COMPARISON OF ANALYTICAL METHODS FOR HXRF GEOCHEMISTRY OF THE PACKSADDLE SCHIST AND TOWN MOUNTAIN GRANITE, LLANO UPLIFT, TEXAS

The bulk geochemistry of crystalline basement rocks from the Llano uplift of central Texas were previously analyzed using a handheld X-ray fluorescence analyzer (hXRF), but gave unexpected and possibly unreliable results. While crystalline rocks are not ideal for hXRF analysis of unprepared samples due to larger grain sizes and inhomogeneities, the ability to analyze samples in situ is helpful in some situations. This study tests variation in composition through well-documented, closely spaced analyses, and compares the results for crystalline and powdered samples. The ~1.3 Ga Packsaddle Schist was deformed and metamorphosed during the Grenville Orogeny, then subsequently intruded by ~1.1 Ga anorogenic magmas of the Town Mountain Granite. Samples of these units were collected at Enchanted Rock State Natural Area in central Texas, from the contact between the Packsaddle Schist and the intruding Enchanted Rock batholith. We analyzed five samples in total, including three igneous rocks and two metamorphic rocks. The Town Mountain Granite hand samples are fine- to coarse-grained phaneritic and pegmatitic granitoids that consist of quartz, orthoclase, plagioclase, and biotite, and are classified as granite or granodiorite. The Packsaddle Schist samples have gneissic to fine-grained schistose foliation, and contain plagioclase, quartz, muscovite, and biotite. The samples were analyzed using a ThermoFisher Niton XL3 Handheld XRF Analyzer for bulk composition. Ten closely spaced analyses on each of the hand samples are used to determine bulk composition and accuracy of the analyses. The samples will also be powdered and analyzed using hXRF. The results will be plotted and compared on an ACF ternary diagram, and compared with previous analyses of these crystalline samples. The results of this comparison will determine whether repeated hXRF analysis of crystalline samples can be reliable, or if they must always be powdered for best results.