Scanning Microscopy


Rock varnish is a microns-thick manganese- and iron-rich coating that forms on exposed rock surfaces in arid and semi-arid environments, and empirical correlations of the varnish cation ratio (K+Ca):Ti with age have been used to estimate ages of geomorphic surfaces. One method of obtaining varnish chemistry for cation-ratio dating involves scanning electron microscope (SEM) energy dispersive x-ray (EDX) analysis of natural varnish surfaces. The chemical analyses of rock varnish with SEM/EDX utilize a sequence of accelerating voltages to vary penetration depths into the sample. Using elemental x-ray maps of natural varnish surfaces obtained by SEM/EDX analysis, penetration into the substrate can be recognized at accelerating voltages where contamination with substrate is inferred from SEM/EDX chemical analyses, illustrating the ability of the SEM method to obtain varnish chemistry with minimal inclusion of substrate. Calculations of theoretical x-ray depth-distribution (¢,(pz)) curves in varnish indicate that at an accelerating voltage of 10 kV most of the emitted x-rays are generated in the upper 0.5 µm of a sample. At a higher voltage of 30 kV most of the signal is still restricted to the upper 2 µm, representing a very small percentage of total varnish volume in many cases. The ability of the SEM method to obtain empirical correlations of the chemistry of the uppermost varnish with varnish age suggests that it is not necessary to average the entire varnish coat, and that surface-biased varnish analyses provide similar results as bulk varnish analyses.

Included in

Geology Commons