Geochronology - Wikipedia
Radiometric clocks are "set" when each rock forms. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock. Using relative and radiometric dating methods, geologists are able to answer The sedimentary rock layers exposed in the cliffs at Zumaia, Spain, are now tilted To establish the age of a rock or a fossil, researchers use some type of clock to . Determining the study these three main types of rocks. Geological Radiometric dating is igneous rock, defending the law of molten material. Thus an axiom.
By the s century, the controversy surrounding evolution prompted new attention. After all, if the Earth were too young for there to have been time for evolution, the evolution debate would be over. In John Jolyacting on suggestion of Edmund Halleyattempted estimate based on the salinity of the ocean. He calculated the amount of salt being transported into the oceans by rivers and compared this to the salinity of sea water, obtaining an age of 90 million years.
Sir William Thomson, Lord Kelvinduring the late 19th century, assumed that the Earth had originally been molten then, using averge melting point of rocks and the laws of thermodynamics, determined that the Earth would completely solidify within 20 million years. Both uniformitarians and evolutionists were uncomfortable, since their notions required a much older Earth, but the quantitative rigor of Thomson's approach made his the most prestigeous estimate of his day.
As it developed, both Joly and Tomson were leaving vital but unknown information out of their equations. Joly missed that salt is removed from the oceans by various processes. Kelvin could not have know that new heat is generated inside the Earth by radioactive decay nuclear fissionbecause the process had not been discovered. The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.
Discovered natural radioactivity In the following years, a large number of radioactive isotopes and their daughter products became known.
Radiometric dating ~ Learning Geology
Pierre and Marie Curie: Discovered that the radioactive element radium continuously releases newly generated heat - radiogenic heat. Radiometric dating of minerals in metamorphic rocks usually indicates the age of the metamorphism. Radioactive decay series A number of elements have isotopes forms of the element that have different atomic masses that are unstable and change by radioactive decay to the isotope of a different element.
Each radioactive decay series takes a characteristic length of time known as the radioactive half-life, which is the time taken for half of the original parent isotope to decay to the new daughter isotope. The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.
If the proportions of parent and daughter isotopes of these decay series can be measured, periods of geological time in millions to thousands of millions of years can be calculated. To calculate the age of a rock it is necessary to know the half-life of the radioactive decay series, the amount of the parent and daughter isotopes present in the rock when it formed, and the present proportions of these isotopes. It must also be assumed that all the daughter isotope measured in the rock today formed as a result of decay of the parent.
This may not always be the case because addition or loss of isotopes can occur during weathering, diagenesis and metamorphism and this will lead to errors in the calculation of the age. It is therefore important to try to ensure that decay has taken place in a 'closed system', with no loss or addition of isotopes, by using only unweathered and unaltered material in analyses. The radiometric decay series commonly used in radiometric dating of rocks are detailed in the following sections.
The choice of method of determination of the age of the rock is governed by its age and the abundance of the appropriate elements in minerals. Practical radiometric dating The samples of rock collected for radiometric dating are generally quite large several kilograms to eliminate inhomogeneities in the rock.
The samples are crushed to sand and granule size, thoroughly mixed to homogenise the material and a smaller subsample selected. In cases where particular minerals are to be dated, these are separated from the other minerals by using heavy liquids liquids with densities similar to that of the minerals in which some minerals will float and others sink, or magnetic separation using the different magnetic properties of minerals.
The mineral concentrate may then be dissolved for isotopic or elemental analysis, except for argon isotope analysis, in which case the mineral grains are heated in a vacuum and the composition of the argon gas driven off is measured directly. Measurement of the concentrations of different isotopes is carried out with a mass spectrometer. In these instruments a small amount micrograms of the sample is heated in a vacuum to ionise the isotopes and these charged particles are then accelerated along a tube in a vacuum by a potential difference.
- Relative dating igneous rocks
- Radiometric dating
Part-way along the tube a magnetic field induced by an electromagnet deflects the charged particles. The amount of deflection will depend upon the atomic mass of the particles so different isotopes are separated by their different masses. Detectors at the end of the tube record the number of charged particles of a particular atomic mass and provide a ratio of the isotopes present in a sample. Potassium—argon and argon—argon dating This is the most widely used system for radiometric dating of sedimentary strata, because it can be used to date the potassium-rich authigenic mineral glauconite and volcanic rocks lavas and tuffs that contain potassium in minerals such as some feldspars and micas.
One of the isotopes of potassium, 40 K, decays partly by electron capture a proton becomes a neutron to an isotope of the gaseous element argon, 40 Ar, the other product being an isotope of calcium, 40 Ca. The half-life of this decay is However, the proportion of potassium present as 40 K is very small at only 0.
Argon is an inert rare gas and the isotopes of very small quantities of argon can be measured by a mass spectrometer by driving the gas out of the minerals. K—Ar dating has therefore been widely used in dating rocks but there is a significant problem with the method, which is that the daughter isotope can escape from the rock by diffusion because it is a gas. The amount of argon measured is therefore commonly less than the total amount produced by the radioactive decay of potassium.
For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established.
On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes.
This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
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.