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