Radiometric Dating - Graphical Method one half-life of the original parent isotope remains, of the sample is now the daughter isotope. By determining the amount of the parent and daughter isotopes present scientists While determining the age of the Earth is intriguing, radiometric dating has. Radiometric dating or radioactive dating is a technique used to date materials such as rocks or In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in.
Absolute Geologic Time
If a new mineral grows in a metamorphic rock, and if that mineral incorporates radioactive isotopes in its crystal structure, then dating of that mineral can provide an estimate of the time of mineral growth metamorphism. Radiometric dating of metamorphic rocks can be successful, but often the results are difficult to interpret, and in many cases are ambiguous. The fundamental assumption in this simplified approach is that there existed no daughter atoms at the time the radiometric clock started.
This assumption is in many cases not valid, as daughter atoms certainly existed in the mineral or rock at the time the radiometric clock started. The solution to this problem can be illustrated using the Rubiduim Rb - Strontium Sr system. Rubidium exists as stable Rb 85 and unstable Rb Rb 87 decays to Sr 87 by beta decay same as C 14with a half-life of 50Ga. Strontium has four isotopes [Sr 84Sr 86Sr 87 and Sr 88 ]. Sr 87 in a given rock may have been produced by the decay of Rb 87 or it may have been present initially as part of the total Sr incorporated before any radioactive decay.
With Sr 87 as the daughter and RB 87 as the parent, the decay equation presented above is: The rock dated is from the Apollo Lunar mission, as is a basalt from a Mare basin.
In addition the whole rock WR was analyzed. All four point fall on a straight line known as an isochron meaning same age. After two half-lives, half of the remaining half will decay, leaving one-quarter of the original radioactive parent atoms.
Those transformed atoms bring the tally of daughter atoms to three-quarters of the crop of parent plus daughter atoms. The ratio of parent to daughter atoms after two half-lives is therefore 1: Successive half-lives reduce the original parent to one-eighth, one-sixteenth, one-thirty-second, and so on. The ratios of parent to daughter isotopes for these are 1: So assuming that when a rock forms it contains an unstable isotope and none of the daughter isotope or a well-known amountand assuming that over geologic time the rock remains a closed system no parent or daughter enters or leaves the rockthen that rock can be accurately dated by determining the ratio of parent to daughter atoms.
The first time this was done was by B. Boltwood inonly eight years after the discovery of radioactivity.
The oldest rocks known on the Earth are about four billion years old. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis.
The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No. 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. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.
In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test.
The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.
Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe. 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.
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. This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample.
Samarium—neodymium dating method[ edit ] Main article: Samarium—neodymium dating This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. Potassium—argon dating This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1.
Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.
Uranium—thorium dating method[ edit ] Main article: Uranium—thorium dating A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years.
It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured. The scheme has a range of several hundred thousand years. A related method is ionium—thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment.
Radiocarbon dating method[ edit ] Main article: