text: Iodine-129 decays with a half-life of 15.7 Ma into Xe, resulting in excess Xe in primitive meteorites relative to primordial Xe isotopic compositions. The property of I can be used in radiometric chronology. However, as detailed below, the age of Earth's formation cannot be deduced directly from I-Xe dating. The major problem is the Xe closure time, or the time when the early Earth system stopped gaining substantial new material from space. When the Earth became closed for the I-Xe system, Xe isotope evolution began to obey a simple radioactive decay law as shown below and became predictable.
  The principle of radiogenic chronology is, if at time t the quantity of a radioisotope is P while at some previous time this quantity was P, the interval between t and t is given by the law of radioactive decay as
  Here  is the decay constant of the radioisotope, which is the probability of decay per nucleus per unit time. The decay constant is related to the half life t, by t= ln(2)/