Radiometric dating of earth
To do this they have selected a certain meteorite, which contained various types of lead (including lead 204, 206, 207 and 208) but no uranium, and they have assumed that this ratio is equivalent to the earth's original lead ratio.
They did this because it is almost certain that these lead isotopes were all present in large quantities when the earth was created.
They consist of measuring the amount of radioactive (mother) element and comparing it to the amount of stable (daughter) element. Uranium is radioactive, which means it is in the process of changing from an unstable element into a stable one. And after 9 billion years there would be 75% lead and 25% uranium, and so on. (an) episode of drastically accelerated decay has ... When the crystal is looked at under a microscope, these discolourations appear as dark ringshence the name "pleochroic halo".
Few people realize it but all radiometric dating methods require making at least three assumptions. Now the magnitude of the radius of a pleochroic halo in a particular mineral depends on the amount of energy that the alpha particle has ... depends on the half-life of the particular decay responsible for this alpha particle emission.
The third assumption is that the sample has remained in a closed system.
This is necessary due to outside influences such as heat and groundwater that can seriously alter the original material.
Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the "begats" since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years.
Kelvin defended this calculation throughout his life, even disputing Darwin's explanations of evolution as impossible in that time period.
By applying the technique to his oldest rock, Holmes proposed that the Earth was at least 1.6 billion years old.
The second assumption is much more speculative since there is no way to verify whether or not some (or most) of the daughter element was already present when the rock solidified. However, in some cases, a few scientists are telling us that they have solved this problem.
For example, with the uranium/lead method scientists have attempted to estimate what the original ratio (of uranium-238 to lead-206) was when the Earth formed.
This week, Cherry Lewis of the University of Bristol presented a talk about the history of dating the Earth as part of the BA Festival of Science in York, England.
Before so-called radiometric dating, Earth's age was anybody's guess.
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In other words, the magnitude of the radius of a pleochroic halo in a particular crystal depends on the half-life of the decay responsible for the alpha particle emission. the radii of pleochroic haloes corresponding to a definite decay in a particular mineral are ...