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?