22 Isotope Geology

M.E.A. Mondal

  1. Introduction

 

1.1         What are Isotopes?

 

 

Isotopes are defined as atoms of same element (hence having the same number of electrons and of protons), but contain different numbers of neutrons, thereby having same atomic number but different atomic mass. Principal processes of nucleosynthesis resulting in formation of elements take place in stars and in supernovae. Some light elements like H and its isotope D, He and Li were created during the Big Bang. Elements with atomic masses up to iron are produced in stars in response to a number of reactions over a range of temperatures. Hydrogen buns in the core of a star to form He, which in turn burns to produce carbon and oxygen (Fig. 1). When hydrogen is exhausted, the star contracts and the temperature rises to the tune of 108K. This is the stage when nuclear reactions start to produce all other elements up to atomic mass of iron (Table 1). Rest of the elements, heavier than iron, are formed during supernova explosion.

Isotopes can be conveniently grouped into two classes: stable and unstable (radioactive). Stable isotopes do not naturally decay but can exist in natural materials in differing proportions in contrast to unstable (radioactive) isotopes that continuously and spontaneously break down/decay in other lower atomic weight isotopes. Earth and other planets still contain the same number of stable isotopes as were present at the time of the formation of the solar system. In that sense, stable isotopes last forever, and their numbers as a whole, do not change through time. In contrast, the number of radioactive isotopes changes through time.

 

Based on their formation/application, isotopes can be conveniently categorized into the following classes:

 

  1. Primordial Isotopes: formed before the formation of Earth. Primordial isotopes were formed by nucleosynthesis in Big Bang and subsequent to it in stars, and were present in the interstellar medium from which the solar system was formed
  2. Cosmogenic Isotopes: form constantly in the Earth’s atmosphere or on the exposed surface of the Earth by interaction of cosmic rays and target element
  3. Anthropogenic Isotopes: generated through nuclear reactions in nuclear reactors or nuclear weapons explosions.
  4. Radiogenic Isotopes: form as a product of radioactive decay
  5. Stable Isotopes: are not product of radioactive decay, they do not naturally decay but can exist in natural materials in differing proportions.
  6. Non-Traditional isotope: these are stable isotopes in addition to traditional stable isotopes (viz. isotopes of H, C, O, S, N). Non-traditional isotopes include isotopes of Ca, Cu, Fe, Cr, Mo, Mg, Sr, Zn etc.

 

Table below (Table 2) lists different classes of isotope.

 

Table 2. Examples of different Classes of isotopes

  1. Importance of Isotopes

 

 

Isotope geochemistry has emerged as an essential tool for understanding many earth processes, particularly in environmental science. Isotopes are extensively being used in understanding physical, chemical and biological processes in contamination studies, atmospheric sciences, climate studies, exploration geochemistry, biogeochemistry, resource management, ecology, microbial activity, geochemical cycles, ocean sciences etc. Isotopes have also been used to obtain age and rate of processes.

 

With the advancement of analytical technology, isotopes will have an increasing overriding importance in environmental studies in the years to come. In addition to traditional stable isotopes, the non-traditional isotopes with many distinct geochemical characteristics, have turned out to be unique tracers of different geo-biological, cosmological and environmental processes. These isotope systems have been extensively employed to evolve sustainable strategies in pollution studies involving multiple sources, both anthropogenic and natural, which otherwise would not be possible utilizing conventional isotopes systems only.

  1. Measurements of Isotopes

 

Based on different properties of isotopes, actual purpose of study, half-life of the radioactive isotopes, abundances of isotopes available in the material to be analyzed, and ratios of the stable isotopes of the element, different techniques are used to measure the isotopes. Radioactive isotopes decay by emitting radiations, and by measuring and quantifying the radiations arising out of radioactive decay, it is possible to have an assessment of abundance by relating it to decay rate. Other method employs direct measurement of isotopes by using mass spectrometry. Thus all the measurement techniques of isotopes revolve around two fundamentally different approaches:

 

  • (1) Counting Methods
  • (2) Mass Spectrometry

 

There has been a quantum jump in the field of isotope geology with the development of state of the art technology in the field of analytical techniques. Current advances in the analytical instrumentation along with miniaturization of instruments has made it possible to analyze many isotopes which were otherwise undetectable a few years ago. With the development of thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), it is now possible to make measurements of isotopic ratios of elements up to uranium. This has also opened new vistas in the applications of isotopes including non-traditional isotopes in the field of exploration, cosmochemistry, geochemistry, atmospheric, climatic and environmental sciences.

 

Counting Methods: These methods are designed to detect alpha, beta and gamma radiation and are known as Alpha Spectrometry, Beta Spectrometry and Gamma Spectrometry, respectively.

 

Mass Spectrometry: These methods are designed to count the isotopes directly. Before counting the isotopes, the separation of isotopes is achieved by allowing the charged particles to pass through a magnetic field (Fig. 2), whereby the isotopes as per their mass to charge ratio are separated (Fig. 3). Different types of mass spectrometers have been designed for different types of elements to achieve better resolution of results. Following mass spectrometric techniques are generally used to measure isotopes in geosciences:

 

  1. Thermal Ionization Mass Spectrometry
  2. Inductively-Coupled Plasma Source Mass Spectrometry
  3. Gas Source Mass Spectrometry (including Isotope Ratio Mass Spectrometery)
  4. Accelerator Mass Spectrometry
  5. Secondary-ion Mass Spectrometry
  1. Reporting Isotopic Data

 

 

The absolute values of concentrations of isotopes and also the difference between two isotopes of an element is generally too small to measure with adequate precision, so it is convenient to use a ratio. By comparing the ratio of the sample to that of a standard, the difference between the two can be determined much more precisely. For radiogenic isotopes, it is convenient to report isotopic ratios by diving by an (stable) isotope which is not produced by radioactive decay, and hence remains constant through time [e.g. 206Pb/204Pb, 206Pb is a radiogenic (stable) isotope, whereas 204Pb is a stable isotope, not produced by radioactive decay]. Conventionally, the stable isotopic ratio of any element is reported in comparison to a standard. Since the variations in isotopic ratios are small, delta (δ) notation is generally used, where δ is defined as:

 

δHE =[(Rsample/Rstandard)-1] X 1000 (expressed as per mill, ‰)

Where R = heavy (H) to light isotope ratio of element E

e.g.  δ18O =[((18O/16O)sample/(18O/16O)standard)-1] X 1000

 

The basic use of radiogenic isotopes has been in age determination which makes use of the constancy of the rate of radioactive decay. Since a radioactive nuclide decays to its daughter at a constant rate which is not dependent on anything, age can be determined by measuring how much of  the nuclide has decayed. Decay follows a statistical rule. The basic law of radioactive decay is that rate of decay of radioactive nuclei is proportional to the amount of the parent isotope (N), and is expressed as:

 

dN/dt = -λN, where λ is decay constant.

 

 

This equation can further be written as:

 

 

N= N0 e–λt where N0 is the number of nuclei present at time t =0 and N is the number of nuclei present at time t.

This equation is considered the fundamental equation of radioactivity. The equation can be further modified to reach to a situation where the parameters are measurable. The equation:

D* = N (eλt -1)

 

has been considered practically useful as it deals with present day situation (D* is the number of radiogenic daughter isotopes).The above equation is further modified as D* cannot be directly measured by instruments. The equation that is used for geochronology is in the form of:

 

Dm= Di + N (eλt -1)

 

where Di is the number of daughter isotopes present initially.

Fig. 4 depicts decay of radionuclide (N) to a stable radiogenic daughter (D*) through time.

 

4.1 Isotope Standards

 

 

The absolute isotopic concentrations of any given element are too small to measure and compare accurately. Conventionally, isotopic ratios of any given element are compared to standard values for that element. By comparing the ratio of a sample to that of a standard, the difference between these two can be determined much more precisely. Different standard materials are used for different stable isotopes. Following are the standards which are mostly used for stable isotope studies:

VSMOW = Vienna Standard Mean Ocean Water  (used for H and O isotopes)

 

PDB = Pee Dee Belemnite, fossil of belemnite from the Pee Dee Formation in Canada (used for C and O isotopes)

 

CDT = Canyon Diablo Troilite, meteorite fragment containing FeS mineral Troilite (used for S isotopes)

 

AIR = Atmospheric air (used for N isotopes).

 

 

  1. Isotopic Fractionation

 

 

Isotopes of an element, having the same number of protons and electrons, and having same electronic configuration, have the same chemical properties and undergo same chemical reactions. Differences in mass, arising out of different number of neutrons, may affect the kinetics of physical, biological and chemical reactions involving different isotopes, mainly for lighter ( mass number <40) isotopes, leading to substantial differences in isotopic ratios of the element. Such partitioning or sorting of isotopes is termed as isotopic fractionation. Molecules react with different rates depending on their different masses because of different isotopic composition. This enables all chemical, physical and biological processes fractionate isotopes. Mass differences give rise to isotopic fractionation during physical processes (e.g. evaporation, condensation, freezing, diffusion, etc.). Fig. 5 depicts the fractionation of oxygen isotopes between water (liquid) and vapour. Isotope fractionation becomes more pronounced for the elements with low atomic mass, large mass difference, largely covalent bonds and multiple oxidation states. Isotopic fractionation is so effective for the stable isotopes at lower temperature that different isotopic reservoirs have come into existence through time.

 

There are some generalizations that are agreed upon:

 

 

  • – Molecules with lighter isotopes diffuse faster and evaporate faster, form weaker bonds, react faster and concentrate in products
  • – Biological processes prefer lighter isotopes in the product
  • – Higher oxidation state is preferred by heavier isotopes
  • – Solid phases and compounds with the strongest bonds are preferred by heavier isotopes

 

Fractionation of isotopes occur mainly by two mechanisms:

 

 

  1. Equilibrium Isotope Fractionation
  2. Kinetic Isotope fractionation

 

Equilibrium Isotope Fractionation: Driving force of this fractionation is the differences in the vibrational energy of the molecules containing different isotopes thereby having different masses.

 

Kinetic Isotope Fractionation: This type of fractionation is transient, occur in unidirectional way (open system, non-equilibrium) and incomplete, and is very effective mechanism for isotope fractionation. This is mainly driven by differences of kinetics of molecules containing different masses.

Fractionation of isotopes is represented by Fractionation Factor (denoted by α), where α is defined as:

  • α = Rreactants/Rproducts (e.g. α18O = (18O/16O)water/(18O/16O)vapour

where R = ratio of heavy to light isotopes

 

 

  1. Applications of Isotopes in Geological & Environmental Geology Studies

Isotope geology deals with the study of the variations in isotopic ratios of various elements in rocks, minerals, soils, sediments, ice, water, natural materials etc. Isotopes can reveal information about the age, genesis and processes involved in the origin and evolution. It is apparent that radiogenic, stable and cosmogenic isotope systems with their unique characteristics can help in understanding many earth science issues.

 

Subject of stable isotope geochemistry is mainly concerned with mass-dependent isotope fractionation, whereas radiogenic isotope mainly deals with the decay products. Stable isotopes are useful in assessing relative contribution of various reservoirs, each with a distinctive isotopic signature. Radiogenic isotopes provide useful information regarding the ages and origins of Earth and earth materials. They are particularly useful to understand petrogenetic processes as the heavy radiogenic isotope ratios are not usually fractionated by physical or chemical processes. So they provide unequivocal clue for the source of the magma. They survive chemical fractionation events, formation and evolution of magma. Isotopes of heavier elements are not separated from each other through crystal-liquid equilibria. Thus, during partial melting, a magma will inherit the isotopic composition of its source and will remain constant during subsequent fractional crystallization provided the magma is not contaminated by material with distinct isotopic ratio. Hence isotopes have edge over trace element geochemistry. Based on isotopic ratios different mantle geochemical components with distinct isotopic signatures have been delineated. Isotopes are very important in understanding the processes like characterization of source, mixing processes, physical and chemical processes and above all age determination. Isotopes have successfully been employed to understand early evolution of the Earth.

Most of the early developed isotopic methods were focused on solving bedrock and core geological problems mainly concerned with providing absolute age and understanding the Earth processes. More recently isotope geology has emerged central to environmental studies in providing powerful tracers, rate monitors and fingerprints for processes in terrestrial, surface and near-surface, atmospheric, ecological and aquatic environment. Isotopes that are widely used in environmental studies include:

 

  • Stable isotopes
  • Cosmogenic isotopes
  • Radiogenic isotopes
  • Anthropogenic isotopes.

 

 

6.1 Stable Isotopes

 

 

Most commonly used stable isotope for environmental sciences include: O, H, N, C, S, Li, Si, B etc. Common fractionation of isotopes of these elements are kinetic processes, equilibrium partitioning and biological processes. O and H isotopes have been successfully utilized to delineate juvenile, meteoric and brine water. δ18O values for mantle rocks significantly differ from that of the surface-reworked sediments, so oxygen isotopic ratios can be employed to evaluate contamination by crustal sediments. Further, secular variation of temperature has been studied by analyzing the oxygen isotopes of carbonate, silica and phosphate. It has been observed that climatic temperature declined from 700C during Archaean time to 30-200C in Paleozoic time in the geological past. It has also been revealed from the study of oxygen isotopes of benthic foraminifera that continental ice sheet in the Antarctica formed during Miocene. Other common examples of applications of stable isotopes have been in the field of groundwater, atmospheric sciences, weathering, hydrocarbon exploration, pollution studies etc. Table 3 shows the common stable isotopes and their applications in Environmental Sciences.

 

 

 

6.2 Cosmogenic Isotopes

 

Isotopes formed from reactions involving cosmic rays within atmosphere and also on the exposed surface, are known as cosmogenic isotopes. These isotopes have been used as atmospheric and ocean circulation tracers, and also in surface processes including geomorphological processes. Cosmogenic isotopes that have accumulated on or near the surface as a result of interaction between the cosmic ray and the target element, can provide information on timing and rate of surface processes including geomorphic processes, fault displacement, burial of sediments, erosion events etc. Table 4 shows the common cosmogenic isotopes and their applications in Environmental Sciences.

Table 4. Common Cosmogenic Isotopes and their application in Environmental Geology

 

 

 

 

6.3 Radiogenic/Radioactive Isotopes

Radiogenic isotopes are practically applied in two different ways: (i) to obtain an absolute age

 

  • (ii) to trace/track the origin of a component. Unlike stable isotopes whose ratios remain unchanged by radioactive processes, a radioactive isotope is unstable and decay to another isotope. Radiogenic isotopes are the products of radioactive processes. Radioactive isotopes, based on their half-life, can conveniently be divided into two groups:

 

  1. Long-lived Radioactive Isotopes
  2. Short-lived Radioactive Isotopes

 

Long-lived isotopes (e.g. 238U-206Pb, 235U-235Pb, 87Rb-86Sr) are generally employed to obtain ages of events in geologic past, whereas the short-lived isotopes are useful in obtaining relatively younger events, processes. Isotopic ratios like 87Sr/86Sr have been used to identify the sources, as the surface rocks vary greatly in 87Sr/86Sr, enabling identification of sources of Sr into soils, surface and groundwater. Some radiogenic isotopes like 210Pb are produced in the atmosphere from the decay of 222Rn, have been used as tracer in atmospheric studies. 210Pb is widely used to determine the age of sediments and also rate of sedimentation.

A number of radiogenic isotopes viz. 3He, 129I, 39Ar which behave in a conservative way may enter the groundwater and provide ideal tracers making them very useful for groundwater studies.

 

6.4 Anthropogenic Isotopes

 

Anthropogenic isotopes are generated either within the nuclear reactors or during nuclear bomb explosions. These isotopes may act as contaminant in environmental studies. Most commonly used anthropogenic isotopes in environmental studies include 14C, 129I, 36Cl, 3H. Conveniently anthropogenic isotopes may be categorized into two groups:

  1. Radionuclides not otherwise present/produced in nature (e.g. 137Cs, 240Pu)
  2. Radionuclides occurring as distinct pulses of otherwise naturally occurring isotopes (e.g. 14C)

Use of nuclear bombs or atmospheric bomb testing in the 1950s produced a spike in the production of anthropogenic isotopes. These higher concentrations can be used as marker. Anthropogenic isotopes have been mostly used in environmental studies.

 

  1. Summary

 

In this lecture we learnt about:

 

  1. What are isotopes?
  2. What are the main classes of isotopes?
  3. How are isotopes useful in understanding geological and environmental processes?
  4. What are the different techniques used for analysis of isotopes?
  5. How are isotope data reported?
  6. What are the different reference material standards used for isotope studies?
  7. What is fractionation of isotopes?
  8. How does isotope fractionation lead to varying isotopic ratios?
  9. Applications of isotopes in solving geological, atmospheric, hydrogeological, climatic and environmental issues.