About Uranium-Lead Dating
- Type of rocks
- Toprolls and geology
- Crustal floating plate
- Weather / climate
- Β. Graduated from the University of New Hampshire University
Andrew Alden is a geologist based in Auckland, California. Active as a research guide for American Geological Survey.
Updated on February 10, 2019
In the isotoped aging measurement method currently used. the uranium-lead This is the oldest, most reliable method if you do it carefully. It is different from other methods. uranium-lead When nature is invaded, it incorporates a natural cross check that shows the grounds.
Basics of Uranium-Lead
The uranium has two isiceable bodies (235U and 238U), both atomic weight of 235 and 238, both of which are unstable radioactive materials, and nuclear particles are released in a chain without stopping until lead (PB).。The 235U is 207PB, the 238U is 206PB, and the two cascades are different. It is convenient to generate at a different speed, as expressed in a hal f-life (half of the atoms collapsed). The half-life of 235U-207PB cascade is 740 million years, and the half-life of 238U-206PB cascade is 4470 million years.
Therefore, it can be effectively regulated when metal grains are formed (specifically when cooling below the trap temperature). the uranium-lead Make “clock” zero. The leading of the leading of the uranium is trapped in the crystal, accumulating the concentration over time. If there is no disturbing particles to release this radioactive lead, the age measurement can be a simple concept. In rocks 740 million years ago, 235U reached a hal f-life, and the 235U and 207PB atoms are the same (PB/U ratio is 1). In the doubl e-length rock, one 235U atoms remain for 3 207 PB atoms (PB/U = 3). In the case of 238U, the PB/U ratio increases slowly with age, but the way of thinking is the same. When the rocks of all ages are collected and the PB/U ratio of the two isotoped pairs is plotted to the graph, the point is a beautiful line called Concordia (see the example of the right column).
Zircon in Uranium-Lead Dating
Popular minerals in U-PB data are Jirkon (ZRSIO)4) There are several reasons.
First, the chemical structure prefers uranium and dislikes lead. The uranium is easily replaced with a zircon, but leads are strongly eliminated. In other words, when the jirkon is formed, the clock is actually zero.
Second, the jirkon has a high catch temperature of 900 ° C, and has a characteristic that clocks are less likely to be disturbed due to geological events such as erosion, density, and gentle modification of sedimentary rocks.
Third, jirkon is widely existed as a main mining in fire rock. Therefore, there is a particular value in the age measurement of these rocks without fossils indicating the age.
Fourth, the zircon is originally hard and has a high density, so it can be easily separated from the crushed stone sample.
Other minerals may be used for uranium-lead It contains two zircon minerals, Monazite, Titanite, and Vaderite and Zirconolite. However, gircons are overwhelmingly popular, so geologists are often simply called “jirkon age measurements.”
However, no matter how good geological methods are, are incomplete. On a date with rock uranium-lead Measure several jirkons and evaluate the quality of the data. Obviously disturbed zircon can be ignored, but in some cases it is difficult to judge. In such a case, the Concoldia Chart is a valuable tool.
Concordia and Discordia
Considering Concordia: As the age of Jirkon goes outward along the curve, imagine that something has been disturbed to escape by some geological event. Then, the jirkon returns to the Zero on the Concordia diagram in a straight line. The straight line takes jirkon from Concordia.
Therefore, multiple jirkon data is important. This disturbance has an uneven impact on the zircon, some of the lead is peeled off, and some are peeled off, and some are left as it is. Therefore, the results of these jirkons draw such a straight line and define what is called discordia.
Here, let’s consider Discoardia. If the rocks of 150 million years ago were disturbed and the fault was formed, and after that, the entire Discordia line would move along the Concordia curve and always the fault age. It will be shown. In other words, Jirkon data tells you not only when the rocks have been created, but also when the rocks have occurred while the rocks were alive.
The oldest jirkon currently found is 4. 4 billion years ago. From such a background the uranium-lead You may be able to understand the content of the research introduced on the Wisconsin University page of Wisconsin, such as the method and date setting record in 2001, such as the 2001 papers.