Correlation of Ash Flow Tuffs Utilizing Laser Induced Breakdown Spectroscopy (LIBS) In Field Applications

Correlation of ash-flow tuffs is typically accomplished using whole-rock geochemistry, paleomagnetism, and geochronology. This study considers the use of a rapid and portable analytical technique, LIBS, for correlation. Because backpack LIBS is under development, it is important to demonstrate that LIBS is accurate enough to be used in the field rather than more expensive and time-consuming methods.

In LIBS analysis, a focused laser pulse ablates atoms from the surface of a substance (solid, liquid, or gas) and forms a plasma. In the plasma, the sample breaks down into individual atoms and electrons move up into their excited states. As the plasma cools, the electrons fall down into their lower orbitals and release photons of light. The light is collected with an optic fiber, diffracted, and recorded on a CCD camera as a spectrum. The spectra contain peaks of essentially all elements in the periodic table. This project utilizes an Ocean Optics LIBS 2500+ with a Big Sky laser operating at 1064 nm. Spectra are analyzed using principal components analysis and the program Unscrambler.

Known magmatic facies of the three Plinian eruptions of the Oligocene Organ Mountains Caldera in southern New Mexico are used to test LIBS as a correlation tool. For Bell Top tuffs 3, 4, and 6, two samples each were collected from the outflow sheets and from their plutonic equivalents. With a laser power of 106 mJ and a Q-switch delay of -2.5µs, 400 shots were taken of each hand sample; four average spectra (100 shots each) were calculated for each sample. Preliminary principal component analysis suggest that each outflow sheet plots as a compositionally unique group, opening the door to in situ chemical analysis and ash-flow tuff correlation.