Dating the Earth’s Rocks

Quantitative Geologic Time Early 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 Cenozoic Era began 80 million years ago Charles Lyell

Early age estimates based on: Sediment deposition rates – 1850s computation that Earth’s age ranges from 1 million to 1 billion years Didn’t account for differences in sedimentation rates in the past. And what about periods of no deposition?

Early age estimates based on: Salinity of sea water – Knowing salinity of oceans and how much salt is added by rivers each year Sir Edmund Halley (Halley’s comet) thought that the earth’s first global ocean was probably slightly younger than the earth and not saline. Although Joly’s resultt was wrong, it supported the idea that the earth was immensely old. 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 studies Reviewing 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 at a fixed rate. When a radioactive isotope, e.g. U-238, is incorporated into a mineral that crystallizes from magma, there is no lead. The radiometric clock starts at this point. Daughter (Pb) is trapped within 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 lead Potassium-40/Argon-40 method: trapped Argon gas derived from decay of potassium 87Rb/86Sr method: decay of rubidium to strontium Carbon-14 method: limitation is the half-life of 5,730 years Nuclear 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. Look at figure 3.3 in text for more radioactive isotopes used

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 Lava is not the only igneous rock used. Volcanic ash can travel much further than lava, and can be found in layers that interbed with fossil bearing layers. Shale and bentonite layers – this provides excellent tools for dating the fossils above and below the bentonite layers. Bracketing Ages

We then use the radiometric dates of the volcanic ash layers (bentonites) to provide upper and lower limits of a particular bed. Volcanic ash tends to kill animals that breathe or ingest it in any way, therefore ash layers are often associated with fossils.

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. Useful for dating recent events (a few centuries old) as well as a few million years old. It fill the gap between C14 and KAR. The track is created when the uranium particle removes electrons from atoms along its track. Those atoms then have a positive charge and are repulsed from each other. 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