Open access peer-reviewed chapter - ONLINE FIRST

Geochemistry of Radioactive Isotopes

By Salih Muhammad Awadh

Submitted: November 7th 2019Reviewed: February 4th 2020Published: April 12th 2020

DOI: 10.5772/intechopen.91616

Downloaded: 64

Abstract

The chapter targeted the geochemistry of radioactive isotopes dealing with multidisciplinary topics and focusing on geochronology and tracer studies. The most common subjects are presented to include the basic principles of radioactive isotopes. The radioactive decay, the parent nuclide, the SI unit of radioactive decay as well as the historical discovery of radioactivity, the neutrons and protons in atomic nuclei, alpha and beta particles, gamma rays, electromagnetic radiation, decay and mode of decay, chain of decay, decay rates, decay timing, principle of dating, radiometric dating, isotope systems, the Rb/Sr System, the U, Th, Pb System, the age of the earth, Sm-Nd dating, K-Ar dating, 14Carbon dating, the geochron, all those were included overall.

Keywords

  • geochemistry
  • isotopes
  • radioactive isotope
  • parent nuclide
  • dating

1. Introduction

The process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves known as radioactive decay that causes the energy loss from the parent nuclide converting it to daughter nuclide [1]. This chapter has been authorized based mainly on published reference focusing on some basic properties and principles of radiation and how to use this phenomenon for the estimation the absolute geological age depending on the isotope half-life and provides brief summary of only a very few examples of dating applications. Geochronology and tracer studies are two principle applications of geochemistry of radiogenic isotope. Geochronology goes to estimate the absolute time based on the radioactive rate decay from the beginning of decay to its daughter by knowing how much nuclides have decayed. Tracer application relies on the variation in ratio of the radiogenic daughter isotope to other isotopes of the element. The purpose of authoring this chapter is to help those who are interested in this field and to provide what is useful and brief in a simplified way away from the complexity.

2. Radioactive decay and natural radioactive isotopes

The radioactive decay (a phenomenon of natural and artificial) means loss of energy that results in an atom named the parent nuclide converting it to an atom of a different type, called the daughter nuclide. The 14C is a parent, emits radiation and transforms to a 14N representing a daughter [2]. Accordingly, it is easy to understand that the radioactivity decay is that process by which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. Radioactive elements and their radiogenic daughters as well as the radiogenic and radioactive are illustrated in Figure 1.

Figure 1.

Periodic table showing the elements of natural radioactive isotopes and their daughters.

3. Unit of radioactive decay and activity measurements

The becquerel (symbol Bq) is typically used asa SI unit of radioactive decay and it is defined as one decay/second. The Bq is just a tiny measure of activity; a small part of tera-becquerel (TBq) or giga-becquerel (GBq) that is commonly used. The curie (Ci) is an another unit of radioactivity that was basically defined as the activity of 1 g of pure radium 226Ra. Currently, The Bq is ordinary equal to number of disintegrations per second; where Ci is equal to 3.7 × 1010 disintegrations per second. Low activities are also measured in disintegrations per minute (dpm) [2]. The name of the unit “becquerel” is originated and belonging to theHenri Becquerel, aFrenchscientist, who discovered radiation while he working onphosphorescentmaterials in 1896. Later, many contributions by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated [2].

4. Alpha particles

They are particles (α) emitted during the radioactive decay from the nucleus consisting of two protons and two neutrons tightly bound together (Figure 2). Such this decay is known as alpha-decay. All chemical elements above Pb, in the Periodic Table have at least one isotope which decays by emitting alpha particles. This process is relatively rare due it requires high energy to release two neutrons and two protons out of a nucleus. The alpha particle is expressed as an identical to a helium nucleus.

Figure 2.

Alpha particle represents the helium atom nuclei.

5. Beta particles

They are also known as beta ray or beta radiation, symbolized by β. Beta has high-energy, high-speed electron or positron emitted during decay process of a nuclei and give β− and β+, which yield electrons and positrons respectively (Figure 3).

Figure 3.

Bata radiation.

6. Gamma ray

Gamma ray is also named gamma radiation symbolized with γ which is an electromagnetic radiation (Figure 4) emitting from the radioactive decay of atomic nuclei [3]. This type of radiation is very common.

Figure 4.

Gamma radiation.

7. Modes of decay

Different decay reactions of the radionuclides, the mass number A and atomic number Z of nucleus defined as A, Z are presented in Table 1. The column (daughter nucleus) represents the difference between the produced nucleus and the parent. Thus, (A–2, Z) means that the mass number is two less than before, but the atomic number is the same as before.

Mode of decayParticipating particlesDaughter nucleus
Decays with emission of nucleons
Alpha decayAn alpha particle (A = 4, Z = 2) emitted from nucleus(A−4, Z−2)
Proton emissionA proton ejected from nucleus(A−1, Z−1)
Neutron emissionA neutron ejected from nucleus(A−1, Z)
Double proton emissionTwo protons ejected from nucleus simultaneously(A−2, Z−2)
Spontaneous fissionNucleus disintegrates into two or more smaller nuclei and other particles
Cluster decayNucleus emits a specific type of smaller nucleus (A1, Z1) smaller than, or larger than, an alpha particle(A−A1, Z−Z1) + (A1, Z1)
Different modes of beta decay
Beta-negative decayA nucleus emits an electron and an antineutrino(A, Z+1)
Positron emission, also beta-positive decayA nucleus emits a positron and a neutrino(A, Z−1)
Electron captureA nucleus captures an orbiting electron and emits a neutrino - The daughter nucleus is left in an excited and unstable state(A, Z−1)
Double beta decayA nucleus emits two electrons and two antineutrinos(A, Z+2)
Double electron captureA nucleus absorbs two orbital electrons and emits two neutrinos - The daughter nucleus is left in an excited and unstable state(A, Z−2)
Electron capture with positron emissionA nucleus absorbs one orbital electron, emits one positron and two neutrinos(A, Z−2)
Double positron emissionA nucleus emits two positrons and two neutrinos(A, Z−2)
Transitions between states of the same nucleus
Gamma decayExcited nucleus releases a high-energy photon (gamma ray)(A, Z)
Internal conversionExcited nucleus transfers energy to an orbital electron and it is ejected from the atom(A, Z)

Table 1.

Detailed radioactive decay reaction after [1].

8. Decay chain and uranium isotopes

The daughter nuclide is a result of the radioactive decay of a certain radioactive element. Daughter is stable or may also be radioactive, so the chain still continues to decay. The resulting second and/or third daughter nuclide may be radioactive leading to sequential radiation, so the process known as decay chain. Uranium is very heavy element has 92 atomic number (Figure 5).

Figure 5.

Configuration of the uranium atom showing the atomic number and mass number.

Three isotopes are most common of uranium; these are with their relative abundance and half-life (t1/2):

  • 238U has relative abundance 99.2739–99.2752% and half-life 4.4683 × 109 years.

  • 235U has relative abundance 0.7198–0.7202% and half-life 703.8 million years.

  • 234U has relative abundance 0.0050–0.0059% and half-life 245,500 years.

Half-life (t) means the time required for a given amount of radionuclide to lose 50% of its activity, and can be expressed as the exponential relationship (Figure 6) represents the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay and spontaneously changes into other element by emitting particles and energy.

Figure 6.

The exponential relationship of half-life with radioactive decay.

The best example for the radioactive decay can be illustrating by the uranium decay chain (Figure 7) [4]. The natural decay chain of 238U which eventually decays to 210Po emitting alpha with a half-life of 140 days to produce finally a stable isotope which is 206Pb.

Figure 7.

Natural radioactive decay series of 238U.

238U234Th234mPa234U230Th226Ra222Rn218Po214Pb214Bi214Po210Pb210Bi210Po206Pb.

9. Isotope systems in geological dating

Many isotope systems are mostly given as examples for dating geologic materials (Table 2).

ParentDaughterHalf-lifeRange availableLithology type
238U206Pb4.47 b.y>10 m.yIgneous & sometimes metamorphic rocks and minerals
235U207Pb707 m.y
232Th208Pb14 b.y
40K40Ar & 40Ca1.28 b.y>10,000 y
87Rb87Sr48 b.y>10 m.y
147Sm143Nd106 b.y
14C14N5730 y100 – 70,000 yOrganic Material

Table 2.

Some radioactive elements with their daughters and dating application.

b.y = billion years; m.y = million years; y = year

It can be done to use this information to date rocks, for example; usually, the amount (N) of an isotope present today, and the amount of a daughter element produced by decay (D*), see Eqs. (1) and (2).

D=N0NE1
N=N0eλtE2

λ, the decay constant

D=NeλtN=Neλt1E3

Consequently, it is possible to calculate the age if you have the number of daughter atoms produced by decay (D*), while the number of parent atoms (N) is known now with should pay attention for the number of daughter atoms that may have been present prior to the start of our clock.

9.1 The Rb/Sr system

We can simplify our isochron equation somewhat by noting that if x is small [7],

ex=1+x+x22!+x33!+x44!+=1+xE4

so that (eλt − 1) = λt, when λt is small.

by β decay. The neutron emits an electron to become a proton.

For this decay reaction, λ = 1.42 × 10–11/y, t1/2 = 4.8 × 1010 y, at present, 27.85% of natural Rb is 87Rb.

If we use this system to plug into Eq. (3), then

87Sr=87Rbeλt1E5

but,

87Srt=87Sr0+87SrE6

or

87Sr=87Srt87Sr0E7

Plugging this into Eq. (5)

87Srt=87Sr0+87Rbeλt1E8

We still do not know 87Sr0, the amount of 87Sr daughter element initially present.

To account for this, we first note that there is an isotope of Sr, 86Sr, that is:

  1. non-radiogenic (not produced by another radioactive decay process),

  2. non-radioactive (does not decay to anything else).

Thus, 86Sr is a stable isotope, and the amount of 86Sr does not change through time.

If we divide Eq. (8) through by the amount of 86Sr, then we get

S87rS86rt=S87rS86r0+Rb87S86rteλt1E9

This is known as the isochron equation. In case of Sr was isotopically homogeneous, the time (t equal 0). For instance, 87Sr/86Sr was the same in the igneous mineral at the time of crystallization. Typically, rock – forming minerals more may be have a different amount of 87Rb [5], and accordingly, those minerals are ordinary have a different 87Rb/86Sr at the crystallization time. During the natural cooling, the 87Rb in each mineral will decay to 87Sr, and each mineral will have a different 87Rb and 87Sr over time [6].

And simplify to:

S87rS86rt=S87rS86r0+Rb87S86rtλtE10

Then time (t) can be computed as:

t=S87rS86rtSr87S86r0Rb87S86rtλE11

The initial ratio (87Sr/86Sr)0, is useful to use as a geochemical tracer because Rb distributed unequally through the Earth over time [7]. The amount of Rb in the earth mantle is typically low (<0.1 ppm). The mantle thus has a low 87Rb/86Sr ratio and would not change its 87Sr/86Sr ratio very much with time, whilst the earth crust has higher amounts of Rb (>20 ppm) and therefore start out with a relatively high 87Rb/86Sr ratio. Over time, this results in crustal rocks having a much higher 87Sr/86Sr ratio than mantle rocks. Thus, it will be expected if the mantle has a 87Sr/86Sr of say 0.7025, melting of the mantle would produce a magma with a 87Sr/86Sr ratio of 0.7025, and all rocks derived from that mantle would have an initial 87Sr/86Sr ratio of 0.7025.

On the other hand, if the crust with 87Sr/86Sr of 0.710 melts, then the resulting magma would have 87Sr/86Sr of 0.710 and rocks derived from that magma would have an initial 87Sr/86Sr ratio of 0.710. So, the rock derived from the mantle or crust can determine its initial Sr isotopic ratio accordingly.

9.2 The U, Th, Pb system

Many Pb isotopes are produced from U and Th isotopes. 238U and 235U and 232Th can produce Pb isotopes during their radioactive decay that can be described as follows:

U2388H4e+P206bby α decayE12
λ238=1.551×1010/y,t1/2=4.47×109yE13
U2357H4e+P207bE14
λ235=9.849×1010/y,t1/2=0.707×109yE15
the present ratio ofU235U238=1137.8E16
Th2326H4e+P208bE17
λ232=4.948×1011/y,t1/2=1.4×1010yE18

232Th does not used in dating.

204Pb is a stable non-radiogenic isotope of Pb, the two isochron equations and get two independent dates from the U-Pb system can be written as:

Pb206Pb204t=Pb206Pb2040+U238Pb2040eλ238t1E19

and

Pb207Pb204t=Pb207Pb2040+U235Pb2040eλ235t1E20

If these two independent dates are concordant, a concordia diagram will show the values of Pb isotopes that would give concordant dates and can plug in t and solve for the be calculated Eqs. (21) and (22) as follows:

Pb206U238=Pb206Pb204tPb206Pb2040U238Pb204t=eλ238t1E21

and

Pb207U235=Pb207Pb204tPb207Pb2040U235Pb204t=eλ235t1E22

The Concordia is particularly useful in dating of zircon, that usually contains a lot U and less amounts of Pb, so we expect it has large amounts of radiogenic Pb that can be produced. Apatite and sphene are the two minerals that are commonly can be used in radiometric dating as well. Zircon from the crystallization time to the present represents a closed system in case no loss or gain of uranium or lead. The age of the zircon can be determined from its position on the Concordia after plotting the 206Pb*/238U and 207Pb*/235U ratios on the Concordia diagram. The discordant dates fall out of the Concordia curve.

The both ends of the Discordia intersect are represented by t0, the older and t*, the younger. Many reasons lead to Pb leakage. Metamorphism for example, could heat the crystal to the point where Pb will become mobile. Another possible reason cause U leakage, where the discordia is represented by the two points that would give two ages −t* representing the possible metamorphic event and t0 representing the initial crystallization age of the zircon.

The Pb-Pb isochrons can be normally concluded from combining the two isochron Eqs. (19) and (20).

Pb207Pb204tPb207Pb2040Pb206Pb204tPb206Pb2040=U235U238eλ235t1eλ238t1E23
U235U238=1137.8E24

Then, and by assuming that the 206Pb and 207Pb dates are the same, then Eq. (23) is the equation have a slope.

m=1137.88eλ238t1eλ235t1E25

that passes through the point.

Pb207Pb2040,Pb206Pb2040E26

10. The age of the earth and dating

The oldest rock found in Canada, with an age of 3.962 b.y ±3 m.y. This provide only a minimum age of the Earth. The age of the earth can be computed based on the chemical concept that is the 235U/238U ratio may have been 1.0 when the elements formed. So, from:

N=N0eλtE27

the

U235U238=U0eλ235t235U2380eλ238tE28

Finally, t is about 6 b.y.

This age represents the maximum age of the Earth. From the Pb-Pb isochron Eq. (23) and based on meteorites that may have been formed at the same time the solar system in which basically the Earth formed as well. The thing to be needed to date meteorites is knowing the initial ratios of the Pb isotopes. Two major types of meteorites are recognized; Fe- meteorites and stony (or chondritic) meteorites. The Fe-meteorites contain troilite (FeS) that has no U. Since the mineral troilite contains no U, all of the Pb present in the troilite is the Pb originally present, and none of it has been produced by U decay. Thus, the troilite in the Fe-meteorites will provide with the initial ratios of 206Pb/204Pb and 207Pb/204Pb.

The Pb ratios can be determined in other meteorites and check if they fall on the isochron of Pb-Pb that passes through the initial ratios determined from troilite in Fe-meteorites. The slope of this isochron (Geochron) estimated the earth age is of 4.55 ± 0.07 × 109 yr. Consequently, the best estimation of the age of the Earth is 4.55 billion years.

10.1 Sm-Nd dating

147Sm decays to 143Nd by alpha decay with half-life of 106 ± 2 b.y. [8], 147Sm, 148Sm, 149Sm, and 144Nd are radioactive, three nuclides accordingly generated 144Nd, 145Nd, and 140Ce [9].

147Sm143NdE29
λ=6.54×1012/yr,t1/2=1.06×1011yE30

The isochron equation is described based on whether the 144Nd is stable and non-radiogenic as:

Nd143Nd144t=Nd143Nd1440+Sm147Nd144teλt1E31

The age of the rock can be estimated later from the isochron equation that basically can be drawn by the determination of the 143Nd/144Nd and 147Sm/144Nd ratios for several minerals.

10.2 K-Ar dating

In the nature, 40K makes up 0.119% of natural K, as it is a radioactive element, its decay can be presented as follows [10]:

40K40CabyβdecayE32

The equation above is not used, because 40Ca can be present as both radiogenic and non-radiogenic Ca [10].

40K40Arbyelectron captureE33

For the combined process,

λ=5.305×1010/y,t1/2=1.31×109yE34

and for the Ar branch of the decay scheme

λe=0.585×1010/yE35

Argon is a gas easily can escape from a magma or liquid, therefore, the percentage of initial 40Ar is expressed as zero; during the rapid cooling of magma, quanitity of the Ar may be trapped. The date consequently obtained will be older than the date at which the magma erupted.

The dating equation used for K-Ar is:

A40r=λeλK40eλt1E36

where λeλ=0.11and refers to fraction of 40K that decays to 40Ar.

Many points need to have attention when use K-Ar dating, the use of minerals like sanidine or biotite is better to use whole rocks because minerals not contain excess Ar. Other thing, some atmospheric argon originated from volcanic eruptions could be absorbed onto the sample surface, 40Ar should be corrected for accordingly. Additionally, most minerals lose Ar during metamorphism due to high temperature, so the date will represent the metamorphic event (Table 3).

DecayDecay factorvalue
40K → 40Ca by β-λβ-4.962 × 10−10 a−1
40K → 40Ar by electron capture and γλe0.572 × 10−10 a−1
40K → 40Ar by electron captureλ’ e0.0088 × 10−10 a−1
combined valueλ = λβ- + λec + λ’ ec5.543 × 10−10 a−1
present day 40K/K0.0001167

Table 3.

Decay constants for K-Ar and Ar-Ar dating [11].

10.3 14C dating

Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms [12]. Radiocarbon (14C) has a short half-life (5730 y), it is therefore only used to date materials younger than about 70,000 years. The ratio of 14C to 14N in the Earth’s atmosphere is constant and the organism have the same ratio of 14C to 14N as the atmosphere. When an organism dies, the 14C decays back to 14N, with a half-life of 5730 years. Measuring the amount of 14C in this dead material thus enables the determination of the time elapsed since the organism died. Bones, teeth, charcoal, fossilized wood, and shells are materials can be used for dating.

10.4 Re-Os dating

Rhenium has stable 185Re and the radioactive 187Re. The latter is the most abundant (62.6%) and decays to 187Os based on beta decay, typically with a half-life of 41.6 × 109 y [13]. Osmium has seven isotopes; only two are the product of natural decay of radioactive isotopes: 186Os is produced from 190Pt by α-decay (half-life 4.7 × 1011 y, [14]) and 187Os by β-decay of 187Re. The two radiogenic isotopes 187Os (∼2%) and 186Os (∼1.6%) are typically normalized to the stable 188Os (13.24%). Rhenium-osmium (Re-Os), an applicable method was first applied to meteorites [15]; it provides a chronometer for directly dating both of sulfides and oxides ore minerals.

11. Discussion and conclusions

This chapter deals with the various types of radiation emitted by radioactive nuclides with principles of radionuclide decay and its radiations. Here an overview of some of the many dating radioactive techniques that play as significant role in our day-to-day lives. The dating techniques developed for defining reliable ages of geologic events other geochronological studies are recording of the isotope concentrations. All radiometric clocks depend on a radioactive “parent” isotope that decays to a daughter stable isotope of another element at a constant rate on geologic timescales. This process may take single step, or it may involve many stages of decay products before reaching the final stable daughter isotope. The half-life of the initial quantity of parent isotope to decay must be on the same order of magnitude as the time span to be measured. The Concordia–Discordia model has been developed for the U and Pb isotopes. The 235U transforms to 207Pb through a chain of radioactive nuclides, releasing (235–207)/4 = 7α-particles with the constant λ5 = 9.8·10–10. The 238U turns to 206Pb releasing (238–206)/4 = 8α-particles with the constant λ8 = 1.55 · 10–10 [16]. Currently, the ratio of 238U/235U (137.88) is growing. Both isotopes of uranium are closely connected to each other in kinetic processes due to the value √238/235 = 1.0063, which is close to 1. The Discordant values can be obtained from the development of the Concordia–Discordia model as an open system with losses of radiogenic lead in accessory minerals such as zircon, monazite, apatite etc. [17]. A very wide time range, not only the 12 b.y. of the Universe age and the 4.5 b.y. of the Earth’s age can be explore, but also the details related paleontology through the history of the Earth and recent events of the last millennia [12]. The K-Ar dating technique developed soon after the discovery of 40K and provided an important dating tool beside U-Pb and U-He dating methods. The half-life (1250 m.y) made this method is most popular for dating geological events [18]. The K-Ar dating are based on the decay of a 40K to an isotope of 40Ar by a branching process; 10.48% of 40K decays to 40Ar by β + decay, and 89.52% decays to 40Ca by β- to the ground state [10]. The age measured by K-Ar techniques reflects the time since radiogenic argon produced by decay of 40K, became trapped in the mineral or rock. The radiogenic noble gas daughter nuclides provide many methods for determining not only the chronology of events but also thermal histories combined with U-Pb and Rb-Sr dating techniques. This technique uses to conclude the cooling history based on use mineral closure or field estimates [19]. It can be applied for dating young volcanic eruptions and for low-temperature phases such as clay minerals like illite. In addition, they can be used for exploring. Despite the Rhenium–osmium (Re-Os), an applicable method was first applied to meteorites [15]; it also provides a chronometer for directly dating both of sulfides and oxides ore minerals. Recently, this technique is developed and become capable to estimate dating via dealing with very low content of Re and Os. The relative abundance of Osmium is the earth’s core and extraterrestrial material with a very leaser amount (ppt) in the mantle and it can be stored in sulfide and oxide minerals in the crust. It 8is best method for dating the age of gold in auriferous pyrites, it also used for dating marine shale containing coal.

It is difficult to obtain good precision and accuracy for radiocarbon due to its abundancy in the environment and it is possible to contaminate from material of a different age. Consequently, the methods for radiocarbon measuring are well tested, reproduced and carefully controlled under specific lab conditions. Recently, the radiocarbon methods have been developed, over the last 30 years to cover most of the materials suitable for radiocarbon measurement. The AMS system at Oxford was built with very high precision and accuracy for radiocarbon dating by High Voltage Engineering Europa BV. For high precious in situ age dating of Pb-U, Hf and U-Th isotope ratios in very small minerals like zircons, it is recommended to use the Thermo Scientific Neptune XT MC-ICP-MS or Thermo Scientific Element 2 and Thermo Scientific Element XR High-Resolution ICP-MS, combined with a laser ablation system. The Thermo Scientific Triton XT Multicollector Thermal Ionization Mass Spectrometer (TIMS), provides the ultimate precision for U-Pb geochronology, while The Triton XT TIMS is an equipment with high-quality age dating for Rb-Sr, Sm-Nd and Re-Os.

To be useful, result must be accurate, so uncertainty must always be taken into account. The geochronological result is influenced by uncertainty. So, if it does not be known well, result is scientifically meaningless. The uncertainty is coming from error in sampling, laboratory procedure; adaption of methods to the problem in question [12]. Overall, the source of uncertainty obtained from:

  1. collecting samples

  2. Parent decay methods

  3. Long half-life parent daughter methods

  4. Age calculations

11.1 Analytical methods

The radiogenic isotopes are typically separated from the no-radiogenic isotopes using spectrometer whenever they used as dating tools or tracers. Sample is ionized normally in thermal-ionization mass spectrometer (TIMS). Recently, the induced coupled plasma (ICP) is technique used for the chemical purification before mass spectrometry method. The laser ablation is also used for analyzing mineral with high concentration of radiogenic elements. However, in samples of whole-rocks, in which the concentration of radiogenic isotope is mostly low, it is necessary to pre-concentrate after dissolution and chemical extraction. The silicate geological samples are routinely dissolved in hot method using either concentrated HF acid or HCLO4 acid at atmospheric pressure; so, the most rock-forming minerals are dissolved, but the resistant minerals like zircon must be dissolved under pressure in a bomb at 220°C. The bomb liner and beaker are made of poly-fluorinated ethylene [20]. The formation of fluoride is the most common problem that may encounter after dissolution in HF as an insoluble in HCl acid. Consequently, the refluxing with HNO3 is needed [21]. The additional adding of HNO3 before completely evaporation of HF leads to promote process [22]. If at some stage, complete digestion is not achieved, decant off the solution is recommended and return to undissolved fraction at the previous stage for a second acid attack [23]. Thereafter, the solution rich in isotope is split for isotope-dilution analysis and for accurate-isotope ratio analysis.

11.2 Dating of igneous rocks

11.2.1 Sr-model ages

The Rb-Sr is used widely as a method provides a great information of igneous rock dating. The naturel process begins from the decay of 87Rb occurred in minerals to 87Sr, so the number of 87Sr daughters produced informs us about t years ago:

Sr87=Sr187+Rb87eλt1E37

Where the 87Sr1 is the number of the initial 87Sr atoms. The given nuclides are of so difficult to measure their absolute abundance, so, it is suitable to find isotope ratio by dividing by 86Sr which is not radiogenic and accordingly still constant with time as follows:

Sr87Sr86p=Sr87Sr86I+Rb87Sr86eλt1E38

Currently, strontium isotope ratio (p) can be measured by mass spectrometry, and the 87Rb/86Sr is calculated from Rb/Sr weigh ratio. From the initial ratio (87Rb/86Sr)I estimated, time (t) can be computed as:

t=1λln1+Sr86Rb87Sr87Sr86pSr86Sr87IE39

Over geological time, Rb-rich minerals “like lepidolite” develop ratio (0.712) of 87Rb/86Sr and may use in chronological studies without error. The Rb-Sr method was extended to include other mineral such as mica (biotite and muscovite) as well as potash feldspar that have lower Rb/Sr ratios. The discordant dates are suggested based on the initial ration (0.712) when the real initial ratio was higher. This expressed as a problem and was overcome by the isochron diagram designed by [24] who developed a new way for treating Rb-Sr data based on the principle of linear equation:

y=c+xmE40

In this way, 87Sr/86Sr (y) is plotted versus 87Rb/86Sr (x), then the intercept cis the initial 87Sr/86Sr ratio (Figure 8).

Figure 8.

Schematic Rb-Sr isochron diagram for a suit of co-magmatic igneous minerals [25].

This figure presents a suite of co-magmatic minerals of same age and initial of 87Sr/86Sr ratio, forming a line called isochron. From slop of isochron, m=eλt1, the mineral age can be determined. The mineral with very low Pb may yield the initial ratio directly on y-axis. On Figure 9, the 87Sr/86Sr increase with decrease 87Rb/86Sr due to decay Rb over time. The slope of ischron is increased accordingly with time. Practically, the y-axis is expanded much to displays rocks of geological time clearly, and the growth lines become vertical accordingly.

Figure 9.

Rb-Sr isochron diagram on axis of equal magnitude showing production of 87Sr as 87Rb is consumed in two hypothetical samples [25].

11.3 Erupted isochro

The original isotopic composition of mantle is basically inherited in the primary basic magma. The alkali ocean-island basalts were investigated for the Rb-Sr system [26]. The results of fourteen wide range type of ocean- island basalt samples plotted on an isochron displayed a proportional correlation to a slop age of about 2B.y representing the time of mantle isolation [25]. This age is known as a mantle isochrones that is also extended to continental igneous rocks [27]. The ratio of 87Sr/86Sr should correct back to initial ratio at the time of magmatism before plotting versus 87Rb/86Sr, so these are termed as pesud-oisochrons. Data of plotting 30 samples from both volcanic and plutonic continental igneous rock suits formed a roughly linear array. The pesud-oisochrons generates from two lines representing crustal contamination of mantle-derived basaltic magma, Scientist replaced that by timing the mantle differentiation events that established mantle domains of different ratio of Rb/Sr in subcontinental lithosphere. This suggestion does not provide reliable results to ascribe age significantly to erupted isochrones. However, the only isotope-isotope mantle isochrones is reliable and can be interpreted and used significantly as tool for dating the age of mantle differentiation events.

How to cite and reference

Link to this chapter Copy to clipboard

Cite this chapter Copy to clipboard

Salih Muhammad Awadh (April 12th 2020). Geochemistry of Radioactive Isotopes [Online First], IntechOpen, DOI: 10.5772/intechopen.91616. Available from:

chapter statistics

64total chapter downloads

More statistics for editors and authors

Login to your personal dashboard for more detailed statistics on your publications.

Access personal reporting

We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. We share our knowledge and peer-reveiwed research papers with libraries, scientific and engineering societies, and also work with corporate R&D departments and government entities.

More About Us