CHARACTERIZING FELDSPAR COMPOSITION IN WICHITA MOUNTAIN IGNEOUS ROCKS USING LASER INDUCED BREAKDOWN SPECTROMETRY

Laser induced breakdown spectrometry (LIBS) is a relatively new method for acquiring rapid compositional data with minimal sample preparation. The method uses a high-powered laser to excite a small volume of a sample, which in turn creates a plasma that emits the photons characteristic of the elements present in the plasma. The method, however, may produce inaccurate results. Our study focused on overcoming the inaccuracies present in the Midwestern State University lab instrument and applying the technique to feldspar crystals in rocks from the Wichita Mountains, Oklahoma. We evaluated the amount of applied power, the spot size of the laser, and the number of laser pulses applied to a sample in an attempt to determine the optimal parameters for repeatability. A moderate power setting (~90 mJ), 70 micrometer spot size, a clean shot followed by one analytical shot on 10 separate points were the parameters that produced the most repeatability. The separate points were averaged for the final peak values. Further spreadsheet manipulation of the compositional data produced an averaged spectrum unique to a specific sample. These conditions were applied to create a database of unique chemical signatures for the feldspars from several of the igneous rock units located in the Wichita Mountains, southern Oklahoma. The Wichita Mountains contain extensive granite, rhyolite, gabbro, and basalt bodies, some with distinctive feldspar compositions. We examined several well documented units, including the Mount Scott Granite, Quanah Granite, Mount Sheridan Gabbro, and the Carlton Rhyolite. Feldspar was chosen for both its abundance in the region and its potential as a geothermometer. This database will aid in future research in both the quantitative analysis of the feldspar components and assessing the geologic history of the Wichita Mountains.