10B - Argon-Argon Dating

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Different minerals have different closure temperatures; is ~300°C, is about 400°C and has a closure temperature of ~550°C. The radioactive potassium-40 decays by two modes, by to 40Ca and by to 40Ar. The amount of 39Ar is proportional to the amount of 39K in the sample, and the ratio of 40K to 39K is constant in nature.

How can such an absurdity be possible? Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs. In broad scale, however, the Stannern pattern is relatively flat and confined to the range 3.

10B - Argon-Argon Dating - This yields a steady plateau age. The quantity of 40Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere.

The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39Ar from 39K. The sample is then degassed in a high-vacuum via a laser or resistance furnace. Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time. The J factor relates to the of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 40K to 40Ar than a less dense one. The method most commonly used to date the primary standard is the. An alternative method of calibrating the used standard is astronomical tuning also known as , which arrives at a slightly different age. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Different minerals have different closure temperatures; is ~300°C, is about 400°C and has a closure temperature of ~550°C. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock. Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. This technique allows the errors involved in K-Ar dating to be checked. Argon—argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified. One problem with argon-argon dating has been a slight discrepancy with other methods of dating. Recent work by Kuiper see also Kerr reports that a correction of 0. Thus the when the dinosaurs died out - previously dated at 65. Similarly, the is now dated at 252.

Argon Argon dating
The least stable is 30Ar with a half-life shorter than 20 nanoseconds. This technique allows the errors involved in K-Ar dating to be checked. The transition to the region of lower closure temperatures gives the timing of Miocene extension, unroofing, and cooling. Oligocene and Miocene extension along low-angle normal faults, associated with metamorphic core complex emplacement, produced the final thermal signal. Because 39Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clean indicator of the potassium content.