STABLE AND RADIOGENIC ISOTOPES
(in Ores and Host rocks)
Geochemistry involves a study of the chemical composition of Earth and other planets. The Chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space are revealed by geochemical studies. Isotope geochemistry is the most important field of geochemistry which involves the determination of the relative and absolute concentrations of the elements and their isotopes in the earth and on its surface. Broadly, the field is divided into two branches: stable and radiogenic isotope geochemistry.
Isotopes of an element are atoms that have the same numbers of protons and electrons but different numbers of neutrons. This means that various isotopes of an element have similar charges but different masses. The superscript number to the left of the element designation indicates the number of protons plus neutrons in the isotope. For example, among the hydrogen isotopes, deuterium (denoted as D or 2H) has one neutron and one proton. This is approximately twice the mass of protium (1H) whereas tritium (3H) has two neutrons and is approximately three times the mass of protium. All isotopes of oxygen have 8 electrons and 8 protons; however, an oxygen atom with a mass of 18 (denoted 18O) has 2 more neutrons than oxygen-16 (16O).
The original isotopic compositions of planetary systems are a function of nuclear processes in stars. Over time, isotopic compositions in terrestrial environments change by the processes of radioactive decay, cosmic ray interactions, and such anthropogenic activities as processing of nuclear fuels, reactor accidents, and nuclear-weapons testing.
Stable and Radiogenic Isotopes
Radioactive (unstable) isotopes are nuclei that spontaneously disintegrate over time to form other isotopes. During the disintegration, radioactive isotopes emit alpha or beta particles and sometimes also gamma rays. The so-called stable isotopes are nuclei that do not appear to decay to other isotopes on geologic timescales, but may themselves be produced by the decay of radioactive isotopes. For example, 14C, a radioisotope of carbon, is produced in the atmosphere by the interaction cosmic-ray neutrons with stable 14N. With a half-life of about 5730 years, 14C decays back to 14N by emission of a beta particle; the stable 14N produced by radioactive decay is called "radiogenic" nitrogen.
Stable isotope compositions of low-mass (light) elements such as oxygen, hydrogen, carbon, nitrogen, and sulfur are normally reported as d values. d values are reported in units of parts per thousand (denoted as ‰ or permil) relative to a standard of known composition. d values are calculated by:
d (in ‰) = (Rx / Rs - 1) ·1000
where R denotes the ratio of the heavy to light isotope (e.g., 34S/32S), and Rx and Rs are the ratios in the sample and standard, respectively. For sulfur, carbon, nitrogen, and oxygen, the average terrestrial abundance ratio of the heavy to the light isotope ranges from 1:22 (sulfur) to 1:500 (oxygen); the ratio 2H:1H is much lower at 1:6410. A positive d value means that the isotopic ratio of the sample is higher than that of the standard; a negative d value means that the isotopic ratio of the sample is lower than that of the standard.
Various isotope standards are used for reporting light stable-isotopic compositions.
Because of their mass differences, different isotopes of an element have slightly different chemical and physical properties. Isotopes of low atomic numbers have mass differences that are large enough for many physical, chemical, and biological processes or reactions to "fractionate" or bring about a change in the relative proportions of various isotopes. As a result of fractionation, waters and solutes often develop unique isotopic compositions (ratios of heavy to light isotopes) that may be indicative of their source or of the processes that formed them. Two different types of processes -- equilibrium and kinetic isotope effects -- cause isotope fractionation.
a) Equilibrium Isotopic Fractionation, and
b) Kinetic Isotopic Fractionation
Equilibrium Isotopic Fractionation: In equilibrium isotope-exchange reactions the isotopes of an element are redistributed among various mineral species. At equilibrium, the forward and backward reaction rates of any particular isotope are identical. This does not mean that the isotopic compositions of two minerals in equilibrium are identical, but that the ratios of different isotopes in each mineral are constant. During equilibrium reactions, the heavier isotope generally becomes enriched (preferentially accumulates) in the mineral or compound with the higher energy state. For example, in coexisting sulfates and sulfides, 34S is enriched in the sulfate relative to the sulfide; consequently, the sulfide is described as depleted in 34S relative to sulfate. During phase changes, the isotopic ratios in molecules in the two phases change. For example, as water vapor condenses (an equilibrium process), the liquid phase becomes enriched in heavier isotopes (18O and 2H) while the vapour phase become enriched in lighter isotopes (16O and 1H).
Kinetic Isotopic Fractionation: Kinetic isotope fractionations occur in systems out of isotopic equilibrium where forward and backward reaction rates are not identical, or the reaction is unidirectional. Some reactions may become unidirectional if the reaction products become physically isolated from the reactants. Reaction rates depend on ratios of the masses of isotopes and their vibration energies. As a general rule, bonds between the lighter isotopes are weaker and hence broken more easily than bonds between the heavy isotopes. Hence, the lighter isotopes react more readily and become concentrated in the products, and the residual reactants become enriched in the heavy isotopes.
In general, biological processes are unidirectional and are excellent examples of "kinetic" isotope reactions. Organisms preferentially use the lighter isotopic species because of the lower energy required for bonding. This results in significant fractionations between the heavier (substrate) and the lighter (biologically mediated) product. The magnitude of fractionation depends on the reaction pathway utilized and the relative energies of the bonds being broken and formed during reactions. In general, slower reactions show greater isotopic fractionation than faster reactions because the organism has time to be more selective. Kinetic reactions can result in fractionations much larger than equivalent equilibrium reaction.
Isotopic fractionation can take place either under purely equilibrium conditions or by kinetic isotopic fractionation. It must be understood that whereas one or the other may be the dominant process, in most cases there is always some contribution by the other. For example, although evaporation can take place under purely equilibrium conditions (i.e., at 100% humidity when the air is still), more typically the products become partially isolated from the reactants (e.g., the resultant vapor is blown downwind). Under these conditions, the isotopic compositions of the water and vapor are affected by an additional kinetic isotope fractionation of variable magnitude.
Use of Various Isotopes Ores and Host Rocks
Sulfur Isotope Geochemistry: A substantial fraction of all economically valuable metal ores are sulfides. These have formed in a great variety of environments and
under a great variety of conditions. Sulfur isotope studies have been very valuable in sorting out the genesis of these deposits.
There are two major reservoirs of sulfur on the Earth that have uniform sulfur isotopic compositions: the mantle, which has δ34S of ~0 and in which sulfur is primarily present in reduced form, and seawater, which has δ34S of +20 ‰ and in which sulfur is present as SO42-. Sulfur in sedimentary, metamorphic, and igneous rocks of the continental crust may have δ34S that is both greater and smaller than these values. All of these can be sources of sulfide in ores, and further fractionation may occur during transport and deposition of sulfides.
Oxygen Isotope Geochemistry: Oxygen isotope studies can be a valuable tool in mineral exploration. Mineralization is very often (though not exclusively) associated with the region of greatest water flux, such as areas of upward moving hot water above intrusions. Such areas are likely to have the lowest values of δ18O.
Lead-Lead Isotope Geochemistry: Lead has four stable isotopes - 204Pb, 206Pb, 207Pb, 208Pb and one common radiogenic isotope 202Pb with a half-life of ~53,000 years. Lead is created in the Earth by the decay of transuranic elements, primarily uranium and thorium.
Lead isotope geochemistry is useful for providing isotopic dates on a variety of materials including ores and their host rocks. Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in igneous rocks or the source of sediments. It has been used to date ice cores from the Arctic shelf, and provides information on the source of atmospheric lead pollution.
Samarium-Neodymium: Samarium-neodymium is an isotope system which can be utilized to provide information on age as well as the source of ore material. 147Sm decays to produce 143Nd with a half life of 1.06x1011 years. This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir - which is an approximation of the chondritic material which formed the solar system. CHUR was determined by analysing chondrite and achondrite meteorites.
Differences in the ratios of the sample relative to CHUR can give information on whether the ore material was supplied by a granitic source (depleted in radiogenic Nd), the mantle, or an enriched source.
Rhenium-Osmium: Rhenium and osmium are chalcophile elements which are present in very low concentrations in the crust. Rhenium undergoes radioactive decay to produce osmium. The ratio of non-radiogenic osmium to radiogenic osmium varies through geological time.
Rhenium enter sulfide minerals more readily than osmium. Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium-osmium ratio from changing appreciably. This locks in an initial osmium ratio of the sample at the time of the melting event. Osmium-osmium initial ratios are used to determine the source characteristic and age of mantle melting events.
3He-4He: 3He was trapped in the earth at the time of its creation. It has been observed that 3He is present in volcanic emissions and oceanic ridge samples. 3He associated with the mantle and is used as a marker of material of deep origin. Due to similarities in helium and carbon in magma chemistry, outgassing of helium requires the loss of volatile components (water, carbon dioxide) from the mantle, which happens at depths of less than 60 km. However, 3He is transported to the surface primarily trapped in crystal lattice of minerals and in fluid inclusions.
4He is produced by decay of uranium and thorium. The continental crust has become enriched with these elements relative to the mantle and thus more 4He is produced in the crust than in the mantle.
The ratio (R) of 3He to 4He is often used to represent 3He content. R is usually given as a multiple of the present atmospheric ratio (Ra).
Common values of R/Ra for geological materials are:
· Old continental crust: < 1
· mid-ocean ridge basalt (MORB): 7 to 9
· Spreading ridge rocks: 9.1 ± 3.6
· Hotspot rocks: 5 to 42
· Ocean and terrestrial water: 1
· Sedimentary formation water: < 1
· Thermal spring water: 3 to 11
3He/4He isotope chemistry is being used to date groundwater, estimate groundwater flow rates, track water pollution, and provide insights into hydrothermal processes, petrogenesis and ore genesis
Isotopic compositions are determined in specialized laboratories using isotope ratio mass spectrometry. The analytical precisions are small relative to the ranges in δ values that occur in natural earth systems. Typical one standard deviation analytical precisions for oxygen, carbon, nitrogen, and sulfur isotopes are in the range of 0.05‰ to 0.2‰; typical precisions for hydrogen isotopes are poorer, from 0.2 to 2.0‰, because of the lower 2H:1H ratio.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)