Using a three-dimensional U-Th-Pb concordia diagram, electron microprobe (EMP) U-Th-Pb analyses are shown to yield isotopic age constraints on isotopically concordant and discordant data that have not been recognized previously. The three-dimensional U-Th-Pb concordia diagram is discussed. It is demonstrated that a date obtained from an EMP analysis is as old as or younger than the 207Pb/235U and 207Pb/206Pb ages, and as old as or older than the 208Pb/232Th age. EMP analyses always yield a minimum age for the oldest Pb component.

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