GARNET-BIOTITE COMPOSITION AS A GEOTHERMOMETER OF SCHIST: BLACK HILLS, SOUTH DAKOTA

The minerals garnet and biotite found in metamorphic rocks can be used to determine temperatures under which a rock has formed, due to a temperature dependent Fe-Mg exchange between these coexisting minerals. As temperatures increase, iron levels should increase within garnets while magnesium levels decrease and vice versa within biotites. The objective of this research is to analyze changes in garnet and biotite compositions along a transect in order to potentially record a change in metamorphic conditions. The amount of Fe and Mg was determined through scanning electron microscope (SEM) analysis of garnet and biotite in one sample of metapelites from the southern Precambrian core of the Black Hills, South Dakota, between Mount Rushmore and Sheridan Lake. The ratio between the two elements was then determined and applied to the garnet-biotite geothermometer (Spear, 2023). The temperature of peak metamorphism found in the sample closest to Mount Rushmore displays temperatures averaging at about 475° C with a standard deviation of 50. The temperature and mineral composition of the sample falls somewhere between the greenschist and amphibolite metamorphic facies.