Absolute Dating of Rocks and Fossils

Early Efforts on Dating the Age of the Earth James Ussher (mid 1600's) 4004 B.C. Georges Buffon (mid 1700’s) 75,000 yr Charles Walcott (1893) 75 m.y. Lord Kelvin (late 1800's) 100 m.y.

II. Constancy of Natural Laws Early Efforts on Dating the Age of the Earth II. Constancy of Natural Laws Catastrophism Georges Cuvier (late 1700’s) Uniformitarianism James Hutton (late 1700's)

Radiometric Dating of Rocks Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay). Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.

Absolute Dating of Rocks The main method used to get the exact age of a rock is called radioactive or radiometric dating. This method studies the decay rates of radioactive isotopes in order to determine the age of the rock. But First What is an atom? What is an isotope?

} ATOMS Protons Neutrons Nucleus Electrons Nucleus Fundamental unit of matter made up of subatomic particles } Protons (positive charge) Nucleus Neutrons (no electrical charge) Electrons (negative charge) Protons Neutrons Nucleus Electrons

ISOTOPES Carbon-12 Carbon-13 Carbon-14 12C 13C 14C 6 6 6 98.9 % 1.1 % < 0.1 % 6 protons 6 protons 6 protons 6 neutrons 7 neutrons 8 neutrons

Types of Radioactive Decay protons alpha particle 4H 2 neutrons electrons 238U 234Th 92 90 ALPHA DECAY alpha decay Parent Nucleus Changes in atomic number and atomic mass number Daughter Nucleus 92 protons 90 protons Atomic Number = - 2 146 neutrons Atomic Mass Number = - 4 144 neutrons Alpha decay occurs when the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a helium nucleus is emitted.

Types of Radioactive Decay protons beta particle p + neutrons electron electrons 137Ba 137Cs 56 55 BETA DECAY beta decay Parent Nucleus Daughter Nucleus Changes in atomic number and atomic mass number 55 protons Atomic Number = + 1 56 protons 82 neutrons Atomic Mass Number = 0 81 neutrons Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted.

Types of Radioactive Decay protons neutrons electrons electron capture 204Po 204Bi 84 83 ELECTRON CAPTURE Changes in atomic number and atomic mass number Parent Nucleus Daughter Nucleus Atomic Number = - 1 84 protons Atomic Mass Number = 0 83 protons 120 neutrons 120 neutrons Electron capture also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron.

U238 Pb206 Series This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

Radioactive Dating During radioactive decay, the original material or “parent material” decays at a set rate into the new material or “daughter material” As the number of parent material decreases, the number of daughter material increases. The amount of time that it takes for exactly one-half of the parent to turn into daughter is known as a half-life. For example – the half-life of Carbon-14 is 5,700 years. Carbon-14 is the parent material and Nitrogen-14 is the daughter material it decays into. Therefore, this means that it will take 5,700 years for one-half of the Carbon-14 to turn into Nitrogen-14.

Radioactive Dating So, why does Radioactive Dating allow geologist to get absolute dates? The half-lives of radioactive isotopes are set in stone and DO NOT change. For example, the half-life of Carbon-14 is always 5,700 years, no matter what. A table of some common half-lives are given in the next slide.

Radioactive Decay Rates Isotope Disintegration Half-Life Iodine-131 131I 131Xe 8.02 days Cobalt-60 60Co 60Ni 5.3 years Carbon-14 14C 14N 5.7 x 103 years 40Ar 40K 1.3 x 109 years Potassium-40 40Ca 4.5 x 109 years Uranium-238 238U 206Pb

Half Life How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay. U238 Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb Po218

Half Life Parent Isotope (100) Daughter Isotope Half-Life = 10 seconds Time (s) Half-Life Parent Daughter 100 10 1 50 50 20 2 25 75 30 3 12.5 87.5 40 4 6.25 93.75 50 5 3.125 96.875

Start with a million parent atoms… 100 50 25 12.5 6.25 3.13 PARENT=1,000,000 DAUGHTER=0

1 Half-Life 100 50 25 12.5 6.25 3.13 1 PARENT=500,000 DAUGHTER=500,000

2 Half-Lives 1 2 PARENT=250,000 DAUGHTER=750,000 100 50 25 12.5 6.25 3.13 1 2 PARENT=250,000 DAUGHTER=750,000

3 Half-Lives 1 2 3 PARENT=125,000 DAUGHTER=875,000 100 50 25 12.5 6.25 3.13 1 2 3 PARENT=125,000 DAUGHTER=875,000

4 Half-Lives 1 2 3 4 PARENT=62,500 DAUGHTER=937,500 100 50 25 12.5 6.25 3.13 1 2 3 4 PARENT=62,500 DAUGHTER=937,500

5 Half-Lives 1 2 3 4 5 PARENT=31,250 DAUGHTER=968,750 100 50 25 12.5 6.25 3.13 1 2 3 4 5 PARENT=31,250 DAUGHTER=968,750

6 Half-Lives 1 2 3 4 5 6 PARENT=15,625 DAUGHTER=984,375 100 50 25 12.5 6.25 3.13 1 2 3 4 5 6 PARENT=15,625 DAUGHTER=984,375

Example #1 100 50 25 20% 12.5 6.25 3.13 1 2 3 4 5 6 2.3 half-lives If a sample has 20% 14C, how old is the sample? Number of parent isotopes Half-Life 14C = 5,700 years Number of daughter isotopes

Example #2 100 50 25 12.5 8% 6.25 3.13 1 2 3 4 5 6 3.6 half-lives A sample is found to have 131I at a concentration of 8%. How old is the sample? Number of parent isotopes Half-Life 131I = 8 days Number of daughter isotopes

Let’s Try Some More… Parent 1 1/2 1/4 1/8 1/16 1/2 3/4 7/8 15/16 Daughter 1 1/2 1/4 1/8 1/16 1/2 1st half-life 3/4 2nd half-life 7/8 3rd half-life 15/16 4th half-life

Once Again C-14 0 % N-14 100% 50% 25% 12.5% 6.25% 50% 1st half-life 75% 2nd half-life 87.5% 3rd half-life 93.75% 4th half-life

Radioactive Dating The smaller the half-life, the less useful the radioactive isotope is for dating really old stuff. For example, Carbon-14 half life is only 5,700 years. Therefore, it can only accurately date rocks no older than about 100,000 years old. Therefore, to date rocks that formed back at the beginning of the Earth, 4.6 billion years ago, we would use an isotope with a much larger half-life. Since Uranium-238 half-life is 4.5 billion years, anything containing it has undergone 1 half-life since the Earth first formed!