September 2, 2017 | Author: andrew | Category: Radioactive Decay, Atoms, Isotope, Nuclear Fission, Radiocarbon Dating

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Geoscience notes...

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Geol 02C Historical Geology J Bret Bennington

By measuring the ratio of parent isotope to daughter isotope in certain rocks, geochronologists are able to determine an estimate of the age of the rock in years. Generally, only igneous and metamorphic rocks can be radiometrically dated because sedimentary usually do not contain the radioisotopes needed for dating. Although igneous rocks occur within the stratigraphic record at relatively few times and places, there enough igneous layers such as ash beds and lava flows to tie the geologic time scale to dates in years over much of its duration.

How Radiometric ages are calculated: A small sample of rock is placed into a mass spectrometer where it is bombarded by neutrons that knock atoms off of the surface of the sample. These atoms are sorted in the mass spectrometer by weight and counted. The information obtained is the total number of remaining parent isotope atoms and the total number of daughter isotope atoms. The equation: Age = ln (daughter / parent + 1) / k k is the decay constant - the probability that an atom of the parent isotope will decay over any given time. First, k must be calculated from the measured half life of the parent isotope:

At t = 1 half life, ratio of parent to daughter is 1:1 T half = ln (1 + 1) / k T half = ln 2 / k k = .693 / T half Now the age can be calculated using the equation at the top. Example: Isotope with a half life of 10,000 years. Measurements with mass spec count 2000 daughter atoms and 400 parent atoms. k= .693 / 10,000 = .0000693 Age = ln (2000 / 400 + 1) / .0000693 Age = 1.79 / .0000693 = 25,855 years

Decay series used in modern radiometric dating Uranium isotopes For measuring the ages of very old rocks (greater than 100 million years) isotopes with very long half lives are used: Thorium 232 - Lead 208 14 Ga Uranium 238 - Lead 206 4.5 Ga Uranium 235 - Lead 207 .7 Ga These isotopes cannot be used on younger rocks because too little daughter product is produced in under a 100 million years to be accurately measured. Possible sources of error It is possible for radiometric methods to yield false dates. Loss of parent isotope: If some parent isotope leaks out of the rock then it will appear as if more parent has decayed than really has. This will increase the apparent age of the sample. Loss of daughter isotope: If some daughter isotope leaks out it will appear as if less parent has decayed than really has. This will decrease the apparent age of the sample.