Metamorphic rocks are not always easy to date using radio-isotopes.Results obtained usually signify the "date" of the metamorphism, but they may also yield the "age" of the original volcanic (or sedimentary) rock.
It is also claimed that the original basalt lavas were erupted between 17 Ma, based on U-Pb dating of "original" zircon grains in metamorphosed felsic (granitic) volcanic layers within the Brahma and Rama Schists.
Twenty-seven Brahma amphibolite samples were collected from various Inner Gorge outcrops as part of the RATE (Radioisotopes and the Age of The Earth) project.
Both laboratories use standard, best-practice procedures on state-of-the-art equipment.
The model K-Ar ages for each of the samples ranged from 405.1±10 Ma to 2574.2±73 Ma.
The technique is invaluable for disclosing the environment of early man's sites and can even, over and series of samples, reveal man's influence on his environment by, for example, .
The sediments most frequently investigated are peat and lake deposits, but the more acid soils, such as podsols, are also analyzed.
This method is fairly quick as only a fraction of the grains present in a sample need be counted.
Its main disadvantage is that percentages can never indicate actual numbers of grains falling to earth, which is solved by Absolute Pollen Counting.
Most people believe that when the different radioisotope dating methods are used on the same rock unit they all yield the same age.