FURTHER INVESTIGATION INTO DISCRIMINATING THE SUBSURFACE PLUTONS OF THE ARBUCKLE MOUNTAINS USING LASER INDUCED BREAKDOWN SPECTROSCOPY

The Southern Oklahoma Aulacogen (SOA) extends from the Dallas area to Amarillo and beyond and contains numerous exposed and buried alkali-feldspar granite plutons. The latter are encountered in numerous basement penetrating holes, most of which produced samples in the form of rotary drill cuttings. These are small (<3 mm on a side) but in most cases polycrystalline. Amphibole compositions within the granites exhibit geochemical variations that reflect changes in the magmas from which they crystallize, and can discriminate between individual plutonic bodies. The large number of samples requires rapid evaluation. Laser Induced Breakdown Spectrometry (LIBS) provides a quick chemical fingerprint potentially useful for discriminating amphibole compositions.

This study examined 1 new well (CHP) and reevaluated samples from 4 others (SR, TF, MW, and UM) from the Arbuckle Mountains of southern Oklahoma. Granite fragments were selected from two or more depths in each well. All cuttings were hand selected and mounted in epoxy discs sanded flat to 600 grit for LIBS analysis; a subset were additionally polished for electron microprobe analysis (EMPA). The LIBS applied a 70um spot size at a fluence of 2000 J/cm² to 5 or more grains at each depth sampled. A clean shot was immediately followed by 25 consecutive pulses at <1 Hz on each point. We averaged the 25 spectra (200nm to 950nm) and normalized each to the N 746.8 peak. The spectra from the points were then averaged to produce a single spetrum for each depth. We compared each depth using peak-ratio plots and whole-spectra principal component analysis (PCA).

Analysis revealed substantial variations in the Na (589.6 nm and 589.0 nm) and Mg (279.6 nm) peaks. EMPA determined the strong outlier data to be chlorite minerals. Without these altered outliers, no significant differences were noted within wells as a function of depth. However, among the wells the amphiboles fell into two population sets with differences in the Na and Mg peaks: west (UM, SR) and east (MW, TF, and CHP). EMPA confirmed that SR amphiboles have higher Na, Si and Mg# relative to MW and TF. The findings suggest that these wells penetrated two distinct plutonic bodies.