The details are best pursued in a dedicated text like McDougall and Harrison. This allows the 39 Ar population to be used as a proxy for the 40 K content of the sample to make possible the calculation of the age for the sample. This simplified conceptual treatment does not give a fair picture of the detailed design and execution of age determinations for a wide variety of types of geological samples.

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But it hopefully makes the point that Ar-Ar dating can take data from small samples based on mass spectrometry. It has contributed to the vast collection of age data for earth minerals, moon samples and meteorites. The Cretaceous-Tertiary boundary in the geological age scale was associated with an iridium-rich layer which suggested that the layer was caused by an impact with an extraterrestrial object. Because that time period, commonly referred to as the K-T boundary, was associated with the extinction of vast numbers of animals in the fossil record, much effort was devoted to dating it with potassium-argon and other methods of geochronology.

The time of 65 million years was associated with the K-T boundary from these studies. Other large impact craters such as the Manson crater in Iowa dated to 74 My were examined carefully as candidates for the cause of the extinction, but none were close to the critical time. Chicxulub was not so obvious as a candidate because much of the evidence for it was under the sea.

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More attention was directed to the Yucatan location after published work by Alan Hildebrand in demonstrated the chemical similarity of Chicxulub core samples with material found distributed in the K-T boundary layer. Carl Swisher organized a team to produce three independent measurements of the age of intact glass beads from the C-1 core drill site in the Chicxulub impact area.



The measurements were done by the argon-argon method. Even this extraordinary matching with the age of the K-T boundary was insufficient to convince many geologists.



The team proceeded to date spherules of glass found in Haiti to provide another bit of evidence. Many pieces of glass ejecta had been found on Haiti, which is over a thousand miles from the impact point currently. But geologists project a much smaller distance between the points at the time of the impact because of measured sea floor expansion.

The Haitian spherules were measured to have age to melting of A third piece of evidence came from age measurements of shocked zircon crystals which were found in the K-T layer as far away as Colorado and Saskatchewan. How Does the Reaction Work?

Potassium K is one of the most abundant elements in the Earth's crust 2. One out of every 10, Potassium atoms is radioactive Potassium K These each have 19 protons and 21 neutrons in their nucleus.

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If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon Ar , an inert gas. For every K atoms that decay, 11 become Ar How is the Atomic Clock Set? When rocks are heated to the melting point, any Ar contained in them is released into the atmosphere.

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Thus the Cretaceous—Paleogene extinction when the dinosaurs died out - previously dated at Similarly, the Permian-Triassic extinction is now dated at From Wikipedia, the free encyclopedia. Retrieved from " https: Views Read Edit View history. This page was last edited on 21 September , at By using this site, you agree to the Terms of Use and Privacy Policy. The Wikibook Historical Geology has a page on the topic of: