New Mexico Geological Society Annual Spring Meeting — Abstracts

Ash flow tuffs represent important marker beds for many types of research, including volcanology, tectonics, archeology, and soil science. Traditional methods for tuff correlation include mapping, mineralogy, ⁴⁰Ar/³⁹Ar age determinations of phenocrysts, and paleomagnetism. Laser-Induced Breakdown Spectroscopy (LIBS) is a relatively new laser ablation technique that can be used to correlate ash flow tuffs by comparing the chemical composition of phenocryst phases. In LIBS, a high-power pulse of laser light ablates and excites ions in a short-lived, high-temperature plasma. As electrons decay to lower-energy orbitals during cooling, they emit light which is collected by a lens or telescope, transmitted to a spectrometer by optic fiber, and diffracted to form a spectrum. Each LIBS spectrum contains information on elemental concentrations for essentially the entire periodic table, forming a detailed chemical fingerprint. Backpack LIBS systems are being designed; in the next few years, geologists will be able to correlate tuffs in the field using this new technology.

In the current study, ash flow tuffs of the Bell Top Formation in southern New Mexico were analyzed by LIBS. Samples of each tuff were collected from at least two different areas. For each sample, 200 LIBS spectra were collected from sanidine and biotite phenocrysts as well as from the whole rock. Using the multivariate technique Projection to Latent Structures (PLS), the spectra from one area were used to train a model to uniquely identify each tuff. The identities of the same tuff units from a different area were then predicted by comparing their spectra to the PLS model. Predictions based on whole-rock and biotite spectra were not successful. However, predictions based on sanidine spectra correctly identified the ash flow tuff units in all cases. Sanidine is the mineral of choice, similar to ⁴⁰Ar/³⁹Ar analysis, because it is more resistant to alteration than biotite and volcanic ash.