Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Evo on 04/10/2015 12:40:34

Title: Radiometric dating - Radionuclides in minerals, where do they come from?
Post by: Evo on 04/10/2015 12:40:34

Hello,

I am currently studying geology and recently we came across techniques to date minerals/rocks. I understand the technique and formulas used to calculate an age.

However in minerals such as garnet X3Y2(Si O4)3, how is it that we have the presence of radioactive nuclides such as Ur or SM present in its crystal lattice in the first place? None of the formulas for any end members of garnet seem to indicate the presence of the elements.

Kind regards

Evo

Title: Re: Radiometric dating - Radionuclides in minerals, where do they come from?
Post by: chiralSPO on 04/10/2015 13:30:37

The radioisotopes are only very minor impurities, often at the ppm level or lower. The crystal is either doped with them, say X₃Y_2–nU_n(SiO₄)₃, where n is 0.00000001 or so, or there are inclusions of other minerals that contain the isotopes, or there are defect vacancies in the original that have been filled with the isotopes (essentially the same as the first example)

Title: Re: Radiometric dating - Radionuclides in minerals, where do they come from?
Post by: evan_au on 05/10/2015 11:39:33

During its fusion-powered lifetime, a large star can generate elements (http://en.wikipedia.org/wiki/Stellar_nucleosynthesis#Key_reactions) up to and including iron.

Many radioactive isotopes are thought to be created and scattered into the interstellar medium by supernova explosions; the presence of some of these (like cobalt 56) can be inferred from the light curve (http://en.wikipedia.org/wiki/Supernova#Light_curves) of the supernova. It is thought that a supernova explosion may have triggered the collapse of the dust cloud that formed the Solar System (and probably several other stellar systems, from the same dust cloud).

These radioactive elements will be incorporated into dust grains, but will then suffer nuclear decay, leaving the daughter nuclei in a place in the crystal structure which is better suited to the electron structure and condensation temperature of the parent atom. Looking at the half-life of the parent isotope, you can place bounds on the time between the supernova and the formation of the dust grain.

It is thought that elements up to around gold can be formed in a supernova explosion. However, while elements beyond gold are relatively rare, they are thought to be too common to be but up from lighter elements in a supernova explosion. Some theories for their origin includes the collision of neutron stars, spraying debris (including heavy nuclei) into space.

Presumably, by studying the half-life of these heavier isotopes, it might be possible to constrain the time since the most recent neutron star merger prior to the formation of the Solar System?