Samarium-neodymium dating

Sm-Nd dating is one of the more recent, yet most common methods of obtaining the absolute ages of geologic materials, especially older minerals and rocks. The method depends on the natural radioactivity of one of the seven isotopes of samarium, samarium-147, which decays to neodymium-143.

Rare-Earth Elements

samarium (Sm) and neodymium (Nd) are both rare-earth elements (REEs, or lanthanides). They occur, commonly in trace amounts, in many of the more widespread minerals and rocks and appear in high concentrations in some rare but economically viable minerals, such as bastnaesite and monazite.

The sixteen REEs, their abundances, and especially their abundance patterns have achieved a remarkable usefulness in geochemistry. Their abundances, usually plotted relative to their abundances in important major reservoirs, especially the average composition of the solar system—as it is estimated from primitive meteorites known as the carbonaceous chondrites—form patterns that have proved exceptionally useful as tracers for a wide variety of cosmic and geologic processes. These include the parentage of igneous, metamorphic, and sedimentary rocks. Rare-earth element concentration patterns, called tracers, are significant in a way that is similar to the importance of the isotopic ratios of radiogenic nuclides such as strontium-87 and strontium-86 or neodymium-143 and neodymium-144, described herein. REEs have a valence (bonding value) of +3 (except for europium and cerium, which may also exist in other valence states), and they have identical outer shell electronic configurations. The reason for this similarity is that the REEs, although exhibiting almost identical geochemical properties, nevertheless vary slightly because they have slightly different ionic radii. Consequently, REEs within a given mineral or other phase act somewhat like isotopes of a given element. Because so much is known about the geochemical behavior of the REEs, the usefulness of the Sm-Nd dating and tracer techniques is great. Samarium and neodymium belong to the light REE part of the lanthanide spectrum and occur next to each other in terms of atomic number and ionic size.

Samarium and Neodymium

Samarium consists of seven natural isotopes as well as several artificial isotopes. The Sm-Nd chronometer is a result of the alpha decay of samarium-147 to neodymium-143, which has an exceptionally long half-life of 106 billion years. Samarium-147 has been known to be unstable for many years. Because of its long half-life and the small dispersion of Sm-Nd ratios in most materials, its use in age dating did not begin until the mid-1970's, with the advent of modern mass spectrometry, high-precision instruments, and the digital collection of data. The radiogenic accumulation of neodymium-143 in most natural minerals and rocks is very slow.

The REE neodymium consists of seven natural isotopes and several unstable species. In order of isotopic abundance, the seven natural isotopes are neodymium-142, -144, -146, -143, -145, -148, and -150. The important radiogenic nuclide is neodymium-143, which forms by the alpha decay of radioactive samarium-147. Like the isotopic composition of strontium (strontium-87 and strontium-86), the abundance of neodymium-143 is measured in a mass spectrometer relative to the neodymium isotope of closest mass and reasonable abundance, neodymium-144. Also like the isotopic composition of strontium, the isotopic composition of neodymium—specifically, the neodymium-143/neodymium-144 ratio—has been of exceptional use in helping scientists to understand a variety of geologic, especially igneous, processes.

Determining Samarium-Neodymium Concentrations and Ratios

Concentrations of samarium and neodymium in minerals, rocks, and other natural substances can be measured using a variety of techniques, although isotopic parameters and precise elemental abundances are commonly measured by mass spectrometry. As the abundances of most of the samarium and neodymium isotopes 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 various methods. These methods include gravimetric analysis, atomic absorption spectrophotometry, X-ray fluorescence spectrophotometry, or microbeam probe analysis. In practice, however, because of low abundances of the REEs in most minerals, the most useful techniques are neutron activation analysis and mass spectrometry. Most modern work consists of determining the relevant isotopic parameters by mass spectrometric isotope dilution after purification by chemical techniques.

A major attribute of the Sm-Nd system is that, thanks to the geochemical affinity of samarium for ultramafic and mafic minerals (minerals low in silica and rich in iron and magnesium), the method can be used to date even comparatively young low-silica rocks, such as peridotite and basalt, that cannot easily be dated by uranium-thorium-lead (U-Th-Pb) or rubidium-strontium (Rb-Sr) techniques. Additionally, in principle, the unusual geochemical behavior of the Sm-Nd ratio in magmatic crystallization or partial melting of source rocks allows contrasting isotopic trends between Sm-Nd and other ratios to be traced.

For example, fractional crystallization of a magma yields residual liquid that becomes increasingly greater in terms of U-Th-Pb and Rb-Sr ratios, whereas Sm-Nd values commonly become lower. Thus, in the last-formed residual rocks, strontium and lead will be more radiogenic, but samarium perhaps less radiogenic, than they were in the beginning magma. The reverse is true for the partial melting of a mantle source rock such as peridotite. It has become useful, therefore, to plot time-of-crystallization (initial) isotopic ratios of lead and especially strontium against that of neodymium. For many rocks, these data form a trend of negative slope, a trend that has been called the “mantle array.”

Preliminary Techniques in Sm-Nd Dating

Commonly, a preliminary determination of samarium and neodymium abundance and the Sm-Nd ratio is made for the materials to be dated, either by a reconnaissance technique or simply by an estimation from known concentrations of these elements in previously measured samples of the same type of mineral or rock. Samples selected on the basis of these determinations are chosen for the optimal conditions for dating. After selection, the samples are crushed, or homogenized if necessary, and a portion is taken which contains enough of the samarium and especially of the more critical neodymium for adequate isotopic analysis. Rarer materials, such as some lunar rocks or meteorites, may not afford enough of a sample for optimal analysis.

The sample of material, if it is the most common 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.” Samples other than silicates may be dissolved in other solvents. The mush is “spiked” with an appropriate amount of purified liquids containing known amounts of samarium and neodymium of known, non-natural 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 samples of samarium, then neodymium, of mixed natural and spiked isotopic composition are collected and evaporated. Achieving the highest accuracy and precision requires that the smallest amounts possible of samarium and neodymium from the laboratory environment (contamination) be included with the completed, purified elements.

Mass Spectrometry

The standard method of mass spectrometry for Sm-Nd chronology involves placing the purified samples as solids (such as oxides or metals) 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 samarium or neodymium ionizes. The charged, ionized sample is accelerated through a series of collimating slits and into a controlled magnetic field, where the beams of ions are separated by charge-mass ratios into beams of separated isotopes. The charge is the same for each atom, so the ions are separated on the basis of mass only. Specific isotopic beams, controlled by the magnetic field, are channeled through more collimating slits into the collector part of the spectrometer.

Commonly, a Faraday cup is used to analyze the number of atoms of each isotope by conversion of each atomic impact into a unit of charge, which is amplified. A digital readout is produced. The actual output of the procedure is isotope ratio measurements—samarium-150/samarium-147, neodymium-143/neodymium-144, and so on—which are converted to the required parameters for determining time by mathematical programs. Because the important quantity of neodymium-143 is relatively close to that of neodymium-144 in most cases, and because it is different by only one mass unit, the standard for reporting the radiogenic component is with the ratio neodymium-143/neodymium-144.

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

Isochron Diagrams

Although the age of the analyzed sample can be calculated using the determined Sm-Nd parameters and the decay constant for samarium-147, it is customary and more useful to determine the age graphically, with an isochron diagram. In the diagram, the actual isotope ratios collected in the spectrometer are used as coordinates. Thus, the parent, unstable component, samarium-147, is designated by reference to the common isotope of neodymium, neodymium-144. The other coordinate—the measure of the radiogenic component—is neodymium-143/neodymium-144. A line connecting points representing samples of equal ages, or an isochron, has a value represented by its slope; 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 samarium-147 isotope. A single mineral or rock would furnish only one point on the diagram, so to draw an isochron, the researcher must know or, more likely, estimate the sample's initial isotopic composition. Ages determined in this way are called “model ages.”

Dating Rock Crystallization

Sm-Nd dating is used to determine the time of crystallization of specific types of igneous rock, the time of formation of comagmatic igneous rocks, and the time of metamorphism of a sequence of rocks of varying composition. The analysis of minerals of equal age but different compositions from a sample of plutonic or volcanic rock would form the points for an isochron whose slope would be proportional to the rock's age of crystallization. A common example is a mafic rock, such as basalt, with minerals of an increasing samarium-147/neodymium-144 ratio, such as plagioclase feldspar and pyroxene. An assumption that is justified in most circumstances is that at the time of crystallization, all the minerals formed have the same isotopic composition of neodymium. If the rock system has not been affected by the introduction of parent or daughter species by metamorphism or weathering (open-system behavior), the points representing these samples will define a perfect line. In practice, however, uncertainties about 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 neodymium at the rock's time of origin is marked by the left-hand or lower intercept of the isochron. Another benefit is that it is easy to see whether one or more points are aberrant or whether a poor fit might indicate an open-system history for the rock. A wide array of terrestrial and extraterrestrial igneous rocks have been dated by this mineral isochron method with good precision.

Dating Rock Series and Sequences

Not only can geologists use this method to date minerals from a single rock, but if a series of rocks of equal age from a common parent are analyzed, their Sm-Nd isotopic parameters should also yield a “whole-rock” isochron proportional to the rocks' age. Consequently, geologists may test for comagmatic properties and ages in a suite of rocks. In practice, however, these properties are often analyzed only with supporting petrologic or geochemical data, if at all.

The time of metamorphism of a sequence of rocks of varying composition also may be determined by the Sm-Nd technique, provided that, at the time of metamorphism, the isotopic composition of neodymium in the rocks is effectively homogenized. The geochemical behavior of the REEs, however, results in much more immobile transport for these elements as compared with, say, rubidium and strontium. Complete homogenization is accomplished through 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 homogenization; the isochron date obtained in such a case would record only some mixture of original events or isotopic compositions. Conversely, if the rocks are permeable, fine-grained, and wet, homogenization may be completed even under low-grade metamorphic conditions. Where conditions have been sufficient to equilibrate Sm-Nd components, whole-rock isochron analysis can reveal this type of metamorphic event.

Dating Materials Precipitated from Seawater

The isotopic composition of neodymium by itself can in principle be used to determine the age of manganese nodules, carbonate rocks, or other materials precipitated from seawater. For this method to work, enough must be known about the isotopic composition of neodymium in that sector of the sea through time—a history that may not readily be available. Strontium dissolved in seawater of a particular geologic episode has the same isotopic composition everywhere in the ocean. Neodymium, however, does not, because the mixing rate of marine neodymium, about one thousand years, is long compared to the average “lifetime” for neodymium atoms in the sea, less than one hundred years. Thus, neodymium of variable isotopic composition washed into the sea from rivers or other sources commonly is deposited on the sea floor before it has become isotopically well mixed. Because neodymium-143 accumulates through time as a result of samarium-147 decay, neodymium on earth becomes more radiogenic through geologic time. One might assume, then, that geologic time could be identified simply by measuring the ratios of neodymium-143 to neodymium-144; unfortunately for the chronologic usefulness of this parameter, however, neodymium is not uniform in the seas, as discussed. Therefore, marine neodymium is used more for an understanding of marine processes, such as water transport, than for chronologic studies. Scientists continue to investigate the potential of marine neodymium as an indicator of time and as a global tectonic tracer.

Role in Understanding Geologic Events

The absolute dating of geologic materials and events has had a profound effect on scientists' understanding of terrestrial and extraterrestrial geologic events. Establishing the age of events in years rather than in relative terms has led to reliable estimates of the earth's age and to calibrated time scales for organic evolution, geomagnetic events, and the plate tectonic cycle. One of the most recent and most useful chronometric methods, the Sm-Nd technique has been extremely important not only for dating but also for tracking of a variety of geologic processes, such as the evolution of seawater. This technique has also helped geologists to understand igneous and metamorphic processes in complex, regionally metamorphosed geologic terrains.

The Sm-Nd method will continue to be of great use in dating Earth's oldest rocks and other solar system materials, including lunar and Martian rocks and meteorites.

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

Duckworth, H. E. Mass Spectrometry. Cambridge, England: Cambridge University Press, 1958. An older work, but it covers well the basic principles of mass spectrometry, the major measurement technique used in conjunction with Sm-Nd dating. For readers with some background in science.

Faure, Gunter. Isotopes: Principles and Applications. 3rd ed. New York: John Wiley & Sons, 2004. Originally titled Principles of Isotope Geology. An excellent introduction to radioactive and stable isotopes and their use in geology. It covers the Sm-Nd technique thoroughly. The work is somewhat technical but well illustrated and indexed. Written for a college-level audience.

‗‗‗‗‗‗‗‗‗‗‗‗‗. 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. Includes data presented in diagrams, more than 400 original drawings, and a long list of references included at the end.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of the Geological Sciences. 2d ed. New York: McGraw-Hill, 1988. This source contains an entry on rock age determination. Not much space is devoted solely to the Sm-Nd method, but other methods are reviewed and the general principles of dating are explained. For general readers.

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. A chapter in this well-written textbook covers mass spectrometry, radioactive decay schemes, and several dating methods. Both rubidium-strontium and samarium-neodymium techniques are discussed. Includes an example of an isochron diagram and a table of trace element abundances. For the reader desiring a clear yet not simplistic source.

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.