ASSESSING AMPHIBOLES IN SUBSURFACE GRANITES FROM THE ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA AULACOGEN

The plutons of the Wichita Granite Group (WGG) of the Southern Oklahoma Aulacogen (SOA) exhibit small geochemical variations partially manifested in their amphibole compositions. Such variations may aid the discrimination of individual plutons within the numerous samples of subsurface SOA granites encountered by basement-penetrating drill holes. Laser Induced Breakdown Spectrometry (LIBS) facilitates rapid evaluation of amphiboles within the small fragments of rocks (cuttings) retrieved from drilling.

We carefully collected drill cuttings from several 10-foot (3.05 m) depth intervals within four well-documented basement-penetrating wells in the Arbuckle Mountains. These materials are part of the catalog at the Oklahoma Petroleum Information Center in Norman: Frankfort Sparks Ranch #1 (three depths), John A. Taylor Morrow #1 (five depths), Union Morton (nine depths), and Hamilton Bros. Turner Falls (9 depths). We mounted hand-selected chips in 1-inch epoxy disks and ground these flat with 600 grit to produce a consistent interface for the LIBS. We used a 70μm spot size to produce a fluence of a little over 2000 J/cm². After collecting spectrometer background values, we hit each spot with a clean shot followed by 25 consecutive pulses at frequencies <1 Hz. The spectrometer collected the intensity of the induced breakdown plasma over wavelengths from 200 nm to 950 nm. Data from the 25 shots were averaged and normalized to the nitrogen peak at 746.8 nm. These averaged data were then evaluated graphically through spectral stacking and through peak ratio plots, and evaluated statistically through principal component analysis (PCA).

The entire sample set produced comparable spectra typical of WGG amphiboles, exhibiting strong peaks at 589.0 nm (Na) and 279.6 (Mg). The set plots on top of the range of values for surface samples of WGG on peak ratio plots of Na/Ca versus Mg/Fe. Most are consistent with ferro-edenite, with a few producing peak ratios comparable to riebeckite. PCA suggests that the samples thus far fall into two populations largely separated by differences in the Na and Mg peaks. The two populations of amphiboles suggest that these wells encountered two or more plutons.