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Elemental concentrations in rock varnish collected from the Cima volcanic field in the Mojave Desert, California, were determined along 72 line profiles through 15 cross sections using scanning electron microscopy and energy dispersive X-ray analysis. The mum-scale analyses allow the recognition of elemental associations in varnish that define the primary Mn, Fe, and silicate phases that constitute the bulk of individual varnish stratigraphic layers. The analyses also allow the recognition of mum-scale variability resulting from the presence of irregularly distributed secondary components, such as detrital mineral grains.

Varnish stratigraphy is primarily defined by variations in the concentrations of Mn, Fe, Si, and Al, each of which typically exceeds 5 wt percent. In general, the concentrations of Fe, Si, and Al are each negatively correlated with Mn concentration. Positive correlations of Al and Si concentration indicate that they usually exist in the same phase, probably the clay component of varnish. Weaker positive correlations of Fe and Si concentration suggest that Fe generally coexists with the clay component, possibly directly associated with the clay structure.

The concentration of many minor elements in Cima varnish layers reflects the stratigraphic variations in major element composition. Ba and Ca are generally associated with Mn in Cima varnish, and K and Mg are generally associated with Si. P and Ti are often associated with Fe, but these associations are less well defined. Unusually high local concentrations of many cations, including Ca, K, Mg, Ti, Si, and Fe, probably result from the incorporation of detrital mineral grains transported onto the surface of the accreting varnish. Locally high concentrations of other cations may be related to authigentc mineralization, including frequent associations of Ba and S in discrete laminae which suggest the in situ formation of a barium sulfate mineral.

The causes of the variations in elemental composition between stratigraphic layers in Cima varnish are poorly understood. Varnish stratigraphy has previously been interpreted in terms of regional paleoenvironmental fluctuations that affect the Mn:Fe ratios in accreting varnish. However, Mn:Fe ratios of the uppermost varnish at Cima are highly variable, and this ratio is thus influenced by factors other than regional environmental conditions. In addition, Mn:Fe ratios provide an incomplete description of the variability between stratigraphic layers, and concurrent use of Mn:Si and Si:Fe ratios would improve definition of stratigraphic variations in varnish composition.