Absolute Dating
J F M A M J J A S O N D J
Earth Forms
Earth Cools
FirstLife
AbundantOxygen
MulticellularOrganisms
Plantsand
Animals
Dinos15-25
Humans11:20 pm
1 second 1/ 31,556,925.974 of the year 1900
9,192,631,770 flips of the magnetic field of a cesium atom
What is Absolute/Numerical Dating?
•The age of an earth material or event in years
Relative age
1st
2nd
3rd
Absolute
1.7 bya95 mya
12 kya
youngest 8 kya
Estimating Earth's Age -Early (failed) Attempt
Bishop Usher – Biblical Interpretation
4004 BC – before the birth of Christ
October 21
9:00 in the morning
Estimating Earth's Age -Early (failed) Attempt
Sedimentation rates - 3 my – 500 my
Halley/Joly - Ocean Salinity – 100 my
Lord Kelvin – Rate of Cooling – 30 my
Radioactive Revolutionaround 1900
• Radioactive decay - spontaneous transformation of an element to another isotope of the same element or another element.
Pieces of an Element
• Protons - positively charged
• Neutrons - no charge
• Electrons - negatively charged
Ne
P
P
e
Isotope - element with different number of neutrons in the nucleus.
P
e
N
P
N
e
Hydrogen - stable Tritium - unstable
Radioactive Decay
N
P
N
e
Tritium
N
P
P
e
e
Helium 3
unstable stable
nuclear
decay
(daughter)(parent)
Alpha Decay – loss of a positively charged He ion
Beta Decay – neutron splits into proton and electron
Radioactive Decay
23892
U 23490
Th +42
He ( 2protons + 2 Neutrons)
Alpha Decay
Beta Decay
23490
Th 23491
Pa + e- Neutron splits into a proton and an electron
Half-life
The fixed period of time during which half the parent atoms present in a closed system decay to form daughter atoms
Half-Life
1 2 3 54
Half-life
Potassium 40 / Argon 40
1 6 / 6
Half-lifes
0 12 grams / 0 grams
2 3 / 9
3 1.5 / 10.5
Half-Life Exercise
Isotope A Isotope BParent Daughter
Sample: 1024 grams
64 grams 960 grams
(10 k) half-life
1st
512 / 512
2nd
256 / 768
3rd
128 / 896
4th
64 / 960
10,000 20,000 30,000 40,000
Parent/Daughter Half Life (yrs) Range (yrs)
238U/206Pb
14C
3H/3He
4.5 billion
5730
12.3
10 million to4.5 billion
100 to 70,000
1 to 50
Parent/Daughter
238U/206Pb
14C
3H/3He
Material Dated
Igneous and Metamorphic rocks
Organic material
Water
Uranium (U) - Thorium (Th) - Lead (Pb)
238U decays to 206Pb (4.5 billion)235U decays to 207Pb (713 million)232Th decays to 208Pb (14.1 billion)
>Rocks containing Uranium provide three possible techniques.>Because all three occur together, it allows a method to cross-check the dates.>Uses zircons, uraninite and uranium ores
Potassium (K) - Argon (Ar) Dating
>Potassium (K) is an extremely common element.>Half-life (t1/2) is 1.3 billion years.>Range is 100,000 to 4.6 billion years.>Useful for relatively young and very old rocks. >Found in muscovite, biotite, orthoclase and glauconite.>Used to date volcanic rocks.
Carbon 14 / Carbon 12
>Cosmic rays hit Nitrogen 14 changing it to Carbon 14.>Carbon 14 is taken in by organisms.>When organism dies, amount of C-14 decreases.
Magnetic Polarity Dating
Dating techniquesthat can be usedon this geologiccolumn:
• absolute dating
• paleomagnetism
• superposition
• fossil correlation
What is the Parent / Daughter ratio of a sample of radiogenic material if the original sample was 200g. and 3 half-lifes have passed. If the half-life is 20,000 years, how old is the sample?
What is the half-life of an isotopic pair if the sample is 2 million years old and contains 625g of parent and 9375g of daughter isotope?