# Isochron Dating

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There are two stable isotopes of carbon: 12 C and 13 C, and one naturally occurring radionuclide: 14 C. The half life of 14 C is only 5, years, which is orders of magnitude shorter than the age of the Earth. Therefore, no primordial radiocarbon remains and all 14 C is cosmogenic see Section 8 for related methods. The main production mechanism is through secondary cosmic ray neutron reactions with 14 N in the stratosphere: 7 14 N n,p 6 14 C. Any newly formed 14 C rapidly mixes with the rest of the atmosphere creating a spatially uniform carbon composition, which is incorporated into plants and the animals that eat them. Prior to the industrial revolution, a gram of fresh organic carbon underwent When a plant dies, it ceases to exchange carbon with the atmosphere and the 14 C concentration decays with time according to Equation 2. This can then be used to calculate the radiocarbon age by rearranging Equation 2. This method was developed by Willard Libby in , for which he was awarded the Nobel Prize in

## Holmes-Houtermans System for Lead Isochrons

The lead-lead isochron method for determining the age of ancient rocks including meteorites is generally thought to be the most reliable and precise method for such dating. Dalrymple calls the lead method “the hourglass of the solar system”. Many years of painstaking research has gone into establishing what is commonly called the Holmes-Houtermans System.

The two series proceed to different final lead isotopes: and there is a third lead isotope, Pb, that is not formed in any of the radioactive processes and can therefore be used as a reference.

An oversight in a radioisotope dating technique used to date everything scientists can back-calculate to determine when the rock was formed. The isochron is then plugged into a model, which uses it to turn the overall.

GSA Bulletin ; : — The Wufeng and Longmaxi organic-rich shales host the largest shale gas plays in China. The fractures hosted in the Longmaxi Formation are mineralized with quartz as the predominant fracture cement, and calcite as an intracementation phase postdating the earlier quartz cement. In contrast, the fractures hosted in the Wufeng Formation are dominantly mineralized by calcite, which occurs either as the only cement present or as a cement phase predating later quartz cement.

REE data equally indicate that the distinguishable Eu anomalies 6. The Sm-Nd isochron ages and fluid inclusion data of fracture cements suggest that fracture opening and calcite precipitation in composite veins within the Wufeng and Longmaxi Formations were triggered by gas generation overpressurization.

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## The Iconic Isochron: Radioactive Dating, Part 2

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.

Dating of “spilitic” basalts from the Proterozoic Seton Formation by the Pb/Pb when the latter ages are calculated with currently accepted decay constants.

Rubidium-strontium isochrons can be used to calculate the last time of complete melting of a rock. The complete melting of the rock is a necessary condition, because that is what accomplishes the equilibrium of the isotopes of strontium. The isotopes of an element are chemically identical , and any chemical process will treat them identically. That’s why we know the ratio of the strontium isotopes in the melt is a horizontal straight line in the illustration above.

The isotope 86 Sr is non-radiogenic in origin and does not change, but 87 Sr is produced by the radioactive decay of 87 Rb. There is no way of anticipating what the 87 Sr is at the time of melt, but if there is 87 Rb present then it will increase with time as the rubidium isotope decays. That is what makes this a useful clock. Rubidium-strontium isochrons will be formed at any time after crystallization of a rock provided the initial conditions are met. Different minerals which make up the rock will in general include different amounts of rubidium 87 Rb in their structures, and those which have more rubidium at the time of crystallization will have more radioactive decays and gain more of the daughter product 87 Sr.

The precise nature of the radioactive decay process predicts that all the minerals should lie along a straight line, an isochron. The longer the time interval, the more the decay and the steeper the slope of the isochron line. The slope of the isochron line gives a measurement of the time since the last complete melting of the rock. It also gives the initial concentrations of strontium at the time of melting by projecting the isochron line to the point of zero 87 Rb concentration.

If the strontium isotope ratios for the various minerals do not form a straight line, then the assumptions of the analysis are invalid.

## K-Ar dating calculation

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 a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating.

Below you will find a text describing the basic principles behind the Planetary Science Institute system of utilizing crater counts and isochron diagrams in order to estimate crater retention ages of surfaces on Mars. As discussed by Hartmann the crater numbers can date the actual formation age of a surface in an ideal case, such as a broad lava flow which forms a one-time eruptive event. The flow accumulates craters and the crater numbers date the time of formation.

In other cases, not uncommon on Mars, the story is more complex. For example a surface may be covered by a few hundred meters of mobile sand dunes; the numbers of craters of diameter D and depth d would give a mean characteristic age of topographic features of the scale of D,d. Smaller craters in mobile dunes would disappear faster and have lower mean ages.

In this sense, the derived age is a size-dependent Acrater retention age B the survival time of craters of given size. Another example is exhumation, which is common on Mars Malin and Edgett , A surface could be formed and covered by sediments at some later time, degrading any craters on the original surface. Much later, the surface may be exhumed, as documented in various cases by Malin and Edgett. In an ideal case, such a surface might then show vestiges of the degraded original craters indicating the duration of exposure of the first surface and a second population of fresh, small, sharp-rimmed craters formed since the recent exhumation event.

Malin and Edgett , , thinking in terms of dating the underlying process, asserted that such processes render Martian crater counts more or less useless for dating. On the contrary, such a situation can give extremely valuable estimates of the timescale of the exhumation processes, not to mention the timescale of exposure of the original underlying surface which in turn is a lower limit to its age.

In general, however, in dating Martian units, we look for simpler situations where a relatively homogenous stratigraphic unit is identified, and which appears not to be contaminated by secondary-ejecta impact craters from any single, nearby, large fresh primary impact crater.

## Isochron dating

Early Earth History Telling time The oldest minerals so far found on earth excluding meteorites are around 4. The earth is expected to be older than this, though, since erosion and tectonic activity destroy rocks over time. The oldest meteorites yield estimated ages of 4. This is taken to be the age of solid material in the solar system, and thus to be the age of the earth. Radiometric dating Radioactive isotopes “decay” over time as particles are lost.

In the preliminary test, the isochron method was used to date two teeth from Bau de l’Aubesier, Provence. Both isochron calculations produced a good estimate.

The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth. The equation is the one which describes radioactive decay:. Solving the equation for “age,” and incorporating the computation of the original quantity of parent isotope, we get:. Some assumptions have been made in the discussion of generic dating, for the sake of keeping the computation simple.

Such assumptions will not always be accurate in the real world. These include:. If one of these assumptions has been violated, the simple computation above yields an incorrect age. Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless. In many cases, there are independent cues such as geologic setting or the chemistry of the specimen which can suggest that such assumptions are entirely reasonable.

However, the methods must be used with care — and one should be cautious about investing much confidence in the resulting age Isochron methods avoid the problems which can potentially result from both of the above assumptions. Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.

For brevity’s sake, hereafter I will refer to the parent isotope as P , the daughter isotope as D , and the non-radiogenic isotope of the same element as the daughter, as D i. In addition, it requires that these measurements be taken from several different objects which all formed at the same time from a common pool of materials.

## The Age of the Earth

Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava.

U-series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform Calculation of Th isochrons, ages, and errors.

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. Then, in , radioactivity was discovered.

Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently. Principles of Radiometric Dating. Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.

The energies involved are so large, and the nucleus is so small that physical conditions in the Earth i.

## Radiometric dating

Radiometric dating is a means of determining the “age” of a mineral specimen by determining the relative amounts present of certain radioactive elements. By “age” we mean the elapsed time from when the mineral specimen was formed. Radioactive elements “decay” that is, change into other elements by “half lives. The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives.

If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula. To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.

Based on the assumptions of basic radioactive dating, the problem of an unknown initial amount of daughter isotope is eliminated by the definition of the isochron.

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. These assumptions include: 1 the initial amount of the daughter isotope is known, 2 neither parent or daughter product has migrated into, or out of, the closed rock system, and 3 decay has occurred at a constant rate over time.

But what if one or some combination of these assumptions is incorrect? Then the computed age based on the accumulation of daughter products will be incorrect Stasson In order to use the valuable information provided by radiometric dating, a new method had to be created that would determine an accurate date and validate the assumptions of radiometric dating. For this purpose, isochron dating was developed, a process “that solves both of these problems accurate date, assumptions at once” Stasson A natural clock must meet four requirements.

Isotope dating satisfies this requirement, as daughter products do not decay back to the original parent element. It has been established through extensive experimentation that radioactive decay occurs at a constant rate. In this case, the initial condition is the amount of daughter isotope in the rock when it was formed. This amount is often unknown and is one of the downfalls of conventional radiometric dating. However, isochron dating bypasses this assumption, as explained below.

The final condition is the number of atoms of parent and daughter isotopes remaining in the rock and can easily be measured in a lab.

## Calculating Rb-Sr Isochrons

An oversight in a radioisotope dating technique used to date everything from meteorites to geologic samples means that scientists have likely overestimated the age of many samples, according to new research from North Carolina State University. To conduct radioisotope dating, scientists evaluate the concentration of isotopes in a material. The number of protons in an atom determines which element it is, while the number of neutrons determines which isotope it is.

to errors in the age calculation. To overcome this prob- lem, subtraction or isochron dating was proposed by. Mejdahl(2). For quartz grains, the internal dose rate.

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.

Like Nimrod of ancient times, they know they must provide an alternative i. One of the indirect evidences that evolutionists universally appeal to is radioactive dating because it appears to supply the deep time their evolutionary models demand. But how accurate is their model, and how scientific is their approach? An isochron is a line on an isotope ratio diagram denoting rock samples. The slope of the line is related to the age of the samples. These two ratios, when plotted on a graph for many different samples from a rock suite, should hypothetically produce a straight line under certain assumed conditions.

The Y intercept of that line will then provide the initial ratio of the daughter isotopes at the time the rock suite formed, and the slope m will provide the age of the rock suite. To find out how much material began the decay process long ago, we need to determine N 0 , the number of parent atoms present when the rock was formed.