Figure 1

(From class notes provided by Craig Jones) U-Pb concordia (curved line) and discordia (straight lines).  When no Pb has diffused out of the crystal or no U has been added to the system, relationships of ²⁰⁶Pb/²³⁸U to ²⁰⁷Pb/²³⁵U will lie along the concordia.  When some amount of Pb has diffused out of the crystal or U has been introduced to the system, relationships of ²⁰⁶Pb/²³⁸U to ²⁰⁷Pb/²³⁵U will lie along the discordia.