These series represent the decay chain products of long-lived primordial U-238, U-235, and Th-232, respectively.
However, each of them also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products.
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
The Eocene volcano-sedimentary units in the southern part of the Eastern Pontides (NE Turkey) are confined within a narrow zone of east–west trending, semi-isolated basins in Bayburt, Gümüşhane, Şiran and Alucra areas.
Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).Various methods have been devised to determine this initial or common Pb, but all involve making unprovable assumptions.Zircon does incorporate initial Pb when it crystallizes. It cannot be proven that the Pb in apparently cogenetic U- or Th-free minerals is only initial Pb, and that it is identical to the initial Pb in the mineral being dated.The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. The longest-lived radioisotopes are The relative abundances of the four stable isotopes are approximately 1.5%, 24%, 22%, and 52.5%, combining to give a standard atomic weight (abundance-weighted average of the stable isotopes) of 207.2(1).Lead is the element with the heaviest stable isotope, Th.Sr, Nd and Pb isotopic systematics imply that the volcanic rocks are derived from a subduction-modified subcontinental lithospheric mantle.