CORRELATION OF ASH FLOW TUFFS FROM THE MOGOLLON-DATIL VOLCANIC FIELD IN SOUTHWESTERN NEW MEXICO USING LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS): AN ANALYSIS OF SANIDINE PHENOCRYSTS

The Mogollon-Datil volcanic field (MDVF) is an approximately 40,000 km² late Eocene to early Miocene volcanic field in southwestern New Mexico that consists of two major caldera clusters with several other scattered calderas (Chapin et al., 2004, NMGS Special Vol.). Thirteen ash flow tuffs from the MDVF were sampled at two to five locations each and correlated by ⁴⁰Ar/³⁹Ar dating (Chapin et al., 2004): Bloodgood Canyon (5 locations), Caballo Blanco (4), Davis Canyon (2), Hell’s Mesa (2), Kneeling Nun (4), La Jencia (2), Rockhouse Canyon (2), South Canyon (2), Vick’s Peak (3), Bell Top Tuff 3 (2), Squaw Mountain (3), Cueva Tuff (4), and Achenback Park (2). Samples were crushed; sanidines were separated using Franz magnetic susceptibility and lithium metatungstate, cleaned, and mounted for LIBS analysis. LIBS is an efficient analytical technique that focuses a high energy laser pulse onto a sample creating a short-lived plasma that, as it cools, releases photons of light that are diffracted and collected by an optical camera device. Multivariate chemometric techniques Principal Component Analysis (PCA) and Partial Least Squares Regression (PLSR) are then used on the LIBS spectra to create a series of binary matching models that successively identify individual tuff units. Models are built using one location for calibration and all other locations for test-set validation. A model using only the four tuffs with the most samples (Bloodgood Canyon, Caballo Blanco, Kneeling Nun, and Cueva Tuff) was created, masking out Li, W, O, C, H, and N peaks as contaminants from sample processing and ambient air and Na, Ca, K, Al, and Si peaks to enable models to focus on trace element differences. The success rate in distinguishing both Caballo Blanco and Kneeling Nun is 100% while Bloodgood Canyon and Cueva Tuff have success rates of 80% and 75%, respectively. The misidentified Cueva Tuff sample has low Ba, which is uncharacteristic of the other Cueva Tuff samples; misidentified Bloodgood Canyon samples have high Ba, compared to the typically low-Ba Bloodgood Canyon sanidines. This may be due to changes in Ba within ash flow tuff units. This study will not only help in doing correlation analyses of volcanic units in the lab at a reduced cost and time, but may also lead to tuff correlation in the field with backpack LIBS instruments.