To determine the age of a rock is a difficult process. If a rock contains certain fossils, approximate age determination can be quick. For example, if you find T. Rex bones in a rock then the rock is from the Late Cretaceous. However, that only constrains the age to between about 65 and 97 Ma. To get an exact age for a rock, isotopic dating must be used.

What is an isotope? Elements are defined by the number of protons and consequently electrons that they contain. The number of neutrons and consequently the atomic weight can vary. Therefore, a single element may have several isotopes with several different atomic weights. Uranium, for example, can have atomic weights of 235, 236, and 238 among others. About 25% of all isotopes are unstable. These parent isotopes undergo spontaneous radioactive decay to produce daughter products that are different elements with different atomic weights.

To get an age Radioactive decay proceeds at a constant rate. Scientists can measure that rate of decay in a laboratory. A rock is analyzed for the amount of parent element and the amount of daughter element by using a mass spectrometer. Mass spectrometers are sophisticated equipment that accurately and precisely measure the mass of elements. By comparing the amount of parent versus daughter and knowing the rate, an age for the rock can be determined. Rates of decay for isotopes are measured in half-lives. This number is the amount of time required for half of the parent to decay to daughter. Decay proceeds at an exponential rate.

What does the age mean?When the typical minerals we see in rocks are forming, temperatures are commonly high. At high temperatures, isotopes can freely exchange among minerals and fluids. At this point, they are not recording ages. When the temperatures cool below a certain "closure temperature" the isotopes are locked into the mineral and the dating clock begins to tick. Different minerals and different isotopes have different closure temperatures. If a mineral with it's clock running is reheated above it's closure temperature, the clock will be completely reset. There will be no record of the earlier history. They will only record the late history. New minerals grown by metamorphic chemical reactions below their closure temperature will record age of growth.

Ar/Ar DatingOne isotopic system for dating minerals uses Ar/Ar. The parent, K 40 (atomic weight) decays to the daughter, Ar 39, a gas. Above the closure temperature of the mineral, Ar flows in and out of the mineral at random. Below the closure temperature, the mineral structure closes up tight and seals it in. For hornblende, this closure temperature is about 500o C. For biotite, it is about 300oC. To analyze the amount of isotopes, first the sample must be irradiated in a nuclear reactor to convert the K to Ar. Then the sample is stepwise heated or probed with a laser in a gas mass spectrometer. A plateau age results if the samples are good.

Interpretation of Hogencamp Mine results Using equipment at Massachusetts Institute of Technology, Dr. Michael Krol determined ages of samples of biotite and hornblende from the Hogencamp mine. The results of Ar/Ar mineral dating from these rocks indicate that the age of the magnetite deposits is about 923 Ma (million years old). The last pulse in the event was the emplacement of the pegmatite and granite dikes into the zone at about the same time. In some areas, activity persisted until 913 Ma. The 834 and 796 Ma ages for the biotite indicate that it took the area 80 to 127 million years to cool 200oC. This is very slow indicating a very quiet period tectonically.