APPLYING THERMOMETRIC TECHNIQUES TO MONTANA SAPPHIRE MINERAL INCLUSIONS

Alluvial Montana sapphire deposits at Rock Creek, Dry Cottonwood Creek, and along the Missouri River have been mined for more than a century. Production of these sapphires continues today, with increased value as gemstones due to heat treatment processes. The P-T formation conditions of corundum at these localities have only been loosely defined through melt and mineral inclusion analysis and interpretation. Rutile inclusions are the most common mineral inclusions in Montana sapphires, found as prismatic crystals and exsolved silk. Prismatic rutile appears as yellow, orange, brown, and black equant to elongate crystals, with color determined by Fe content. Rutile is a carrier of high field strength elements, and its Zr content is buffered by the coexistence of zircon and quartz in a system. Alternatively, the absence of quartz can be calibrated for by using an estimated silica activity of the melt. In this preliminary study, we measure the Zr concentrations of ~20 crystalline syn-genetic rutile inclusions in corundum from Rock Creek, Missouri River, and Dry Cottonwood Creek locations with LA-ICPMS and apply the Zr-in-rutile thermometer with pressure dependency - and using a revised calibration for silica activity - to establish minimum rutile formation temperatures. Zr concentrations in rutile inclusions range from 194 ppm to 1340 ppm, with Missouri River samples indicating higher formation temperatures (~750°C on average) than Rock Creek samples (~660°C). To test the applicability of this methodology on rutile included in corundum, we further analyze rutile inclusions from corundum localities with better-constrained P-T conditions. Zircon inclusions in Montana sapphires have also been investigated to determine the agreement between the Ti-in-zircon thermometer and the Zr-in-rutile thermometer. The combination of these calibrations provides the most complete estimation of temperature conditions during the formation of alluvial Montana sapphires, while delivering the first results of applying the Zr-in-rutile thermometer to rutile inclusions within corundum.