Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. is then interpreted to be the date that the system became closed, and the younger date, t*, the age of an event (such as metamorphism) that was responsible for Pb leakage.
Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay.
Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.
If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.
This is also actually kind of trivial and easily determined in the lab. Let’s see what the Missing Universe Museum thinks are the assumptions of radioactive dating methods. I guess we have to start at the top and work our way down… During an organisms life, it takes in CO have the common 6 protons and 6 neutrons. However, due to some interesting nuclear chemistry (which I’ll go into if requested), there’s another version of carbon (called an isotope) that has 6 protons and 8 neutrons. Note that if the number of protons change, then the atom is no longer carbon. Amazingly (and unlike what is claimed by the creationists), scientists have known about a variety of methods that create carbon-14 and how those methods have varied over time. Well, we take a carbon sample from a material of a known age and date that. Basically, the calibration curves are off by no more than 16 years over the historical range (6,000 years or so) and no more than 163 years over the last 20,000 years.The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. If this happens, then the date obtained will be older than the date at which the magma erupted.For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.An age could be estimated by measuring the amount of carbon-14 present in the sample and comparing this against an internationally used reference standard.The impact of the radiocarbon dating technique on modern man has made it one of the most significant discoveries of the 20th century.No other scientific method has managed to revolutionize man’s understanding not only of his present but also of events that already happened thousands of years ago.Archaeology and other human sciences use radiocarbon dating to prove or disprove theories.After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*. The first is that atoms have always decayed at the same rate.And this isn’t really an assumption as the decay rates have been tested in the laboratory for a hundred years or so, we have an example of a natural nuclear reactor where we can measure the various products and determine the decay rates (and the fine structure constant), and we can observe the past by looking deep into the past of the universe. The sigh isn’t for the effort of writing, it’s for the effort of finding all the references.