Radiometric Dating – Closure Temperature

In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature at which a system has cooled so that there is no longer any exchange of parent or daughter isotopes with the external environment. This temperature varies broadly between different minerals and also differs depending on the parent and daughter atoms being considered. It is specific to a particular material and isotopic system.

These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. 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 isotopes. 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. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to blocking temperature.

These temperatures can also be determined in the field by comparing them to the dates of other minerals with well-known closure temperatures.

Closure temperatures are used in geochronology and thermochronology to date events and determine rates of processes in the geologic past.

via Closure temperature

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4 Responses to Radiometric Dating – Closure Temperature

  1. alfy says:

    I too read this wikipedia article and was somewhat bemused by the diagram. As you know, I am just a simple country boy and believe in the conservation of mass and energy, particularly with increasing years.

    Let me explain. In the diagram we begin with a Pb atom, AW=212, AN = 82 which transforms to a BI atom, AW=212, AN = 83. This means the original atom has gained a proton from some unspecified source, (AN gone up from 82 to 83) and lost a neutron (AW stays constant).
    I stopped at this point. The whole thing clearly needs to be better explained.

    I appeal to George or Steve. Can you understand the diagram? Is there something I have missed?

  2. Phil Krause says:

    It’s decaying by beta decay. You can have beta plus or beta minus decay where a neutron decays into a proton or visa versa. In the process an electron and antineutrino is emitted. Have a look on wiki.

  3. Steve B says:

    The first stage of decay is that Pb212 loses a negative Beta particle, which is in fact an electron. How this is achieved is that a neutron is converted into a proton and the emitted electron/beta particle. So, Pb212 becomes Bi213 its atomic number increasing by one due to the creation of the additional proton. The Atomic Weight of the Bi isotope is the same as the Pb isotope however.

  4. Deskarati says:

    Pictured changed as suggested by Steve B.

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