The Origin of Sapphires: U-Pb Dating of Zircon Inclusions Sheds New Light

Robert R. Coenraads, F. Lin Sutherland and Peter D. Kinny
School of Earth Sciences, Macquarie University, NSW, 2109, Australia
Division of Earth Sciences, The Australian Museum, 6–8 College Street, Sydney, NSW 2000, Australia
Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia

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.

Keywords: sapphire • uranium-lead dating • inclusions • zircon • Central Province, Australia

Mineralogical Magazine; March 1990 v. 54; no. 374; p. 113-122; DOI: 10.1180/minmag.1990.054.374.13
© 1990, The Mineralogical Society
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