Analysis of Rock Varnish from East-Central California by Handheld Laser-Induced Breakdown Spectroscopy

Rock varnish is a dark-colored coating of intercalated clay minerals, Mn-Fe oxyhydroxides, and silica that commonly forms on rock surfaces in arid regions exposed to sunlight, moisture, and manganese microbial redox cycling. Varnish gradually accumulates through time, which provides the possibility for its use as a paleoclimate proxy. Previous research on rock-varnish chemistry has used microanalytical approaches only possible in the laboratory. We are investigating the innovative approach of laser-induced breakdown spectroscopy (LIBS) for rapid analysis of varnish in the field. In-situ element detection and microchemical mapping has been undertaken for rock varnish formed on desert pavements covering two alluvial terraces of different age in the Deep Springs Valley of east-central California. The most abundant elements recorded by handheld LIBS devices are Mn, Fe, Si, Al, Na, K, Mg, Ca, and Ba, with spectral peaks for H, Li, Ti, Cr, and Sr also observed. Microscale maps of relative elemental abundances were generated by rastering the laser beam across 2x2 mm areas on multiple samples. Our results are consistent with those from previous laboratory studies and note the following trends: Al distributions are coherent with those for Si; Mn distributions are unrelated to those for Si, Al, and Fe; there is strong coherent covariation of Ba with Mn; and Li distributions are independent of those for other elements. The LIBS technique is a reliable way to investigate rock-varnish elemental composition in the field. If matrix-matched calibration curves can be generated for elements of interest, then determination of absolute elemental concentrations will be possible.