DISCRIMINATING AMPHIBOLE POPULATIONS IN SOUTHERN OKLAHOMA AULACOGEN GRANITES USING LASER INDUCED BREAKDOWN SPECTROMETRY

The units of the Wichita Granite Group (WGG) are the shallow intrusive products of Eocambrian Southern Oklahoma Aulacogen magmatism. This includes 11 plutonic bodies exposed in the Wichita Mountains, each mapped as individual units based on geography, geochemical composition, mineral assemblage, and texture. The WGG is also found in the subsurface within basement-penetrating petroleum exploration wells along the northwest-southeast SOA trend. Although the occurrence of WGG in the subsurface is extensively documented, the group has not been divided into individual plutonic (lithodemic) units, save for outcrop-proximal samples. Determining the nature and distribution of plutons in the subsurface would further enlighten our view of Aulocogen magmatism.

Much of the subsurface samples are fragments (“chips”) produced by drilling. Although these are typically no larger than a centimeter on an edge, many of the WGG granites are of sufficiently fine texture that each chip captures several grains. WGG rocks are alkali-feldspar granites, many are similar in texture and mineral content, but the mafic mineral composition differs among the units of the WGG, including the mode and composition of amphiboles. Assessment of the amphiboles might provide a means to discriminate granite chip samples as functions of depth and location.

Although small, the chips are near continuously sampled in10 foot (3.05 m) intervals, providing a large number for evaluation. Laser Induced Breakdown Spectrometry (LIBS) offers an easy-to-use, low-cost assessment of mineral composition that could quickly discriminate amphibole populations. To test this application, I evaluated a diverse population of amphiboles of known composition (a training set), comparing these to a set of WGG amphiboles (evaluated by petrography). Thus far, principal component analysis of sets of entire spectra is able to discriminate between individual amphibole types in the training set and the amphiboles from individual WGG units. Peak ratio plots of Na 589.0 nm / Ca 393.4 nm and Mg 279.6 nm / Fe 259.9 nm are useful for discrimination and limited classification of amphiboles from both the training set and the WGG samples. Both techniques have been successfully applied to chip sample sets from a small number of wells.