Potassium isotope dating
True granites are relative latecomers on the geologic scene as they required a number of recycles of crustal material to differentiate and concentrate potassium.
In an earlier edition of , Lorence Collins (March/April, 1999) provided a thorough overview of the origin and nature of granitic rocks.
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.
The crystals develop an interlocking texture with some of the trace minerals becoming completely surrounded by later forming crystals.
Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye.
From these observations early attempts were made to use the dimensions of haloes as an age dating technique.
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.
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).
Granites, for example, have more than 10% quartz and abundant potassium feldspar.