Earth History GEOL 2110

Lecture 9Absolute 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 (preeminent 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.

Henri Becquerel (1852-1908)

Marie Curie (1867-1934)

Pierre Curie (1859-1906)

1896 – Reported evidence for radioactivity by showing that photographic film became exposed when adjacent to uranium minerals

Came to identify two new radioactive elements – radium and polonium which came to be recognized as intermediate element 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 – Rutheford 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

Bertram Boltwood(1875 -1927)

1905 – Boltwood proved that lead (Pb) was the stable (daughter) product of uranium (U) radioactive decay

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 measured 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-life – the time it takes for half of the original amount of parent isotopes to decay; shows decay to an exponential functionThe 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.

Half-lives0 1 2 3 4

Decay of Radioactive Isotopes

Beta decay of Rb87 to Sr87

Isotopic Systems used in Age Dating

ZirconZrSiO4

U substitutes for Zr, but Pb does not.

Isotopic Systems used in Age DatingPb-Pb Age DatingTh232 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 DatingC14 Dating

Half-life – 5,730 yrs

Applications for:

• Late Pleistocene and Holocene events

• Archeology• Dating organic

material

Isotopic Systems used in Age DatingFission 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 ClockBlocking 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 Clock87Rb 87Sr; 86Sr is stableRb – chemically substitutes for KSr – chemically substitutes for Ca

87Rb/86Sr

87Sr/86Sr

Ca-plagioclaseGranite whole rockK-feldsparBiotite (K-rich)

t=087Sr/86Sr°

t=1

Metamorphismat t=1

t=2Isochron

87Sr/86Sr°(m)

Next Lecture

Origin and Early Evolution of the EarthPart 1: Accretion and Differentiation