2 Quantitative Geologic Time Early estimates Calculation based on:Old Testament – Earth is 6,015 years oldJames Ussher ( )1658 Earth formed onSunday, 23 October, 4004 BC
3 Early age estimates based on: Thermodynamics I –Cooling of the earth from molten material based on melting point of PbGeorges Louis de Buffon ( )1770s Earth cooled from molten ball of magma - 75,000 years old(Major contradiction to Ussher)
4 Early age estimates based on: Evolutionary rates – Organisms change over timeCharles LyellCenozoic Era began80 million years agoCharles Lyell
5 Early age estimates based on: Sediment deposition rates –1850s computation that Earth’s age rangesfrom 1 million to 1 billion yearsDidn’t account for differences in sedimentation rates in the past. And what about periods of no deposition?
6 Early age estimates based on: Salinity of sea water –Knowing salinity of oceansand how much salt is addedby rivers each yearSir 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 ( )1899 Earth’s seasdeveloped 90 million years ago
7 Early age estimates based on: Thermodynamics II –Melting temperatures and cooling rates of rocksWilliam Thomson Kelvin ( )1890s Earth formed million years agoLord Kelvin
8 Finally, Radioactivity! Radioactive Decay -Provides a more accurate method of dating rocksHenri Becquerel (1896)Detected the phenomenon of radioactive decay
9 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.
11 Methods and concepts:Radioactivity is natural, spontaneous breakdown of the nuclear structure of atomsParent nuclide = daughter product + particle expelled
12 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
13 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 isno lead. The radiometric clock startsat this point. Daughter (Pb) is trappedwithin the crystal lattice.
14 How can we use this information to tell how old a rock is? Crystallization of minerals locks in an original quantity of radioactive atomsRadiometric dating of a crystal possible because daughter products are retainedMass spectrometer is used to measure minute amounts of isotopes
15 Incorporation of Radioactive Atoms (Igneous Rock) Minerals in igneous rocks give the most reliable age dates.
16 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
17 Half-lives differ among elements 235 U million yearsdinosaur paleontology14 C 5,700 yearsice-age paleontology, archaeology238 U billion yearsdetermining age of earth, asteroids, etc.Look at figure 3.3 in text for more radioactive isotopes used
18 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.
19 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
20 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 GreenlandAge of meteorites: U/Pb and Rb/Sr dating yields 4.6 billion yearsMoon rocks: U/Pb and Rb/Sr dates range from 3.3 to 4.6 billion years
21 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
22 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.
23 Putting it all together (Using radiometric dating to find the ages of index fossils)
24 Fission Track DatingFission-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)
25 Fission Track Dating Can be used to determine: age of meteoritesage of formation of obsidian (volcanic glass)source of clastic sedimentssource of material for archaeological artifactsCeremonial blades embedded in skull - AztecChondritic meteorite
26 Carbon-14 DatingUsed 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.