Here's a breakdown:

* Isochron: An isochron is a line on a graph that represents points of equal age. This is often used in geochronology, the study of determining the age of rocks and minerals. Isochrons are created by plotting the ratios of different isotopes (versions of the same element with different numbers of neutrons) against each other.

* Magnetic Field: A magnetic field is a region around a magnet or an electric current where magnetic forces can be felt. Changes in the magnetic field occur due to various factors like movement of charged particles, changing currents, or even magnetic storms.

In summary: Isochrons are about age determination, while magnetic fields are about magnetic forces. They are distinct concepts.