How are parent isotopes used in radiometric dating
Radiometric dating , radioactive dating or radioisotope 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. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. 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.
8.4 Isotopic Dating Methods
Radioactive Dating Radioactive Dating principle sources: A relative age simply states whether one rock formation is older or younger than another formation. The Geologic Time Scale was originally laid out using relative dating principles. The geological time scale is based on the the geological rock record, which includes erosion, mountain building and other geological events. Over hundreds to thousands of millions of years, continents, oceans and mountain ranges have moved vast distances both vertically and horizontally.
For example, areas that were once deep oceans hundreds of millions of years ago are now mountainous desert regions. How is geological time measured? The earliest geological time scales simply used the order of rocks laid down in a sedimentary rock sequence stratum with the oldest at the bottom. However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata.
After Charles Darwin's publication Origin of Species Darwin himself was also a geologist in , geologists realised that particular fossils were restricted to particular layers of rock. This built up the first generalised geological time scale. Once formations and stratigraphic sequences were mapped around the world, sequences could be matched from the faunal successions. These sequences apply from the beginning of the Cambrian period, which contains the first evidence of macro-fossils.
Fossil assemblages 'fingerprint' formations, even though some species may range through several different formations. This feature allowed William Smith an engineer and surveyor who worked in the coal mines of England in the late s to order the fossils he started to collect in south-eastern England in He noted that different formations contained different fossils and he could map one formation from another by the differences in the fossils.
As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels. By matching similar fossils in different regions throughout the world, correlations were built up over many years. Only when radioactive isotopes were developed in the early s did stratigraphic correlations become less important as igneous and metamorphic rocks could be dated for the first time.
Geological divisions Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms. For example, the Devonian Period is known as the 'Age of Fishes', as fish began to flourish at this stage. However, the end of the Devonian was marked by the predominance of a different life form, plants, which in turn denotes the beginning of the Carboniferous Period.
The different periods can be further subdivided e. This is the latest version of the time scale, as revised and published in Stratigraphic succession: Radiometric Radioactive Dating The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium-lead dating, the concordia diagram is used which also decreases the problem of nuclide loss.
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. 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. The precision of a dating method depends in part on the half-life of the radioactive isotope involved. 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 can not 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.
For example, decay of the parent isotope Rb Rubidium produces a stable daughter isotope, Sr Strontium , while releasing a beta particle an electron from the nucleus. Numerical ages have been added to the Geologic Time Scale since the advent of radioactive age-dating techniques. Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by: It illustrates how the amount of a radioactive parent isotope decreases with time.
This amount is a percentage of the original parent amount. Time is expressed in half-lives. Experiment by dragging on the graph. Note that this half-life can be obtained from the graph at the point where the decay and growth curves cross. Determine the half-lives for the other three isotopes and enter your estimate into the text fields below each graph.
Note the differences in scale between the various graphs Re-setting the Clock - 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 diffusion, setting 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. Radiocarbon Dating The radiocarbon dating method was developed in the 's by Willard F. Libby and a team of scientists at the University of Chicago.
It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50, years in age. The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology. The Rb-Sr method. Rubidium occurs in nature as two isotopes: Rb decays with a half-life of Which minerals and rocks can be dated with the Rb-Sr method?
The minerals must contain Rb, which is a rather rare element. Examples include the mica family biotite and muscovite and the feldspar family plagioclase and orthoclase. What steps are involved in Rb-Sr dating? Select a fresh, unweathered rock sample. Sample Selection A geologist collects a fresh, unweathered hand sample for age dating.
Fresh is the key word here, and means that the chemistry of the sample has NOT been changed since the sample formed. Weathering alters the chemistry of rocks including their isotopic compositions. Therefore, a highly weathered rock may yield unreliable age information. Crush the rock and separate the Rb-bearing minerals. Getting a Rock Sample Ready for the Mass Spectrometer For reliable age determination, careful sample preparation is an important and often tedious process.
The rock is mechanically crushed into small fragments. Fragments of the Rb-bearing minerals are then separated from the whole rock using a variety of methods, such as a magnetic separator. These materials are then used to prepare a "whole-rock" sample and several "mineral separate" samples. The whole rock sample will yield the weighted average isotopic composition of all the minerals in the rock. Each mineral separate will yield the composition of that particular mineral.
Other Steps There are other steps that must be carried out to prepare a sample for analysis by a mass spectrometer, such as converting the sample to a solution by dissolving the mineral separates in selected acids, using techniques of column chemistry to increase the concentration of the small amounts of Rb and Sr in the solution and then precipitating the concentrated solution as a "salt" compound.
It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis. Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer. A Mass Spectrometer is used to Measure Isotopic Ratios The gas source mass spectrometer includes three fundamental parts, 1 a "source" of positively charged ions or molecular ions, 2 a magnetic analyzer, and 3 ion collectors.
Once formed, the ions are accelerated and focused by charged plates into a beam that enters a flight tube. Multiple ion detectors are arranged to collect the ion beams of interest. These collectors measure each beam as a current that can be amplified and determined with high precision. A Mass Spectrometer is a very powerful and sophisticated instrument.
Many types exist. Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument. Understanding how a mass spectrometer functions is beyond the level of this activity. But you should know that it measures the amounts of various isotopes present in specially prepared samples of rocks and minerals as well as other materials.
Understanding the isochron diagram is the key to determining the age of a rock using the Rb-Sr method. More Parent-Daughter Relationships Thermoluminensnce Thermoluminescence TL dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated lava, ceramics or exposed to sunlight sediments.
As a crystalline material is heated during measurements the process of thermoluminescence starts. Thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating.
A technician of the U.S. Geological Survey uses a mass spectrometer to determine Precise dating has been accomplished since The parent isotopes and corresponding daughter products most commonly used to. Isotopes Commonly used for Radiometric Dating. Isotopes, Half-life (years), Effective Dating Range (years). Dating Sample, Key Fission Product. Lutetium-
All absolute isotopic ages are based on radioactive decay , a process whereby a specific atom or isotope is converted into another specific atom or isotope at a constant and known rate. Most elements exist in different atomic forms that are identical in their chemical properties but differ in the number of neutral particles—i. For a single element, these atoms are called isotopes.
Radioactive Dating Radioactive Dating principle sources:
When a rock cools from the molten to the solid state, its radioactive isotopes are immobilized in mineral crystal lattices and then decay in place. Knowing the rate of decay of one nuclear species nuclide into another, scientists can, in….
RADIOMETRIC TIME SCALE
Science in Christian Perspective. Radiometric Dating. A Christian Perspective. Roger C. Wiens has a PhD in Physics, with a minor in Geology.
Dating Fossils – How Are Fossils Dated?
Originally fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the different organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers. Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time. One of the isotope pairs widely used in geology is the decay of 40 K to 40 Ar potassium to argon It has a half-life of 1. In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral. One good example is granite, which normally has some potassium feldspar Figure 8.
A technician of the U. Geological Survey uses a mass spectrometer to determine the proportions of neodymium isotopes contained in a sample of igneous rock.
.How Carbon Dating Works