Argon–argon dating

Scientists want to know how old rocks are. They use a special way to find out. They look for a tiny gas inside the rock. This gas is called argon. The more gas there is, the older the rock. Scientists use heat to let the gas out. They use a hot tool to melt small bits. This helps them see the gas clearly. This way is very good and fast. It helps us learn about the past.

Scientists use a way called argon-argon dating to find rock ages. This method is very accurate. It is better than the old potassium-argon way. The old way needed two separate tests. The new way only needs one tiny piece of rock.

First, experts pick small mineral grains from a rock. They put these grains in a nuclear reactor. This step uses neutrons to change potassium into argon. We call this step irradiation.

Next, they heat the sample with a laser or a furnace. The heat melts the mineral. This lets out trapped gases. One gas is argon. The amount of argon tells us how old the rock is. More argon means the rock is older.

To get the right age, scientists use a standard. A standard is a sample with a known age. They test it at the same time as the new sample. This helps them check their work. This method helps us study how rocks change over time. It even helped find the right age for when dinosaurs died.

Scientists use a special way to find the age of rocks. It is called argon-argon dating. This method is very accurate. It was made to be better than an older way called potassium-argon dating. The old way was hard because it needed two separate tests. You had to split your rock sample into two parts. One part was for potassium and one was for argon. The new argon-argon method is much simpler. It only needs one single rock fragment or one tiny mineral grain.

How does this new way work? First, experts crush a rock sample. They hand-select tiny crystals or mineral grains. These grains go into a nuclear reactor for irradiation. This step uses neutrons to turn stable potassium into a new kind of argon. Next, the sample is heated with a laser or a furnace. This heat melts the mineral and releases trapped gases. One of these gases is argon. The amount of this argon tells us how old the rock is. More argon usually means the rock is older.

To get the right age, scientists need a standard. A standard is a sample with an age we already know. They test the standard at the same time as the unknown sample. This helps them calculate a number called the J-factor. This factor relates to how many neutrons hit the sample. Scientists also use the K/Ar technique to find the age of these standards. Some people even use astronomical tuning to check the ages.

This method is great for dating certain minerals. It works well for metamorphic and igneous minerals. Different minerals have different closure temperatures. This is the temperature where they start trapping gas. For example, biotite closes at 300 degrees Celsius. Muscovite closes at 400 degrees Celsius. Hornblende closes at 550 degrees Celsius. This helps scientists see the thermal history of a rock.

This tool helps us understand big moments in Earth's history. It even helped fix the date for the extinction of the dinosaurs. Before, people thought it happened 65 or 65.5 million years ago. Now, we know it was closer to 66.0 or 66.1 million years ago. This change happened after scientists found a small error. They found that a correction of 0.65% was needed. Now our history of the Earth is even more clear.

Argon–argon dating is a highly precise method of radiometric dating. Scientists use this technique to determine the age of various geological materials. It was specifically invented to improve upon an older technique called potassium–argon (K/Ar) dating. The older K/Ar method was difficult because it required splitting a single sample into two parts. One part was measured for potassium, and the other was measured for argon. In contrast, the argon–argon method requires only one single rock fragment or mineral grain. This allows for a single measurement of argon isotopes to provide the necessary data.

The mechanism of this method relies on a process called neutron irradiation. First, scientists crush a rock sample to find tiny mineral crystals. They hand-select these crystals for a process inside a nuclear reactor. Inside the reactor, neutrons hit the sample to trigger an (n-p) reaction. This reaction converts stable potassium-39 (39K) into radioactive argon-39 (39Ar). This step is vital because it allows scientists to measure argon isotopes directly. By using this method, they can calculate the ratio between different types of argon.

Once the crystals are irradiated, they undergo a process called degassing. Scientists place the sample in a high-vacuum mass spectrometer. They use either a laser or a resistance furnace to heat the sample. This intense heat causes the crystal structure of the mineral to degrade. As the mineral melts, it releases trapped gases that were held inside. These gases can include water, nitrogen, or carbon dioxide from the atmosphere. Most importantly, the heat releases radiogenic argon, which is argon produced by radioactive decay.

To calculate a specific age, scientists must use a process called relative dating. The argon–argon method cannot determine an absolute age on its own. Instead, it must be compared to a standard of a known age. This standard is irradiated at the same time as the unknown sample. This allows scientists to determine the J-factor, which is a parameter of the irradiation. The J-factor represents the fluence, or the density, of the neutron bombardment. A denser flow of neutrons converts more potassium-39 into argon-39.

This dating method is most useful for metamorphic and igneous minerals. It is particularly effective for studying the thermal history of rocks. Different minerals have different closure temperatures, which are temperatures where they begin to trap gas. For example, biotite has a closure temperature of approximately 300°C. Muscovite has a higher closure temperature of about 400°C. Hornblende has an even higher closure temperature of around 550°C. Because of these differences, a single granite might show several different ages. These ages reflect when each mineral cooled below its specific temperature.

While powerful, the method requires careful interpretation and specific assumptions. Scientists assume that a rock has retained all of its radiogenic 40Ar after cooling. They also assume the sample was properly collected during the analysis. The abundance of radiogenic argon increases as a sample gets older. However, this increase follows an exponential decay based on the half-life of potassium-40. The half-life of potassium-40 is approximately 1.248 billion years. This long duration makes the method useful for studying very old geological events.

Recent scientific work has even led to the recalibration of historical dates. Researchers like Kuiper et al. discovered that a small correction was necessary. They found that a correction of 0.65% was needed for certain calculations. This adjustment changed our understanding of the Cretaceous–Paleogene extinction event. Previously, scientists thought the dinosaurs died out 65.0 or 65.5 million years ago. Thanks to these refinements, the event is now dated to 66.0–66.1 Ma. This shows how constant refinement makes our geological history more accurate.