Lead isotopes

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

Thus, the ratios of lead isotopes 204,206,207 and 208 can vary markedly depending on the source of the lead. One use of these ratios lies in determination of the ages of rocks from the abundances of the various isotopes and the half-lives of their precursor radioactive isotopes. 

Ratios of lead isotopes depend on the source of the lead. They vary because lead is an end product of radioactive decay from elements of greater atomic number. 

Trace-element analysis of metals can give indications of the geographic provenance of the material. Both emission spectroscopy (84) and activation analysis (85) have been used for this purpose. Another tool in provenance studies is the measurement of relative abundances of the lead isotopes (86,87). This technique is not restricted to metals, but can be used on any material that contains lead. Finally, for an object cast around a ceramic core, a sample of the core material can be used for thermoluminescence dating. 

Lead isotopes. Sato and Sasaki (1973) concluded on the basis of a remarkable narrow range in lead isotopic composition of Kuroko ores that lead of Kuroko ore came from deep-seated source which originated from subducting pelagic sediments. 

Lead isotopic data on Kuroko deposits, vein-type deposits in Honshu and volcanic rocks are summarized and plotted in Fig. 1.44 (Fehn et al., 1983). 

Lead isotopic data support this interpretation namely, these data clearly indicate leaching of lead from the rocks (Pehn et al., 1983). 

Lead isotopic data on the epithermal deposits together with Kuroko deposits are plotted in Fig. 1.116 (Sato and Sasaki, 1973 Sato et al., 1973, 1981 Sato, 1975 Sasaki et al., 1982 Sasaki, 1987 Fehn et al., 1983). It is evident that lead isotopic compositions of epithermal vein ores are more scattered than Kuroko ores, although averaged values are similar to the Kuroko ores. This variation seems to be due to the difference in crustal materials underlying the ore deposits Lead isotopic compositions of different ore deposits which formed at different ages in the same district show the same values (Sasaki, 1974). 

Figure 1.116. Lead isotopic variation in Japanese Neogene ores. The majority of data fall in a relatively narrow range which is no more than twice the experimental uncertainty indicated by the replicate analyses of NBS-SRM-981 standard (Sasaki et al., 1982).

Sato, Kazuo and Sasaki, A. (1973) Lead isotopes of the black ore ( Kuroko ) deposits from Japan. Econ. GeoL, 68, 547-552. 

Sato, Kazuo, Delevaux, M.H. and Doe, B.R. (1981) Lead isotope measurements on ores, igneous, and sedimentary rocks from the Kuroko mineralization area. Geochem. J., 15, 135-140. 

Halbach et al. (1997) reported lead isotope data on volcanic rocks, sediments and ores from the hydrothermal JADE field in the Okinawa Trough and pointed out that lead isotopic compositions of Okinawa JADE ores are very similar to Kuroko ores (Fig. 2.31) and both sediments and volcanic rocks contributed comparable amounts of lead to the deposit. 

It is shown in Fig. 2.57 that the lead isotopic variation of the Besshi-subtype is similar to that of midoceanic ridge basalt, suggesting the lead in the Besshi-subtype was derived from mantle. The data from the Shimokawa, and Yanahara deposits (Group B) are slightly more radiogenic than Group A, suggesting that crustal lead was involved in the formation of the Shimokawa deposit, and lead isotopic values for the Shimokawa and Yanahara plot between MORB and Cretaceous-Tertiary deposits in Japan (Kuroko, skarn, vein-type deposits). 

Halbach, H.P., Hansmann, W., Koppel, V. and Proajus, B. (1997) Whole-rock and sulfide lead-isotope data from the hydrothermal JADE field in the Okinawa back-arc trough. Mineralium Deposita, 32, 70-78. 

James WD, Boothe PN, Presley BJ (1998) Compton suppression garmna-spectroscopy in the analysis of radium and lead isotopes in ocean sediments. J Radioanal Nucl Chem 236 261-265 Jarvis KE, Gray AL, Houk RS (1992) Handbook of Inductively Coupled Plasma Mass Spectrometry, Blackie, Glasgow... 

Oversby VM, Gast PW (1968) Lead isotope composition and uranium decay series disequilibrimn in recent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

Krishnaswami S, Graustein WC, Turekian KK, Dowd F (1982) Radium, thorium, and radioactive lead isotopes in groundwaters application to the in-situ determination of adsorption-desorption rate constants and retardation factors. Water Resour Res 6 1663-1675 Krishnaswami S, Bhushan R, Baskaran M (1991) Radium isotopes and Rn in shallow brines, Kharaghoda (India). Chem Geol (Isot Geosci) 87 125-136 Kronfeld J, Vogel JC, Talma AS (1994) A new explanation for extreme " U/ U disequilibria in a dolomitic aquifer. Earth Planet Sci Lett 123 81-93... 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

As a guide to the evolutionary history of the continents, Patterson decided to measure the lead isotope ratios of Earth s crust as a whole. As rocks erode, their minerals are collected and mixed in the oceans, where they eventually settle in layers of sediment. Patterson organized a formidable series of experiments to measure the lead isotopes on land, in various layers of ocean water, and in sediments on the sea floor. 

In a climax to his sediment studies, Patterson reported tersely that we have found the composition of lead in snow to be very different from the composition of lead which has been deposited on the ocean floors during the past 100,000 years. The lead in Lassen Volcanic National Park had a signature mix of lead isotopes, a characteristic fingerprint identifying it as a... 

Tsaihwa J. Chow and C. C. Patterson. The Occurrence and Significance of Lead Isotopes in Pelagic Sediments. Geochimica et Cosmochimica Acta. 26 (Feb. 1962) 263-308. 

---