Download Historical Geology - Radiometric Dating

Need a list of minerals and radioactive parent/daughter isotopes
Radiometric Dating
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We can determine the age of the Earth by using the decay of radioactive
elements or isotopes. The elements most frequently used are Uranium 238 and
Lead 206 (from the mineral Zircon); Potassium 40 and Argon 40 (from the Mica
minerals); Rubidium 87 and Strontium 87 (from Feldspar minerals). These sets
of elements are called Parent-Daughter elements.
These radioactive pairs or Parent-Daughter elements have well defined decay
rates and from this, scientists have determined the Half-life of these elements.
Basically, when the prescribed amount of time has passed, one half of the
original radioactive element (the Parent element) has decayed creating the
Daughter element. This gives us half Parent isotopes and half Daughter isotopes
that help to comprise certain minerals.
The half-life of Uranium-Lead is 4.5 Billion Years. This means that after 4.5
BY the original amount of naturally radioactive Uranium 238 is converted to Lead
206 via the radioactive decay process. This process has protons and neutron
lost or transformed and gamma radiation emitted. The Potassium-Argon half-life
is 1.3 Billion Years, and the Rubidium-Strontium half-life is 47 Billion Years. To
successfully use these half-life elements, you must first have the minerals
containing them in the rock that you are studying.
Then you must be aware that different readings in metamorphosed areas are
common. This is because the half-life can be reset if the original minerals were
metamorphosed into new minerals. This means that the original amount of the
Parent isotope is unknown. The dating process can only register the time of
metamorphism. At this point in time, new minerals were created using the
available atoms and isotopes.
The White Tank Mountains is an example of “resetting” of the radiometric
clock. One part of the mountain range dates back to the Proterozoic, 1.7 Billion
Years, and another part dates back 30 Million Years to the Tertiary, based on
radiometric readings. There is over a billion years difference in the age of these
rocks, because some were metamorphosed and their radiometric date shows an
age much younger than the original Proterozoic rock.
If we look at Rubidium 87 and Strontium 87, we see that the Rubidium will
loose one neutron to create a new element, Strontium. On the periodic table of
the elements, we see that an element is defined by the number of protons that it
has. If this number changes, the element has changed. As a radioactive
element decays, it gives off gamma radiation and neutrons that yield protons and
electrons. This is how Rubidium can start off with 37 protons and 50 neutrons,
and with decay, can be transformed to Strontium with 38 protons and 49
neutrons.
Need a list of minerals and radioactive parent/daughter isotopes
But how does this decay actually give us an absolute or radiometric date? It
is done by a geologist. A rock sample is taken and the type of minerals, and
those likely to contain radioactive elements are determined. The sample is taken
to the lab and placed into a mass spectrometer. This machine is calibrated to
count specific isotopes. The reading will yield a count for the Rubidium 87
isotopes and the Strontium 87 isotopes. From this a simple mathematical
formula is used to determine how much Rubidium 87 was originally in the sample
and how much is left. This will allow a determination of how much time has
passed.
Example
This may sound difficult at first, but calculating the half-life is quite easy. Let’s
say that we tested a rock sample containing abundant feldspar. We then look to
count the Rubidium and Strontium 87 isotopes that are in the sample. The
machine is calibrated and the reading comes out with 120000 Rubidium 87 and
100 Strontium 87 isotopes.
From this we know that originally there were 120100 Rubidium 87 isotopes,
prior to the isotopic decay. That is because there is a one-for-one decay of
Rubidium 87 to Strontium 87.
Parent element count plus the Daughter element count equals the original total
Parent element.
120000 Rb87 + 100 Sr87 = 120100 Rb87 prior to decay
If the sample had reached the first half-life there would be equal Rubidium 87
and Strontium 87. That would be 60050 Rb87 and 60050 Sr87. Since our
sample has 120000 Rb87 and 100 Sr87, we know that it has not had time to
decay to the initial half-life of 47 Billion years with an effective dating range of 10
million to 4.6 million years. So the half-life number is 60050 isotopes.
From this we take the number of Strontium 87 (100) and divide it by the halflife number of isotopes, which is 60050.
100 Sr87
------= .0167
60050 Rb/Sr half-life number for that sample
This means that .0167 is the ratio showing the portion of radioactive decay of
Rubidium to Strontium in relation to the half-life of 47 billion years. Now we use
this ratio to determine the actual total number of years that Rubidium 87 has
been decaying to Strontium 87.
47 Billion Years times .0167 = .785 Billion years or 785,000 Million Years
The sample is just over three-quarters of a billion years old.
Need a list of minerals and radioactive parent/daughter isotopes
Exercises – Show all work.
1. A sample of Hornblende shows a spectrographic reading for Potassium 40
at 700 and a reading for Argon 40 at 150. The half-life of Potassium 40 is
1.3 Billion years. How much time has elapsed since this rock unit formed?
2. If a sample is 3 Billion years old, which Parent-Daughter isotope set(s)
is/are best to used to determine its age?
Uranium-Lead with a range of 10 million to 4.6 billion and a half-life of 4.5
billion? Potassium 40 –Argon 40 with a range from 50,000 to 4.6 billion
years and a half-life of 1.3 billion? Or Rubidium 87 – Strontium 87 with a
range of 10 million to 4.6 billion years and a half-life of 47 billion? Which
one and WHY?
Need a list of minerals and radioactive parent/daughter isotopes
3. We have a sample with Uranium 238 counting at 8900 and Lead 206
reading at 100. What minerals are likely present? What is the age of the
rock sample? (Reference the half-life numbers in number two and page
201 of your text.)
4. From your text, list minerals that are associated with U238-Pb206 decay,
Rb87-SR87 decay, and K40-Ar40 decay.
5. Reiterate why metamorphism affects radiometric dating.
6. From a rock sample there is a K40-Ar40 ratio of .5. What is the age of the
rock sample in million years?