isotope geochemistry

isotope geochemistry Isotope geochemistry has provided the basis for some of the most important advances in the Earth sciences. Studies of the distribution of radiogenic isotopes in rocks and minerals are the only accurate and precise way we have of measuring the passage of time in geological processes. This has implications for fields as diverse as dating the age of the Earth to measuring rates of evolutionary change in planktonic microfossils. Radiogenic and stable isotopes can also be used to trace the sources of elements in processes such as crust–mantle interactions and the factors that control the composition of sea water. The temperature-dependence of stable isotope fractionation can also be used to recover information about low-temperature phenomena, such as climate change between glacial and interglacial intervals as well as processes at higher temperatures, such as the crystallization of granite magmas. In short, there is almost no field of geology that has not been touched by the study of natural isotope variations.

General principles

The nucleus of an atom is made up of neutrons and protons. By definition, all atoms of a particular element have the same number of protons, but most elements have atoms with differing numbers of neutrons. These different types of atom of the same element are termed isotopes. For example, carbon has three isotopes of interest to Earth scientists: carbon-12 (¹²C), carbon-13 (¹³C), and carbon-14(¹⁴C). All these isotopes contain six protons in the nucleus, but each has a different number of neutrons. For example, the nucleus of ¹²C also contains six neutrons, to give a total mass number of 12. Of these three carbon isotopes, ¹²C and ¹³C do not undergo radioactive decay, and are termed stable isotopes. In contrast, ¹⁴C is unstable and decays to ¹⁴N (nitrogen-14). The process by which unstable isotopes spontaneously decay to other isotopes (which may themselves be unstable) is called radioactivity. During radioactive decay a parent isotope decays by one of several different mechanisms to form a daughter isotope of a different atom. The isotopes that are formed from radioactive decay are termed radiogenic isotopes. The variations measured in the isotopic composition of elements in different reservoirs on the Earth are due to two separate processes. Different amounts of radiogenic isotopes arise from variations in the parent/ daughter ratios of different reservoirs and the passage of time, which leads to the steady addition of radiogenic isotopes to a reservoir. Isotope variations can also arise among stable isotope compositions if one isotope is preferentially incorporated in one of two phases that interact with one another. These two processes form a natural division in isotope geology between radiogenic isotope studies and stable isotope studies.

Radiogenic isotopes

There are many natural radioactive isotopes. Those of special interest to geological studies are ⁴⁰K/⁴⁰Ar (potassium– argon), ⁸⁷Rb/⁸⁷Sr (rubidium–strontium), ¹⁴⁷Sm/¹⁴³Nd (samarium–neodymium), ²³²Th/²⁰⁸Pb (thorium–lead), ²³⁵U/²⁰⁷Pb, and ²³⁸U/²⁰⁶Pb (uranium–lead). Details of these systems are given in Table 1. The half-life of a radiogenic isotope is the time it takes for half a given number of atoms of the parent isotope (e.g. ⁴⁰K) to decay to the daughter isotope (e.g. ⁴⁰Ar).

Each of the isotope systems has slightly different applications in the Earth sciences. Isotopic dating is among the most important (see isotopic dating). The essential provisions for this dating technique to be valid are that the minerals were formed at the same time in equilibrium with one another and that they have not gained or lost any rubidium or strontium since they were formed. Various isotope systems can be used for dating igneous rocks, but differences in their geochemical behaviour mean that they are applied to different dating problems according to how abundant the parent or daughter isotopes are in the rocks of interest and how susceptible the isotope system is to varying degrees of metamorphic alteration.

Table 1 Natural radioactive isotopes of special geological interest

Isotope system	Half-life (t1/2)	Isotope ratio of interest
40K → 40Ar	1.40 × 109years	40Ar / 36Ar
87Rb → 87Sr	4.89 × 1010years	87Sr / 86Sr
147Sm → 143Nd	1.06 × 1011years	143Nd / 144Nd
232Th → 208Pb	1.40 × 1010years	208Pb / 204Pb
235U → 207Pb	7.04 × 108years	207Pb / 204Pb
238U → 206Pb	4.47 × 109years	206Pb / 204Pb

Table 2 Stable isotopes of special geological interest

Isotopes	Isotope ratios	Notation
1H, 2H	2H/1H	δD
10B, 11B	11B/10B	δ11B
12C, 13C	13C/12C	δ13C
16O, 17O, 18O	18O/16O	δ18O
32S, 33S, 34S, 36S	34S/32S	δ34S

Different reservoirs in the Earth have evolved different radiogenic isotope compositions over time as a result of varying parent/daughter ratios. For example, rubidium is preferentially partitioned into the crust relative to strontium. The Rb/Sr ratio of continental crust is consequently higher than that of the mantle, and on average the ⁸⁷Sr/⁸⁶Sr ratios of crustal rocks are higher than those of rocks derived from the mantle.

Stable isotopes

Variations in the stable isotope composition of an element can arise during several different kinds of chemical reactions and physical processes. This is a consequence of the fact that certain thermodynamic properties of molecules are dependent on the masses of their component atoms. In practice, this stable isotope fractionation is significant only in isotopes with masses of less than 40. The major stable isotopes of interest in geological applications are shown in Table 2.

Stable isotope variations are generally referred to in terms of the del-notation, rather than by their isotope ratios. For example, oxygen isotope compositions are given by: Hence, the more positive the δ¹⁸O value, the heavier the oxygen isotopic composition; that is, the higher the ¹⁸O/¹⁶O. The extent of stable isotope fractionation between two different species is generally dependent on temperature in a relatively simple way, the magnitude being greater at lower temperatures. Other general rules of stable isotope fractionation are: the light isotope of an element is preferentially partitioned into reduced species (e.g. the δ³⁴S of SO₄²⁻ is heavier than that of coexisting H₂S); biota tend to enrich the light isotope of an element (e.g. the δ¹³C values of plants are lighter than that of atmospheric CO₂); and the light isotope of an element will favour the heavier cation in two minerals formed in equilibrium (e.g. the δ³⁴S of galena, PbS, is lighter than that of coexisting sphalerite, ZnS).

Other applications of isotope geochemistry in the Earth sciences

The stable isotope composition of rocks and minerals derived from the mantle (e.g. mid-ocean ridge basalts (MORB) and ocean island basalts (OIB)) do not generally show large variations; this is because the high temperatures in the mantle preclude significant stable isotope fractionation. However, radiogenic isotopes (mostly, strontium (Sr), neodymium (Nb), and lead (Pb)) show considerable variations that indicate that the mantle is not a homogeneous body, but contains reservoirs with different isotope compositions that reflect processes within the mantle and exchange of material between the crust and mantle. Some portions of the mantle have isotope compositions similar to those expected of a homogeneous Earth that has not undergone any differentiation. In contrast, some mantle-derived rocks have isotope compositions that indicate that they have had substantial portions of crust extracted from them. Other portions of the mantle have isotope compositions that appear to reflect recycling of continental material back into the mantle or movement of metasomatic fluids within the mantle, or both.

Destructive plate margins such as island arcs are major sites of interaction between the crust and the mantle. In some instances they have stable isotope compositions (particularly oxygen and boron) and radiogenic isotope composition (mostly strontium and lead) that extend outside the variation shown by MORB and OIB, indicating that they contain an additional source of some elements. The isotope composition of this additional source is similar to that of the altered oceanic crust and sediments that are carried down by the subducting plate beneath the island arc. Hence, isotope studies provide some of the clearest evidence that at least some subducted material is driven off the subducted slab (either by melting or dehydration reactions) and incorporated into the overlying volcanic rocks.

The nature and the growth and evolution of the continental crust is one of the most fundamental aspects of the study of the Earth. There has been some disagreement over whether virtually all the continental crust formed very early in the Early's history and has remained relatively constant in size since that time, or whether it has been growing at a more or less constant rate since the Earth formed. Isotope studies have provided partial answers to this question through the direct dating of portions of the continental crust (primarily using U–Pb dating of zircons) and by determining the average age of sediments weathered from the continents (from the Sm–Nd isotope system). These studies appear to show that there have been periods in Earth history when there has been more rapid continental growth (particularly around 2.7Ga (2.7 × 10⁹ years)) and that the continental crust has remained relatively constant in size over the past 2 Ga.

Granites comprise a significant portion of the continental crust. Isotopes (and other geochemical tracers) indicate that a high proportion of granites have compositions that are intermediate between two end members: I- and S-types. The S-type end members tend to have isotope compositions (in particular for oxygen and strontium) that are very similar to old continental crust and clastic sediments. It is therefore thought that S-type granites are derived from melting of these materials. In contrast, I-type end members have isotope compositions that are closer to those of mantle-derived rocks. They are accordingly thought to represent melts derived from the upper mantle without significant subsequent interaction with the crust.

Many types of ore deposits are thought to have formed from the circulation of aqueous fluids through heated rocks. During this process chemical and isotopic exchange takes place between the reservoirs and is recorded in the isotope composition of hydrothermal minerals. Isotope studies (particularly of boron, carbon, sulphur, strontium, and lead) yield information about the source of the elements in the mineral deposit (for example, whether an element was derived from sea water or from rocks). Oxygen and hydrogen isotopes are especially useful in determining the origin of the fluids and the hydrology of the ore deposit (for example, whether the fluids were sea water, brines, or meteoric water, and how extensive the reactions were between the fluids and the rock).

When plankton precipitate calcium carbonate shells the oxygen isotope composition of their shells records the temperature and isotopic composition of the sea water in which they grew (which is largely dependent on ice volume). When they die, their shells accumulate in sediments at the sea floor. Analyses of microfossils from shells of different ages can thus be used to reconstruct the climatic history of the oceans. These studies have revealed regular variations in the Earth's climate, and it is apparent that our climate is linked to changes in the Earth's orbit and in the tilt of its axis of rotation (Milankovich cycles).

M. R. Palmer

Bibliography

Faure, G. (1986) Principles of isotope geology (2nd edn). John Wiley and Sons, New York.