Dating the Earth’s Rocks

Quantitative Geologic TimeEarly estimates

Calculation based on: Old Testament – Earth is 6,015 years old

James Ussher (1581-1656)1658 Earth formed on

Sunday, 23 October, 4004 BC

Early age estimates based on:

Thermodynamics I – Cooling of the earth from

molten material based on melting point of Pb

Georges Louis de Buffon (1707-1788) 1770s Earth cooled from molten ball

of magma - 75,000 years old(Major contradiction to Ussher)

Early age estimates based on:

Evolutionary rates – Organisms change over time

Charles Lyell

Charles LyellCenozoic Era began 80 million years ago

Early age estimates based on:Sediment deposition rates –

1850s computation that Earth’s age ranges from 1 million to 1 billion years

Early age estimates based on:

Salinity of sea water – Knowing salinity of oceans and how much salt is added

by rivers each year

John Joly (1857-1933)1899 Earth’s seas

developed 90 million years ago

Early age estimates based on:

Thermodynamics II –Melting temperatures and cooling

rates of rocks

William Thomson Kelvin (1824-1907)

1890s Earth formed 24-40 million years ago

Lord Kelvin

Finally, Radioactivity!

Radioactive Decay - Provides a more accurate method of dating rocks

Henri Becquerel (1896)Detected the phenomenon of radioactive decay

Modern radioactive-isotope studiesReviewing atoms:

• Basic unit of matter of elements.

• Each atom has a nucleus containing protons and neutrons. Orbiting the nucleus are electrons.

• The atomic number of an atom is the number of protons in the nucleus.

• The mass number is the number of protons plus the number of neutrons.

• Isotopes are variants of the same atom, but with a different number of neutrons (and different mass number). Eg: uranium’s nucleus always has 92 protons, but can have different number of neutrons. Three common isotopes are U-234, U-235, and U-238.

Components of an Atom

Methods and concepts:

Radioactivity is natural, spontaneous breakdown of the nuclear structure of atoms

Parent nuclide = daughter product + particle expelled

Rate of nuclear decay is constant

Radioactive isotopes have a unique rate & mode of decay. The rate is typically called a half life representing the time it takes for ½ the parent material to decay.

Half-life = span of time needed for one half of original atom to decay to daughter product

• Unstable radioactive isotopes (parent) decay and give rise to stable (daughter) atomic products.

All radioactive isotopes decay ata fixed rate.

When a radioactive isotope, e.g.U-238, is incorporated into a mineralthat crystallizes from magma, there is no lead. The radiometric clock startsat this point. Daughter (Pb) is trappedwithin the crystal lattice.

How can we use this information to tell how old a rock is?

• Crystallization of minerals locks in an original quantity of radioactive atoms

• Radiometric dating of a crystal possible because daughter products are retained

• Mass spectrometer is used to measure minute amounts of isotopes

Incorporation of Radioactive Atoms (Igneous Rock)

Minerals in igneous rocks give the most reliable age dates.

Principal Geologic Timekeepers: Radioactive Parents & Stable Daughters

Uranium-lead: decay of U235 or U238 to leadPotassium-40/Argon-40 method: trapped Argon gas derived from decay of potassium87Rb/86Sr method: decay of rubidium to strontiumCarbon-14 method: limitation is the half-life of 5,730 yearsNuclear fission-track counting in crystals

Half-lives differ among elements

235 U 704 million years

dinosaur paleontology

14 C 5,700 years

ice-age paleontology, archaeology

238 U 4.5 billion years

determining age of earth, asteroids, etc.

Radiometric Dating I:

• Currently there are five isotopes that are extensively used in radiometric dating ancient rocks: rubidium-87, thorium-232, uranium-235, uranium-238, and potassium-40.

• The mineral must remain a closed system. Chemical or physical processes can affect the parent/daughter ratio.

•Only fresh, unweathered rock is used, and several widely spaced samples are taken for analysis.

Metamorphism “resets” the atomic clock.

Metamorphic rocks will appear younger due to

the loss of daughter isotopes.

Can be used to determine time of

metamorphism

Radiometric dating II: establishes absolute ages of certain rock and

sedimentary layers (within statistical limits)

• Oldest known Earth material is 4.36 billion years (based on zircon crystals from western Australia)

• Other very old rocks (over 4 billion years) come from Canada and Greenland

• Age of meteorites: U/Pb and Rb/Sr dating yields 4.6 billion years

• Moon rocks: U/Pb and Rb/Sr dates range from 3.3 to 4.6 billion years

Radiometric Dating of Rocks

Bracketing Ages

We then use the radiometric dates of the volcanic ash layers (bentonites) to provide upper and lower limits of a particular bed.

Putting it all together (Using radiometric dating to find the ages of index fossils)

Fission Track Dating

• Fission-track dating measures the number of microscopic linear tracks left by the decay of U-238. Useful for dating samples from 1 million to 2 billion years old.

• The age of the sample is determined from the number of fission tracks present and the amount of uranium the sample contains: the older the sample, the greater the number of tracks.

• Naturally occurring glass and minerals like garnet, zircon, mica, epidote are datable.

The length of the track can provide information on how fast the rock cooled (long tracks = rapid cooling; short tracks = slow cooling)

Fission Track Dating Can be used to determine:

• age of meteorites

• age of formation of obsidian (volcanic glass)

• source of clastic sediments

• source of material for archaeological artifacts

Ceremonial blades embedded in skull - Aztec

Chondritic meteorite

Carbon-14 Dating

• Used to date relatively recent events since half-life is 5,730 years (can be use to date up to 75,000 years).

• Can only be used to date material of biogenic origin (plants and animals – things that were once alive).

• Carbon-14 is created in upper atmosphere when neutrons are absorbed by nitrogen atoms (causing nitrogen to eject a proton).

• The isotope is incorporated into carbon dioxide and is absorbed by living matter.

• As long as organism is alive carbon-14 is continually replaced – ratio of carbon-14/carbon-12 is constant.

• When organism dies carbon-14 is converted to nitrogen-14 via beta emission.

Radiometric dating vs. Historical records

Dendrochronology – “Tree-Ring” Analysis

Dendrochronology (dating using tree rings )

Correlation of Tree-Ring Sections