Clinopyroxene-corundum assemblages from alkali basalt and alluvium, eastern Thailand: constraints on the origin of Thai rubies

An inclusion of corundum (ruby) was found in a clinopyroxene xenocryst in a lkali basalt from the late-Cenozoic Chanthaburi-Trat volcanics of eastern T hailand. 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+Fe2 +) > 0.9)) and is chemically similar to clinopyroxene inclusions in rubies from nearby alluvial gem deposits, suggesting a common origin for the two t ypes of occurrence. Sapphirine (Mg/(Mg+Fe2+) = 0.91-0.94) and garnet (py(56 -67)alm(11-18)grs(18-23)) also occur as inclusions in alluvial rubies. Ther modynamic calculation of the equilibrium 2 di + 2 crn = 2 cats + en constra ins the temperatures of clinopyroxene + corundum crystallization to between 800 and 1150 +/- 100 degreesC. Use of other equilibria as stability limits places the pressures of crystallization between 10 and 25 kbar, implying d epths 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 > simila r to 63 km). The xenocrystic clinopyroxene could have coexisted in equilibrium with garn et of similar composition to the observed inclusions at the deduced tempera tures of crystallization. The rubies probably crystallized in rocks of mafi c composition, i.e. garnet-clinopyroxenites or garnet-pyriclasites, within the upper mantle.