CORRELATION OF ASH-FLOW TUFFS AND LIMESTONES USING LASER-INDUCED BREAKDOWN SPECTROSOPY (LIBS): MOVING TOWARDS REAL-TIME GEOCHEMISTRY IN THE FIELD

Correlation of rock units is fundamental for many geologic studies. Because correlation methods often use sophisticated laboratory instruments for geochronology, fossil identification, paleomagnetism, or detrital zircon ages, field studies are sometimes lengthened by waiting for lab results. This study presents data on the use of LIBS spectra to correlate two different rock types: ash-flow tuffs and limestones. The purpose of this study is to evaluate LIBS as a field tool for correlation.

LIBS is a laser ablation spectroscopic technique in which a pulsed laser forms a short-lived, high-temperature plasma that contains atoms from the material. As the plasma cools, electrons decay from exicted orbitals to lower-energy levels and emit the energy difference as photons of light. The light is collected by fiber optic, diffracted, and recorded as a spectrum. LIBS spectra contain the intensities of wavelengths between 200 and 1000 nm; every element emits light in this range, making LIBS spectra rich fingerprints of rock and mineral composition.

Mid-Tertiary ash-flow tuffs of the Bell Top Formation, previously correlated by Ar/Ar geochronology and paleomagnetism, were used to test LIBS as a correlation tool. Spectra from whole-rocks, biotites, and sanidines from Bell Top tuffs 3, 5, and 6 collected from two different locations were used. Spectra from one location were used to train a PLS-1 (partial least squares regression) multivariate model; the model was then used to predict the identities of the tuffs from the second location. Spectra from the whole-rocks and biotite phenocrysts unsuccessfully correlated the tuffs, probably because of different post-eruptive processes in the two locations. However, spectra from sanidine phenocrysts successfully correlated the tuffs from the two areas.

To test LIBS as a tool for limestone correlation, nine limestone beds from a quarry in Franklin County, KS, were analyzed. Half of the spectra from each sample were used to train a matching algorithm, using a series of binary PLS-1 models. The other half of the spectra were then fed into the matching algorithm to see whether they would be correlated to the correct bed. The beds were correlated with 100% success. These results suggest that LIBS has enormous potential as a field-based geochemical tool.