U [sbnd]Pb dating of Fe-rich phases using a sequential leaching method

Abstract

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.