Dating Methods



Absolute dating

Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done oldest dating method, 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. Uranium comes in two common isotopes with atomic weights of and we'll call them U and U. Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't oldest dating method until they become lead Pb.

The oldest dating method cascades are different—U becomes Pb and U becomes Pb. What makes this fact useful is that they occur at different rates, as expressed in their half-lives the time it takes for half the atoms to decay. The U—Pb cascade has a half-life of million years and the U—Pb cascade is considerably slower, with a half-life of 4.

So when a mineral grain forms specifically, when it first cools below its trapping temperatureit effectively sets the uranium-lead "clock" to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. The favorite mineral among U-Pb daters is zircon ZrSiO 4for several good reasons.

First, its chemical structure likes uranium and hates lead. Uranium easily substitutes for zirconium while lead is strongly excluded. This means the clock is truly set at zero when zircon forms. Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocksnot even moderate metamorphism. Third, zircon is widespread in igneous rocks as a primary mineral.

This makes it especially valuable for dating these rocks, which have no fossils to indicate their age. Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density. Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite. However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating.

But even the best oldest dating method methods are imperfect. Dating a rock involves uranium-lead measurements on many zirconsthen assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram.

The straight line takes the zircons off the concordia. This is where data from many zircons is important. The disturbing event affects the zircons oldest dating method, stripping all the lead from some, only part of it from others and leaving some untouched. The results from these zircons therefore plot along that straight line, establishing what is called a discordia.

Now consider the discordia. If a million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the oldest dating method. This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life. The oldest zircon yet found dates from 4. With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's " Earliest Piece of the Earth " page, including the paper in Nature that announced the record-setting date.

Search the site GO. Updated February 28, Basics of Uranium-Lead Uranium comes in two common isotopes with atomic weights of and we'll call them U and U. Learn Something New Every Day Email Address Sign Up. There was an error. Please enter a valid email address. Follow Us Facebook Twitter Pinterest. Science, Tech, Math Humanities Arts, Music, Recreation Resources About Us Advertise Privacy Policy Careers Contact Terms of Use.


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