Glassy Melt Inclusions in Sapphires from Montana
Two-phase inclusions are common in many sapphires. Physical and chemical analysis of the gas and liquid phases present in these inclusions can shed light on the formation conditions of alluvial sapphires, even in the absence of evidence of source rock lithology. We report here some initial observations of a different type of two-phase inclusion, seen in alluvial sapphires from the Rock Creek and Missouri River deposits in the state of Montana.
At first glance, these inclusions appeared to be ordinary gas-liquid inclusions (left image); however, polishing through the inclusions demonstrated that the “liquid” component was actually a glassy solid. The identification of the solid as a glass rested with the phase’s optically isotropic nature and the lack of an identifiable Raman spectroscopic signal attributable to any crystalline phase. Glassy melt inclusions in Montana sapphires were previously documented by John Koivula (R.W. Hughes, Ruby & Sapphire, RWH Publishing, Boulder, Colorado, 2007, pp. 468–469). Electron microprobe analysis (EPMA) of glass inclusions from several samples showed them to be dacitic to trachydacitic in composition. In this sense the inclusions are ordinary two-phase inclusions in which the original liquid phase was a silicic magma that quenched to form a silicate glass. The gas phase is presumed to have originally been dissolved in the silicate magma. Later exsolution produced a separate gas phase as volatile solubility in the magma decreased due to falling pressure and/or temperature. Volatile exsolution likely also led to the discoid fractures commonly seen around melt inclusions, which are filled with silicate liquid and/or the volatiles themselves (again, see left image). The center image captures a snapshot of the process of volatile exsolution and the “bursting” of the two-phase inclusions, in which multiple gas bubbles seem to have precipitated instantaneously as the inclusion exploded out into the surrounding corundum. Due to the high relief of the silicate glass, these discoid fractures often have a “flux”-like appearance. In some cases, the fractures seem to have partially healed, leaving hexagonal geometric patterns as evidence of the crystallographic orientation of the sapphire host (right image). Further work is being carried out to characterize these inclusions and to decipher the story they have to tell about the genesis of alluvial Montana sapphires.
Aaron Palke is a postdoctoral research associate, and Nathan Renfro is the analytical manager of the gem identification department and analytical microscopist in the inclusion research department, at GIA in Carlsbad, California. Richard Berg is an emeritus senior research geologist and curator of the Mineral Museum with the Montana Bureau of Mines and Geology of Montana Tech in Butte.