Isochron dating is a common technique of radiometric dating and is applied to date certain events, such as crystallization , metamorphism , shock events, and differentiation of precursor melts, in the history of rocks. Isochron dating can be further separated into mineral isochron dating and whole rock isochron dating ; both techniques are applied frequently to date terrestrial and also extraterrestrial rocks meteorites. The advantage of isochron dating as compared to simple radiometric dating techniques is that no assumptions are needed about the initial amount of the daughter nuclide in the radioactive decay sequence. Indeed, the initial amount of the daughter product can be determined using isochron dating. This technique can be applied if the daughter element has at least one stable isotope other than the daughter isotope into which the parent nuclide decays. All forms of isochron dating assume that the source of the rock or rocks contained unknown amounts of both radiogenic and non-radiogenic isotopes of the daughter element, along with some amount of the parent nuclide.
Some isotopic systems based on short-living extinct radionuclides such as 53 Mn , 26 Al , I , 60 Fe and others are used for isochron dating of events in the early history of the Solar System.
However, methods using extinct radionuclides give only relative ages and have to be calibrated with radiometric dating techniques based on long-living radionuclides like Pb-Pb-dating to give absolute ages. Isochron dating is useful in the determination of the age of igneous rocks , which have their initial origin in the cooling of liquid magma.
It is also useful to determine the time of metamorphism, shock events such as the consequence of an asteroid impact and other events depending of the behaviour of the particular isotopic systems under such events. From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
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Isochron dating method
Why evolution works and creationism fails. Retrieved from " https: Articles needing additional references from May All articles needing additional references. Views Read Edit View history. This page was last edited on 11 June , at Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists.
These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. In this case, it is a big advantage to present the data in a form in which the abundance of both the parent and daughter isotopes are given with respect to the abundance of the initial background daughter.
The incremental additions of the daughter type can then be viewed in proportion to the abundance of parent atoms. In mathematical terms this is achieved as follows. When some daughter atoms are initially present designated D 0 , the total number D is the sum of radiogenic and initial atoms, so that.
To establish the condition that both parent and daughter abundances should be relative to the initial background, a stable isotope S of the daughter element can be chosen and divided into all portions of this equation; thus,. This term is called the initial ratio. The slope is proportional to the geologic age of the system. In practice, the isochron approach has many inherent advantages.
With time, each would then develop additional daughter abundances in proportion to the amount of parent present. If a number of samples are analyzed and the results are shown to define a straight line within error, then a precise age is defined because this is only possible if each is a closed system and each has the same initial ratio and age.
The uncertainty in determining the slope is reduced because it is defined by many points. A second advantage of the method relates to the fact that under high-temperature conditions the daughter isotopes may escape from the host minerals. In this case, a valid age can still be obtained, provided that they remain within the rock. Should a point plot below the line, it could indicate that a particular sample was open to migration of the dating elements or that the sample was contaminated and lay below the isochron when the rock solidified.
Rubidium-strontium Rb-Sr dating was the first technique in which the whole-rock isochron method was extensively employed. Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not.
Some studies have shown that rubidium is very mobile both in fluids that migrate through the rock as it cools and in fluids that are present as the rock undergoes chemical weathering. Similar studies have shown that the samarium - neodymium Sm-Nd parent-daughter pair is more resistant to secondary migration but that, in this instance, sufficient initial spread in the abundance of the parent isotope is difficult to achieve.
When an igneous rock crystallizes, a wide variety of major and trace minerals may form, each concentrating certain elements and radioactive trace elements within the rock. By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed.
In this case, a graph can be set up in which the slope of the line may be computed from an assumed value for the initial ratio, and it is usually possible to show that uncertainties related to this assumption are negligible. This is possible in potassium-argon K-Ar dating , for example, because most minerals do not take argon into their structures initially.
In rubidium-strontium dating , micas exclude strontium when they form but accept much rubidium. In uranium-lead U-Pb dating of zircon , the zircon is found to exclude initial lead almost completely. Minerals too are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered. A rock, on the other hand, may contain minerals formed at more than one time under a variety of conditions.
Under such circumstances the isolation and analysis of certain minerals can indicate at what time these conditions prevailed. If a simple mineral is widespread in the geologic record , it is more valuable for dating as more units can be measured for age and compared by the same method. However, if a single parent-daughter pair that is amenable to precise analysis can be measured in a variety of minerals, the ages of a wide variety of rock types can be determined by a single method without the need for intercalibration.
In some cases the discovery of a rare trace mineral results in a major breakthrough as it allows precise ages to be determined in formerly undatable units. For example, the minerals baddeleyite , an oxide of zirconium ZrO 2 , and zirconolite CaZrTi 2 O 7 , have been shown to be widespread in small amounts in mafic igneous rocks i.
Here, a single uranium-lead isotopic analysis can provide an age more precise than can be obtained by the whole-rock isochron method involving many analyses. When single minerals are analyzed, each grain can be studied under a microscope under intense side light so that alterations or imperfections can be revealed and excluded.
If minerals are used for dating, the necessary checks on the ages are achieved by analyzing samples from more than one location and by analyzing different grain sizes or mineral types that respond differently to disturbing events. It can be said that minerals provide a high degree of sample integrity that can be predicted on the basis of experience gained through numerous investigations under a variety of geologic conditions.
An ideal mineral is one that has sufficient parent and daughter isotopes to measure precisely, is chemically inert, contains little or no significant initial daughter isotopes, and retains daughter products at the highest possible temperatures. A specific datable mineral like rutile , which can be linked to a specific event such as the formation of a mineral deposit , is especially important. Since Earth was formed, the abundance of daughter product isotopes has increased through time.
For example, the ratio of lead of mass relative to that of mass has changed from an initial value of about 10 present when Earth was formed to an average value of about 19 in rocks at the terrestrial surface today. This is true because uranium is continuously creating more lead.
This would be called a model age. No parent-daughter value for a closed system is involvedórather, just a single isotopic measurement of lead viewed with respect to the expected evolution of lead on and in Earth. Unfortunately, the simplifying assumption in this case is not true, and lead model ages are approximate at best. Other model ages can be calculated using neodymium isotopes by extrapolating present values back to a proposed mantle-evolution line.
In both cases, approximate ages that have a degree of validity with respect to one another result, but they are progressively less reliable as the assumptions on which the model is calculated are violated. The progressive increase in the abundance of daughter isotopes over time gains a special significance where the parent element is preferentially enriched in either the mantle or the crust.
In contrast, modern volcanic rocks in the oceans imply that much of the mantle has a value between about 0. Should crustal material be recycled, the strontium isotopic signature of the melt would be diagnostic. Fossils record the initial, or primary, age of a rock unit. Isotopic systems, on the other hand, can yield either the primary age or the time of a later event, because crystalline materials are very specific in the types of atoms they incorporate, in terms of both the atomic size and charge.Decay scheme of K-Ar, U-Pb and Sm-Nd, petrogenetic implications-part B
An element formed by radioactive decay is quite different from its parent atom and thus is out of place with respect to the host mineral. All it takes for such an element to be purged from the mineral is sufficient heat to allow solid diffusion to occur. Each mineral has a temperature at which rapid diffusion sets in, so that, as a region is slowly heated, first one mineral and then another loses its daughter isotopes.
This is the temperature below which a mineral becomes a closed chemical system for a specific radioactive decay series. Accordingly, the parent-daughter isotope ratio indicates the time elapsed since that critical threshold was reached. In this case, the host mineral could have an absolute age very much older than is recorded in the isotopic record. The isotopic age then is called a cooling age.
It is even possible by using a series of minerals with different blocking temperatures to establish a cooling history of a rock bodyói. When this happens, the age has little to do with the cooling time. Another problem arises if a region undergoes a second reheating event. Certain minerals may record the first event, whereas others may record the second, and any suggestion of progressive cooling between the two is invalid.
This complication does not arise when rapid cooling has occurred. Identical ages for a variety of minerals with widely different blocking temperatures is unequivocal proof of rapid cooling. Fortunately for geologists, the rock itself records in its texture and mineral content the conditions of its formation.
A rock formed at the surface with no indication of deep burial or new mineral growth can be expected to give a valid primary age by virtue of minerals with low blocking temperatures. On the other hand, low-blocking-point minerals from a rock containing minerals indicative of high temperatures and pressures cannot give a valid primary age.
Such minerals would be expected to remain open until deep-level rocks of this sort were uplifted and cooled. Given these complicating factors, one can readily understand why geochronologists spend a great deal of their time and effort trying to see through thermal events that occurred after a rock formed. The importance of identifying and analyzing minerals with high blocking temperatures also cannot be overstated.
Minerals with high blocking temperatures that form only at high temperatures are especially valuable. Sufficient contamination can produce any isochron pattern regardless of the true isochron. It is even possible to get a negative slope. This would be equivalent to a negative or future date.
When you look at actual isochron plots such as the ones at the above source, there seems to be room for subjectivity. Some are better than others but there is often room for multiple plot lines. Even uniformitarian geologists recognize the existence of false isochrons, so how do they distinguish good data from bad? The answer is where the sample fits in the Geologic Column assumption quite prevalent within evolutionary science today.
This is the isochron presented by the author of the paper cited above. It is a fairly good five point isochron. It is also supported by additional isotope ratios. In addition the data points were the five out of ten samples that, "were interpreted to be in the more or less closed-system. So this otherwise good isochron was rejected because it disagreed with the fossils or essentially the pre-determined view of where those fossils are buried within the Geologic column.