U-Th-Pb Geochronology by Ion Microprobe

There would be few geological studies in which, at some stage, there did not arise a question of timing. The answer is often to be found through direct observation; the principles of superposition and crosscutting relationships apply in determining the order of events on all scales from the microscopic to the macroscopic, from crystallization history to continental assembly. By augmenting those principles with the means to establish sequence and correlation provided by palaeontology, the geologist has the capability, through observation and logical reasoning alone, to determine the relative ages of a great range of geological processes. However, while these techniques make it possible to place geological events in time order, they do not provide an absolute measure of time itself.The measurement of absolute time in geology— geochronology—requires a quantifiable physical process that takes place continuously at a known rate from the time of the event to be dated to the present day. Some cyclic processes, such as the passage of the seasons, leave their imprint in parts of the geological record and can provide detailed, accurate measurements of elapsed time intervals, but they do not permit the measurement of absolute time (age) unless the record is unbroken to the present day or the age of one of the cycles is known by some independent means. The number of annual growth bands in a fossil coral, for example, tells how long that coral once lived, but not when. To measure absolute geologic time, one needs a process that is continuous and unidirectional. The most widely utilized of such processes is natural radioactivity.The concept behind radioisotope geochronology is quite simple. Some of the elements in rocks and minerals have isotopes (atoms of the same atomic number but different mass numbers) that are naturally radioactive-the nuclei of those isotopes are unstable, and liable to break down spontaneously (decay) to an isotope of a different element. If the newly-formed isotope is also unstable, the process continues until a stable nucleus forms.Radioactive decay occurs at rates characteristic of each element and isotope. As far as is known, those rates are independent of any chemical or physical parameters (e.g., pressure, temperature, chemical state etc. ) . The probability that a given nucleus of a given isotope will decay in any given time period is a constant, so the number of decays occurring per unit time is proportional to the number of atoms of that