Preliminary Oxygen Isotope Values for Montana Sapphires

Montana is the largest historic producer of sapphires in the U.S. and the origin of these gemstones has long been a matter of debate. Below we report new stable isotope data on δ¹⁸O (analytical uncertainty ± 0.1%) of 12 sapphires from four locations in central and western Montana. These sapphires have δ¹⁸O values that overlap with many specimens worldwide, but are depleted in ¹⁸O compared to the global average for this mineral (7.8 ± 3.8%, Giuliani et al., 2005, Geology 33, 240-252). Sapphires from the Rock Creek alluvial deposit form a distinct cluster with some of the lowest δ¹⁸O values reported in the literature. Previous workers have concluded that Montana sapphires were transported to the surface as xenocrysts in lamprophyre (Yogo deposit) or other igneous melts. Regional differences in the isotopic composition of Montana sapphires probably have little to do with the chemistry of the transporting magma, but instead are likely attributed to: 1) derivation from crustal or upper mantle source rocks with different O isotope composition; 2) different temperatures of isotopic equilibration with source rocks of similar isotopic composition; or 3) different degrees of isotopic re-equilibration with magma during ascent and emplacement.

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Location	δ18O	Location	δ18O
Silver Bow	4.6, 6.7	Rock Creek	2.7, 2.6
Dry Cottonwood	6.6, 6.5	"	3.4, 2.7
Yogo	4.3, 4.6	"	2.7, 2.3