In-field analysis of rock varnish with SciAps LIBS

Publication: Russell S. Harmon, Daria Khashchevskaya, Michelle Morency, Lewis A. Owen, Morgan Jennings, Jeffrey R. Knott and Jason M. Dortch. “Analysis of Rock Varnish from the Mojave Desert by Handheld Laser-Induced Breakdown Spectroscopy,” Molecules 26, 5200 (2021).

This study investigated the chemical composition of rock varnish using SciAps Z-300 handheld LIBS to determine LIBS utility for three applications: elemental detection and identification, micro-chemical mapping, and subsurface compositional profiling. In situ LIBS analysis of rock varnish and visually similar coatings on granite outcrops and boulders across the Mojave Desert climatic region recognized the most abundant elements in rock varnish—Mn, Fe, Si, Al, Na, K, Mg, Ca and Ba, along with the primary spectral lines for H, Li, Ti, V, Sr and Rb.

Typical use of handheld LIBS during fieldwork. (b) Discontinuous varnish on basalt at Fossil Falls, Owens Valley. (c) Desert varnish on a granite outcrop, Alabama Hills, Owens Valley. Residual desert varnish “patches” on granite boulders on the (d) Lone Pine and (e) Fish Springs alluvial fans of Owens Valley. (f) Strongly varnished clasts in the desert pavement on an alluvial terrace in Deep Springs Valley.

‍Abstract: Laser-induced breakdown spectroscopy (LIBS) is a form of optical emission spectroscopy that can be used for the rapid analysis of geological materials in the field under ambient environmental conditions. We describe here the innovative use of handheld LIBS for in-situ analysis of rock varnish. This thinly laminated and compositionally complex veneer forms slowly over time on rock surfaces in dryland regions and is particularly abundant across the Mojave Desert climatic region of east-central California (USA). Following the depth profiling examination of a varnished clast from colluvial gravel in Death Valley in the laboratory, our in-situ analysis of rock varnish and visually similar coatings on rock surfaces was undertaken in the Owens and Deep Spring valleys in two contexts, element detection/identification and microchemical mapping. Emission peaks were recognized in the LIBS spectra for the nine elements most abundant in rock varnish—Mn, Fe, Si, Al, Na, Mg, K, Ca and Ba, as well as for H, Li, C, O, Ti, V, Sr and Rb. Focused follow-up laboratory and field studies will help understand rock varnish formation and its utility for weathering and chronological studies.

‍Keywords: laser-induced breakdown spectroscopy (LIBS); rock varnish; geochemical fingerprinting; microchemical mapping; desert pavement

‍Access to publication: https://doi.org/10.3390/molecules26175200

‍About this journal: Molecules is an international, peer-reviewed, open access journal of chemistry, published semimonthly online by MDPI.