Instruments and procedures
Use of mass spectrometers
The age of a geologic sample is measured on as little as a billionth of a gram of daughter isotopes. Moreover, all the isotopes of a given chemical element are nearly identical except for a very small difference in mass. Such conditions necessitate instrumentation of high precision and sensitivity. Both these requirements are met by the modern mass spectrometer. A high-resolution mass spectrometer of the type used today was first described by the American physicist Alfred O. Nier in 1940, but it was not until about 1950 that such instruments became available for geochronological research (see also mass spectrometry).
For isotopic dating with a mass spectrometer, a beam of charged atoms, or ions, of a single element from the sample is produced. This beam is passed through a strong magnetic field in a vacuum, where it is separated into a number of beams, each containing atoms of only the same mass. Because of the unit electric charge on every atom, the number of atoms in each beam can be evaluated by collecting individual beams sequentially in a device called a Faraday cup. Once in this collector, the current carried by the atoms is measured as it leaks across a resistor to ground. Currents measured are small, from only 10−11 to 10−15 ampere, so that shielding and preamplification are required as close to the Faraday cup as possible. It is not possible simply to count the atoms, because all atoms loaded into the source do not form ions and some ions are lost in transmission down the flight tube. Precise and accurate information as to the number of atoms in the sample can, however, be obtained by measuring the ratio of the number of atoms in the various separated beams. By adding a special artificially enriched isotope during sample dissolution and by measuring the ratio of natural to enriched isotopes in adjacent beams, the number of daughter isotopes can be readily determined. The artificially enriched isotope is called a “spike.” It is usually a highly purified form of a low-abundance natural isotope, but an even better spike is an isotope with a mass not found in nature at all. Lead-205 produced in a type of particle accelerator called a cyclotron constitutes such an ideal spike.
As the sample is heated and vaporizes under the vacuum in the source area of the mass spectrometer, it is commonly observed that the lighter isotopes come off first, causing a bias in the measured values that changes during the analysis. In most cases this bias, or fractionation, can be corrected if the precise ratio of two of the stable isotopes present is known. Today’s state-of-the-art instruments produce values for strontium and neodymium isotopic abundances that are reproducible at a level of about 1 in 20,000. Such precision is often essential in the isochron method (see above) because of the small changes in relative daughter abundance that occur over geologic time.
Technical advances
The ability to add a single artificial mass to the spectrum in a known amount and to determine the abundances of other isotopes with respect to this provides a powerful analytical tool. By means of this process, known as isotope dilution, invisibly small amounts of material can be analyzed, and, because only ratios are involved, a loss of part of the sample during preparation has no effect on the result. Spike solutions can be calibrated simply by obtaining a highly purified form of the element being calibrated. After carefully removing surface contamination, a precisely weighted portion of the element is dissolved in highly purified acid and diluted to the desired level in a weighed quantity of water. What is required is dilution of 1 cubic cm to 1 litre (0.23 cubic inch to 1 gallon) from which a second cubic centimetre is again diluted to a litre to approach the range of parts per million or parts per billion typically encountered in samples. In this way, a known number of natural isotopes can be mixed with a known amount of spike and the concentration in the spike solution determined from the ratio of the masses. Once the calibration has been completed, the process is reversed and a weighed amount of spike is mixed with the parent and daughter elements from a mineral or rock. The ratio of the masses then gives the number of naturally produced atoms in the sample. The use of calibrated enriched isotopic tracers facilitates checks for contamination, even though the process is time-consuming. A small but known amount of tracer added to a beaker of water can be evaporated under clean-room conditions. Once loaded in a mass spectrometer, the contamination from the beaker and the water is easily assessed with respect to the amount of spike added. Contamination as small as 10−12 gram can be detected by this method.
The materials analyzed during isotopic investigations vary from microgram quantities of highly purified mineral grains to gram-sized quantities of rock powders. In all cases, the material must be dissolved without significant contamination. The spike should be added before dissolution. Most of the minerals in rocks can be dissolved in a day or so at a temperature near 100 °C (212 °F). Certain minerals that are highly refractory both in nature and in the laboratory (e.g., zircon) may require five days or more at temperatures near 220 °C (428 °F). In this case, the sample is confined in a solid Teflon (trade name for a synthetic resin composed of polytetrafluoroethylene) metal-clad pressure vessel, introduced by the Canadian geochronologist Thomas E. Krogh in 1973.
The method just described proved to be a major technical breakthrough as it resulted in a reduction in lead-background contamination by a factor of between 10,000 and nearly 1,000,000. This means that a single grain can now be analyzed with a lower contamination level (or background correction) than was possible before with 100,000 similar grains. Advances in high-sensitivity mass spectrometry of course were essential to this development.
Once dissolved, the sample is ready for the chemical separation of the dating elements. This is generally achieved by using the methods of ion-exchange chromatography. In this process, ions are variously adsorbed from solution onto materials with ionic charges on their surface and separated from the rest of the sample. After the dating elements have been isolated, they are loaded into a mass spectrometer and their relative isotopic abundances determined.
The abundance of certain isotopes used for dating is determined by counting the number of disintegrations per minute (i.e., emission activity). The rate is related to the number of such atoms present through the half-life. For example, a certain amount of carbon-14 (14C) is present in all biological components at Earth’s surface. This radioactive carbon is continually formed when nitrogen atoms of the upper atmosphere collide with neutrons produced by the interaction of high-energy cosmic rays with the atmosphere. An organism takes in small amounts of carbon-14, together with the stable (nonradioactive) isotopes carbon-12 (12C) and carbon-13 (13C), as long as it is alive. Once it dies, however, no additional carbon-14 is acquired and the level of radiocarbon in the organism’s tissue decreases progressively as a function of half-life. The time that has passed since the organism was alive can be determined by counting the beta emissions from a tissue sample. The number of emissions in a given time period is proportional to the amount of residual carbon-14.
The introduction of an instrument called an accelerator mass spectrometer has brought about a major advance in radiocarbon dating. Unlike the old detector (e.g., the Geiger counter) that counts the few decay particles emitted from a large amount of carbon, the new instrument counts directly all of the carbon-14 atoms in a sample. This increase in instrument sensitivity has made it possible to reduce the sample size by as much as 10,000 times and at the same time improve the precision of ages measured. (For a detailed discussion of radiocarbon age determination, see Carbon-14 dating and other cosmogenic methods.)
In a similar development, the use of highly sensitive thermal ionization mass spectrometers is replacing the counting techniques employed in some disequilibrium dating. Not only has this led to a reduction in sample size and measurement errors, but it also has permitted a whole new range of problems to be investigated. Certain parent-daughter isotopes are extremely refractory and do not ionize in a conventional mass spectrometer. To solve this problem, researchers are developing new instruments in which a small amount of material can be evaporated from the surface with a pulse of energy and ionized with a pulse of laser light.
A major advance in geochronology and isotope geochemistry involves the analysis of mineral grains in place without chemical dissolution. This type of analysis uses the sensitive high-resolution ion microprobe (SHRIMP), a double-focusing secondary ion mass spectrometer, in which a focused beam of ions is directed at a spot 5–30 microns (1 micron [micrometre] equals 0.00004 inch) in diameter on a mineral sample. This process blasts atoms from the surface, and, after a 15 to 20 minute analysis, a pit approximately 1 micron deep is created. The liberated secondary ions are filtered and focused in an electrostatic analyzer and measured according to their mass and energy. Uranium-lead dating of zircon using this method was pioneered by William Compston at the Australian National University. Although this method is not as precise as chemical dissolution methods, it permits spatial resolution on the order of several microns. Thus, it is possible to date both the timing of crystallization of igneous rocks and the age of the magma-enveloped rock crystals on which the igneous zircon rims grew.
Another recent analytical advance in zircon dating is the application of laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) coupled to a laser system. The laser produces a beam of ions focused on a spot as small as 10 microns in diameter, which during the analysis produces a pit of between 2 and 1,200 microns deep. The ions produced during ablation are analyzed in the coupled mass spectrometer according to mass and energy. Although analytical uncertainty is typically 1 to 4 percent—somewhat higher than chemical dissolution methods but comparable to SHRIMP methods—the LA-ICP-MS technique allows a large number of single zircon grains to be dated rapidly. The method is commonly used to establish the source of detrital grains forming sedimentary rocks, a task that requires analysis of more than 100 individual grains.
Major methods of isotopic dating
Isotopic dating relative to fossil dating requires a great deal of effort and depends on the integrated specialized skills of geologists, chemists, and physicists. It is, nevertheless, a valuable resource that allows correlations to be made over virtually all of Earth history with a precision once only possible with fossiliferous units that are restricted to the most recent 12 percent or so of geologic time. Although any method may be attempted on any unit, the best use of this resource requires that every effort be made to tackle each problem with the most efficient technique. Because of the long half-life of some isotopic systems or the high background or restricted range of parent abundances, some methods are inherently more precise. The skill of a geochronologist is demonstrated by the ability to attain the knowledge required and the precision necessary with the least number of analyses. The factors considered in selecting a particular approach are explored here.
Uranium–lead method
As each dating method was developed, tested, and improved, mainly since 1950, a vast body of knowledge about the behaviour of different isotopic systems under different geologic conditions has evolved. It is now clear that with recent advances the uranium–lead method is superior in providing precise age information with the least number of assumptions. The method has evolved mainly around the mineral zircon (ZrSiO4). Because of the limited occurrence of this mineral, it was once true that only certain felsic igneous rocks (those consisting largely of the light-coloured, silicon and aluminum-rich minerals feldspar and quartz) could be dated. Today, however, baddeleyite (ZrO2) and zirconolite (CaZrTi2O7) have been found to be widespread in the silica-poor mafic igneous rocks. In addition, perovskite (CaTiO3), a common constituent of some ultramafic igneous rocks, has been shown to be amenable to precise uranium–lead dating. As a result of these developments, virtually all igneous rocks can now be dated. This capability, moreover, has been enhanced because the most advanced geochronological laboratories are able to analyze samples that weigh only a few millionths of a gram. This amount can be found in a comparatively large number of rocks, whereas the amount previously required (about 0.1 gram) cannot. Age determinations also can now be made of low-uranium trace minerals such as rutile (TiO2), a common constituent found in mineral deposits, adding still further to the number of entities that are datable by the uranium–lead method. Other minerals commonly employed to date igneous and metamorphic rocks include titanite, monazite, and even garnet in certain favourable cases. Additional minerals have been tried with varying success.
Double uranium-lead chronometers
The reason why uranium–lead dating is superior to other methods is simple: there are two uranium–lead chronometers. Because there exist two radioactive uranium atoms (those of mass 235 and 238), two uranium–lead ages can be calculated for every analysis. The age results or equivalent daughter–parent ratio can then be plotted one against the other on a concordia diagram. If the point falls on the upper curve shown, the locus of identical ages, the result is said to be concordant, and a closed-system unequivocal age has been established. Any leakage of daughter isotopes from the system will cause the two ages calculated to differ, and data will plot below the curve. Because each of the daughters has a different half-life, early leakage will affect one system more than the other. Thus, there is a built-in mechanism that can prove or disprove whether a valid age has been measured. Historically, it had been observed that the uranium–lead systems in the mineral zircon from unmetamorphosed rocks were almost invariably disturbed or discordant but yielded a linear array on the concordia diagram. Given a set of variably disturbed samples, an extrapolation to zero disturbance was possible. More recently, it has been found that of all the grains present in a rock a very few still retain closed isotopic systems but only in their interior parts. Thus, grains with a diameter comparable to that of a human hair, selected under a microscope to be crack-free and of the highest possible quality, have been found to be more concordant than cracked grains. In addition, it has been shown that most such grains can be made much more concordant by mechanically removing their outer parts using an air-abrasion technique. Of course, the ability to analyze samples weighing only a few millionths of a gram was essential to this development. As noted earlier, this in turn was possible solely because the lead background contamination had been reduced from 1 × 10−6 gram to almost 1 × 10−12 gram per analysis. The methods of selection and abrasion used to locate grains with closed isotopic systems could be worked out only because the uranium–lead method has the inherent ability to assess with a single analysis whether or not a closed isotopic system has prevailed.
The presence of two radioactive parents provides a second major advantage because, as daughter products, lead atoms are formed at different rates and their relative abundance undergoes large changes as a function of time. Thus,the ratio of lead-207 to lead-206 changes by about 0.1 percent every two million years. Since this ratio is easily calibrated and reproduced at such a level of precision, errors as low as ±2 million years at a confidence level of 95 percent are routinely obtained on lead-207–lead-206 ages. By contrast, errors as high as ±30 to 50 million years are usually quoted for the rubidium–strontium and samarium–neodymium isocron methods (see below Rubidium–strontium method; Samarium–neodymium method).
