ACCURATE AND PRECISE MEASUREMENT OF ROCK VARNISH CHEMISTRY USING SEM/EDS

We describe important refinements to the Harrington-Whitney technique by which an energy-dispersive spectrometer attached to a scanning electron microscope (SEM/EDS) is used to measure the chemical composition of rock varnish, a Si-/Al-/Fe-/Mn-rich material coating rock surfaces in arid regions. In particular, we have developed a calibration procedure which generates accurate, quantitative SEM/EDS analyses. This procedure differs from that typically followed for SEM/EDS because the reference spectrum for each element is calibrated using multiple standards. Analyses of 27 standards of varied composition, including three synthetic rock varnish standards, demonstrate the accuracy of our measurement technique. Repeated analyses of the same sample quantify the precision of our measurements and suggest that, if a high beam current and pulse processing rate are used, 1000 s is the optimal acquisition time for material similar in composition to rock varnish. Our data show that a spectral analysis program, using stored reference spectra, is capable of deconvoluting the severely overlapping Ba-L(alpha) and Ti-K(alpha) X-ray peaks and accurately determining the abundance of Ti and Ba, both of which are present in rock varnish. The accuracy and precision of SEM/EDS analyses of intentionally roughened synthetic varnish standards compare favorably with those of polished standards. This finding indicates that surface roughness does not introduce a significant error into SEM/EDS analyses of in situ rock varnish. Our data imply that carefully calibrated SEM/EDS can accurately measure rock varnish chemistry and determine varnish cation ratios, (Ca + K) /Ti, which have been used as a calibrated geochronometer by previous investigators.