Isochron dating process

Isochron dating process

Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists.

Isochron dating

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 involved—rather, 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.

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 body—i.

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. The mineral zircon datable by the uranium-lead method is one such mineral. Successively higher blocking temperatures are recorded for another mica type known as muscovite and for amphibole , but the ages of both of these minerals can be completely reset at temperatures that have little or no effect on zircon.

Vast areas within the Canadian Shield , which have identical ages reflecting a common cooling history, have been identified. These are called geologic provinces. The age of a geologic sample is measured on as little as a billionth of a gram of daughter isotopes. Moreover, all the isotopes of a given chemical element are nearly identical except for a very small difference in mass. Such conditions necessitate instrumentation of high precision and sensitivity.

Both these requirements are met by the modern mass spectrometer. A high-resolution mass spectrometer of the type used today was first described by the American physicist Alfred O. Nier in , but it was not until about that such instruments became available for geochronological research see also mass spectrometry. For isotopic dating with a mass spectrometer, a beam of charged atoms, or ions, of a single element from the sample is produced. This beam is passed through a strong magnetic field in a vacuum , where it is separated into a number of beams, each containing atoms of only the same mass.

Because of the unit electric charge on every atom, the number of atoms in each beam can be evaluated by collecting individual beams sequentially in a device called a Faraday cup. Once in this collector, the current carried by the atoms is measured as it leaks across a resistor to ground. It is not possible simply to count the atoms, because all atoms loaded into the source do not form ions and some ions are lost in transmission down the flight tube. Precise and accurate information as to the number of atoms in the sample can, however, be obtained by measuring the ratio of the number of atoms in the various separated beams.

By adding a special artificially enriched isotope during sample dissolution and by measuring the ratio of natural to enriched isotopes in adjacent beams, the number of daughter isotopes can be readily determined. Lead produced in a type of particle accelerator called a cyclotron constitutes such an ideal spike. As the sample is heated and vaporizes under the vacuum in the source area of the mass spectrometer, it is commonly observed that the lighter isotopes come off first, causing a bias in the measured values that changes during the analysis.

In most cases this bias, or fractionation, can be corrected if the precise ratio of two of the stable isotopes present is known. Such precision is often essential in the isochron method see above because of the small changes in relative daughter abundance that occur over geologic time. The ability to add a single artificial mass to the spectrum in a known amount and to determine the abundances of other isotopes with respect to this provides a powerful analytical tool.

By means of this process, known as isotope dilution , invisibly small amounts of material can be analyzed, and, because only ratios are involved, a loss of part of the sample during preparation has no effect on the result. Spike solutions can be calibrated simply by obtaining a highly purified form of the element being calibrated.

After carefully removing surface contamination, a precisely weighted portion of the element is dissolved in highly purified acid and diluted to the desired level in a weighed quantity of water. What is required is dilution of 1 cubic cm to 1 litre 0. In this way, a known number of natural isotopes can be mixed with a known amount of spike and the concentration in the spike solution determined from the ratio of the masses.

Dating - The isochron method: Many radioactive dating methods are based on By means of this process, known as isotope dilution, invisibly small amounts of. And there are known processes which can yield an incorrect isochron age. Does this leave room to discard isochron dating as entirely.

The Bible is quite clear about the origin and timeframe for the creation of Earth and the cosmos. If Scripture is inaccurate in this, then how can it be trusted in anything else? Some evolutionists throw out theistic evolution God using evolution as His creative process as a philosophical panacea, with the goal of leading people to conclude that Genesis is a myth.

Radioactive decay has become one of the most useful methods for determining the age of formation of rocks. However, in the very principal of radiometric dating there are several vital assumptions that have to be made in order for the age to be considered valid.

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 Iconic Isochron: Radioactive Dating, Part 2

However, it is not clear exactly when that happened. Whether it happened during a Flood can be reasonably questioned. In order for a change in the decay constant to be helpful to a creationist arguing for a short age for fossiliferous strata, the decay constant would have to change during the Flood. However, this would mean that the radioactive elements inside the bodies of Noah, his family, the animals in the ark, and whatever animals survived outside the ark were spared from the otherwise general increase in radioactive decay, or that their bodies proved resistant to the effects of the increased radioactivity. This is not impossible, but does require extra intervention or more change in the usual laws of physics. The percentage of 87 Sr varies between 6.

Isochron dating is a common radiometric dating technique applied to date natural events like the crystallization of minerals as they cool, changes in rocks by metamorphism, or what are essentially naturally occurring shock events like meteor strikes. Minerals present in these events contain various radioactive elements which decay and the resulting daughter elements can then be used to deduce the age of the mineral through an isochron. The appeal of isochron dating is that it does not presuppose the initial amount of the daughter nuclide in the decay sequence. Indeed, the initial amount is not important because it can be found through this type of dating. Isochron dating began when scientists recognized difficulties with the assumptions of radiometric dating, especially how much of the daughter products might have been present when the mineral first formed. Isochron dating has been developed in an attempt to solve such problems. According to theory, the sample starts out with daughter isotopes present at constant ratios in relation to one another, but with the parent isotope the ratio is arbitrary. As a result it can be displayed in the form of a straight horizontal line on a graph. As the parent decays to daughter the ratios change and the straight line remains but becomes angled. The slope of the line equals the number of half-lives the parent isotope has passed since solidification.

The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.

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Radioactive Dating
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