Radiometric dating of metamorphic rocks names

radiometric dating of metamorphic rocks names

Radiometric dating is a method of determining the age of an artifact by assuming that on average decay rates have been constant (see below for the flaws in that assumption) and measuring the amount of radioactive decay that has occurred. [1] Radiometric dating is mostly used to determine the age of rocks, though a particular form of radiometric dating—called Radiocarbon dating —can date wood, cloth, skeletons, and other organic material.

Because radiometric dating fails to satisfy standards of testability and falsifiability , claims based on radiometric dating may fail to qualify under the Daubert standard for court-admissible scientific evidence. It is more accurate for shorter time periods (e.g., hundreds of years) during which control variables are less likely to change.

One key assumption is that the initial quantity of the parent element can be determined. With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample. One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect (too old).

In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C 14 to C 12 , but the same principle otherwise applies.

Recognizing this problem, scientists try to focus on rocks that do not contain the decay product originally. For example, in uranium-lead dating, they use rocks containing zircon (ZrSiO 4 ), though it can be used on other materials, such as baddeleyite . [2] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium , but strongly reject lead. Zincon has a very high closure temperature, is very chemically inert, and is resistant to mechanical weathering. For these reasons, if a rock strata contains zircon, running a uranium-lead test on a zircon sample will produce a radiometric dating result that is less dependent on the initial quantity problem.

It is important that the sample not have had any outside influences. One example of this can be found in metamorphic rocks. [5] This does not mean that all rock samples are unreliable, but it is possible to account for a process which throws off the data for metamorphic rocks.

For example, with Uranium-lead dating with the crystallization of magma, this remains a closed system until the uranium decays. As it decays, it disrupts the crystal and allows the lead atom to move. Likewise, heating the rock such as granite forms gneiss or basalt forms schist . This can also disrupt the ratios of lead and uranium in the sample.

Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. [1] The use of radiometric dating was first published in 1907 by Bertram Boltwood [2] and is now the principal source of information about the absolute age of rocks and other geological features , including the age of fossilized life forms or the age of the Earth itself, and can also 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 . [3] Among the best-known techniques are radiocarbon dating , potassium-argon dating and uranium-lead dating . By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement (except as described below under "Dating with short-lived extinct radionuclides"), the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope ratio mass spectrometry . [9]

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating can not be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. [10]

The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. [12] [16] However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.

Uranium-lead radiometric dating involves using Uranium-235 or Uranium-238 to date a substance's absolute age. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. [13] [18] An error margin of 2–5% has been achieved on younger Mesozoic rocks. [19]

Uranium-lead dating is often performed on the mineral zircon (ZrSiO 4 ), though it can be used on other materials, such as baddeleyite , as well as monazite (see: monazite geochronology ). [20] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium , but strongly reject lead. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques. [21]



Age of the Earth - Wikipedia

Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. [1] The use of radiometric dating was first published in 1907 by Bertram Boltwood [2] and is now the principal source of information about the absolute age of rocks and other geological features , including the age of fossilized life forms or the age of the Earth itself, and can also 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 . [3] Among the best-known techniques are radiocarbon dating , potassium-argon dating and uranium-lead dating . By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement (except as described below under "Dating with short-lived extinct radionuclides"), the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope ratio mass spectrometry . [9]

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating can not be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. [10]

The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. [12] [16] However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.

Uranium-lead radiometric dating involves using Uranium-235 or Uranium-238 to date a substance's absolute age. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. [13] [18] An error margin of 2–5% has been achieved on younger Mesozoic rocks. [19]

Uranium-lead dating is often performed on the mineral zircon (ZrSiO 4 ), though it can be used on other materials, such as baddeleyite , as well as monazite (see: monazite geochronology ). [20] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium , but strongly reject lead. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques. [21]