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- W344703178 abstract "Rock varnish is a microns-thick manganese- and iron-rich coating that forms on exposed rock surfaces in arid and semi-arid environments. Empirical correlations of the varnish cation ratio (K+Ca)/Ti with age have been used to estimate ages of geomorphic surfaces, with varnish chemistry generally acquired by either proton-induced x-ray emission (PIXE) analysis of natural varnish surfaces. 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 with SEM/EDX, penetration into the substrate can be recognized at accelerating voltages where contamination with substrate is inferred from SEM/EDX chemical analyses. This demonstrates the ability of the SEM method to obtain varnish chemistry with minimal inclusion of substrate. Calculations of theoretical x-ray depth-distribution ({phi}({rho}z)) curves in varnish indicate that at an accelerating voltage of 10 kV most of the emitted electrons are generated in the upper 0.5 micron of a sample. At a higher voltage of 30 kV most of the signal is still restricted to the upper 2 microns, 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 ofmore » the uppermost varnish with varnish age suggests that significant time-dependent changes in chemistry occur in the upper one- to two-microns of rock varnish. 11 refs., 5 figs., 2 tabs.« less" @default.
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- W344703178 date "1990-04-02" @default.
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- W344703178 title "Scanning electron microscopic analysis of rock varnish chemistry for cation-ratio dating : an examination of electron beam penetration depths" @default.
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