clinopyroxenites or garnet-pyriclasites, within the upper mantle.

Mineral inclusions and coexisting minerals of gem corundum from the Thailand area analysed by SEM-EDS in this study can be divided into sapphire and ruby suites. The sapphire suite includes alkali feldspar (sanidine), nepheline, zircon, hercynite-spinel series and magnetite-hercynite series. The ruby suite includes garnet (pyrope), high-Al diopside ('fassaite') and sapphirine. The mineral chemistry and other data suggest that Thai rubies could have crystallized in high-pressure metamorphic rock of ultramafic/ mafic composition. In contrast, Thai area sapphires could have crystallized in high-grade metamorphic (gneissic) rock or from magmas of highly alkaline composition, located at shallower levels in the lithosphere than those hosting the Thai rubies.

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