Absolute Time

• How do you measure time?

• How do geologists measure time?
• Requirements
• Process that occurs at a constant rate
• Method of keeping a cumulative record of that process

• Fundamental question:

• Biological Processes
• Tree rings - bristlecone pine, ~3000 years
• Geological Processes
• Varved sedimentation
• Varves are alternating dark and light layers deposited annually
• Bradley (1929)
• 790 m of shale (Green River Formation, Tertiary) represents 6.5 m.y. of deposition

• 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.

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

• Half-life
• Each radioactive isotope (radionuclide) has a unique half-life (time required for half the radionuclides to decay)

• Half life of an isotope is calculated from its decay constant (amount which 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
• Original amount of parent should equal amount of parent measured plus amount of daughter, if
• Approach (cont.)
• No daughter was present at the time of crystallization
• No parent or daughter was added after crystallization (except from radioactive decay)
• No parent or daughter was lost after crystallization (except from radioactive decay)
 
• Approach (cont.)
• Use mineral that is rich in parent isotope, poor in daughter isotope
• Use minerals that will not lose daughter or gain parent through diagenesis or metamorphism
• Use isotope pair appropriate for age of rock
• Age is usually the time of crystallization or formation (except sometimes for K-Ar dating, which is reset with metamorphism)

• Techniques based on specific parent-daughter pairs
• ²³⁸U-²⁰⁶Pb: t_1/2 = 4.5 b.y.
• Measured in
• Zircon (ZnSiO₄) - U⁴⁺ substitutes for Zn⁴⁺; Pb²⁺ excluded
• Uraninite (mostly UO₂)
• 10 m.y. to 4.6+ b.y.
 
• ²³⁵U-²⁰⁷Pb: t_1/2 = 0.7 b.y.
• Same as ²³⁸U-²⁰⁶Pb
• ⁸⁷Rb-⁸⁷Sr: half-life (t_1/2 ) = 47 b.y.
• Rb⁺ substitutes for K⁺
• Sr²⁺ substitutes for Ca²⁺
• Used with minerals low in Ca, rich in K (muscovite, biotite, K-spar)
• 10 m.y. to 4.6+ b.y.
• ⁴⁰K-⁴⁰Ar, ⁴⁰Ca: t_1/2 = 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

• ¹⁴C-¹⁴N: t_1/2 = 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
• Used to date groundwater
• Tritium (³H) -³He : t_1/2 = 12.3 years
• Natural plus nuclear bomb product
• Used to date groundwater (H₂O)
• Determines if age less than 50 years

• ¹⁴C age vs. Historical age

• ¹⁴C age vs. Uranium series age (adapted from Bard and others, 1990, Nature 345:405)

• Correcting for preexisting Pb using ²⁰⁴Pb (which is not produced by radioactive decay) results in good agreement between different methods.

• Limitations

• Radiometric dating not as precise as biostratigraphic correlation
• Usually can’t directly date sedimentary rocks
• Must date ashes or cross-cutting dikes to constrain age of sediments