Geologic Time

Absolute Time

• How do we measure time?
• Rates of natural processes
• revolution of Earth
• rotation of Earth
• How do we measure geologic time?
• Same way - rates of natural processes
• Requirements
• Process that occurs at a constant rate
• method of keeping a cumulative record of that process

Geologic Clocks

• Key question:

What is the age of the Earth?

• Biological Processes
• Tree rings - bristlecone pine, ~3000 years
• Geological Processes
• varve sedimentation
• Bradley (1929)
• 790 m of shale (Green River Formation, Tertiary) represents 6.5 m.y. of deposition

Geologic Clocks (cont.)

• Geochemical Processes
• Salt in the sea
• John Joly (1899)
• total salt content (g) ÷ rate of salt addition (g/yr) = age = 90 m.y.
• Geophysical Processes
• cooling of earth from a molten state by conduction and radiation; Lord Kelvin (1899): 20-100 m.y.

Geologic Clocks (cont.)

• Chemical Processes
• Weathering of rocks: difficult - temperature dependant
• Nuclear Reactions
• Do not involve chemical bonds
• Not temperature dependant at conditions which rocks form
• Radioactive isotope (parent) spontaneously gains or loses a particle, causing it to become another element (daughter)

Radiometric Dating
General Principles

• Half-life
• each radioactive isotope (radionuclide) has a unique half-life (time required for half the radionuclides to decay)
• The half life of an isotope is calcuated from its decay constant (amount which decays per unit time)

1) measure amount of radioactive isotope in sample (mass spectrometer)
2) count disintegrations per unit time (radioactivity counter)
3) calculate fraction of isotope that decays per unit time
Decay constant for ²³⁵U = 9.85 x 10^-10  per year

Radiometric Dating
General Principles (cont.)

• Approach
• Compare amount of daughter isotope to amount of parent originally there
• Age is usually the time of crystallization or formation (except sometimes for K-Ar dating)

Radiometric Dating
General Principles (cont.)

• Approach (continued)
• Compare amount of daughter isotope to amount of parent originally there
• Use mineral that is rich in parent, poor in daughter
• Use minerals that will not lose daughter or gain parent through diagenesis or metamorphism
• Use isotope pair appropriate for age of rock

Radiometric Dating Techniques

• Techniques based on specific parent-daughter pairs
• ²³⁸U-²⁰⁶Pb: t1/2 = 4.5 b.y.
• Measured in zircon (ZnSiO4), uraninite
• 10 m.y. to 4.6+ b.y.
• ²³⁵U-²⁰⁷Pb: t1/2 = 0.7 b.y.
• same as ²³⁸U-²⁰⁶Pb

Radiometric Dating Techniques (cont.)

• ⁸⁷Rb-⁸⁷Sr: half-life (t1/2 ) = 47 b.y.
• Rb⁺ substitutes for K⁺
• Sr²⁺ substitutes for Ca²⁺
• Used with minerals low in Ca, rich in K (muscovite, biotite, K-spar, whole rock)
• 10 m.y. to 4.6+ b.y.

Radiometric Dating Techniques (cont.)

• ⁴⁰K-⁴⁰Ar, ⁴⁰Ca: t1/2 = 1.3 b.y.
• Use with same minerals as Rb-Sr method (muscovite, biotite, hb, whole volcanic rock)
• Used to date ocean basalts
• 50,000 to 4.6 b.y.
• Can also date time of metamorphism

Radiometric Dating Techniques (cont.)

• ¹⁴C-¹⁴N: t1/2 = 5730 years
• ¹⁴C formed in atmosphere
• measure any carbon bearing material
• measure ¹⁴C/¹²C ratio and compare to ratio in "modern materials"
• good for ages younger than 70,000 yr
• Quality control: Comparison of methods
• ¹⁴C age vs. historical age

 
• ¹⁴C age vs. Uranium series age

Radiometric Dating
Limitations

• Limitations
• Not as precise as biostratigraphic correlation. Want to compare strata deposited at same time in different areas.
• usually can't use for sedimentary rocks
• must use age of ashes or cross-cutting dikes to constrain age of sediments