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Radiometric dating tells about past .ppt
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- 2. The Key tothe Past Relative Time- “this rock is older than that” Principles Used to Determine Relative Age • Unconformities • Correlation • The Standard Geologic Time Scale • Index Fossils Absolute Time- “this rock is 28 million years old” Principles of radioactive decay
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- 8. Radiometric dating Radiometricdating determines Absolute Age. It uses the half-life of atoms to figure out the age of the rock layers the atoms are in. Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.
- 9. Radioactive decay, alsoknown as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting radiation. A material that spontaneously emits such radiation — which includes Alpha particles, Beta particles, Gamma rays — is considered radioactive.
- 10. • Isotopes arevariants of a particular element which differ in Atomic Weight/ Mass/ Mass Number (Neutron + Proton), but have the same Atomic number (Protons). • The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons. • An atom of a given element may have a wide range in its number of neutrons. • Each isotope of a given element has a different Mass number. • For example, Carbon-12, Carbon-13 and Carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6 , which means that every carbon atom has 6 protons, so that the number of neutrons in these isotopes are 6, 7 and 8 respectively.
- 11. • Potassium naturallyoccurs in 3 isotopes: • 39 K (93.2581% , 40 K (0.0117%) , 41 K (6.7302%) • The 40 K isotope is radioactive; it decays with a half-life of 1.248×109 years to 40 Ca and 40 Ar. • K–Ar dating, is a radiometric dating method based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, etc. In these materials, the decay product 40 Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). • Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining. The long half-life of 40 K allows the method to be used to calculate the absolute age of samples older than a few thousand years. • The quickly cooled lavas make nearly ideal samples for K–Ar dating. • Potassium-40 has a half-life of 1.3 billion years. It can be used to date rocks older than 100,000 years.
- 12. • Rubidium occursin nature as the isotopes 86 Rb and 87 Rb. • The present relative abundance of these isotopes ie. 72.17% 86 Rb and 27.83% 87 Rb is the same in all rocks and minerals, regardless of age. • Apparently, these heavy isotopes were thoroughly mixed in the primeval Earth and have not experienced fractionation since then regardless of the geologic processes that have acted upon them. • Rb readily substitutes for K in micas and K-feldspar. • Rocks and minerals that have high concentrations of K also tend to have relatively high Rb, although the K/Rb ratio is not uniform in all materials, ranging over more than four orders of magnitude.
- 13. • Strontium hasfour stable isotopes, 88 Sr, 87 Sr, 86 Sr, and 84 Sr, whose relative abundance is 82.5%, 7.0%, 9.9%, and 0.6%, respectively. • But because 87 Sr is a decay product of 87 Rb, its exact abundance in a rock or mineral depends not only upon the amount of 87 Sr present when the material formed, but also upon the concentration of Rb and the age. • Materials rich in Rb, such as micas and alkali feldspars, will obviously contain considerable 87 Sr, especially if they are old. • As isotopic ratios are more accurately measured by mass spectrometers than the absolute amount of a single isotope, the abundance of 87 Sr is conventionally expressed as the ratio 87 Sr/86 Sr.
- 14. • The numberof atoms of 86 Sr in a mineral is constant, because it is a stable isotope not formed as a decay product of any other naturally occurring radioactive isotope. • The relationships among the present day measurable (87 Sr/86 Sr ) present ratio; the initial ratio (87 Sr/ 86 Sr)0, when the rock or mineral formed at time zero; its present day measurable 87 Rb/86 Sr ratio; the age in t years since the formation of the rock or mineral at time zero; and the decay constant λ for 87 Rb, is expressed by the equation (87 Sr/86 Sr)present = (87 Sr/86 Sr)0 +(87 Rb/87 Sr)*(e λt - 1)
- 15. • This isa linear equation of the form y = mx +c • A plot of x = 87 Rb/86 Sr and y = 87 Sr/86 Sr measured on separated minerals from one igneous rock, or on a group of genetically related whole rocks from a single igneous or metamorphic body that has behaved as a closed system since t = 0, yields a straight line called an isochron. (87 Sr/86 Sr)present = (87 Sr/86 Sr)0 +(87 Rb/87 Sr)*(e λt - 1) The intercept (c) of the isochron on the y axis is the initial ratio (87 Sr/86 Sr)0. From the slope of the line m = (e λ t - 1), the age of the rock from the time of crystallization can be calculated.
- 16. Limitations • The rockmust not have undergone any metasomatism (i.e. the chemical alteration of a rock by hydrothermal and other fluids) which could have disturbed the Rb-Sr system either thermally or chemically. • One of the major drawbacks of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids. • Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot hydrothermal fluids present during metamorphism
- 17. URANIUM – LEADDATING METHOD Uranium- Lead dating is one of the oldest and if done properly one of the most accurate. Uranium comes as two common isotopes: 235 U and 238 U. Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb). The two cascades are different—235 U becomes Pb207 (half life- 704 million years) and 238 U becomes 206 Pb (half life- 4.47 billion years). 238 U can be used to date rocks older than 10 million years. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time; helping us in dating. The favourite mineral among U-Pb dates is zircon (ZrSiO4), for several good reasons. Uranium- lead dating works only for metamorphic and igneous rocks.
- 18. • Lead (Pb)has four stable isotopes: 204 Pb, 206 Pb, 207 Pb, 208 Pb. 204 Pb is entirely a primordial nuclide and is not a radiogenic nuclide. • Lead isotopes Abundances • 204 Pb 1% (non-radiogenic) • 206 b 24% • 207 Pb 23% • 208 Pb 52% • 206 Pb, 207 Pb and 208 Pb are decay products of U and Th decay chains. • 238 U ---› 206 Pb • 235 U ---› 207 Pb • 232 Th ---›208 Pb • U-Pb isochron • Use of a single decay scheme (usually 238 U to 206 Pb) leads to the U–Pb isochron dating method, analogous to the rubidium- strontium dating method.
- 19. • The methodwas developed in the late 1940s at the by Willard Libby. • Carbon has Three Isotopes 12 C, 13 C and 14 C • 99% of all natural carbon is 12 C (Stable) • 13 C is also a stable nucleus. 1% of all natural carbon is 13 C . • Carbon with 6 protons and 8 neutrons is called Carbon-14 (14 C). • This is an unstable radioactive isotope. About 1 in 1012 carbon atoms in the atmosphere is 14 C. • The half-life of the decay of 14 C to nitrogen is 5730 years so the concentration halves every 5730 years. • Radiocarbon (14 C) is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. • Generation of radioactive 14 C occurs primarily in the upper troposphere, but C (mostly as CO2 ) mixes thoroughly in the atmosphere, and is incorporated into living organisms.
- 20. • The proportionof 14 C to 12 C in living tissue is comparable with the proportion in the atmosphere (for terrestrial organisms), or to a water body for aquatic organisms. • Animals get their 14 C dose from the food that they consume. • When the animal or plant dies, it stops exchanging carbon with its environment, and thereafter the amount of 14 C it contains begins to decrease. It decays back to nitrogen (14 N) by emitting an electron. • Measuring the amount of 14 C in a sample from a dead plant or animal, such as a piece of wood or a fragment of bone, provides information that can be used to calculate when the animal or plant died. • By measuring 14C content, you can estimate how long ago the tissue died. • The older a sample is, the less 14 C there is to be detected, and because the half-life of 14 C is about 5,730 years, the oldest dates that can be reliably measured by this process date to approximately 50,000 years ago, although special preparation methods occasionally make accurate analysis of older samples possible.
- 21. • Samples thatca be radiocarbon dated include charcoal, wood, twigs, seeds, bones, shells, leather, peat, lake mud, soil, hair, pottery, pollen, wall paintings, corals, blood residues, fabrics, paper or parchment, resins, and water, among others. Despite its usefulness, radiocarbon dating has a number of limitations. • It is assumed that the ratio of 12 C to 14 C has remained the same since last 50,000 Years. • First, the older the object, the less carbon-14 is there to measure. • Radiocarbon dating is therefore limited to objects that are younger than 50,000 years.