radiometric dating of rocks 1896-1898 radioactivity discovered by: henri becquerel and marie and...
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Radiometric Dating of Rocks
1896-1898
Radioactivity discovered by:
Henri Becquerel and Marie and Pierre Curie.
Phenomenon of Radioactivity
Some elements, such as uranium (U) and thorium (Th), are unstable:
They decay spontaneously.
The Atom
Uranium Nucleus
spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).
Alpha Particle
emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).
Uranium - Thorium Decay
U He + Th238
92
4
2
234
90
spontaneous
decay
“parent” “daughter product”
alpha particle =2 protons + 2 neutrons = positively charged ion of Helium
Thorium:90 protons + 144 neutrons
Beta Particle Emission
But, Th is also unstable, and it emits a beta particle …
234
90
Th + Pa234
90
234
91
Thorium - Protactinium Decay
beta particle
beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron
Protactinium:91 protons + 143 neutrons
U PbSeries
This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).
U PbSeries
The rate at which this process occurs is measured in terms of the “half life”.
Half Life
Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb
The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:
(1) As radioactive elements decay in the Earth, they heat up the surrounding rocks.
(2) Radiometric Dating
Radiometric Dating
Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed.
This is an example of uniformitarianism.
The Principle of Uniformitarianism
The physical, chemical and biological laws that operate today have also operated in the geologic past.
To estimate the age of a rock:
D = amount of daughter product. P = amount of parent.
For a particular radioactive element in a rock, determine the present ratio = D/P.
D
P
Rate of decay (from theory and measurement)
Make assumptions about original ratios (from theory of geochemistry).
As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified).
D
P
D
P
Half-Lives of Radioactive Isotpes
Examples of Very Old Rocks Found on Earth:
3.4 billion year old granites - South Africa.
3.7 billion year old granites - Greenland.
Oldest terrestrial rocks > 4.6 billion years.Central Canada and Central Australia
Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.
Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.