Radioactive dating rates

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Figure 1 shows a typical discoloration halo around a radioactive mineral inclusion in the mineral pyroxene.

The zone of damage is roughly spherical around a central mineral inclusion or radioactive source.

Granite is a well-known type of plutonic igneous rock, but there are many others as well.

Geologists distinguish these types of rock based on their chemical and mineralogical composition.

These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles.

In numerous papers published in scientific journals in the 1970s and 1980s, Gentry built the case that the different alpha decay energies of various naturally occurring radioactive isotopes resulted in distinctly different halo diameters.

The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity.

Through the cycles of burial, folding, faulting, and subduction of crustal plates, rocks get pushed and dragged down to depths where - under heat and pressure - changes take place.Plutonic rocks on the other hand cool very slowly, on the order of a million years or more for some deeply buried and insulated magmas.The mineral grains in these rocks can grow very large and are readily distinguished in hand samples.This was never fully successful as the size/intensity of an observed damage halo was also a function of the abundance of radionuclides present in the inclusion, and the crystalline structure of the host mineral. First is his contention that the granitic rocks from which samples reportedly came constitute the "primordial" crust of the Earth.Within these rocks are biotite (an iron-bearing form of mica) and fluorite crystals which bear a relatively uncommon class of tiny, concentric discoloration "haloes" (figure 2).

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