I recently wrote a piece on Carbon dating in part response to an ignorant creationist who maintained that it couldn’t tell the age of the universe1. It isn’t supposed to do so, but he was too wilfully ignorant to realise that. This article continues on that theme and looks at another radioisotopic dating method.
One of the most widely used dating methods in modern geology is the Uranium-Lead (U-Pb) method, and this is most often used on the very refractory and resilient mineral Zircon2. Zircon is a mineral whose nominal chemical formula is ZrSiO4, and in English that is Zirconium Silicate, a compound of zirconium, silicon and oxygen. The crystal structure of Zircon is such that atoms of other elements can often replace the Zirconium in the crystal. One of these elements is Uranium and that has been an enormous boon to geology. Another bonus is that Zircon is very resistant to corrosion and weathering3.
Zircon is formed in silicon-rich magma deep in the earth’s crust and it is these magmas which commonly form what are termed felsic igneous rocks such as granite and rhyolite3. When these magmas erupt, they often do so explosively, and they create large ‘ash’ clouds that spread across the countryside and deposit a fine ash everywhere that covers the land like a blanket. These ash beds therefore represent an instant in time, geologically speaking, and they contain minute crystals of zircon which can be dated.
Uranium has an atomic number of 92, which means it has 92 protons in its nucleus. In nature, Uranium is found as three isotopes 238U (~99.27%), 235U (~0.72%) and 234U (~0.01%). The superscript numbers refer to the number of nucleons (protons and neutrons) in the Uranium nucleus. As a consequence, 238U has 146 neutrons (238-92=146) in its nucleus, 235U has 143 neutrons, and 234U has 142 neutrons. 238U has a half-life (over which half of the atoms will decay) of 4.468 billion years and 235U has a half-life of 703.8 million years, so both are useful for radioisotopic dating. 234U has a very short half-life of 245,500 years and is not used for dating, partly because of this short half-life, but also because of its scarcity4. It keeps being found in nature because it is generated by the decay of 238U. The long half-life of 238U allows it to be used to date rocks which are as old as the planet itself, i.e. up to 4,600 million years old.
The decay chain is the series of steps through which a radioactive isotope goes as it decays, and that of 238U is very complex, with 14 steps, but the final result of its decay chain is 206Lead, or 206Pb5,6. The decay chain of 235U is slightly less complex, with only 11 steps, and the result of this decay chain is 207Lead, or 207Pb7.
There are two techniques used in U-Pb dating. The first is the coupled use of the 238U and 235U decay chains, producing what is called a ‘concordia diagram’ which graphs the ratio of 206Pb/238U (y-axis) in the sample against the ratio of 207Pb/235U (x-axis). This is a way of coping with a common problem of radiogenic lead loss. The second is what is called the U-Pb isochron dating method in which the ratio of 206Pb/238U alone is used to provide a date2.
As noted above, one of the most difficult problems to overcome with U-Pb dating of zircons was the loss of the radiogenic Lead (i.e. the lead generated from the decay of Uranium; 206Pb) from the crystal lattice. This is usually fairly limited and tended to be restricted to the outside of the zircon crystals, so some acid etching and physical abrasion techniques were developed to minimise this, but they were never entirely successful. A new technique of high temperature annealing of the crystal followed by high pressure and temperature acid etching (chemical abrasion) seems to have sorted out this problem almost entirely. Prior to this new technique being used, error margins used to be around 1-2%, so a rock dated as 250 million years in age would be at best 250 +/- 2.5 million years (i.e. 247.5 to 252.5 million years in age). The new technique has dramatically decreased the error, with such a sample described above giving a date of 250 +/- 0.1 million years (i.e. 249.9 to 250.1 million years in age). This dramatically improved precision has completely changed the utility of radioisotopic dating. It used to possible to date significant thicknesses of rock, such that a rock unit, say 400 metres thick, could be ‘dated’ to provide a rough estimate of its numerical age. Now it is possible to date numerous individual beds throughout such a thick rock unit. A recent paper demonstrates the application of this more precise technique to date individual fossil zones8.
Science is always advancing, correcting itself and finding ways to overcome problems or hindrances, something that creationism cannot do. It only pretends to do so.