U-PB DATING OF FE-RICH PHASES USING A SEQUENTIAL LEACHING METHOD

A sequential leaching method was developed for U-Pb dating of Fe-rich phases which uses a series of successively stronger acids. The method was applied to date Fe-rich phases of magmatic origin in the Kinsman Q uartz Monzonite, New Hampshire, USA, and of sedimentary origin in the Dales Gorge Member of the Brockman Iron Formation, Hamersley basin, Au stralia. Using the composition of major element released by the acid l eaching and combining it with X-ray and SEM analyses of the residues w e were able to identify the relatively pure phases released and use th ese samples for dating. Magnetite separates from the Kinsman Quartz Mo nzonite yielded U-Pb age of 372 +/- 34 Ma, which is consistent with Sm -Nd and a Rb-Sr isochron ages determined in previous studies. Leached whole-rock samples of the Brockman Iron Formation yielded a Pb-207-Pb- 206 age of 2213 +/- 250 Ma which agrees with previously determined Pb- Pb, Sm-Nd, and Rb-Sr isochron ages. This age is interpreted as the tim e of redistribution of U and Pb associated with the Fe-rich phases dur ing regional metamorphism. While the potential of the sequential leach ing method is demonstrated, more work is needed in order to improve th e precision of the data. This requires further investigation of the na ture of the pure phases. Copyright (C) 1997 Elsevier Science Ltd.