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When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings.Simply counting the number of rings will give one a fairly good idea of the age of the tree.This can be seen in the following figure from White (2005).The crust has an increasing ratio of Sr-87/Sr-86 over time compared to the bulk earth composition, while the mantle has a decreasing ratio.
"Near the end you imply that low petrologic type chondrites are the most easily dated.
For all practical purposes, the original isotope is considered extinct after 6 half-life intervals. A small portion of a meteorite is vaporized in the device forming ions.
These ions are accelerated in an electric field through collimating slits and subject to a magnetic field which causes the ions to follow a curved path. By adjustment of the strength of the magnetic field and suitable placement of an ion collector, the different isotopes can be measured with precision.
Actually, meteorites that formed by melting, e.g., the various types of achondrites, usually give more precise ages.
Certain natural phenomena or processes, such as Earth's year-long solar orbit, and the resulting annual climatic variations that govern the growth of tree rings, can be used as "natural clocks." If we can find and date a rock that we know has been around since the Earth formed, we can measure the age of the Earth.26Al and some other nuclides not mentioned are also used in this way.