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In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques.

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.

At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes.

This temperature is what is known as blocking temperature and represents the temperature below which the mineral is a closed system to measurable diffusion of isotopes.

The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.

An error margin of 2–5% has been achieved on younger Mesozoic rocks.

Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the blocking temperature.

These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.

One key assumption is that the initial quantity of the parent element can be determined.

With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample.

As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy.

This temperature is what is known as blocking temperature and represents the temperature below which the mineral is a closed system to isotopes.




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