Radiogenic

Lead occurs naturally as a mixture of four non-radioactive isotopes, and Pb, as well as the radioactive isotopes ° Pb and Pb. All but Pb arise by radioactive decay of uranium and thorium. Such decay products are known as radiogenic isotopes. 

Thorinm-232 is the only non-radiogenic thorium isotope of the U/Th decay series. Thorinm-232 enters the ocean by continental weathering and is mostly in the particulate form. Early measurements of Th were by alpha-spectrometry and required large volume samples ca. 1000 T). Not only did this make sample collection difficult, but the signal-to-noise ratio was often low and uncertain. With the development of a neutron activation analysis " and amass spectrometry method " the quality of the data greatly improved, and the required volume for mass spectrometry was reduced to less than a liter. Surface ocean waters typically have elevated concentrations of dissolved and particulate 17,3 7,62... 

Surface water enrichment is the result of aeolian and fluvial inputs, which are thought to be the most important sources of Th to the ocean. Thorinm-232 has been proposed as a link between the radiogenic thorium isotopes and trace metals and anthropogenic pollutants. " While the pathways are very different for the radiogenic thorium isotopes, Th is delivered to the ocean in a fashion similar to many pollutants and trace metals. For example, Guo et found Th distributions in the Gulf of Mexico and off Cape Hatteras in the North Atlantic Ocean agreed well with the general distribution pattern of aluminum. 

Fig. 2.57. Pb isotopic compositions in volcanogenic Cu sulfide deposits in Japan. Diamond shape K], K2 and K3 indicate average values of Kuroko-type deposits Ki Taro (Cretaceous) K2 Northeast Japan (Miocene) K3 southwest Japan (Miocene). Note that the Shimokawa ores (solid square, Besshi subtype) and Yanahara ores (open square, Hitachi subtype), closely associated with sediments, tend to have slightly more radiogenic values than the others (solid circles) (Sato and Kase, 1996).

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). 

Analytical solutions. Following the approach of Spiegelman and Elliott (1993), the effects of transport of melt through a melting column on the chemistry of radiogenic isotopes can be separated from melting by expressing the concentration of an element as ... 

Albarede F (2001) Radiogenic ingrowth in systems with multiple reservoirs applications to the differentiation of the mantle-crust system. Earth Planet Sci Lett 189 59-73... 

Criteria for successful U-Th-Pb dating are similar to those of standard U-series techniques, but in addition (1) sufficient amounts of ° Pb and ° Pb must have accumulated by radiogenic ingrowth to be distinguished from the initial Pb that is always present at the time of formation and (2) the decay scheme must be closed for all intermediate daughters from °Th and Pa to stable ° Pb and ° Pb, respectively. 

Despite the uncertainty about the rate of incorporation of °Thxs into Mn deposits, Th chronology can still provide some useful information about their growth rates. For instance, °Thxs has been used to suggest short-term changes in growth rates of Mn crusts (Eisenhauer et al. 1992) and to check the Be chronologies of crusts used to reconstruct the radiogenic isotope history of seawater (Abouchami et al. 1997 Frank 2002). 

Two important geochemical characteristics make Ra isotopes potentially useful as an estuarine tracer 1) having highly particle reactive Th isotopes residing largely in sediments as its direct radiogenic parents, which ties Ra directly to bottom sediments, and 2) exhibiting vastly different environmental behavior in fresh water and saltwater systems. 

Knowing the total amounts of potassium-40 and of radiogenic argon-40 in a sample enables one to calculate first, their relative weight amounts and then, from these values, the age of the sample. The reliability of a potassium-argon date is generally expressed as a plus or minus value after the calculated date. 

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