# Isochron dating problems for short The age of the earth is normally estimated by radiometric dating - which gives And OE Christians (theistic evolutionists) see no problem with this dating whilst . This claim is countered by the fact that the origin of short-period comets is still . Hands-on introduction to using the isochron method to determine radiometric ages. Rb-Sr isochron problem (Microsoft Word kB Feb8 12). The isochron technique of radiometric dating is often presented as overcoming problems with earlier methods. However, it is easy to imagine scenarios that.

K-Ar Dating 40K is the radioactive isotope of K, and makes up 0.

• The Age of the Earth
• Isochron dating

Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40K is important in dating rocks. But this scheme is not used because 40Ca can be present as both radiogenic and non-radiogenic Ca.

Note that this is not always true. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped. 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. Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years. The dating equation used for K-Ar is: Some of the problems associated with K-Ar dating are Excess argon. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals should not contain any excess Ar because Ar should not enter the crystal structure of a mineral when it crystallizes.

Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende. If none of these are present, then the only alternative is to date whole rocks.

### Isochron Dating

Some 40Ar could be absorbed onto the sample surface. This can be corrected for. Most minerals will lose Ar on heating above oC - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock.

If only partial loss of Ar occurs then the age determined will be in between the age of crystallization and the age of metamorphism.

### Problems with Isochrons

If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event. The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred. Suppose X is a parent element that decays in to Y and Z is another isotope of Y not produced by radioactive decay. Let x, y, and z refer to their concentrations. Since Y and Z are isotopes, we would assume they have similar chemical properties. Let's assume that initially, the ratio of y and z is constant, and then X begins decaying to Y. If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them. However, we can imagine situations in which such a linear relationship could be produced without indicating a true age.

Suppose A is very old or appears very old and B is very young. Suppose A and B become thoroughly mixed. Their perceived radiometric age would then be between that of A and B.

## Isochron Dating

Now, suppose a mixture of Y and Z penetrates this mixture of A and B, in some places more than in others, but with a constant ratio of Y and Z. This will then yield a beautiful isochron, but the age given will be meaningless. This can also happen if water removes a constant fraction of X but no Y from A, making A appear older, and then the mixture of Y and Z enters.