Potassium argon k ar dating
These each have 19 protons and 21 neutrons in their nucleus.
If one of these protons is hit by a beta particle, it can be converted into a neutron.
method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass).
One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40).
This is approximately 2,500 times as much Ar as is found in natural muscovite.
Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. envisage noble gases from the mantle (and the atmosphere) migrating and circulating through the crust, so there should be evidence of excess in crustal rocks and their constituent minerals could well be the norm rather than the exception.
However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?That is, a fresh mineral grain has its K-Ar "clock" set at zero.The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.For example, in the Middle Proterozoic Musgrave Block (northern South Australia), a wide scatter of K-Ar mineral "ages" was found, ranging from 343Ma to 4493Ma due to inherited (excess) , permitting inclusion of the gas in the crystallizing minerals.Likewise, when Ar "dating" was attempted on Proterozoic granulite-facies rocks in the Fraser Range (western Australia) and Strangways Range (central Australia), it was found that garnet, sapphirine, and quartz contained excess was probably incorporated at the time of the formation of the minerals, and calculations suggested a partial pressure of ~0.1 atm Ar in the Proterozoic lower crust of Australia, which extends over half the continent.