Rubidium-strontium dating

Rubidium-strontium (Rb-Sr) dating is one of the most common methods of obtaining absolute (numerical) ages of geologic materials, especially older minerals and rocks.

Rubidium

In 1904, British chemist and physicist Ernest Rutherford proposed that geologic time might be measured by the breakdown of uranium in uranium-bearing minerals. A few years later, American chemist Bertram Boltwood published the absolute, or numerical, ages of three samples of such minerals. The ages, which approximated 500 million years, indicated the antiquity of some earth materials, a finding developed by British geologist Arthur Holmes in his classic The Age of the Earth (1913). Holmes's early time scale for the earth was not immediately accepted by most of his peers, but it helped to set the stage for the eventual use of absolute ages as the prime quantitative components in the study of geology and its many subdisciplines. After the early study of the isotopes of uranium, including uranium-series transition isotopes, came the discovery of other unstable isotopes and the formulation of the radioactive decay schemes that have become essential to geochronology, including the rubidium-strontium method.

Rubidium (Rb), an alkali-group (lithium, sodium, potassium, rubidium, cesium, and francium) element with a valence (bonding value) of +1, is a trace element in terrestrial and solar system materials. It is not a necessary component in any known mineral; it substitutes for the major element potassium in common, rock-forming minerals such as the alkalic feldspars and micas. Because of their similar ionic size and geochemical behavior, the ratio of potassium to rubidium is a useful petrologic parameter.

Rubidium consists of two natural isotopes: rubidium-87 and rubidium-85. Several artificial isotopes also are known. Rubidium-87 has been known to be unstable since 1940, but its use in age dating did not begin until the advent of modern mass spectrometry in the 1950's. It decays by the emission of a beta particle to the stable nuclide strontium-87. The half-life for this decay is not precisely known, because of the low energy spectrum of the emitted beta particles. Half-life values range from 47 to 50 billion years. Although the commonly accepted value is 47 billion years, a comparison of Rb-Sr dates for samples of meteorites and lunar rocks with dates for the same samples yielded by other techniques indicates that a value of 48.8 billion years is more correct. In any event, the long half-life and the low parent-daughter ratio mean that the radiogenic accumulation of strontium-87 in most natural minerals and rocks is very slow.

Strontium

Strontium—which, along with beryllium, magnesium, calcium, barium, and radium, belongs to the alkaline-earth group of elements—has a valence of +2. Though more abundant than rubidium in materials of the solar system, it also is a trace element. It forms its own mineral phases in the form of strontianite, a strontium carbonate, and celestite, a strontium sulfate, but it occurs more significantly as a substitute for the major species calcium in common, rock-forming minerals such as plagioclase feldspar, calcium-rich pyroxenes and amphiboles, apatite, and calcium carbonate minerals. Like the potassium-rubidium ratio, the calcium-strontium ratio has use in the understanding of various geologic processes. Strontium comprises four naturally occurring isotopes: strontium-88, -87, -86, and -84. There are also several artificial isotopes, the best known being the nuclear-fission-produced strontium-89 and -90.

Purification of Samples

Concentrations of rubidium and strontium in minerals, rocks, and other natural substances can be determined by a variety of techniques, although isotopic parameters and precise elemental abundances are commonly determined by mass spectrometry. As the abundances of rubidium-85, strontium-88, strontium-86, and strontium-84 all are precise percentages of the total for each of these elements and do not vary as a function of nuclear instability, each can be calculated by taking the appropriate percentage of the total elemental concentrations, as determined by gravimetric analysis, atomic absorption spectrophotometry, X-ray fluorescence spectrophotometry, microbeam probe, or another type of analysis. Most modern work, however, focuses on determining the relevant isotopic parameters by mass spectrometry, after purification by chemical techniques.

Commonly, a preliminary determination of rubidium and strontium abundance and the Rb-Sr ratio is made or estimated for the minerals, rocks, or other materials to be dated by a reconnaissance technique, such as X-ray fluorescence. Samples selected on the basis of these determinations are chosen for dating. After selection, the samples are crushed—homogenized, if necessary—and a fraction is taken which contains enough of the rubidium and especially the more critical strontium for adequate isotopic analysis. Rarer materials, such as meteorites and lunar rocks, may not afford enough of a sample for optimal analysis. The fraction of material—if, as is most common, it is in the geologic form of aluminosilicate compounds, perhaps with some organic material—is dissolved in a mixture of hydrofluoric and perchloric acids and reduced by evaporation to a concentrated “mush” of material. Samples other than silicates may be dissolved in other, more appropriate solvents. The concentrated mush is spiked with an appropriate amount of purified liquids containing known amounts of rubidium and strontium of known, nonnatural isotopic composition. This material is dissolved in a small amount of hydrochloric acid and placed on calibrated ion exchange columns. The columns are washed with hydrochloric acid, and purified portions of rubidium, then strontium, of mixed natural and spiked isotopic composition, are collected and evaporated. Achieving the highest accuracy and precision requires that the smallest amounts of rubidium and strontium from the laboratory environment (contamination) be included with the completed, purified elements.

Mass Spectrometry

The standard method of mass spectrometry for Rb-Sr dating involves placing the purified samples as solids on metal filaments and installing the loaded filaments in solid-source mass spectrometers. The spectrometer source regions, evacuated to very low pressures, are constructed so that the metal filaments can be heated until the rubidium or strontium ionizes. The charged, ionized sample is accelerated through a series of collimating slits into a controlled magnetic field, where the beams of ions are separated by charge-mass ratios into beams of separate isotopes; as the charge of the elements is the same for each atom, the ions are separated on the basis of mass only. Specific isotopic beams, controlled by the magnetic field, are channeled through more collimating slits to the collector part of the spectrometer. Commonly, a Faraday cup is used to analyze the number of atoms of each isotope by the conversion of each atomic impact into a unit of charge, which is then amplified. The actual output is isotope ratio measurements—rubidium-87/rubidium-85, strontium-88/strontium-86, and so on—which are converted using mathematical programs into the required parameters for determining time. Because strontium-87 is closest in abundance to strontium-86 in most cases, and because it differs by only one mass unit, the standard for reporting the radiogenic component is with the ratio strontium-87/strontium-86.

By mixing precise amounts of “spikes” of strontium and rubidium, whose isotopic compositions differ markedly from natural isotopic compositions, with the strontium and rubidium in the natural sample, a combined mass spectrum is obtained. From these data can be calculated the precise abundances of rubidium and strontium in the natural material (a process known as isotope dilution) and the critical isotopic composition of the natural strontium.

Isochron Diagrams

Although the age of the analyzed sample can be calculated using the determined Rb-Sr parameters and the decay constant for rubidium-87, it is customary and more useful to determine the age graphically, with the use of an isochron diagram. In the diagram, the actual isotope ratios collected in the spectrometer are used as coordinates. Thus, the parent, unstable component, rubidium-87, is designated by reference to a common isotope of strontium, strontium-86. The other coordinate, the measure of the radiogenic component, is strontium-87/strontium-86. A line connecting points representing samples of equal ages, an isochron, has an age value that is represented by its slope on the figure; a horizontal isochron has an age value of zero, and positive slopes of successively greater degree have increasingly greater ages, given in terms of the isochron slope and the half-life of the parent rubidium-87 isotope. A single mineral or rock would furnish only one point on the diagram, so for an isochron to be drawn, there must be knowledge of or, more likely, an estimate of the sample's initial isotopic composition. Ages calculated in this way are termed “model ages.”

Analysis of minerals of equal age but different compositions from a sample of plutonic volcanic rock would allow the construction of an isochron, whose slope would be proportional to the age of crystallization of the rock. An assumption that is justified in most circumstances is that at the time of crystallization from a magma or lava, all minerals formed have the same isotopic composition of strontium. The basis for this assumption is that, unlike isotopes of elements with a mass of less than about 40, there is no measurable fractionation of isotopes of an element at the physical-chemical conditions of the liquidus material. If the rock system has not been affected by open-system behavior—for example, by the introduction of parent or daughter species by metamorphism or weathering—the points representing these samples will define a perfect line. In practice, uncertainties in measuring each of the parent-daughter parameters, and perhaps some open-system behavior, result in imperfect isochronism and, therefore, uncertainties in the calculated age.

A benefit of the isochron method is that the isotopic composition of strontium at the time of origin of the rock, or strontium-87/strontium-86, is marked by the left-hand or lower intercept of the isochron, where rubidium-87/strontium-86 is equal to zero. As discussed above, for a model age calculated from a single mineral or rock analysis, this parameter would have to be estimated. Another benefit is that one can readily see whether one or more points are aberrant or whether a poor fit might indicate an open-system history for the rock. A wide variety of terrestrial and extraterrestrial igneous rocks have been dated by this mineral isochron method with good precision.

As with minerals from a single rock, if a series of rocks of equal age from a common parent (such as fractionally differentiated rocks from a single magma or similar rocks which resulted from different degrees of partial melting of a common source) are analyzed, their Rb-Sr isotopic parameters should yield an isochron proportional to the unique age of the rocks, or a whole-rock isochron. Consequently, one may test for ages and for comagmatic properties in a suite of rocks. In practice, however, it is common for these properties to be unraveled only with supporting petrologic or geochemical data, if at all.

Uniqueness of the Method

The most spectacular benefit of the Rb-Sr isochron technique and one that is unique to the method is the ability, under some circumstances, to identify both the original time of crystallization of an igneous rock, such as a granite, and a later time of metamorphism resulting, for example, in a granitic gneiss. These events may be traced by constructing both mineral and whole-rock isochrons from several, chemically differing types of the gneiss. Points representing mineral components, along with points representative of their whole-rock mixtures, may form an isochron proportional to the age of metamorphism. These mineral isochrons should have the same slopes and therefore ages. However, a whole-rock isochron constructed only from the whole-rock points will be steeper, with a slope proportional to the earlier time of intrusion of the granite. This seemingly peculiar behavior results from a reequilibration of strontium isotopes at the time of metamorphism in the vicinity of the rock sampled; on a more regional scale, however, whole-rock parameters were not homogenized.

The method requires, in addition to reasonably closed-system behavior, that the later metamorphic event be capable of reequilibrating the local rock systems completely. In practice, that is accomplished either through fairly high-grade metamorphism, through the availability of sufficient water to effect the isotopic exchange, or, most likely, through both. Dry metamorphism, even if high-grade, may not result in reequilibration; the mineral isochron date obtained will therefore record only the magmatic event. Conversely, if the rocks are permeable, fine-grained, and wet, rehomogenization may be completed even under low-grade metamorphic conditions. Where conditions of economic mineralization have been sufficient to equilibrate Rb-Sr components, whole-rock isochron analysis can reveal this type of metamorphic event. If the event results in the formation of new minerals, these minerals may record the time of metamorphism. Unfortunately, in some studies, the multiple possibilities of incomplete rehomogenization, open-system behavior, and poor precision of measurement result in poor isochronism and age data of questionable or no value.

Use with Marine Components

Because of a lack of fossils with determinable ages in many sediments and sedimentary rocks, absolute age analysis by several isotopic methods, including Rb-Sr dating, has been tested. The technique works well only for certain minerals for which model ages can be calculated by assuming the ratio between rubidium-87 and strontium-86; for marine components, this assumed parameter may be reasonably determined, thanks to the effective homogenization of strontium in seawater during a given geologic time. The most commonly dated material is glauconite, although the closed-system requirement may be violated by the unavoidable inclusion of detrital components of varying strontium-isotopic composition. Other materials of limited usefulness are minerals such as phillipsite and some illite. Evaporitic minerals containing enough rubidium, such as sylvite, also have been dated by this technique, although omnipresent recrystallization may perturb the isotopic systematics.

In some cases, the isotopic composition of strontium by itself can be used to determine the age of carbonate rock. As previously stated, strontium dissolved in seawater of a particular geologic episode has the same isotopic composition everywhere in the ocean. That is because the mixing rate of marine strontium—about one thousand years—is short compared with the average “lifetime” for strontium atoms in the sea—about two million years. Thus, strontium of variable isotopic composition washed into the sea from rivers or other sources is well mixed. Because strontium-87 accumulates through time as a result of rubidium-87 decay, terrestrial strontium, including marine strontium, becomes more radiogenic through geologic time. Theoretically, then, marine strontium of any geologic time should have a unique value, and the time could be identified simply by measuring the ratio of strontium-87 to strontium-86. Unfortunately, from the strictly chronologic perspective, strontium is not monotonic; strontium of some periods, for example, is less radiogenic than that for the preceding and succeeding periods. Careful work has delimited the marine growth curve for strontium, however, and the technique has met with considerable success.

Role in Understanding Geologic Phenomena

The absolute dating of geologic materials and events has had an unprecedented influence on the understanding of geologic events on Earth and of solar system minerals and rocks. The ability to establish events in terms of actual years, rather than in relative terms such as “older than” or “younger than,” has led to a realistic estimate of the earth's age and to calibrated time scales for organic evolution, geomagnetic events, and the structural development of the earth's crust. One of the earliest and most useful chronometric schemes for the oldest rocks, the Rb-Sr technique has been of exceptional value, not only for dating but also for the use of strontium-isotopic composition as an indicator or tracer for a variety of geologic processes, such as the evolution of seawater. Additionally, the technique has had much success in the unraveling of igneous and metamorphic processes in complex, regionally metamorphosed geologic terrains.

Because of its usefulness in the precise determination of the ages of very old rocks, the Rb-Sr method will continue to be of major use in dating Earth's oldest rocks and extraterrestrial solar system materials—for example, lunar rocks and meteorites, including meteorites from the moon and possibly from Mars.

Principal Terms

absolute age: the numerical timing of a geologic event, as contrasted with relative, or stratigraphic, timing

geochronology: the study of the absolute ages of geologic samples and events

half-life: the time required for a radioactive isotope to decay by one-half of its original weight

isochron: a line connecting points representing samples of equal age on a radioactive isotope (parent) versus radiogenic isotope (daughter) diagram

isotope: a species of an element having the same number of protons but a different number of neutrons and therefore a different atomic weight

mass spectrometry: the measurement of isotope abundances by separating the isotopes by mass and charge in an evacuated magnetic field

radioactive decay: a natural process by which an unstable, or radioactive, isotope transforms into a stable, or radiogenic, isotope

Bibliography

Faure, Gunter. Origin of Igneous Rocks: The Isotopic Evidence. New York: Springer, 2010. Descriptions of multiple radioactive isotope dating methods contained within this book. Principles of isotope geochemistry are explained early, making this book accessible to undergraduates. Data presented in diagrams, more than 400 original drawings, and a long list of references included at the end.

‗‗‗‗‗‗‗‗‗‗. Principles of Isotope Geology. 2d ed. New York: John Wiley & Sons, 1986. An excellent, though technical, introduction to the use of radioactive and stable isotopes in geology, including a thorough treatment of the Rb-Sr technique. The work is well illustrated and well indexed. Suitable for college-level readers.

Levin, Harold L. The Earth Through Time. 9th ed. Philadelphia: Saunders College Publishing, 2009. This college-level text contains a brief, clear description of the Rb-Sr method. A diagram of a whole-rock isochron is included. Five other radiometric dating techniques are discussed, and background information on absolute age and radioactivity is provided. Includes review questions, a list of key terms, and references.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of the Geological Sciences. 2d ed. New York: McGraw-Hill, 1988. This source contains entries on radioactivity and radioactive minerals. The entry on dating methods includes a brief section on the Rb-Sr method. Includes the formula for radioactive decay and a table of principal parent and daughter isotopes used in radiometric dating. The entry on rock age determination has a longer discussion of the Rb-Sr dating and includes an isochron diagram. For college-level audiences.

Smith, David G., ed. The Cambridge Encyclopedia of Earth Sciences. Cambridge, England: Cambridge University Press, 1981. Organized as a compilation of high-quality and authoritative scientific articles rather than a typical encyclopedia. Chapter 8, “Trace Elements and Isotope Geochemistry,” covers mass spectrometry, igneous processes, radiogenic isotopes, and radioactive decay schemes. Rb-Sr decay is used to illustrate principles of geochronology. An Rb-Sr isochron diagram is provided and explained. A more technical discussion than the one in Levin's book. Suitable for the reader with some background in science.

Walther, John Victor. Essentials of Geochemistry. 2d ed. Jones & Bartlett Publishers, 2008. Contains chapters on radioisotope and stable isotope dating and radioactive decay. Geared more toward geology and geophysics than toward chemistry, this text provides content on thermodynamics, soil formation, and chemical kinetics.

Zalasiewicz, Jan. The Planet in a Pebble: A Journey into Earth's Deep History. New York: Oxford University Press, 2010. An easily accessible account of Earth's formation and history, written for the layperson. Summarizes many studies in geology, explaining basic physics and chemistry, and even delving in to radiometric dating. This text is indexed and also provides further readings and bibliographies for each chapter.