Absolute Dating
Absolute dating assigns fixed dates to the age of an object, people or intangible concepts, such as human language development.It means absolute dating methods produce specific chronological dates for objects and occupations. Absolute dating largely relies on scientific developments of the 20th century, but it also can derive absolute dates from history and archaeology.These dating methods provides a computed numerical age in contrast with relative dating which provides only an order of events.In archeology, absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans.This type of dating employs many dating techniques* like atomic clocks, carbon dating, annual cycle methods, and trapped electron method.
*After 1950, the physical sciences contributed a number of absolute dating techniques that had a revolutionary effect on archaeology and geology. These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism). Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
Prior to the discovery of radiometric dating which provided a means of absolute dating in the early 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques to determine the age of geological events.Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes.The Law of Superposition was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
The most popular method of radio dating is radiocarbon dating which is possible because of the presence of C-14, an unstable isotope of carbon. C-14 has a half life of 5730 years which means that only half of the original amount is left in the fossil after 5730 years while half of the remaining amount is left after another 5730 years. This gives away the true age of the fossil that contains C-14 that starts decaying after the death of the human being or animal. Dendrochronology is another of the popular method of finding the exact age through growth and patterns of thick and thin ring formation in fossil trees. It is clear then that absolute dating is based upon physical and chemical properties of artifacts that provide a clue regarding the true age.
The absolute dating methods most widely used and accepted are based on the natural radioactivity of certain minerals found in rocks. Since the rate of radioactive decay of any particular isotope is known, the age of a specimen can be computed from the relative proportions of the remaining radioactive material and its decay products. By this method the age of the earth is estimated to be about 4.5 billion years old. Some of the radioactive elements used in dating and their decay products (their stable daughter isotopes) are uranium-238 to lead-206, uranium-235 to lead-207, thorium-232 to lead-208, samarium-147 to neodymium-143, rubidium-87 to strontium-87, and potassium-40 to argon-40. Each radioactive member of these series has a known, constant decay rate, measured by its half-life, that is unaffected by any physical or chemical changes. Each decay element has an effective age range, including uranium-238 (100 million to 4.5 billion years) and potassium-40 (100,000 to 4.5 billion years).
Radiometric dating - Principles and History
In 1896, French physicist Henri Becquerel discovered radioactivity: the spontaneous emission of particles and energy from unstable nuclei of elements.The atoms of some chemical elements have different forms, called isotopes. These break down over time in a process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
Isotope: A version of an atom that differs from other atoms of the same element only in the number of neutrons. Different isotopes of an element have similar chemical properties (undergo similar chemical reactions) but have different physical properties (such as evaporation rates).
Stable Isotope: An isotope that persists forever because it has a “stable” ratio of protons to neutrons. For example, carbon-12 is a stable isotope.
Radioactive (or unstable) Isotope: An isotope that decays into another element because it has an “unstable” ratio of protons to neutrons. For example, carbon-14 is a radioactive isotope.
During radioactive decay, the radioactive parent isotope changes to a stable daughter isotope giving off heat in the process. There are 3 types of radioactive emissions:
Alpha ray: Equivalent to two protons and two neutrons (essentially a helium nucleus).
Beta ray: A free electron is released when a neutron converts to a proton.
Gamma ray: Consists of a photon (a packet of energy).
Some radioactive parent isotopes decay directly to a daughter isotope. However, some radioactive atoms decay to the daughter atom through a series of intermediate steps (called a decay series). The U238 decay series is a good example.The half-life is the amount of time required for one half of the parent to decay to daughter.Initially, there are many radioactive parent atoms so there are more radioactive emissions. As decay proceeds and there are fewer parent atoms and fewer emissions. By the 1st half life, 50% of the parent atoms will have decayed to daughter. By the 2nd half life, another 50% of the remaining parent will have decayed (leaving 25% parent and 75% daughter).
Absolute age dating is based upon the decay of radioactive (unstable) isotopes.Since the decay rate is constant over time, the parent:daughter ratio can be used to calculate the age of the mineral or rock.
Dating basically depends upon 3 measurements:
1) the amount of unstable parent isotope in the mineral
2) the amount of stable daughter isotope in the mineral
3) the decay constant (l) of the particular radioactive parent isotope.
Radiometric dating (also called radioactive 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 (In radiometry, the rate of radioactive decay of a specific element provides an absolute date).The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14 .Carbon-14 dating is probably one of the best-known dating methods, but the half-life of Carbon-14 is approximately 5730 years, plus or minus 40 years. Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old. Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.
The atmospheric C 14 is incorporated into carbon dioxide molecules (CO2). Organisms acquire C 14 from the air and water (along with 13C and 12C), and they acquire the environmental ratios of these isotopes. However, when organisms die, they stop acquiring any carbon and the C 14starts to decay back to N 14 via beta decay. The C 14 :N 14 ratio decreases over time, and this ratio can be used to calculate a material's age.All organic matter (bones, shells, wood, charcoal, cloth, and limestone) contains C-14and can be dated with this technique.
Carbon-14 has a relatively short half life of 5,730 years. It is good for dating young rocks and artifacts. Beyond 60,000 - 80,000 years, there is too little Carbon-14 left in the sample and this technique cannot be used.
Potassium-argon dating is another absolute dating method that is used to determine the age of igneous or sedimentary rocks. This, in turn, should provide some evidence for the dates of the fossils within the rocks. In this method, an absolute date is determined by measuring the amount of decay of potassium-40, a radioactive isotope of the element potassium that has transformed into the stable isotope argon-40.Potassium-40, for example, decays into Argon-40 with a half-life of 1.25 billion years, so that after 1.25 billion years half of the Potassium-40 in a rock will have become Argon-40. This means that if a rock sample contained equal amounts of Potassium-40 and Argon-40, it would be 1.25 billion years old.
Fission-track dating is a more recent application of the decay of radioisotopes, but this technique does not use the ratio of parent to daughter isotope to obtain an age.
Most U 238 undergoes alpha decay. However, a very small proportion of U 238 nuclei undergo fission and the nucleus splits to form two smaller but very energetic nuclei that move away from each other. When this happens in a mineral, the two departing nuclei leave behind a trail of destruction in the crystal lattice. The trail is called a fission track.
The density of fission tracks in a mineral increase with age and can be used to calculate the mineral's age.
Fission track dating is ideal for samples from “recent” times back to 100,000,000 years. Beyond 100,000,000 years, the density of the tracks becomes so great (saturated) that they cannot be counted reliably.
Fission tracks can “anneal” or heal with reheating, and so this method is affected by metamorphism.
Note :
Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock. Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in. However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.
All radiometric dating methods measure isotopes in some way. Most directly measure the amount of isotopes in rocks, using a mass spectrometer. Others measure the subatomic particles that are emitted as an isotope decays. Some measure the decay of isotopes more indirectly. For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes. Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.
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