Radioactive dating - The Australian Museum
It was only in the early part of the 20th century, when isotopic dating methods were Radiocarbon dating can be used on sediments or sedimentary rocks that . Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot . Radiometric dating is largely done on rock that has formed from solidified lava. .. to have been derived from silicic volcanic ash from Jurassic island arcs at the .
All ordinary matter is made up of combinations of chemical elementseach with its own atomic numberindicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopeswith each isotope of an element differing in the number of neutrons in the nucleus.
A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture.
Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-lifeusually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product.
In many cases, the daughter nuclide itself is radioactive, resulting in a decay chaineventually ending with the formation of a stable nonradioactive daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.
This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric 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 datingthe 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.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. 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.
Note that because of the position of the dated beds, there is room for improvement in the time constraints on these fossil-bearing intervals e. A continuous vertical stratigraphic section will provide the order of occurrence of events column 1 of Figure 2. These are summarized in terms of a "relative time scale" column 2 of Figure 2. Geologists can refer to intervals of time as being "pre-first appearance of species A" or "during the existence of species A", or "after volcanic eruption 1" at least six subdivisions are possible in the example in Figure 2.
For this type of "relative dating" to work it must be known that the succession of events is unique or at least that duplicate events are recognized -- e. Unique events can be biological e. Ideally, geologists are looking for events that are unmistakably unique, in a consistent order, and of global extent in order to construct a geological time scale with global significance.
Some of these events do exist. For example, the boundary between the Cretaceous and Tertiary periods is recognized on the basis of the extinction of a large number of organisms globally including ammonites, dinosaurs, and othersthe first appearance of new types of organisms, the presence of geochemical anomalies notably iridiumand unusual types of minerals related to meteorite impact processes impact spherules and shocked quartz.
These types of distinctive events provide confirmation that the Earth's stratigraphy is genuinely successional on a global scale. Even without that knowledge, it is still possible to construct local geologic time scales. Although the idea that unique physical and biotic events are synchronous might sound like an "assumption", it is not.
It can, and has been, tested in innumerable ways since the 19th century, in some cases by physically tracing distinct units laterally for hundreds or thousands of kilometres and looking very carefully to see if the order of events changes.
Geologists do sometimes find events that are "diachronous" i. Because any newly-studied locality will have independent fossil, superpositional, or radiometric data that have not yet been incorporated into the global geological time scale, all data types serve as both an independent test of each other on a local scaleand of the global geological time scale itself. The test is more than just a "right" or "wrong" assessment, because there is a certain level of uncertainty in all age determinations.
For example, an inconsistency may indicate that a particular geological boundary occurred 76 million years ago, rather than 75 million years ago, which might be cause for revising the age estimate, but does not make the original estimate flagrantly "wrong".
It depends upon the exact situation, and how much data are present to test hypotheses e. Whatever the situation, the current global geological time scale makes predictions about relationships between relative and absolute age-dating at a local scale, and the input of new data means the global geologic time scale is continually refined and is known with increasing precision.
This trend can be seen by looking at the history of proposed geologic time scales described in the first chapter of [Harland et al,p. The unfortunate part of the natural process of refinement of time scales is the appearance of circularity if people do not look at the source of the data carefully enough. Most commonly, this is characterised by oversimplified statements like: When a geologist collects a rock sample for radiometric age dating, or collects a fossil, there are independent constraints on the relative and numerical age of the resulting data.
Stratigraphic position is an obvious one, but there are many others. There is no way for a geologist to choose what numerical value a radiometric date will yield, or what position a fossil will be found at in a stratigraphic section. Every piece of data collected like this is an independent check of what has been previously studied.
The data are determined by the rocks, not by preconceived notions about what will be found. Every time a rock is picked up it is a test of the predictions made by the current understanding of the geological time scale.
The time scale is refined to reflect the relatively few and progressively smaller inconsistencies that are found. This is not circularity, it is the normal scientific process of refining one's understanding with new data.
It happens in all sciences. If an inconsistent data point is found, geologists ask the question: However, this statistical likelihood is not assumed, it is tested, usually by using other methods e. Geologists search for an explanation of the inconsistency, and will not arbitrarily decide that, "because it conflicts, the data must be wrong.
The continued revision of the time scale as a result of new data demonstrates that geologists are willing to question it and change it. The geological time scale is far from dogma. If the new data have a large inconsistency by "large" I mean orders of magnitudeit is far more likely to be a problem with the new data, but geologists are not satisfied until a specific geological explanation is found and tested. An inconsistency often means something geologically interesting is happening, and there is always a tiny possibility that it could be the tip of a revolution in understanding about geological history.
Admittedly, this latter possibility is VERY unlikely. There is almost zero chance that the broad understanding of geological history e.
The amount of data supporting that interpretation is immense, is derived from many fields and methods not only radiometric datingand a discovery would have to be found that invalidated practically all previous data in order for the interpretation to change greatly. So far, I know of no valid theory that explains how this could occur, let alone evidence in support of such a theory, although there have been highly fallacious attempts e.
When Radiometric Dating "Just Works" or not A poor example There are many situations where radiometric dating is not possible, or where a dating attempt will be fraught with difficulty. This is the inevitable nature of rocks that have experienced millions of years of history: The real question is what happens when conditions are ideal, versus when they are marginal, because ideal samples should give the most reliable dates.
If there are good reasons to expect problems with a sample, it is hardly surprising if there are! It contains a mixture of minerals from a volcanic eruption and detrital mineral grains eroded from other, older rocks. If the age of this unit were not so crucial to important associated hominid fossils, it probably would not have been dated at all because of the potential problems.
After some initial and prolonged troubles over many years, the bed was eventually dated successfully by careful sample preparation that eliminated the detrital minerals. Lubenow's work is fairly unique in characterising the normal scientific process of refining a difficult date as an arbitrary and inappropriate "game", and documenting the history of the process in some detail, as if such problems were typical.
Another example is "John Woodmorappe's" paper on radiometric datingwhich adopts a "compilation" approach, and gives only superficial treatment to the individual dates. Among other problems documented in an FAQ by Steven Schimmrichmany of Woodmorappe's examples neglect the geological complexities that are expected to cause problems for some radiometrically-dated samples.
A good example By contrast, the example presented here is a geologically simple situation -- it consists of several primary i. It demonstrates how consistent radiometric data can be when the rocks are more suitable for dating. For most geological samples like this, radiometric dating "just works".
Consider this stratigraphic section from the Bearpaw Formation of Saskatchewan, Canada Baadsgaard et al. Modified from Baadsgaard et al. The section is measured in metres, starting with 0m at the bottom oldest. This section is important because it places a limit on the youngest age for a specific ammonite shell -- Baculites reesidei -- which is used as a zonal fossil in western North America. It consistently occurs below the first occurrence of Bacultes jenseni and above the occurrence of Baculites cuneatus within the upper part of the Campanian, the second to last "stage" of the Cretaceous Period in the global geological time scale.
The biostratigraphic situation can be summarized as a vertically-stacked sequence of "zones" defined by the first appearance of each ammonite species: About 40 of these ammonite zones are used to subdivide the upper part of the Cretaceous Period in this area. Dinosaurs and many other types of fossils are also found in this interval, and in broad context it occurs shortly before the extinction of the dinosaurs, and the extinction of all ammonites. The Bearpaw Formation is a marine unit that occurs over much of Alberta and Saskatchewan, and it continues into Montana and North Dakota in the United States, although it adopts a different name in the U.
The numbers above are just summary values. Other examples yield similar results - i. The results are therefore highly consistent given the analytical uncertainties in any measurement. Eberth and Braman described the vertebrate paleontology and sedimentology of the Judith River Formation, a dinosaur-bearing unit that occurs stratigraphically below the Baculites reesidei zone the Judith River Formation is below the Bearpaw Formation. It should therefore be older than the results from Baadsgaard et al.
An ash bed near the top of the Judith River Fm. Again, this is compatible with the age determined for the Baculites reesidei zone and its relative stratigraphic position, and even with the relative position of the two samples within the same formation.
How do these dates compare to the then current geological time scale? Here are the numbers they applied to the geological boundaries in this interval, compared to the numbers in the newer studies: Comparison of newer data with the Harland et al.
As you can see, the numbers in the rightmost column are basically compatible. Skeptics of radiometric dating procedures sometimes claim these techniques should not work reliably, or only infrequently, but clearly the results are similar: Most of the time, the technique works exceedingly well to a first approximation. However, there are some smaller differences. The date for the Baculites reesidei zone is at least 0. Well, standard scientific procedure is to collect more data to test the possible explanations -- is it the time scale or the data that are incorrect?
Obradovich has measured a large number of high-quality radiometric dates from the Cretaceous Period, and has revised the geological time scale for this interval.
Specifically, he proposes an age of This is completely compatible with the data in Baadsgaard et al. Conclusions Skeptics of conventional geology might think scientists would expect, or at least prefer, every date to be perfectly consistent with the current geological time scale, but realistically, this is not how science works. The age of a particular sample, and a particular geological time scale, only represents the current understanding, and science is a process of refinement of that understanding.
More Bad News for Radiometric Dating
In support of this pattern, there is an unmistakable trend of smaller and smaller revisions of the time scale as the dataset gets larger and more precise Harland et al. If something were seriously wrong with the current geologic time scale, one would expect inconsistencies to grow in number and severity, but they do not.
The same trend can be observed for other time periods. Palmer and Harland et al. The latter includes an excellent diagram summarizing comparisons between earlier time scales Harland et al. Sincethere have been still more revisions by other authors, such as Obradovich for the Cretaceous Period, and Gradstein et al.
A recent geological time scale, based on Harland et al. This is not uncommon. Besides the papers mentioned here, there are hundreds, if not thousands, of similar papers providing bracketing ranges for fossil occurrences. The synthesis of work like this by thousands of international researchers over many decades is what defines geological time scales in the first place refer to Harland et al.
Although geologists can and do legitimately quibble over the exact age of a particular fossil or formation e. The data do not support such an interpretation. The methods work too well most of the time. In addition, evidence from other aspects of geology e. Prior to the availability of radiometric dating, and even prior to evolutionary theory, the Earth was estimated to be at least hundreds of millions of years old see above. Radiometric dating has simply made the estimates more precise, and extended it into rocks barren of fossils and other stratigraphic tools.
The geological time scale and the techniques used to define it are not circular. They rely on the same scientific principles as are used to refine any scientific concept: There are innumerable independent tests that can identify and resolve inconsistencies in the data. This makes the geological time scale no different from other aspects of scientific study.
Refuting the conventional geological time scale is not an exercise in collecting examples of the worst samples possible. A critique of conventional geologic time scale should address the best and most consistent data available, and explain it with an alternative interpretation, because that is the data that actually matters to the current understanding of geologic time.
References also refer to " Other sources " Baadsgaard, H. Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan Campanian-Maastrichtian stage boundary? Canadian Journal of Earth Sciences, v.
Stratigraphy, sedimentology, and vertebrate paleontology of the Judith River Formation Campanian near Muddy Lake, west-central Saskatchewan. Bulletin of Canadian Petroleum Geology, v. A Triassic, Jurassic and Cretaceous time scale. A Geologic Time Scale: