2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 29
Presentation Time: 8:00 AM-6:00 PM

Preliminary Oxygen Isotope Values for Montana Sapphires

BERG, Richard B., Montana Bureau of Mines and Geology, Montana Tech of the U. of Montana, 1300 W. Park St, Butte, MT 59701, GAMMONS, Christopher H., Geological Engineering, Montana Tech of The Univ of Montana, 1300 West Park Street, Butte, MT 59701 and AREHART, Greg B., Geological Sciences, Univ of Nevada, Reno, MS-172, Reno, NV 89557, dberg@mtech.edu

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.

Locationδ18O Locationδ18O
Silver Bow4.6, 6.7Rock Creek2.7, 2.6
Dry Cottonwood6.6, 6.5"3.4, 2.7
Yogo4.3, 4.6"2.7, 2.3