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 Quartz Monzonite, New Hampshire, USA, and of sedimentary origin in the Dales Gorge Member of the Brockman Iron Formation, Hamersley basin, Australia. Using the composition of major element released by the acid leaching and combining it with X-ray and SEM analyses of the residues we were able to identify the relatively pure phases released and use these samples for dating. Magnetite separates from the Kinsman Quartz Monzonite 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 207Pb-206Pb 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 time of redistribution of U and Pb associated with the Fe-rich phases during regional metamorphism. While the potential of the sequential leaching method is demonstrated, more work is needed in order to improve the precision of the data. This requires further investigation of the nature of the pure phases.
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Acknowledgments--The authors wish to thank A.F. Trendall for providing the Brockman Iron Formation sample and to M. Bar-Matthews and L. Halicz of the Geological Survey of Israel for the use of the SEM and ICP-MS. The authors also wish to thank K. Mezger, J.M. Mattinson, and J. N. Aleinikoff for their thorough reviews. Support for this work was provided by a NSF grant EAR-9205767 to JDB and YE, and by a Hebrew University internal grant to YE.