Trace metals and natural radioisotopes are measured in an unusually thick and presumed ancient desert varnish from the Colorado Plateau in Utah. Uranium and thorium concentrations in the sequence: varnish-altered rind-heartrock (Shinarump formation sandstone) indicate that uranium with little accompanying thorium is derived from external sources. The varnish forms a closed system for ²³⁰Th and ²³¹Pa with equilibrium values for both ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U. Selective leaching of the ferromanganese oxides followed by analysis of both the leachate and silicate residue is proposed to allow age determinations.
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Knauss et al. (1980) studied this question.
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