GEO 103 Lecture Notes - Lecture 7: Chert, Clay Minerals, Mass Spectrometry

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Principle of inclusions
Principle of fossil succession
Relative Dating Principles
Half
-
life
of radioactive isotope is time it takes for one half of the atoms in the original unstable
parent isotope
daughter isotopes
.
Half
-
life of specific radioactive isotope is constant and can be measured
These half
-
life times can vary quite a lot.
Time of half
-
lives for different isotopes of different elements can vary from less than 1/billionth of
a second to 49 billion years.
Disregarding how many atoms exists in the first place, the half
-
life is always the same.
Radioactive decay is a
geometric
process, not linear, so is a curved line on a graph of amount vs
time.
Half
-
Lives
Water drips into the glass
The rate is constant.
In this example of uniform linear change
Uniform Linear Change
In radioactive decay during each equal time unit of one half
-
life, proportion of parent atoms
decreases by 1/2.
Geometric Radioactive Decay
Geologists can use this to calculate the age of sample containing the radioactive element.
By measuring the
parent/daughter ratio
and knowing half
-
life of the parent element (determined
in the laboratory)
Instrument that measures proportions of atoms with different masses.
This device brings atoms around a curve and ions are pulled based on the mass of the
atoms.
Parent/daughter ratio usually determined by a
mass spectrometer
.
If a rock has parent/daughter ratio of 1:3 = a parent proportion of 25% (1+3 = 4 which is the
total)
And half
-
life is 57 million years,
How old is the rock?
25% means that it is 2 half
-
lives old.
The rock is 57 million * 2 = 114 million years old.
For example:
Determining Age
Most radiometric dates obtained from igneous rocks.
As magma cools and crystallizes, radioactive parent atoms separate from previously formed
daughter atoms.
Because they fit, some radioactive parents included in crystal structure of certain minerals in the
cooling magma. In these crystals, only the specific atom exists.
Daughter atoms are different elements with different sizes, and therefore, do not generally fit into
the same minerals as the parent.
Geologists can use crystals containing parent atoms to date crystallizations.
What Materials can be dated?
Lecture 7
-
Dating Methods
Monday, February 6, 2017 9:03 AM
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