The existence of two ` parallel ' uranium-lead decay routes allows several dating techniques within the overall U-Pb system . The existence of two ` parallel ' uranium-lead decay routes ( 238U to 206Pb and 235U to 207Pb ) leads to multiple dating techniques within the overall U-Pb system . The term ` U-Pb dating ' normally implies the coupled use of both decay schemes in the ` concordia diagram ' ( see below ) .