Controlled leaching of monazites

The difficulties of obtaining a clear-cut geological interpretation of calculated isotopic ages are often increased by the presence of discordant age patterns. Although such patterns have been reported for most uranium-thorium bearing minerals, age determinations on monazites may exhibit distinctly more pronounced discrepancies. This is effectively illustrated by age patterns for several monazite occurrences ranging in age from 500 to 2550 m.y., including new analyses of specimens from Houtenbek in the Bushveld Complex, Bandolierkop, (N.E. Transvaal), Kakamas (Namaqualand), and Mount Darwin (Rhodesia). Acid-washing experiments of a semiquantitative nature shed new light on the nature of monazite age discordance. The results indicate substantial excesses of acid-soluble radiogenic Pb206 over radiogenic Pb208 for samples with low thorium-lead ages. Excesses of acid-soluble radiogenic Pb206 and Pb207 are accompanied by excesses of soluble uranium to thorium. The acid-soluble uranium and the acid-soluble Pb206 and Pb207 are evidently situated in a different phase, within the monazite, than the radiogenic Pb208. Common lead is more leachable than radiogenic lead. The soluble common lead may also be cited in the phase containing the acid-soluble uranium and the acid-soluble Pb206 and Pb207. The problem of the cause of monazite age discordance is discussed with the aid of these data. Both the acid leach data and the ‘intersection point’ on concordia diagrams are consistent with very recent alterations, presumed to coincide with weathering.