Lead isotopes (geochemistry)
The study of the isotopic composition of stable and radioactive lead in geological and environmental materials to determine their ages or origins. See also: Lead
Lead isotope geochemistry provides the principal method for determining the ages of old rocks and the Earth itself, as well as the sources of metals in mineral deposits and the evolution of the mantle.
Lead (Pb) has four stable isotopes of mass 204, 206, 207, and 208. Three are produced by the radioactive decay of uranium (U) and thorium (Th) [reactions ((1)–(3)),
where t1/2 is the half-life of the isotope and α and β denote alpha and beta particles, respectively]. The lead produced by the decay of uranium and thorium is termed radiogenic. Since 204Pb is not produced by the decay of any naturally occurring radionuclide, it can be used as a monitor of the amount of initial (nonradiogenic) lead in a system. This will include all of the 204Pb and variable amounts of 206Pb, 207Pb, and 208Pb. See also: Alpha particles; Beta particles; Radioactivity; Thorium; Uranium
It is possible to calculate the isotopic composition of lead at any time t in the past by calculating and deducting the amount of radiogenic lead that will have accumulated, provided a mineral or rock represents a closed system. A closed system is one in which there has been no chemical transfer of uranium, thorium, or lead in or out of the mineral or rock since it formed. All calculations for uranium-lead dating should yield the same age; this is a unique and powerful property. The ratio of radiogenic 207Pb to 206Pb is simply a function of age, not the U/Pb ratio. Certain minerals such as zircon, monazite, and uraninite are particularly well suited for dating because of extremely high concentrations of uranium or thorium relative to initial lead. However, the degree to which they behave as closed systems can vary. For samples having concordant U-Pb ages, data lie along a curved line called the concordia, the line defined by the daughter/parent ratios of each isotopic system that have equal ages (Fig. 1). There are several uranium-rich minerals that commonly yield concordant ages, the most useful being the rare-earth phosphate monazite, a common accessory mineral in crustal rocks. However, there are many minerals with appreciable radiogenic lead which have discordant ages, indicating they have not been a closed system throughout their history as a discrete phase. Zircon (ZrSiO4), a common accessory mineral in many types of crustal rocks, has been used more than any other phase for U-Pb dating. However, the data are normally discordant. Data for a series of zircons from the Little Belt Mountains, Montana, lie on a well-defined straight line that intersects the concordia at two points (Fig. 1). It has been shown that phases subject to lead loss (or uranium gain) during a period of time that is short compared with the age of the phase yield daughter/parent ratios defining a straight line termed a discordia. The lower intersection of the discordia with the concordia indicates the time of the episodic bulk lead loss, while the upper intersection represents the age of the phase. Discordance in zircons is more pronounced in uranium-rich varieties and is caused by the severe damage to the lattice produced by recoiling alpha particles. See also: Monazite; Rock age determination; Zircon
Fig. 1 Systematics of Uranium-lead (U-Pb) dating. For a closed system containing uranium but no primary lead, the ratios of 206Pb/238U and 207Pb/235U will vary with the age of the sample, as shown by the concordia line. Ages indicated by marks along each line are in units of 1 billion years. The data for uraninites are consistent with this. For systems where episodic losses of lead have occurred in the past, values may lie along a discordia, as shown for zircons from the Little Belt Mountains of Montana.
Even if a rock or mineral contains appreciable initial lead, it may still be dated by using isochron methods. Since the amount of radiogenic lead relative to nonradiogenic lead is a function of the U/Pb ratio and time, the slope on a plot of 206Pb/204Pb against 238U/204Pb is proportional to age. An isochron is a line on a graph defined by data for rocks of the same age with the same initial lead isotopic composition, the slope of which is proportional to the age. In practice, the 238U/204Pb ratio may well have been disturbed by recent alteration of the rock because uranium is highly mobile in near-surface environments. For this reason it is more common to combine the two uranium decay schemes and plot 207Pb/204Pb against 206Pb/204Pb; the slope of an isochron on this plot is a function of age.
Isochron dating has been used to determine an age of 4.55 billion years for the Earth and the solar system by dating iron and stony meteorites (Fig. 2). The position of data along the isochron is a function of the U/Pb ratio. The iron meteorites are particularly important for defining the initial lead isotopic composition of the solar system since they contain negligible uranium. The meteorite isochron is commonly termed the Geochron. See also: Earth, age of; Geochronometry; Meteorite
Fig. 2 Plot of 207Pb/204Pb versus 206Pb/204Pb for troilite primordial lead and selected stone meteorites. The slope of the primary isochron (Geochron) for modern lead indicates an age of 4.55 million years for these materials. The white rectangular area illustrates the range of variation in most terrestrial leads and corresponds approximately to the region detailed in Fig. 3.
Geochemistry of Earth
By using the position of data for typical continental crustal rocks and samples of basalt that are derived as magmas from the mantle as shown on the Geochron (Fig. 2), the indication is that the silicate Earth has a U/Pb ratio of about 0.1. This is high relative to chondritic meteorites, commonly considered the best representative of primitive unprocessed preplanetary solar system material. A significant fraction of the Earth's total lead inventory could be in the metallic core. Also, lead is extremely volatile and may have been lost at the temperatures that inner solar system objects may have experienced in their accretionary history.
The Earth's mantle has been depleted by repeated melting during its 4.55-billion-year history, and the loss of such melts should leave the mantle with a low U/Pb ratio. However, close inspection reveals that the lead isotopic compositions of most mantle-derived magmas plot to the right of the Geochron (Fig. 3), implying a higher U/Pb ratio since the Earth formed. Originally it was thought that this discrepancy was caused by late accretion of the Earth or late core formation, either of which would displace the mantle to the right of the Geochron. However, there is independent isotopic evidence that the Earth did not accrete late, and there are theoretical reasons why the Earth's core almost certainly formed very early. A more likely explanation is that the mantle has been modified throughout its history by the subduction of ocean-floor basalt enriched in uranium and depleted in lead by low-temperature seawater alteration. The basalt lavas of some ocean islands such as St. Helena have especially radiogenic lead, thought to reflect an extreme example of such reenrichment. See also: Subduction zones
Fig. 3 Lead isotopic compositions of most ocean-floor and ocean-island basalts plot to the right of the Geochron defined by meteorite data (Fig. 2). The composition is the opposite of that predicted from the effects of depletion of the Earth's mantle by partial melting and suggests reenrichment by uranium-enriched subducted ocean floor.
Lead isotopes can serve as tracers in the lithosphere, atmosphere, and hydrosphere. Lead isotopes are commonly used to trace the sources of constituents in continental terranes, granites, ore deposits, and pollutants. For example, the class of low-temperature hydrothermal lead-zinc (Pb-Zn) mineralization known as Mississippi Valley type ore deposits have extremely variable 206Pb/204Pb ratios in their galenas, ranging up to 100. These variations reflect the time-integrated U/Pb ratio of the source of the lead, and they can be used to identify specific geological units from which the lead was scavenged. Similarly, some granites such as those of the Isle of Skye in northwest Scotland have very unradiogenic lead, indicating that the magmas were derived by melting portions of the lower continental crust that were depleted in uranium about 3 billion years ago. See also: Ore and mineral deposits
The industrialized countries of the world use large tonnages of lead annually, about one-third of which is widely distributed in the air, water, soil, and vegetation of the environment. Isotopic composition of lead in various environmental samples has identified sources and pathways of lead pollution. Most of the lead in the atmosphere originates from the combustion of gasoline containing alkyl lead antiknock compounds. The second-largest emission source of atmospheric lead is coal combustion. Lead aerosols eventually fall to the ground as precipitation or as dust and accumulate in topsoil and in surface water, where they may be incorporated into terrestrial or aquatic life. Lead isotopes have been used to trace contaminant dispersion in the environment. Lead isotope studies, for example, have helped support the contention that high concentrations of lead near roadways are the result of local deposition of large aerosols from automobile exhaust. Similarly, the isotopic composition of lead in natural waters and in sediments has been useful in identifying the extent to which sources are anthropogenic. See also: Air pollution; Water pollution
While there are at least 11 known radioactive isotopes of lead, only 212Pb, 214Pb, and especially 210Pb have been of interest geochemically. The usefulness of these isotopes stems from the unique mechanism by which they are separated from parent isotopes in the uranium or thorium decay series.
Unlike their noble-gas parents, the radioactive lead isotopes as well as other daughter products have a strong affinity for atmospheric aerosols. On formation, the daughter products exist as small positive ions associated with polarized air or water molecules; they form light aggregate particles within periods of tens of seconds. Both 212Pb and 214Pb have been used to study the process of diffusion of ions in gases and the mechanism of attachment of small ions to aerosols. Measurement of the distribution of radon (Rn) daughter product activities (212Pb and 214Pb) with respect to aerosol size has been important in the development of theoretical models of ion-aerosol interactions. The short half-lives of 212Pb and 214Pb also make these isotopes suitable for studies of near-ground atmospheric transport processes.
While the short-lived lead isotopes disappear from the atmosphere primarily by radioactive decay, 210Pb, because of its longer half-life, is removed mainly by precipitation and dry deposition. Its horizontal and vertical distributions are the result of the integrated effects of the distribution and intensity of sources, the large-scale motions of the atmosphere, and the distribution and intensity of removal processes. The inventory of 210Pb in the air is about a thousand times lower than expected, given the amount of its parent 222Rn, a measure of the efficiency with which aerosols are removed from the atmosphere. Numerous measurements of 210Pb as well as other daughter products indicate a tropospheric aerosol residence time of under 1 week. Since the residence time of 210Pb is so short and the oceans are not a significant source, the isotope sometimes can be used to distinguish between air masses originating over the continents and over the oceans. See also: Aerosol; Air mass; Atmospheric chemistry; Radon
An important use of 210Pb is as a particle tracer in aquatic systems. Concentrations of 210Pb in surface waves of the oceans generally show the same latitude variations as seen in air and rain. Concentrations in surface waters are roughly 20 times less than expected if there were no removal mechanisms other than radioactive decay. On entering ocean waters, 210Pb is incorporated into microscopic marine organisms, particularly zooplankton, whose remains eventually sink, rapidly conveying 210Pb to underlying waters. Using 210Pb as a tracer has helped explain the mechanisms by which various substances, including pollutants, are removed from the oceans. See also: Seawater
Most of the 210Pb in deep ocean waters is produced from the decay of dissolved 226Ra. The activity of 210Pb is as low as 20% of the activity of radium, indicating the operation of a deep-water scavenging mechanism acting preferentially on 210Pb. Measurements of 210Pb concentrations in deep ocean water indicate a scavenging residence time of around 40 years. The removal process appears to involve horizontal transport of 210Pb to selected areas of intense scavenging by sediments.
One of the most important uses of 210Pb is for dating recent coastal marine and lake sediments. As the isotope is rapidly removed from water to underlying deposits, surface sediments often have a considerable excess of 210Pb. The excess is defined as that present in addition to the amount produced by the decay of radium in the sediments. When the sedimentation rate is constant and the sediments are physically undisturbed, the excess 210Pb decreases exponentially with sediment depth as a result of radioactive decay during burial. The reduction in activity at a given depth, compared with that at the surface, provides a measure of the age of the sediments at that depth. Typically, excess 210Pb can be measured for up to about five half-lives or about100 years, and it is therefore ideally suited for dating sediments that hold records of human impact on the environment. See also: Radioisotope (geochemistry); Sedimentology
R. E. Criss, Principles of Stable Isotope Distribution, 1999
Ali Fazeli = egeology.blogfa.com
A. P. Dickin, Radiogenic Isotope Geology, 1997
Ali Fazeli = egeology.blogfa.com
G. Faure and T. M. Mensing, Isotopes: Principles and Applications, 3d ed., 2004
Ali Fazeli = egeology.blogfa.com