|- by Allan Fraser|
establishment of geological age has always fascinated me, mostly the very
precise way in which the element uranium decays to lead and how science
has used this fact to determine geological age. The element Lead has the
best memory of all the elements in the periodic table. This is an unusual
trait of lead that makes it the most precise timepiece on earth. All that
is required to date the time of formation of a mineral is to quantify its
lead isotopic content.
The aim of Geology is to obtain an understanding of the structure and history of the earth and the processes, which have given our planet the form it is today. The early geologist obtained a perspective of the history and age of the earth by the fossils of animals and plants found in sedimentary rocks. A relative scale of age was formulated on this basis. However, there was no adequate way of deciding the absolute ages to be attached to this relative time scale.
Parents and Daughters
It was the invention of the Mass Spectrometer by English Physicist F.W.Aston in 1919 and a redesign by Alfred Neir in the 1930's that lead to the discovery that lead has four isotopes and uranium two. Naturally occurring uranium is a mixture of isotopes of mass 235 and 238. Similarly, naturally occurring lead is a mixture of all four of its isotopes, lead-204, 206, 207, and 208. Both uranium isotopes decay at different rates. Uranium-235 decays via a complicated chain of transformations into other radioactive isotopes, until finally reaching stable lead-207. Uranium-238 decays twenty times slower than uranium-235 and follows a similar decay scheme to uranium-235, ultimately arriving at lead-206. Lead-208 forms from the decay of the element, thorium. Lead-204 is not derived from the decay of Uranium or any other radioactive source. Therefore, a method exists for determining the absolute ages of minerals by means of natural radioactivity. Lead is being produced from Uranium and Thorium by radioactive decay at a characteristic rate, which is followed by a well-defined law. Therefore the amount of the Lead daughter product relative to the amounts of uranium and thorium parents provides a measure of the time available for the decay of the parent element since it's time of incorporation in the rock or one of the constituent minerals.
Common Lead and Age Determination
Today, sophisticated analytical techniques such as Mass Spectrometry provides accurate measurements of all of these isotopes, with the result that three independent radiometric ages (the number of years since the mineral crystallised) could be obtained for a particular mineral, one from the ratio of lead-206/uranium-238, another from lead-207/uranium-235 and another from lead-208/thorium-232. The lead-207/lead-206 also gives an age but is not independent of the others. Most uranium and thorium minerals incorporate ordinary lead into themselves at the time of their formation, and this complicates the determination of the lead produced by radioactive decay. However, the lead-204 isotope can be used to estimate the amount of ordinary lead contaminating the mineral, since this isotope is not produced by the radioactive decay of uranium or thorium. "Common" lead contains the isotopes lead-204, lead-206, lead-207 and lead-208 and the ratios are regionally variable. If a uranium or thorium mineral contains common lead it is necessary to analyse isotopically, lead from a uranium-free mineral such as galena (lead sulphide) which is associated with the radioactive mineral, and the proportion of lead-204 isotope to correct for the common lead which was incorporated into the mineral during crystallisation.
Disagreement in the Numbers
In favourable circumstances four ages can be obtained for a single uranium and thorium mineral, but, unfortunately, these frequently disagree amongst themselves. Such "discordant" ages are not due to analytical error but due to complicated physiochemical processes that have acted on the mineral in the course of geologic time that change the parent and daughter isotopes. Only the rare mineral uraninite tends to give concordant ages, that is, agreement between the four ratios; when this is the case it must be the true age of crystallisation.
Zircons are forever
The minerals whose ages have been determined by lead/uranium and lead/thorium ratios fall into two groups, one made up of a few minerals with high percentages of uranium and thorium, the other made up of zirconium and rare earth minerals that will substitute uranium or thorium in a particular structural position in the crystal lattice of the mineral. In the second group consisting of the minerals zircon (zirconium silicate) and monazite (a rare-earth phosphate), the radioactive elements have been partially replaced by elements of similar chemical bonding characteristics. For example, in monazite some of the zirconium positions in the crystal lattice have been replaced by uranium. In monazite, the element thorium is abundant. Minerals of high uranium content commonly weather easily. Zircon, however, is extremely resistant to weathering and monazite moderately so. The uranium atoms in zircon and monazite appear to be well protected as zircons persist through many environmental changes, including weathering, transportation, and deposition in a sedimentary bed.
Nowadays Mass Spectrometry methods be used with laser technology and this has made it possible to analyse very small samples of zircon ranging down to single grains and achieve very high accuracy and precision. An improvement in the precision allows the geologist to place a greater certainty on the age measurement of an individual zircon or a population of zircons from a particular deposit.