assignment radio metrics

8/2/2019 Assignment Radio Metrics

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INTRODUCTION

Rocks are made up of many individual crystals, and each crystal is usuallymade up of at least several different chemical elements such as iron,magnesium, silicon, etc. Most of the elements in nature are stable and do notchange. However, some elements are not completely stable in their naturalstate. Some of the atoms eventually change from one element to another by aprocess called radioactive decay. If there are a lot of atoms of the originalelement, called the parent element, the atoms decay to another element,called the daughter element, at a predictable rate. The passage of time can becharted by the reduction in the number of parent atoms, and the increase inthe number of daughter atoms.

Radiometric dating can be compared to an hourglass. When the glass is turnedover, sand runs from the top to the bottom. Radioactive atoms are likeindividual grains of sand--radioactive decays are like the falling of grains fromthe top to the bottom of the glass. You cannot predict exactly when any oneparticular grain will get to the bottom, but you can predict from one time tothe next how long the whole pile of sand takes to fall. Once all of the sand hasfallen out of the top, the hourglass will no longer keep time unless it is turnedover again. Similarly, when all the atoms of the radioactive element are gone,the rock will no longer keep time (unless it receives anew batch of radioactive atoms).


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Figure 1 . The rate of loss of sand from from the top of an hourglass compared to exponential type of decay ofradioactive elements. Most processes that we are familiar with are like sand in an hourglass. In exponential decathe amount of material decreases by half during each half-life. After two half-lives one-fourth remains, after threhalf-lives, one-eighth, etc. As shown in the bottom panel, the daughter element or isotope amountincreases rapidly at first and more slowly with each succeeding half life.


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Unlike the hourglass, where the amount of sand falling is constant right upuntil the end, the number of decays from a fixed number of radioactive atomsdecreases as there are fewer atoms left to decay (see Figure 1) . If it takes acertain length of time for half of the atoms to decay, it will take the sameamount of time for half of the remaining atoms, or a fourth of the original

total, to decay . In the next interval, with only a fourth remaining, only oneeighth of the original total will decay . By the time ten of these intervals, orhalf-lives, has passed, less than one thousandth of the original number of radioactive atoms is left . The equation for the fraction of parent atoms left isvery simple . The type of equation is exponential, and is related to equationsdescribing other well-known phenomena such as population growth . Nodeviations have yet been found from this equation for radioactive decay.

Also unlike the hourglass, there is no way to change the rate at whichradioactive atoms decay in rocks . If you shake the hourglass, twirl it, or put itin a rapidly accelerating vehicle, the time it takes the sand to fall will change .

But the radioactive atoms used in dating techniques have been subjected toheat, cold, pressure, vacuum, acceleration, and strong chemical reactions tothe extent that would be experienced by rocks or magma in the mantle, crust,or surface of the Earth or other planets without any significant change in theirdecay rate.

In only a couple of special cases have any decay rates been observed to vary,and none of these special cases apply to the dating of rocks as discussed here .

An hourglass will tell time correctly only if it is completely sealed . If it has ahole allowing the sand grains to escape out the side instead of going through

the neck, it will give the wrong time interval . Similarly, a rock that is to bedated must be sealed against loss or addition of either the radioactivedaughter or parent . If it has lost some of the daughter element, it will give aninaccurately young age . As will be discussed later, most dating techniqueshave very good ways of telling if such a loss has occurred, in which case thedate is thrown out (and so is the rock!).

An hourglass measures how much time has passed since it was turned over .(Actually it tells when a specific amount of time, e.g., 2 minutes, an hour, etc.,has passed, so the analogy is not quite perfect.) Radiometric dating of rocksalso tells how much time has passed since some event occurred . For igneous

rocks the event is usually its cooling and hardening from magma or lava . Forsome other materials, the event is the end of a metamorphic heating event (inwhich the rock gets baked underground at generally over a thousand degreesFahrenheit), the uncovering of a surface by the scraping action of a glacier,the chipping of a meteorite off of an asteroid, or the length of time a plant oranimal has been dead.


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INTRODUCTION TO RADIOMETRIC DATING METHODS

Radiometric dating methods are the strongest direct evidence that geologists have for the age of thEarth. All these methods point to Earth being very, very old -- several billions of years old. YoungEarth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old are fond of attacking radiometric dating methods as being full of inaccuracies and riddled wisources of error. When I first became interested in the creation-evolution debate, in late 1994,looked around for sources that clearly and simply explained what radiometric dating is and whyoung-Earth creationists are driven to discredit it. I found several good sources, but none thseemed both complete enough to stand aloneand simple enough for a non-geologist to understandthem. Thus this essay, which is my attempt at producing such a source.

THEORY OF RADIOMETRIC DATING

What is radiometric dating?

Simply stated, radiometric dating is a way of determining the age of a sample of material using thdecay rates of radioactive nuclides to provide a 'clock.' It relies on three basic rules, plus a coupof critical assumptions. The rules are the same in all cases; the assumptions are different for eacmethod. To explain those rules, I'll need to talk about some basic atomic physics.

There are90 naturally occurring chemical elements. Elements are identified by their atomicnumber , the number of protons in the atom's nucleus. All atoms except the simplest, hydrogen-1have nuclei made up of protons and neutrons. Hydrogen-1's nucleus consists of only a singl proton. Protons and neutrons together are callednucleons , meaning "particles that can appear inthe atomic nucleus."

A nuclide of an element, also called anisotope of an element, is an atom of that element that has aspecific number of nucleons. Since all atoms of the same element have the same number o protons, different nuclides of an element differ in the number of neutrons they contain. Foexample, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutronUranium-238 contains 92 protons and 146 neutrons, while uranium-235 contains 92 protons an143 neutrons. To keep it short, a nuclide is usually written using the element's abbreviationUranium's abbreviation is U, so uranium-238 can be more briefly written as U238.

Many nuclides arestable -- they will always remain as they are unless some external force changesthem. Some, however, areunstable -- given time, they will spontaneously undergo one of theseveral kinds of radioactive decay, changing in the process into another element.

There are two common kinds of radioactive decay,alpha decay andbeta decay .

i) Alpha decay


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In alpha decay, the radioactive atom emits analpha particle . An alpha particle contains two protons and two neutrons, and is usually represented by a lowercase Greek letter alpha, . Aftemission, it quickly picks up two electrons to balance the two protons, and becomes an electricalneutral helium-4 (He4) atom. When a nuclide emits an alpha particle, its atomic number drops b2, and its mass number (number of nucleons) drops by 4. Thus, an atom of U238 (uranium, atom

number 92) emits an alpha particle and becomes an atom of Th234 (thorium, atomic number 90Physicists usually write this asU238 --> Th234 + .

ii) Beta decay

In beta decay, the radioactive atom emits abeta particle . A beta particle is just an electron, and isusually represented by a lowercase Greek letter beta, . To balance the electron's negative electriccharge, a neutron inside the nucleus is transformed to a proton. The mass number doesn't change,

but the atomic number goes up by 1. Thus, an atom of carbon-14 (C14), atomic number 6, emits a beta particle and becomes an atom of nitrogen-14 (N14), atomic number 7. Physicists usually writhis asC14 --> N14 + .

iii)Electron absorption

A third, very rare type of radioactive decay is calledelectron absorption . In electron absorption, a proton absorbs an electron to become a neutron. In other words, electron absorption is the exareverse of beta decay. The atomic number goes down by 1, but the mass number doesn't changSo an atom of potassium-40 (K40), atomic number 19 can absorb an electron to become an atom argon-40 (Ar40), atomic number 18.

Half-life

Thehalf-life of a radioactive nuclide is defined as the time it takes half of a sample of the elemento decay. A mathematical formula can be used to calculate the half-life from the number o breakdowns per second in a sample of the nuclide. Some nuclides have very long half-livemeasured in billions or even trillions of years. Others have extremely short half-lives, measured tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristiof a nuclide. They don't change at all. That's the first axiom of radiometric dating techniques: thhalf-life of a given nuclide is a constant. (Note that this doesn't mean the half-life of anelement is aconstant. Different nuclides of the same element can have substantially different half-lives.)

The half-life is a purely statistical measurement. It doesn't depend on the age of individual atomA sample of U238 ten thousand years old will have precisely the same half-life as one ten billioyears old. So, if we know how much of the nuclide was originally present, and how much therenow, we can easily calculate how long it would take for the missing amount to decay, and therefo


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how long it's been since that particular sample was formed. That's the essence of radiometridating: measure the amount that's present, calculate how much is missing, and figure out how lonit would take for that quantity of the nuclide to break down. Because it's a statistical measure-menthere's always a margin of error in the age figure, but if the procedure is done properly, the margiis very small.

Obviously, the major question here is "how much of the nuclide was originally present in ousample?" In some cases, we don't know. Such cases are useless for radiometric dating. Wemust know the original quantity of the parent nuclide in order to date our sample radiometricallyFortunately, there are cases where we can do that.

In order to do so, we need a nuclide that's part of a mineral compound. Why? Because there's basic law of chemistry that says "Chemical processes like those that form minerals candistinguish between different nuclides of the same element." They simply can't do it. If an elemehas more than one nuclide present, and a mineral forms in a magma melt that includes thaelement, the element's different nuclides will appear in the mineral in precisely the same ratio th

they occurred in the environment where and when the mineral was formed. This is the seconaxiom of radiometric dating.

The third and final axiom is that when an atom undergoes radioactive decay, its internal structurand also its chemical behavior change. Losing or gaining atomic number puts the atom in different row of the periodic table, and elements in different rows behave in different ways. Thnew atom doesn't form the same kinds of chemical bonds that the old one did. It may not form thsame kinds of compounds. It may not even be able to hold the parent atom's place in the compounit finds itself in, which results in an immediate breaking of the chemical bonds that hold the atoto the others in the mineral.

Why not? you might ask. Well, an atom's chemical activity pattern is a result of its electron shestructure. (The exact details of this are rather complicated, so I won't go into them here.) When thnumber of electrons change, the shell structure changes too. So when an atom decays and changinto an atom of a different element, its shell structure changes and it behaves in a different wachemically.

Some Naturally Occurring Radioactive Isotopes and their half-lives


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RadioactiveIsotope

(Parent)

Product

(Daughter)

Half-Life

(Years)

Samarium-147 Neodymium-143

106 billion

Rubidium-87 Strontium-87 48.8 billion

Rhenium-187 Osmium-187 42 billion

Lutetium-176 Hafnium-176 38 billion

Thorium-232 Lead-208 14 billion

Uranium-238 Lead-206 4.5 billion

Potassium-40 Argon-40 1.26 billion

Uranium-235 Lead-207 0.7 billion

Beryllium-10 Boron-10 1.52 million

Chlorine-36 Argon-36 300,000

Carbon-14 Nitrogen-14 5715

Uranium-234 Thorium-230 248,000

Thorium-230 Radium-226 75,400

Most half-lives taken from Holden, N.E. (1990) Pure Appl. Chem. 62 , 941-958.

History of Radiometric Dating

At the time that Darwin'sOn theOrigin of Species was published, the

The radioactivity of Potassium 40 is unusual, in that twoprocesses take place:

b-decay:88.8%

electron capture: 11.2%

Radioactive Parent Stable Daughter Half lifePotassium 40 Argon 40 1.25 billion yrsRubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billionyears

Uranium 235 Lead 207 704 millionyears

Uranium 238 Lead 206 4.47 billionyearsCarbon 14 Nitrogen 14 5730 years


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earth was "scientifically" determined to be 100 million years old. By 1932, it was found to be 1 billion years old. In 1947, science firmly established that the earth was 3.4 billion years oldFinally in 1976, it was discovered that the earth is "really" 4.6 billion years old. The study ogeology grew out of field studies associated with mining and engineering during the sixteenth nineteenth centuries. In these early studies the order of sedimentary rocks and structures were us

to date geologic time periods and events in a relative way. At first, the use of "key" diagnostfossils was used to compare different areas of the geologic column. Although there were attempto make relative age estimates, no direct dating method was available until the twentieth century.

However, before this time some very popular indirect methods were available. For example, LoKelvin had estimated the ages of both the Earth and the Sun based on cooling rates. The answer 25 million years deduced by Kelvin was not received favorably by geologists. Both the physicgeologists and paleontologists could point to evidence that much more time was needed to produwhat they saw in the stratigraphic and fossil records. As one answer to his critics, Kelvin producea completely independent estimate -- this time for the age of the Sun. His result was in closagreement with his estimate of the age of the earth. The solar estimate was based on the idea th

the energy supply for the solar radioactive flux is gravitational contraction. These two independeand agreeing dating methods for of the age of two primary members of the solar system formedstrong case for the correctness of his answer within the scientific community.

This just goes to show that just because independent estimates of age seem to agree with each othdoesn't mean that they're correct - despite the fact that this particular argument is the very same onused to support the validity of radiometric dating today. Other factors and basic assumptions mualso be considered.

Of course, Kelvin formed his estimates of the age of the Sun without the knowledge of fusion the true energy source of the Sun. Without this knowledge, he argued that, "As for the future, w

may say, with equal certainty, that inhabitants of the Earth cannot continue to enjoy the light anheat essential to their life, for many million years longer, unless sources now unknown to us ar prepared in the great storehouse of creation." This last statement was prophetic. There were inde powerful and unknown sources of energy fueling the Sun's energy output.

The same is true of the basis of Kelvin's estimate of the age of the Earth. It was based on the idethat no significant source of novel heat energy was affecting the Earth. He believed this evethough he did admit that some heat might be generated by the tidal forces or by chemical actioHowever, on the whole, he thought that these sources were not adequate to account for anythinmore than a small faction of the heat lost by the Earth. Based on these assumptions he at firsuggested an age of the Earth of between 100 Ma and 500 Ma. This estimate was actually reduce

over his lifetime to between 20 Ma and 40 Ma and eventually to less than 10 Ma.Of course, later scientists, like John Perry and T. H. Huxley challenged Kelvin's assumptionPerry, in particular, a noted physicists and former assistant to Kelvin, showed that coolingcalculations using different but equally likely assumptions and data resulted in ages for the Earth as much as 29 Ga. After this came to light, Kelvin admitted that he might just as well have set horiginal upper limit on the age of the Earth at 4,000 Ma instead of 400 Ma. Of course, this was close as Kelvin ever came to publicly recanting his position. Later, after radioactivity had bee


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proven to be a significant source of the Earth's internal heat, he did privately admit that he mighave been in error.

AssumptionsDating rocks by radioactive timekeepers is simple in theory, but almost all of the different method

(except for the isochron methods - see below) rely on these few basic assumptions: Beginning Conditions Known Beginning Ratio of Daughter to Parent Isotope Known (zero date problem) Constant Decay Rate No Leaching or Addition of

Parent or Daughter Isotopes All Assumptions Valid for

Billions of Years There is also a difficulty in

measuring precisely very small

amounts of the variousisotopes.

There is, of course, one radiometricdating method that appears toovercome the vital "zero date

problem". The isochron dating method theoretically overcomes the need toknow the initial ratio of parent and daughter isotopes. It will be covered in more detail below. For now, we will

look at those methods that do fall under the above assumptions.

Interweaving the relative time scalewith the atomic time scale posescertain problems because only certaintypes of rocks, chiefly the igneousvariety, can be dated directly byradiometric methods; but these rocksdo not ordinarily contain fossils. Igneous rocks are those such as granite and basalt, whiccrystallize from molten material called "magma". There is even some valid question as to granite could be formed from magma at all since this has never, to my knowledge, been observeor duplicated in the lab. Radio-halos from rapidly decaying radioactive isotopes in granite seem indicate that the granites were formed almost instantly.

Most sedimentary rocks such as sandstone, limestone, and shale (which do contain fossils) arrelated to the radiometric time scale by bracketing them within time zones that are determined bdating appropriately selected igneous rocks in lava flows, or weathered from lava flows.Potassiu
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- Argon and Argon - Argon dating are based on the current understanding that radioactivPotassium-40 decays to the stable form, Argon-40 with a half-life of approximately 1.25 billioyears. The same principle holds true for the other isotope dating methods.

K/Ar and 40Ar/ 39Ar Methods

Isotopes of Potassium and Argon

The isotopes the KAr system relies on are Potassium (K) and Argon (Ar). Potassium, an alkametal, the Earth's eighth most abundant element is common in many rocks and rock-forminminerals. The quantity of potassium in a rock or mineral is variable proportional to the amount silica present. Therefore, mafic rocks and minerals often contain less potassium than an equamount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or minerathrough alteration processes. Due to the relatively heavy atomic weight of potassium, insignificafractionation of the different potassium isotopes occurs. However, the40K isotope is radioactiveand therefore will be reduced in quantity over time. But, for the purposes of the KAr datinsystem, the relative abundance of 40K is so small and its half-life is so long that its ratios with theother Potassium isotopes are considered constant.

Natural abundance of potassium and argon39K = 93.2581 0.0029%40K = 0.01167 0.00004%41K = 6.7302 0.0029%

40Ar = 99.60%38

Ar = 0.063%36Ar = 0.337% (40Ar/36Ar = 295.5)

Argon, a noble gas, constitutes approximately 0.1-5% of the Earth's present day atmosphereBecause it is present within the atmosphere, every rock and mineral will have some quantity oArgon. Argon can mobilized into or out of a rock or mineral through alterationand thermal processes. Like Potassium, Argon cannot be significantly fractionated in nature. However,40Ar isthe decay product of 40K and therefore will increase in quantity over time. The quantity of 40Ar produced in a rock or mineral over time can be determined by substracting the amount known to bcontained in the atmosphere. This is done using the constant40Ar/36Ar ratio of atmospheric Argon.

This ratio is 295.5.Radioactive decay of parent isotope to daughter isotope

The nuclei of naturally occurring40K is unstable, decaying at a constant rate (half-life = 1.25 billion years). The decay scheme is electron capture and positron decay. About 89% of the40K atoms will decay to40Ca. For the K/Ar dating system, this decay scheme to calcium isotopes is


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ignored. The remaining 11% of the40K atoms decay to40Ar. It is this scheme that makes the K/Ar method work.

The buildup of radiogenic40Ar (40Ar*) in a closed system can be expressed by the equation:

The K/Ar Dating Technique

General Assumptions For The Potassium-Argon Dating System

Certain assumptions must be satisfied before the age of a rock or mineral can be calculated witthe Potassium-Argon dating technique. These are:

The material in question is a closed system. In other words, no radiogenic40Ar has escapedfrom the rock/mineral since it formed. In the case of a volcanic mineral, this means rapicooling. Likewise, potassium has not been gained or lost.

A correction is made for atmospheric argon (40Ar from the 40Ar/36Ar ratio = 295.5subtracted).

No non-atmospheric40Ar was incorporated into the rock/mineral during or after itsformation.

The isotopes of potassium in the rock/mineral have not fractionated, except by40K decay. The decay constants of 40K are accurately known. The quantities of 40Ar and potassium in the rock/mineral are accurately determined.

The K/Ar Age Determination

Once the40Ar and potassium in a rock/mineral are accurately measured, the amount of 40K (basedon the relative abundance of 40K to total potassium) and40Ar* (radiogenic40Ar) must be calculated.The K/Ar method uses a spike (known quantity) of 38Ar mixed with the argon extracted from therock/mineral to determine the quantity of 40Ar*. The resulting40Ar* and 40K can be plugged intothe age equation as follows:


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Problems And Limitations Of The K/Ar Dating Technique

Because the K/Ar dating technique relies on the determining the absolute abundances of both40Ar and potassium, there is not a reliable way to determine if theassumptionsare valid. Argon loss andexcess argon are two common problems that may cause erroneous ages to be determined. Argoloss occurs when radiogenic40Ar (40Ar*) produced within a rock/mineral escapes sometime after itsformation. Alteration and high temperature can damage a rock/mineral lattice sufficiently to allo40Ar* to be released. This can cause the calculated K/Ar age to be younger than the "true" age othe dated material. Conversely, excess argon (40Ar E) can cause the calculated K/Ar age to be older than the "true" age of the dated material. Excess argon is simply40Ar that is attributed to radiogenic40Ar and/or atmospheric40Ar. Excess argon may be derived from the mantle, as bubbles trapped ina melt, in the case of a magma. Or it could be a xenocryst/xenolith trapped in a magma/lava durinemplacement.

The 40Ar/ 39Ar Dating Technique

Principles of the 40Ar/ 39Ar method

The40Ar/39Ar dating technique is a more sophisticated variation of the K/Ar dating technique. Bottechniques rely on the measurement of a daughter isotope (40Ar) and a parent isotope. While theK/Ar technique measures potassium as the parent, the40Ar/39Ar technique uses39Ar.

Because the relative abundancesof the potassium isotopes are known, the39Ar K (produced from39K by a fast neutron reaction) can be used as a proxy for potassium. Therefore, unlike thconventional K/Ar technique, absolute abundances need not be measured. Instead, the ratios of tdifferent argon isotopes are measured, yielding more precise and accurate results. Additiona

advantages of the single isotopic measurements of the40

Ar/39

Ar technique are decreased effects of sample inhomogeneity and the use of smaller sample sizes.

Sample Irradiation / Production Of 39Ar

Because39Ar K can only be produced by a fast neutron reaction on39K [ 39K(n,p)39Ar ], all samplesdated by the40Ar/39Ar technique must be irradiated in the core of a nuclear reactor. The amount of39Ar K produced in any given irradiation will be dependant on the amount of 39K present initially,
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the length of the irradiation, the neutron flux density and the neutron capture cross section for 39K.However, because each of these parameters is difficult to determine independantly, a minerastandard, or monitor, of known age is irradiated with the samples of unknown age. The monitoflux can then be extrapolated to the samples, thereby determining their flux. This flux is known the 'J' and can be determined by the following equation:

In addition to39Ar production from39K, several other 'interference' reactions occur duringirradiation of the samples. Other isotopes of argon are produced from potassium, calcium, argoand chlorine. These are:

As the table above illustrates, several "undesirable" reactions occur on isotopes present withievery geologic sample. These reactor produced isotopes of argon must be corrected for in order

determine an accurate age. The monitoring of the interfering reactions is performed through the uof laboratory salts and glasses. For example, to determine the amount of reactor produced40Ar from40K, potassium-rich glass is irradiated with the samples. The40Ar/39Ar ratio of the glass is thenmeasured in the mass spectrometer to determine the correction factor that must be applied to threst of the samples in that irradiation. CaF is also routinely irradiated and measured to determinthe 36Ar/37Ar and39Ar/37Ar correction factors. The "desirable" production of 37Ar from40Ca allowsus determine how much36Ar and39Ar to correct for, as well as the K/Ca ratio of the sample. Thedesirable production of 38Ar from37Cl allows us to determine how much chlorine is present in our


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samples. A salt of KCl is irradiated to determine the38Ar/39Ar production ratio which can then beapplied to other samples to determine K/Cl ratios.

40Ar/ 39Ar Age Determination

Once the J (neutron flux parameter),40

Ar* and39

Ar K have been determined (ie. subtractingatmospheric argon, system blank and interferring reactor produced isotopes), they can be includein the40Ar/39Ar age equation:

Because the40Ar/39Ar technique relies on ratios instead of absolute quantities, we are able toextractand measure multiple aliquots of argon from a single sample. Multiple argon extractioncan be performed on a sample in several ways. Step-heating is the most common way and involveither afurnaceor a laser to uniformily heat the sample to evolve argon. The individual ages from

each heating step are then graphically plotted on anage spectrumor an isochron. Mechanicalcrushing is also a technique capable of releasing argon from a single sample in multiple steps.

Laser probesalso allow multiple ages to be determined on a single sample aliquot, but do so usingaccurate and precise spatial control. For example, laser spot sizes of 100 microns or less allow user to extract multiple argon samples from across a small mica or feldspar grain. The results froma laser probe can be plotted in several graphical ways, including a map of a grain showing laterargon distribution.

40Ar/39Ar total fusion of a sample is comparable to a K/Ar age determination in that it relies owholesale release of argon at one time. However, unlike conventional K/Ar,40Ar/39Ar total fusion

measures ratios, making it ideal for samples known to be very argon retentive (eg. sanidine). Totfusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms.

Some Problems With The 40Ar/ 39Ar Technique .

Standard Inter-calibration
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In order for an age to be calculated by the40Ar/39Ar technique, the J parameter must be known. For the J to be determined, a standard of known age must be irradiated with the samples of unknowage. Because this (primary) standard ultimately cannot be determined by40Ar/39Ar, it must be firstdetermined by another isotopic dating method. The method most commonly used to date th primary standard is the conventional K/Ar technique. The primary standard must be a mineral th

is homogeneous, abundant and easily dated by the K/Ar and40

Ar/39

Ar methods. Traditionally, this primary standard has been a hornblende from the McClure Mountains, Colorado (a.k.a. MMhb-1Once an accurate and precise age is determined for the primary standard, other minerals can bdated relative to it by the40Ar/39Ar method. These secondary minerals are often more convenient todate by the40Ar/39Ar technique (e.g. sanidine). However, while it is often easy to determine the ageof the primary standard by the K/Ar method, it is difficult for different dating laboratories to agreon the final age. Likewise, because of heterogeneity problems with the MMhb-1 sample, the K/Aages are not always reproducible. This imprecision (and inaccuracy) is transferred to the secondaminerals used daily by the40Ar/39Ar technique. Fortunately, other techniques are available to re-evaluate and test the absolute ages of the standards used by the40Ar/39Ar technique. Some of theseinclude other isotopic dating techniques (e.g. U/Pb) and the astronomical polarity time scal

(APTS).

Decay Constants

Another issue affecting the ultimate precision and accuracy of the40Ar/39Ar technique is theuncertainty in the decay constants for 40K. This uncertainty results from 1) the branched decayscheme of 40K and 2) the long half-life of 40K (1.25 billion years). As technology advances, it islikely that the decay constants used in the40Ar/39Ar age equation will become continually morerefined allowing much more accurate and precise ages to be determined.

J Factor

Because the J value is extrapolated from a standard to an unknown, the accuracy and precision othat J value is critical. J value uncertainty can be minimized by constraining the geometry of thstandard relative to the unknown, both vertically and horizontally. The NMGRL does this birradiating samples in machined aluminum disks where standards and unknowns alternate everother position. J error can also be reduced by analyzing more flux monitor aliquots per standalocation.

39Ar Recoil

The affects of irradiation on potassium-bearing rocks/minerals can sometimes result ianomalously old apparent ages. This is caused by the net loss of 39Ar K from the sample by recoil(the kinetic energy imparted on a39Ar K atom by the emission of a proton during the (n,p) reaction).Recoil is likely in every potassium-bearing sample, but only becomes a significant problem witvery fine grained minerals (e.g. clays) and glass. For multi-phase samples such as basaltic whorocks, 39Ar K redistribution may be more of a problem than net39Ar K loss. In this case,39Ar mayrecoil out of a low-temperature, high-potassium mineral (e.g. K-feldspar) into a high-temperatur


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low potassium mineral (e.g. pyroxene). Such a phenomenon would great affect the shape of the agspectrum.

Rubidium-Strontium Dating

Rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The nuclidrubidium-87 (Rb87) decays to strontium-87 (Sr87) with a half-life of 47 billion years. Strontiuoccurs naturally as a mixture of several nuclides. If three minerals form at the same time indifferent regions of a magma chamber, they will have identical ratios of the different strontiumnuclides. (Remember, chemical processes can't differentiate between nuclides). The totalamount of strontium might be different in the different minerals, but theratios will be the same. Now,suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-betweeamount. That means that when the minerals crystallize there is a fixed ratio of Rb87:Sr87. As timgoes on, atoms of Rb87 decay to Sr-87, resulting in a change in the Rb87:Sr87 ratio, andalso in achange in the ratio of Sr87 to other nuclides of strontium. The decrease in the Rb87:Sr87 ratio exactly matched by the gain of Sr87 in the strontium-nuclide ratio. It has to be -- the two sides o

the equation must balance.If we plot the change in the two ratios for these three minerals, the resulting graph comes out asstraight line with an ascending slope. This line is called anisochron . The line's slope thentranslates directly into a figure for the age of the rock that contains the different minerals. Th power of the Rb/Sr dating method is enormous, for it provides multiple independent ways overifying the accuracy of the isochron. Whenevery one of four or five different minerals from thesame igneous formation matches the isochron perfectly, it can safely be said that the isochron correct beyond a reasonable doubt. Contaminated or otherwise bad samples stand out like lighthouse beacon, because they don't show a good isochron line.


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Figure (i) A rubidium-strontium three-isotope plot. When a rock cools, all its minerals have thesame ratio of strontium-87 to strontium-86, though they have varying amounts of rubidium. Asthe rock ages, the rubidium decreases by changing to strontium-87, as shown by the dottedarrows. Minerals with more rubidium gain more strontium-87, while those with less rubidiumdo not change as much. Notice that at any given time, the minerals all line up--a check toensure that the system has not been disturbed.

Figure( ii) The original amount of the daughter strontium-87 can be precisely determinedfrom the present-day composition by extending the line through the data points back torubidium-87 = 0. This works because if there were no rubidium-87 in the sample, thestrontium composition would not change. The slope of the line is used to determine the age of the sample.

U238/U235/Th232 Series


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U238 and U235 are both nuclides of the element uranium. U235 is well known as the majofissionable nuclide of uranium. It's the primary active ingredient of nuclear power plant reactocores. It has a half-life of roughly 700 million years. U238 is more stable, with a half-life of 4 billion years. Th232 is the most common nuclide of the element thorium, and has a half-life of 13 billion years.

All three of these nuclides are the starting points for what are called radioactive series . Aradioactive series is a sequence of nuclides that form one from another by radioactive decay. Thseries for U238 looks like this:

U238 --> Th234 + Th234 --> Pa234 + Pa234 --> U234 + U234 --> Th230 + Th230 --> Ra226 + Ra226 --> Rn222 + Rn222 --> Po218 + Po218 --> Pb214 + Pb214 --> Bi214 + Bi214 --> Po214 + Po214 --> Pb210 + Pb210 --> Bi210 + Bi210 --> Po210 + Po210 --> Pb206 +

Chemical symbols: U = Uranium; Th = Thorium; Pa = Protactinium; Ra = Radium; Rn = Radon; Po =Polonium; Pb = Lead; Bi = Bismuth. = alpha particle (alpha decay); = beta particle (beta decay).

We can calculate the half-lives of all of these elements. All the intermediate nuclides betweeU238 and Pb206 are highly unstable, with short half-lives. That means they don't stay around verlong, so we can take it as given that these nuclides don't appear on Earth today except as the resuof uranium decay. We can find out the normal distribution of lead nuclides by looking at a lead orthat doesn't contain any uranium, but that formed under the same conditions and from the samsource as our uranium-bearing sample. Then any excess of Pb206 must be the result of the decaof U238. When we know how much excess Pb206 there is, and we know the current quantity U238, we can calculate how long the U238 in our sample has been decaying, and therefore holong ago the rock formed.

Th232 and U235 also give rise to radioactive series -- different series from that of U238

containing different nuclides and ending in different nuclides of lead. Chemists can apply similtechniques to all three, resulting in three different dates for the same rock sample. (Uranium anthorium have similar chemical behavior, so all three of these nuclides frequently occur in the samores.) If all three dates agree within the margin of error, the date can be accepted as confirme beyond a reasonable doubt. Since all three of these nuclides have substantially different half-livefor all three to agree indicates the technique being used is sound.


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But even so, radioactive-series dating could be open to question. It's always possible that migratioof nuclides or chemical changes in the rock could yield incorrect results. The rock being datemust remain a closed system with respect to uranium, thorium, and their daughter nuclides for thmethod to work properly. Both the uranium and thorium series include nuclides of radon, an inegas that can migrate through rock fairly easily even in the few days it lasts. To have a radiometrdating method that is unquestionably accurate, we need a radioactive nuclide for which we can gabsolutely reliable measurements of the original quantity and the current quantity. Is there any sucnuclide to be found in nature? The answer is yes.

Uranium-lead dating method

Theuranium-lead radiometric datingscheme has been refined to the point that the error margin indates of rocks can be as low as less than two million years in two-and-a-half billion years. An errmargin of 25 % has been achieved on younger Mesozoicrocks.

Uranium-lead dating is often performed on themineral zircon(ZrSiO4), though it can be used onother materials, such as baddeleyite.Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead. It has a very high closuretemperature, is resistant to mechanical weathering and is very chemically inert. Zircon also formmultiple crystal layers during metamorphic events, which each may record an isotopic age of thevent. In situ micro-beam analysis can be achieved via laser ICP-MSor SIMStechniques.

A concordia diagram as used in uranium-lead dating , with data from the Pfunze Belt ,Zimbabwe . All the samples show loss of lead isotopes, but the intercept of the
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errorchron (straight line through the sample points) and the concordia (curve) showsthe correct age of the rock.

One of its great advantages is that any sample provides two clocks, one based on uranium-235decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238

decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck thallows accurate determination of the age of the sample even if some of the lead has been lost. Thcan be seen in the concordia diagram, where the samples plot along an errochron (straight linewhich intersects the concordia curve at the age of the sample.

Lead-lead dating

Lead-lead dating is a method for dating geological samples, normally based on 'whole-rock'samples of material such as granite. For most dating requirements it has been superseded buranium-lead dating(U-Pb dating), but in certain specialized situations (such as dating meteoritesand theage of the earth) it is more important than U-Pb dating.

Decay equations for common Pb-Pb Dating

There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium anthorium in nature; they are206Pb, 207Pb, and208Pb. 204Pb is the only non-radiogenic lead isotope,therefore is not one of the daughter isotopes. These daughter isotopes are the final decay producof U and Th radioactive decay chains beginning from238U,235U and 232Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constarate. This shifts the ratio of radiogenic Pb versus non-radiogenic204Pb (207Pb/204Pb or 206Pb/204Pb) infavor of radiogenic207Pb or 206Pb.

This can be expressed by the following decay equations:

where the subscripts P and I refer to present-day and initial Pb isotope ratios, 235 and 238 are decayconstants for 235U and238U, and t is the age.

The concept of common Pb-Pb dating (also referred to as whole rock lead isotope dating) wadeduced through mathematical manipulation of the above equations. It was established by dividinthe first equation above by the second, under the assumption that the U/Pb system waundisturbed. This rearranged equation formed:
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where the factor of 137.88 is the present-day238U/235U ratio. As evident by the equation, initial Pbisotope ratios, as well as the age of the system are the two factors which determine the present daPb isotope compositions. If the sample behaved as a closed system then graphing the differenc between the present and initial ratios of 207Pb/204Pb versus206Pb/204Pb should produce a straight line.The distance the point moves along this line is dependent on the U/Pb ratio, whereas the slope the line depends on the time since Earths formation. This was first established by Nier et al., 194

The Formation of the Geochron

The development of the Geochron was mainly attributed toClair Cameron Pattersons applicationof Pb-Pb dating on meteorites in 1956. The Pb ratios of three stony and two iron meteorites we

measured. The dating of meteorites would then help Patterson in determining not only the age othese meteorites but also the age of Earths formation. By dating meteorites Patterson was directdating the age of various planetesimals. The process of isotopic differentiation is identical on Earthas it is on other planets, therefore the core of these planetesimals would be depleted of Uraniumand Thorium, while the crust and mantle would contain higher U/Pb ratios. As planetesimals collided, various fragments were scattered and produced meteorites. Iron meteorites weridentified as pieces of the core, while stony meteorites were segments of the mantle and crustunits of these various planetesimals.

Iron Meteorite found in Canyon DiabloMeteorite Impact

Samples of iron meteoritefrom Canyon Diablo(Meteor Crater ) Arizona were found to have theleast radiogenic composition of any material in the solar system. The U/Pb ratio was so low that nradiogenic decay was detected in the isotopic composition

As illustrated in figure 1, this point defines the lower (left) end of the isochron. Therefore troilitfound in Canyon Diablo represents the primeval lead isotope composition of the solar systemdating back to 4.55 +- 0.07 Byr. The stony meteorites however, exhibited very high207Pb/204Pbversus 206Pb/204Pb ratios, indicating that these samples came from the crust or mantle of the planetesimal. These samples define an isochron in figure 1, whose slope gives the age ometeorites as 4.55 Byr. Patterson also analyzed terrestrial sediment collected from the ocean flooThis was representative of the Bulk Earth composition, and was plotted on the isochron. Thisotope composition of this sample was found to lie on the meteorite isochron, therefore givin
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good evidence that earth had the same origin as the meteorites, as well as the same age, thereforsolving the age of the Earth and giving rise to the name geochron.

Lead isotope isochron diagram used by C. C. Patterson to determine the age of the Earth in 195Animation shows progressive growth over 4550 million years (Myr) of the lead isotope ratios f
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two stony meteorites (Nuevo Laredo and Forest City) from initial lead isotope ratios matchinthose of the Canyon Diablo iron meteorite.

Helium diffusion

Helium diffusion is one type of nuclear decay dealing with the emission of Heliumnuclei knownas an alpha emission.Elementslikeuraniumandthoriumproduce helium inzirconsas a biproductof their radioactivity. This helium seeps out of zircons quickly over a wide range of temperatures.If the zircons really are about 1.5 billion years old (the age which conventional dating giveassuming a constant decay rate), almost all of the helium should have dissipated from the zirconlong ago. But there is a significant amount of helium still inside thezircons, showing their ages to be 6000 +/- 2000 years.Accelerated decaymust have produced a billion years worth of helium inthat short amount of time.

A Helium Diffusion Model Proposed

As early as the 1970s,Robert Gentrypointed out that rocks appeared to be retaining more Heliumthan they should using standard models.Russell Humphreys, Steven Austin, Andrew Snelling, andJohn Baumgardner , the first three professors at theInstitute for Creation Researchand the last, ascientist at Los Alamos National Laboratory, conceived a model using what was known regardinhelium diffusion inzircons. Their report suggested that the amount of helium in the zircon waswell over what would be observed in an old universe, citing a 5 order of magnitude erroFollowing thescientific method, they used their model to predict a collection of diffusion rates athitherto untested temperatures. Humphreys claims these predictions were validated in a separaexperiment reported in his 2004 report.

Helium atom
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Kevin Henke, an instructor at the University of Kentucky, spent 25000 words challenging thesresults. Humphreys responded in a 2005 report. It should be noted that Henke's attack was on th2003 model, not the putatively correct predictions found after the original report.

Helium in the Atmosphere

There is also insufficient mass of helium in earth's atmosphere to account for 4.6 billion years oradioactive decay. Helium is a noble gas which doesn't combine with any other element, but theis not enough of it to account for the radioactive decay which should have occurred in anold earth scenario. Malcolm Davis gives a careful treatment of the mathematics involved in calculating thrate at which helium escapes, and notes that the rate at which Helium is being produced is thirttimes higher than its rate of escape. If the earth were very old, much more Helium should havaccumulated in the atmosphere. Malcolm mentions some of the processes scientists have offered explain the difference such as periods of higher than average temperature, but notes these have n been observed.

"Also, the rate of helium buildup would be slower now than in the past, because the radioactivsources have decayed. This would put an even lower upper limit on the age of the earth.

The only way around this problem is to assume that the helium is escaping into space. But for thto happen, the helium atoms must be moving fast enough to escape the earths gravity (i.e., abovthe escape velocity). Collisions between atoms slow them down, but above a critical height (thexobase) of about 500 kilometres (300 miles) above the earth, collisions are very rare. Atomcrossing this height have a chance of escaping if they are moving fast enoughat least 10.7kilometres per second (24,200 miles per hour).3 Note that although helium in a balloon will floahelium when unenclosed will just mix evenly with all the other gases, as per normal ga behaviour."

Uniformitarians suggest models for which helium can escape due to thesolar wind.

Rhenium-osmium dating

Rhenium-Osmium dating is a form of radiometric datingbased on the beta decayof theisotope 187Re to 187Os. This normally occurs with ahalf lifeof 41.6 109 y, but studies using fully ionised187Re atoms have found that this can decrease to only 33 y, Both rhenium and osmium are stronglsiderophilic(iron loving) andchalcophilic(sulfur loving) making them useful in datingsulfide oressuch as gold and Cu-Ni deposits.

This dating method is based on an isochron calculated based on isotopic ratios measured using NTIMS (Negative Thermal Ionization Mass Spectrometry).

Rhenium-Osmium isochron

Rhenium-Osmium dating is carried out by theisochron datingmethod. Isochrons are created byanalysing several samples believed to have formed at the same time from a common source. Th
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Re-Os isochron plots the ratio of radiogenic187Os to non-radiogenic188Os against the ratio of the parent isotope187Re to the non-radiogenic isotope188Os. The stable and relatively abundant osmiumisotope188Os is used to normalize the radiogenic isotope in the isochron.

The Re-Os isochron is defined by the following equation:

where:

t is the age of the sample,

is the decay constant of 187 Re,

(e t -1) is the slope of the isochron which defines the age of the system.

A good example of an application of the Re-Os isochron method is a study on the dating of agold deposit in theWitwatersrandminingcamp,South Africa.

Carbon-14 Dating

The element carbon occurs naturally in three nuclides: C12, C13, and C14. The vast majority carbon atoms, about 98.89%, are C12. About one atom in 800 billion is C14. The remainder arC13. Of the three, C12 and C13 are stable. C14 is radioactive, with a half-life of 5730 years. C14 also formed continuously from N14 (nitrogen-14) in the upper reaches of the atmosphere. Ansince carbon is an essential element in living organisms, C14 appears in all terrestrial (landbounliving organisms in the same proportions it appears in the atmosphere.

Plants and protists get C14 from the environment. Animals and fungi get C14 from the plant animal tissue they eat for food. When an organism dies, it stops taking in C14. The C14 already the organism doesn't stop decaying, so as time goes on there is less and less C14 left in thorganism's remains. If we measure how much C14 there currently is, we can tell how much thewas when the organism died, and therefore how much has decayed. When we know how much hdecayed, we know how old the sample is. Many archaeological sites have been dated by applyinradiocarbon dating to samples of bone, wood, or cloth found there.

Radiocarbon dating depends on several assumptions. One is that the thing being dated is organic origin. Radiocarbon dating does not work on anything inorganic, like rocks or fossils. Only thinthat once were alive and now are dead: bones, teeth, flesh, leaves, etc. The second assumption that the organism in question got its carbon from the atmosphere. A third is that the thing haremained closed to C14 since the organism from which it was created died. The fourth one is thwe know what the concentration of atmospheric C14 was when the organism lived and died.

That last one is more important than it sounds. When Professor William Libby developed the C1dating system in 1949, he assumed that the amount of C14 in the atmosphere was a constan
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However, after a few years a number of scientists got suspicious of this assumption, because dateobtained by the C14 method weren't tallying with dates obtained by other means.

A long series of studies of C14 content produced an equally long series of corrective factors thmust be taken into account when using C14 dating. So the dates derived from C14 decay had to

revised. One reference on radiometric dating lists an entire array of corrective factors for thchange in atmospheric C14 over time. C14 dating serves as both an illustration of how usefuradiometric dating can be, and of the pitfalls that can be found in untested assumptions.


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The Samarium-Neodymium, Lutetium-Hafnium, and Rhenium-OsmiumMethods

All of these methods work very similarly to the rubidium-strontium method. They all use three-isotope diagrams similar to Figure 4 to determine the age. The samarium-neodymium method is the most-often used of these three. Ituses the decay of samarium-147 to neodymium-143, which has a half-life of 105 billion years. The ratio of the daughter isotope, neodymium-143, toanother neodymium isotope, neodymium-144, is plotted against the ratio of the parent, samarium-147, to neodymium-144. If different minerals from thesame rock plot along a line, the slope is determined, and the age is given bythe same equation as above. The samarium-neodymium method may bepreferred for rocks that have very little potassium and rubidium, for which thepotassium-argon, argon-argon, and rubidium-strontium methods might bedifficult. The samarium-neodymium method has also been shown to be moreresistant to being disturbed or re-set by metamorphic heating events, so forsome metamorphosed rocks the samarium-neodymium method is preferred.For a rock of the same age, the slope on the neodymium-samarium plots willbe less than on a rubidium-strontium plot because the half-life is longer.However, these isotope ratios are usually measured to extreme accuracy--several parts in ten thousand--so accurate dates can be obtained even forages less than one fiftieth of a half-life, and with correspondingly small slopes.

The lutetium-hafnium method uses the 38 billion year half-life of lutetium-176decaying to hafnium-176. This dating system is similar in many ways tosamarium-neodymium, as the elements tend to be concentrated in the sametypes of minerals. Since samarium-neodymium dating is somewhat easier, thelutetium-hafnium method is used less often.

The rhenium-osmium method takes advantage of the fact that the osmiumconcentration in most rocks and minerals is very low, so a small amount of theparent rhenium-187 can produce a significant change in the osmium isotoperatio. The half-life for this radioactive decay is 42 billion years. The non-radiogenic stable isotopes, osmium-186 or -188, are used as the denominatorin the ratios on the three-isotope plots. This method has been useful for datingiron meteorites, and is now enjoying greater use for dating Earth rocks due todevelopment of easier rhenium and osmium isotope measurement techniques.

Fission Track Dating

Fission track dating is a radioisotopic dating method that depends on the tendency of uranium(Uranium-238) to undergo spontaneous fission as well as the usual decay process. The largamount of energy released in the fission process ejects the two nuclear fragments into thsurrounding material, causing damage paths called fission tracks. The number of these trackgenerally 10-20 in length, is a function of the initial uranium content of the sample and of tim


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These tracks can be made visible under light microscopy by etching with an acid solution so thecan then be counted.

Spontaneous Fission Tracks Induced Fission Tracks

The usefulness of this as a dating technique stems from the tendency of some materials to lostheir fission-track records when heated, thus producing samples that contain fission-track produced since they last cooled down. The useful age range of this technique is thought to rangfrom 100 years to 100 million years before present (BP), although error estimates are difficult tassess and rarely given. Generally it is thought to be most useful for dating in the window betwee30,000 and 100,000 years BP.

A problem with fission-track dating is that the rates of spontaneous fission are very slow, requirinthe presence of a significant amount of uranium in a sample to produce useful numbers of trackover time. Additionally, variations in uranium content within a sample can lead to large variationin fission track counts in different sections of the same sample.

Because of such potential errors, most forms of fission track dating use a form of calibration o"comparison of spontaneous and induced fission track density against a standard of known agThe principle involved is no different from that used in many methods of analytical chemistrwhere comparison to a standard eliminates some of the more poorly controlled variables. In th
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zeta method, the dose, cross section, and spontaneous fission decay constant, and uranium isotoratio are combined into a single constant."

Of course, this means that the fission track dating method is not an independent method oradiometric dating, but is dependent upon the reliability of other dating methods. The reason f

this is also at least partly due to the fact that the actual rate of fission track production. Somexperts suggest using a rate constant of 6.85x10-17 yr -1 while others recommend using a rate of 8.46x10-17 yr -1 (G. A. Wagner, Letters to Nature , June 16, 1977). This difference might not seemlike much, but when it comes to dates of over one or two million years, this difference amounts about 25-30% in the estimated age value. In other words, the actual rate of fission track productioisn't really known, nor is it known if this rate can be affected by various concentrations of U238 or other physical factors. For example, all fission reactions produce neutrons. What happens fission from some other radioactive element, like U235 or some other radioisotope, producestracks? Might not these trackways be easily confused with those created by fission of U238?

The human element is also important here. Fission trackways have to be manually counted. This

problematic since interpreting what is and what is not a true trackway isn't easy. Geologisthemselves recognize the problem of mistaking non-trackway imperfections as fission track"Microlites and vesicles in the glass etch out in much the same way as tracks."45 Of course, thereare ways to avoid some of these potential pitfalls. For example, it is recommended that one choosamples with as few vesicles and microlites as possible. But, how is one to do this if they are seasily confused with true trackways? Fortunately, there are a few other "hints". True tracks arstraight, never curved. They also tend to show characteristic ends that demonstrate "younging" the etched track. True tracks are thought to form randomly and have a random orientationTherefore, trackways that show a distribution pattern tend not to be trusted as being "true". Certacolor and size patterns within a certain range are also used as helpful hints. Yet, even with athese hints in place, it has been shown that different people count the same trackways differentlyup to 20% differently.44 Add up the human error with the error of fission track rate and we aresuddenly up to a range of error of 50% or so.

This is yet another reason why calibration with other dating techniques is used in fission tracdating. It just isn't very reliable or accurate by itself. And, it gets even worse. Fairly recentlyRaymond Jonckheere and Gunther Wagner (American Mineralogist, 2000) published resulshowing that there are two kinds of real fission trackways that had "not been identifie previously." The first type of trackway identified is a "stable" track and the second type produced through fluid inclusions. As it turns out, the "stable tracks do not shorten significanteven when heated to temperatures well above those normally sufficient for complete annealing fission tracks." Of course, this means that the "age" of the sample would not represent the timsince the last thermal episode as previously thought. The tracks through fluid are also interestinThey are "excessively long". This is because a fission fragment traveling through a fluid inclusidoes so without appreciable energy loss. Such features, if undetected, "can distort the temperaturtime paths constructed on the basis of confined fission-track-length measurements." Again, thauthors propose measures to avoid such pitfalls, but this just adds to the complexity of this datin"method".


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These problems have resulted in several interesting contradictions, despite calibration. Foexample, Naeser and Fleischer (Harvard University) showed that, depending upon the calibratiomethod chosen, the calculated age of a given rock (from Cerro de Mercado, Mexico in this cascould be different from each other by afactor of "sixty or more " - - "which give geologicallyunreasonable ages. In addition, published data concerning the length of fission tracks and th

annealing of minerals imply that the basic assumptions used in an alternative procedure, the lengreduction-correction method, are also invalid for many crystal types and must be approached wicaution unless individually justified for a particular mineral." Now that's pretty significant - being off by a factor of sixty or more?! No wonder the authorsrecommend only going with results that do not provide "geologically unreasonable ages".

Circular Calibration MethodsThere is a methodological problem connected with the manner in which geologists infer the argoretention abilities of different minerals. Concerning the suitability of different minerals for K-Adating, Faure (1986, p. 72) writes "The minerals beryl, cordierite, pyroxene, and tourmalinfrequently contain excess 40Ar, while hornblende, feldspar, phlogopite, biotite, and sodalitcontain such excess 40Ar only rarely ... ." And how is this known? By comparing the K-Ar datyielded by such minerals with the expected ones. Thus the correctness of the geologic time scaleassumed in deciding which minerals are suitable for dating. For example, concerning the use glauconies for K-Ar dating, Faure (1986, p. 78) writes, "The results have been confusing becauonly the most highly evolved glauconies have yielded dates that are compatible with th biostratigraphic ages of their host rocks whereas many others have yielded lower dates. ThereforK-Ar dates of 'glauconite' have often been regarded as minimum dates that underestimate thdepositional age of their host." All of the choices are made in order to obtain dates that are more agreement with each other.

It is also interesting that Faure (1986, pp. 345-6) mentions that fission track dating is calibrate(the "zeta calibration") using rocks of "known" ages. However, if these "known" ages arincorrect, then fission track dating that is based on these ages is also incorrect. Thus fission tracdating is not an independent test that helps to verify the accuracy of other tests. The result is thradiometric dating in general is in danger of being based on circular reasoning.

Dating with short-lived extinct radionuclides

Absolute radiometric dating requires a measurable fraction of parent nucleus to remain in thsample rock. For rocks dating back to the beginning of the solar system, this requires extremelong-lived parent isotopes, making measurement of such rocks' exact ages imprecise. To be able distinguish the relative ages of rocks from such old material, and to get a better time resolutiothan that available from long-lived isotopes, short-lived isotopes that are no longer present in throck can be used.

At the beginning of the solar system, there were several relatively short-lived radionuclides lik26Al, 60Fe, 53Mn, and129I present within the solar nebula. These radionuclidespossibly produced by the explosion of a supernovaare extinct today, but their decay products can be detected very old material, such as that which constitutesmeteorites. By measuring the decay products of
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extinct radionuclides with amass spectrometer and using isochronplots, it is possible to determinerelative ages of different events in the early history of the solar system. Dating methods based oextinct radionuclides can also be calibrated with the U-Pb method to give absolute ages. Thus bothe approximate age and a high time resolution can be obtained. Generally a shorter half-life leadto a higher time resolution at the expense of timescale.

The 129I - 129Xe chronometer

129I beta-decays to129Xe with a half life of 17 million years. Since xenon is a volatile noble gas itcan be assumed that there wasn't much of it in the rock to begin with. Since it is much rarer thaiodine, it can be assumed that most of the129Xe present in the rock is a by-product of 129I decay. Byusing the solar system's average xenon content as thenatural abundance, the excess of 129Xe to theabundance of 129I ratio can be derived.

The 26Al - 26Mg chronometer

Another example of short-lived extinct radionuclide dating is the26Al - 26Mg chronometer, whichcan be used to estimate the relative ages of chondrules. 26Al decays to26Mg with ahalf-lifeof 720000 years. The dating is simply a question of finding the deviation from thenatural abundanceof 26Mg (the product of 26Al decay) in comparison with the ratio of the stable isotopes27Al/24Mg.

The excess of 26Mg (often designated26Mg* ) is found by comparing the26Mg/27Mg ratio to that of other Solar System materials.

The 129I - 129Xe chronometer gives an estimate of the time period for formation of primitivemeteorites of about 20 million years. Since some xenon might have escaped the rocks thiformation period might be even shorter.

The26Al - 26Mg chronometer on the other hand estimates the formation time to only a few millionyears (1.4 million years for Chondrule formation).

Airborne Radiometric (Gamma Ray Spectrometry) Surveys

Radiometric surveys detect and map natural radioactive emanations, called gamma rays, fromrocks and soils. All detectable gamma radiation from earth materials come from the natural dec products of only three elements, i.e. uranium, thorium, and potassium. In parallel with thmagnetic method, that is capable of detecting and mapping only magnetite (and occasionall pyrrhotite) in soils and rocks, so the radiometric method is capable of detecting only the presenof U, Th, and K at the surface of the ground.

The basic purpose of radiometric surveys is to determine either the absolute or relative amounts U, Th., and K in the surface rocks and soils. Before considering the geologic implications of thinformation, we will discuss how gamma rays are affected by the natural environment and hothey are measured. No other geophysical method, and probably no other remote sensing metho
http://en.wikipedia.org/wiki/Mass_spectrometerhttp://en.wikipedia.org/wiki/Natural_abundancehttp://en.wikipedia.org/wiki/Chondruleshttp://en.wikipedia.org/wiki/Half-lifehttp://en.wikipedia.org/wiki/Natural_abundancehttp://en.wikipedia.org/wiki/Mass_spectrometerhttp://en.wikipedia.org/wiki/Natural_abundancehttp://en.wikipedia.org/wiki/Chondruleshttp://en.wikipedia.org/wiki/Half-lifehttp://en.wikipedia.org/wiki/Natural_abundance


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requires us to consider so many variables in order to reduce the observational data to a form that useful for geological interpretation. Meteorological conditions, the topography of the survey arethe influence of the planets cosmic environment, the height of the sensor above ground and thspeed of the aircraft are just a few of the variables which affect radiometric measurements, anwhich can bias our analysis unless we deal with them very thoroughly. We will consider thos

variables that are important when designing the specifications for, and interpreting the datobtained from, airborne surveys in as non-mathematical a manner as possible.

A few of the benefits that we can expect from the interpretation of radiometric surveys include:

Changes in the concentration of the three radioelements U, Th., and K accompany mosmajor changes in lithology, hence the method can be used as a reconnaissance geologimapping tool in many areas.

Variations in radioelement concentrations may indicate primary geological processes suchas the action of mineralizing solutions or metamorphic processes.

These variations also characterize secondary geological processes like supergene alteratio

and leaching. Radiometric surveys are capable of directly detecting the presence of uranium. This data can also assist in locating some intrusive related mineral deposits.

In appropriate areas, when used as a reconnaissance technique for mapping geology and fo prospecting, the cost/benefit ratio for airborne radiometric surveying is nearly as good as that fairborne magnetometer surveying.

Basic Principles

Gamma rays are tiny bursts of very high frequency, hence high energy, electromagnetic waves thare spontaneously emitted by the nuclei of some isotopes of some elements. They have mucshorter wavelengths than most other electromagnetic rays, including X-rays, and therefore, are le penetrating. Only a limited number of isotopes of the natural elements emit gamma rays; anamong these, there are only three which are common enough within earth materials to make thegeologically useful. These three are Bi214, Tl208, and K 40. Bi214 comes from the decay of U238 and is,therefore, an indication of the concentration of uranium in the earth materials that lie within thrange of the detector. Tl208 comes from the decay of Th232 and is an indicator of thorium content;and K 40 is one of the minor natural isotopes of potassium and the only isotope of K that isradioactive. It makes up only .012% of the total potassium in rocks and soils, but because thfraction remains quite constant, even during weathering and metamorphism, the gamma radiatiofrom it is a good indicator of changes in the amount of potassium in rocks.

Gamma rays are defined by their energies, measured in electron volts, or eV. One eV is the amounof kinetic energy that a single electron would acquire in falling through an electrical potentiadifference of 1 volt. The gamma rays from Tl208, the Th indicator, have an energy of 2.62 millionelectron volts or 2.62 MeV. We can understand the physical meaning of 2.62 MeV by noting thathis amount of energy is sufficient to lift a speck of dust having a mass of one microgram distance of 1/25 millimeter. The gamma rays from Bi214 have an energy of 1.76 MeV; while thofrom K40 have an energy of 1.46 MeV. All three of these energies are constant; they never chang


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they therefore constitute well defined peaks in the energy spectrum emanating from rocks. Figu4.1-1 shows an example of the natural gamma ray spectrum of a typical felsic intrusive rocmeasured at a terrain clearance of 120 meters.

Figure: A typical gamma ray spectrum from felsic intrusive rock measured at 120 metres terraiclearance.

In order to emphasize the smaller peaks, the spectrum in this figure is shown on a logarithmiscale. Note that there a many peaks but the three that are mentioned above are the most importaones. We also notice a sharp cut-off just beyond the Tl208 peak. This cut-off occurs because thereare no radioactive isotopes in the natural environment which emit gamma rays having energiehigher than 2.62 MeV. The general increase in the background level of the spectrum towards thlower energies is primarily due to Compton scattering which we will discuss shortly. The heighof the peaks are proportional to the amounts of the respective radioactive isotopes that are presein the rock. Thus, in principle, if we measure gamma ray spectra over different regions of exposerock, and compare them, we should be able to translate changes in the heights of the 1.46, 1.76 an2.62 MeV peaks into corresponding variations in the concentrations of potassium, uranium, anthorium within the different rock types.

Some of the oldest rocks on earth are found in Western Greenland. Because of their great age, thehave been especially well studied. The table below gives the ages, in billions of years, from twel


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different studies using five different techniques on one particular rock formation in WesterGreenland, the Amitsoq gneisses.

Technique Age Range (billion years)

uranium-lead 3.600.05

lead-lead 3.560.10

lead-lead 3.740.12

lead-lead 3.620.13

rubidium-strontium 3.640.06






lutetium-hafnium 3.550.22

samarium-neodymium 3.560.20

(compiled from Dalrymple, 1991)

Note that scientists give their results with a stated uncertainty. They take into account all th possible errors and give a range within which they are 95% sure that the actual value lies. The tnumber, 3.600.05, refers to the range 3.60+0.05 to 3.60-0.05. The size of this range is every bit important as the actual number. A number with a small uncertainty range is more accurate thannumber with a larger range. For the numbers given above, one can see thatall of the ranges overlapand agree between 3.62 and 3.65 billion years as the age of the rock. Several studies also showedthat, because of the great ages of these rocks, they have been through several mild metamorphheating events that disturbed the ages given by potassium-bearing minerals (not listed here). A


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pointed out earlier, different radiometric dating methods agree with each other most of the timover many thousands of measurements.

Name : Muhammad Bilal

Class: BS 4 th year

Dept : Geology


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CONTENTS

Introduction Introduction to radiometric dating methods Theory of radiometric dating

i) Alpha decayii) Beta decay

iii) Electron absorption

Half-lifeHistory of Radiometric Dating

Assumptions


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K/Ar and 40Ar/ 39Ar Methods

o Isotopes of Potassium and Argon

o Natural abundance of potassium and argono Radioactive decay of parent isotope to daughter isotopeo The K/Ar Dating Techniqueo General Assumptions For The Potassium-Argon Dating Systemo The K/Ar Age Determinationo Problems And Limitations Of The K/Ar Dating Technique

The 40Ar/ 39Ar Dating Technique

o Principles of the 40Ar/ 39Ar methodo Sample Irradiation / Production Of 39Aro 40Ar/ 39Ar Age Determinationo Some Problems With The 40Ar/ 39Ar Technique.

Standard Inter-calibrationDecay ConstantsJ Factor39Ar Recoil

Rubidium-Strontium Dating U238/U235/Th232 Series Uranium-lead dating method Lead-lead dating

Decay equations for common Pb-Pb Dating

The Formation of the Geochron

Uranium-lead dating method Helium diffusion method

A Helium diffusion Model proposedHelium in the Atmosphere

Rhenium-osmium dating


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Rhenium-Osmium isochron

Carbon-14 Dating The Samarium-Neodymium, Lutetium-Hafnium, and Rhenium-Osmium Methods Fission Track Dating Circular Calibration Methods Dating with short-lived extinct radionuclides

The 129I - 129Xe chronometerThe 26Al - 26Mg chronometer

Airborne Radiometric (Gamma Ray Spectrometry) SurveysBasic Principles

References


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References:

1. PRINCIPLES OF ISOTOPE GEOLOGY , 2nd Ed.Gunter Faure , c. John Wiley & Sons, 1986

2. EARTH THEN AND NOW , 2nd Ed.Carla W. Montgomery and David Dathec. Wm C. Brown, Publishers, 1994

3. http://www.answersingenesis.org/articles/nab4. Sean D. Pitman M.D. July 2001 ,Radiometric Dating Methods , Updating

October 2008.5. McDougall, I., and Harrison, T.M., 1999,Geochronology and thermochronology

by the40Ar/39Ar method: New York, Oxford University Press, xii, 269 p.6. www.radiometric methods/radate.htm7. http://en.wikipedia.org/wiki/Lead-lead_dating8. http://www.miningbasics.com/radiometric-methods-prospecting9. http://creationwiki.org/Helium_diffusion10. en.wikipedia.org/wiki/ Radiometric _dating
http://www.answersingenesis.org/articles/nabhttp://en.wikipedia.org/wiki/Lead-lead_datinghttp://www.miningbasics.com/radiometric-methods-prospectinghttp://creationwiki.org/Helium_diffusionhttp://www.answersingenesis.org/articles/nabhttp://en.wikipedia.org/wiki/Lead-lead_datinghttp://www.miningbasics.com/radiometric-methods-prospectinghttp://creationwiki.org/Helium_diffusion

assignment radio metrics

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