Earth History
GEOL 2110
Lecture 9
Absolute Dating of the Earth
Major Concepts
• The discovery of radioactivity in the early 1900’s and the
recognition that radioactive decay (a statistical event)
occurs at a constant average rate for particular unstable
isotopes has provided a means of determining the absolute
ages of earth materials
• Different radioactive isotopes decay at different
characteristic rates, which are portrayed as the half-life of
the isotope.
• For the radioactive decay of unstable parent isotopes into
stable daughter isotopes to provide useful ages requires
that the minerals hosting these isotopes remains
chemically closed. Weathering, mechanical alteration, or
significant reheating can reset the apparent age.
Early Ideas about the Age of the Earth
• Genesis – 6,000 yrs
• Archbishop Usser (1654) 9AM, October 26, 4004 BC
• Buffon (1760) – 75,000 yrs
• Post-diluvian geologists of the mid-1800 - 100’s of
millions
• Darwin, 1859 – 300 million based on rates of erosion
“requires unlimited drafts upon the bank of antiquity”
George Scropes (1827, geologist, political economist)
• Lord Kelvin (pre-eminent physicist) – 1846 calculated
the age of the earth assuming its origin by cooling
from a molten state - his estimates ranged from 400
million to 20 million
“The most brilliant argument is no better than its weakest assumption”
Prothero and Dott, p. 98
The Discovery of Radioactivity
.
1896 – Reported evidence for radioactivity
by showing that photographic film became
exposed when adjacent to uranium minerals
Henri Becquerel (1852-1908)
Marie Curie
(1867-1934)
Pierre Curie
(1859-1906)
Came to identify two new
radioactive elements – radium and
polonium which came to be
recognized as intermediate
elements formed from the
radioactive decay of uranium
All three won the 1903 Nobel Prize
in Physics.
The Discovery of Radioactivity
.
Ernest Rutherford
(1871-1937)
Fredrick Soddy
(1877-1956)
1902 – Rutherford and Soddy recognized
that the total amount of radiation emitted
from radium was proportional to the number
of unstable (radioactive) isotopes present.
They reasoned that the emissions must
decrease (decay) in a regular fashion over
time - thus was born the idea that
radioactive decay could be used as a means
of dating minerals.
The Discovery of Radioactivity
1905 – Boltwood proved that lead (Pb) was the
stable (daughter) product of uranium (U)
radioactive decay
Bertram Boltwood
(1875 -1927)
1907 – Took Rutherford’s suggestion that
radioactive decay in uranium-bearing minerals
could be used to date the crystallization age of
the mineral if the rate of decay was known.
AGE = Amt of daughter Isotope (Pb) /Amt of parent isotope (U) * decay rate (1010yr)
With the decay of U  Pb being imperceptibly slow and involving
intermediate unstable isotopes, he used the relatively fast decay
rate of radium. Inaccurate , but OK first order estimate.
Calculate ages ranging from 410 – 2200 Ma for 10 global samples
Decay of Radioactive Isotopes
The chemical behavior of an atom is controlled by the number of -electrons, which is the
same as the number +protons in order to maintain charge balance. The number of
protons in the nucleus (atomic # ) defines the type of element the atom is.
Neutrons have no charge and therefore do not affect the chemical behavior of
elements. Neutrons (and protons) do have mass, however, and therefore affect atomic
weight of the element
Decay of Radioactive Isotopes
Decay of Radioactive Isotopes
Decay of Radioactive Isotopes
Half-lives
0
1
2
3
4
Half-life – the time it takes for half of the original amount of parent
isotopes to decay; shows decay to an exponential function
The rate of radioactive decay is a statistical average for the entire
population of parent isotopes – gives the probability that a given
unstable atom will decay in a given time period.
Decay of Radioactive Isotopes
Beta decay of Rb87 to Sr87
Isotopic Systems used in Age Dating
Zircon
ZrSiO4
U substitutes for Zr, but
Pb does not.
Isotopic Systems used in Age Dating
Pb-Pb Age Dating
Th232  Pb208 (14Ga)
U238  Pb206 (4.5Ga)
U235  Pb207 (0.7Ga)
Pb204 is stable,
abundance constant
Plotting these three
ratios on Pb evolution
curves yields precise
ages and an internal
check on closure
Isotopic Systems used in Age Dating
C14 Dating
Half-life – 5,730 yrs
Applications for:
• Late Pleistocene
and Holocene
events
• Archeology
• Dating organic
material
Isotopic Systems used in Age Dating
Fission Track Dating
U238 decay involves rare
fission of the nucleus
rather than alpha decay
Each fission event leave a
path of destruction – a
track
The density of tracks for a
given abundance of U238 is
a function of time
Resetting the Isotopic Clock
Blocking Temperatures
Different minerals have
different temperatures at
which they behave as
closed systems
whereupon they preserve
the progression of isotopic
ratio evolution with time
Blocking temperatures
also vary by isotopic
system
Resetting the Isotopic Clock
 87Sr; 86Sr is stable
Rb – chemically substitutes for K
Sr – chemically substitutes for Ca
87Rb
Isochron
t=2
t=1
87Sr/86Sr
Metamorphism
at t=1
87Sr/86Sr°(m)
t=0
87Sr/86Sr°
87Rb/86Sr
Ca-plagioclase
Granite whole rock
K-feldspar
Biotite (K-rich)
Next Lecture
Origin and Early Evolution of the
Earth
Part 1: Accretion and Differentiation