The origin of sapphires: U–Pb dating of zircon inclusions sheds new light

Abstract

Uranium-lead isotope dating of two zircon inclusions in sapphires from the Central Province, NSW. gives ages of 35.9+ 1.9 and 33.7_+2.1 million years (Ma). These ages fall within the range of basalt potassium-argon ages of 19 to 38Ma and zircon fission track ages of 2 to 49Ma for the timing of volcanism of the Central Province, NSW. These data, combined with the observation that corundum is found associated with many alkali basaltic provinces, indicate a genetic link between the growth of large corundum crystals and the processes involved in alkali basaltic magma generation. The reported failure of experimental attempts to grow corundum from a corundum-bearing basaltic composition, and more significantly, the abundance of incompatible elements such as U, Th, Zr, Nb and Ta in inclusion minerals indicate that the crystallization process is not simple. Corundum and the other minerals found as its inclusions (zircon, columbite, thorite, uranium pyrochlore, alkali feldspar etc,) could not have crystallized from most basaltic compositions. A more complex process must occur in which crystallization takes place when there are high proportions of incompatible elements and volatiles in the melt. These crystallization products are then carried to the surface by upward movement of later magmas. The extent of this process presumably determines whether a particular basaltic province carries sufficient corundum to be worked into economic concentrations of sapphire.

An unusual sapphire–zircon–magnetite xenolith from the Chanthaburi Gem Province, Thailand

“…The arguments against lead loss are discussed by Coenraads et al (1990). This zircon shows moderately high U (194-519 ppm) and Th (216-519 ppm) with a U/Th ratio of <1 (0.69-0.94).…”

“…Knowing the decay rate and the amount of daughter lead isotopes present, the zircons can be used for age determination and hence provide an age of the associated sapphire formation. The U-Pb analyses were made with the Sensitive High Mass Resolution Ion Microprobe (SHRIMP) at the Australian National University, Canberra, as outlined in Coenraads et al (1990). Young Cainozoic zircons require special treatment of the data.…”

Sapphires from Antsiranana Province, Northern Madagascar

“…The following have been identified as typical mineral inclusions observed in magmatic sapphires: assemblages of feldspars (albite, calcic plagioclase, Na-and/or K-rich sanidine), zircon, columbite, rutile, hematite (sometimes in solid solution with ilmenite), spinel (hercynite, magnetite, gahnospinel, and/or cobalt-spinel), uraniumpyrochlore, pyrrhotite, thorite, and uraninite; lowSi and Fe-rich glassy inclusions have also been identified (see, e.g., Phillips, 1984, 1986;Coldham, 1985Coldham, , 1986Gübelin and Koivula, 1986;Irving, 1986;Kiefert, 1987;Kiefert and Schmetzer, 1987;Coenraads et al, 1990Coenraads et al, , 1995Aspen et al, 1990;Guo et al, , 1994Guo et al, , 1996aSutherland, 1996;Sutherland and Coenraads, 1996;Krzemnicki et al, 1996;Sutherland et al, 1998a and b;Maliková, 1999).…”

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

“…A further observation leading these authors to favor syenites as parental melts is that repeatedly melt inclusions of syenitic composition were reported from magmatic sapphires (e.g., Pakhomova et al 2006;Zaw et al 2006;Izokh et al 2010). Up until now, carbonatites have not been considered as potential parental melts, although many authors (e.g., Coenraads et al 1990;Van Long et al 2004;McGee 2005;Nechaev et al 2009;Izokh et al 2010;Palke et al 2017) reported that CO 2 must have played a significant role in the formation of these sapphires, as CO 2 -rich fluid inclusions are commonly observed in many magmatic sapphires. Palke et al (2017) interpreted the presence of analcime-calcite inclusions in alluvial magmatic sapphires from Montana, USA as an indication for that the sapphires crystallized from a melt that simultaneously exsolved a carbonatite melt, and fractions of that carbonatite melt were trapped by the growing sapphire and later recrystallized to the analcime-calcite paragenesis.…”

“…Several hypotheses have been proposed to explain the igneous crystallization of sapphires, the most prominent being: (1) Crystallization from highly evolved melts such as syenites (e.g., Irving 1986;Coenraads et al 1990;Aspen et al 1990;Giuliani et al 2009). (2) Crystallization from alkaline Si-and Al-rich melts formed by partial melting of amphibole-bearing lithospheric mantle (Sutherland 1996;Sutherland et al 1998b).…”