Absolute Age

1
Absolute Age

• Isotopic Dating
• Isotopes and Radioactive Decay
• Isotopes - Differing number of neutrons, but same
  number of protons
• Radioactive Decay - Spontaneous nuclear
  disintegration of unstable isotopes
• Daughter Product
• Half-life
• the time required for half of the radioactive
  isotopes to change
• Various techniques (e.g. U-Pb, K-Ar, 14C
  radiocarbon dating)

2
(No Transcript)
3
(No Transcript)
4
Isotopic Decay

• Unstable Nucleus
• spontaneously change into another type of atom
• Decay through Beta emission (RbgtSr), Positron
  emission(NgtC), Electron capture (KgtAr), Alpha
  emission (UgtTh), Spontaneous fission
• Decay Rate is Constant, it depends on the
  particular isotope

5
Radioactive Decay Series
6
Isotopic Dating Methods

• Rubidium-Strontium
• Whole Rock, Feldspar, Mica, Clays gt100my
• Half-Life 48.8 by
• Potassium-Argon
• Volcanics, Feldspar, Mica, Clays gt100,000my
• Half-Life 1.25 by
• Uranium-Thorium-Lead 238Ugtgt206Pb 235Ugtgt207Pb
• Whole Rock, Zircon
• Half-Life 4.47by 0.704by
• Samarium-Neodymium
• Whole Rock, Silicate Minerals
• Half-Life 106 by
• Fission Track-

7
Decay Curve for Potassium-40
8
Isotopic Dating Methods

• Fission Track-
• Discovered in 1960s
• Small tunnels seen with a Scanning Electron
  Microscope (SEM)
• Due to spontaneous fission of Uranium isotopes
• Used to determine the the number of years elapsed
  since uranium bearing mineral formed
• Counts of the tracks produced, the original
  number of Uranium atoms and the decay rate of
  Uranium
• Used to date rocks a few 100 years to billions of
  years old
• Used preferentially for time periods between
  50,000 and 1,000,000 years

9
Fission Tracks
10
Other Isotopic Dating Methods

• Carbon-14
• Ocean Mixing, Dating Organic Remains
• Half-life 5730 years
• Tritium
• Ground and Surface Water Mixing, Glacial Growth
• Half-Life 12.3 years
• Silicon-32
• Ocean Mixing
• Half-Life 280 years
• Beryllium-10
• Sedimentation Rates, Subduction of Sediments
• Half-Life 1.5 x 106 years
• Chlorine-36
• Young Volcanics, glaciation, erosion surfaces
• Half-Life 3.1 x 105 years

11
Non Isotopic Dating Techniques

• Thermoluminescence- emission of light when a
  substance is heated
• Time a sample was buried Archeological
• Ceramics, Glass time of last heating
• lt100 years
• Racemization- change in amino acids from living
  to dead organisms
• Bones, Teeth burial of organic matter
• lt106 years
• Hydration Rinds- 50um thick layers produced when
  volcanic glass is exposed to the atmosphere
• Need constant climate
• Obsidian artifact manufacture
• lt105 years

12
Absolute Age

• Uses of Isotopic Dating
• Igneous Rocks
• Eruption or Intrusion of Magma
• Metamorphic Events
• reset clock gives time of metamorphism
• Organic remains- fossils (70,000 years)
• Reliability of Isotopic Dating
• Accuracy (true age) and precision
  (reproducibility of measurements) of analytical
  instrumentation

13
Combining Relative and Absolute Ages

• Using various relative age principles, as well
  as-
• Isotopic dating
• Usually of igneous rocks
• Metamorphic rocks give age of metamorphism (reset
  the clock) (younger age)
• Sedimentary rocks give the age of the formation
  of minerals within the rock (older age)

14
(No Transcript)
15
The Geomagnetic Time Scale

• Times of normal (North magnetic pole) vs. reverse
  polarity (South magnetic pole)
• Last time polarity flipped was 20,000 y.b.p.
• Polarity Intervals- segments of time in which the
  magnetic field is reversed or normal
• Magnetic data from volcanic rocks and deep sea
  sediments

16
(No Transcript)
17
Rhythmic Bands

• Deposited in response to changes in environmental
  factors
• seasons, daily fluctuations in sunlight, tides
• Types
• Varves
• glacial lakes, winter and summer, clay and silt
• tidal couplets
• dated back to 20,000 years in the Baltic
  Recognized in older rocks of 3.5by, tidal effects
  recognized in rocks 2.5 by
• Tree Rings- dendrochronology
• summer and spring small cells vs. large cells
• extend 8,000 years in some localities
• Corals and Bivalve Shells
• Seasonal changes, daily changes in light
• Ice Layers
• 3000m long cores date back 65,000 years,
  atmospheric and climate studies

18
Shell Bands and Tree Rings
19
Age of the Earth

• Early speculation based on
• rates of sedimentation
• saltiness of the ocean
• cooling of the earth
• Dating of meteorites and moon rocks
• Between 4.5 4.6 billion years old
• Newest evidence oldest mineral dated on earth
  4.4 by
• Comprehending Geologic Time

20
(No Transcript)
21
(No Transcript)
22
GEOLOGIC HISTORY
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)