A RECONNAISSANCE OF TRACE ELEMENT SIGNATURES FROM WEST TEXAS GYPSUM DEPOSITS

The Permian strata of the western Texas’ rolling plains are host to numerous deposits of gypsum. We analyzed four samples from four horizons using laser induced breakdown spectrometry (LIBS). In chronostratigraphic order, the samples are: an alabaster nodule fragment from the upper Clear Fork Group in Hardeman County, massive alabaster from the lower portion of the Blaine Formation in Foard County, bedded alabaster from the lower portion of the Blaine Formation in Cottle County, and a yellowish selenite layer from the Whitehorse Group from Hall County.

We prepared the alabaster samples with a flat surface using a water-cooled rock saw, whereas we cleaved the selenite with a hammer. All samples were ablated with a 266 nm Nd:YAG laser, with a spot size focus of 70 um at a fluence near 2000 J/cm². We applied these conditions to 10 spots on each sample, firing a cleaning shot before each of the 10 spectra-collecting shots. The intensities of the light emitted from the resulting plasma in atmosphere were collected over the 200-950 nm wavelengths and corrected for background. We averaged the 10 background-subtracted spectra and then normalized by the intensity of the N 746.8 nm peak. The integrated values above background for the prominent emission peaks of Sr, Ba, Mg, Fe, Na, and K were then compared.

The Whitehorse group selenite produced integrated values below the other samples for all of the investigated elements, with the exception of Mg, which was similar to the lower Blaine alabaster. The Clear Fork alabaster is identical to the Whitehorse selenite except with larger Na and lower Mg values. The Blaine formation samples exhibited striking differences, particularly in respect to Fe and the heavier alkali-earth elements. The lower Blaine alabaster yielded relatively elevated Na, Fe, and K integral peak values when compared to the Whitehorse selenite, and is the only sample with detectable Sr. The upper Blaine mimics the Whitehorse selenite except for lower Mg values, but has elevated Fe peak values that are similar to those in the lower Blaine, and is the only sample with detectable Ba.