U-Th-Pb Dating

Abstract

General. Three independent ages may be obtained in the U-Th-Pb system: ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U or ²⁰⁷Pb/²⁰⁶Pb, and ²⁰⁸Pb/²³²Th. Emphasis has been placed on U-Pb dating because the value ²³⁸U/²³⁵U is a physical constant that permits internal treatment of the data not found in any other dating system. This treatment helps to eliminate the assumption that the phase being dated has remained closed to changes in the parent-daughter system. The theoretical systematics are expressed in Fig. 1. Wetherill (1956a, b) showed that a phase,

which is subject to no lead loss or uranium gain (a closed system), will have ²⁰⁷Pb/²³⁵U ages equal to ²⁰⁶Pb/²³⁸U ages and that the data will lie along a curved line called concordia. In addition, he showed that phases subject to lead loss or uranium gain during a period of time that is short compared with the age of the phase (episodic bulk daughter losses or parent gains), recently or in the distant past, will have data that will lie along a straight line called discordia. The lower intersection of discordia with concordia represents the time of the episodic event and the upper intersection represents the age of the phase. Nicolaysen (1957) suggested that diffusion of lead out of a phase might take place at a constant rate over the entire history of the phase (continous diffusion), and Tilton (1960) showed that continuous diffusion with bulk lead loss or uranium gain also closely approaches a straight line.