Lead-207 Isotope Ratios

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Lead is a toxic element with an extremely long environmental half-life. It is found in a wide variety of naturally occurring compounds, including the oxides: lead monoxide, PbO, in which lead is in the +2 oxidation state; lead dioxide, PbO2, in which it is in the +4 oxidation state; and trilead tetroxide, Pb(C2H3O2)2, which occurs as a yellow solid. In aqueous solution, it forms lead(II) ions and the water-soluble lead ion chelate, Pb(CH3COO)2.

Among the 43 known natural isotopes of lead (Pb), five are most important to geochemical research: 204Pb, 206Pb, 207Pb, and 208Pb. Of these, the last three are primordial nuclides; they are at the ends of decay chains originating in long-lived 238U, 235U, and 232Th isotopes.

The isotopic composition of lead in rocks depends on the amount of uranium and thorium present, so lead-isotope ratios are used to estimate the age of geological specimens. This information is also useful for the study of tectonic processes that affect the distribution of uranium and thorium in the Earth’s crust.

In the late 1960s, airborne particulates from burning leaded gasoline were analyzed for their organolead composition by gas chromatography with inductively coupled plasma-mass spectrometry. The lead isotope ratios of the different organolead species measured in these airborne samples from the same city exhibited considerable variability that could not be explained by analytical scatter. The results are illustrated in Figure 18.7.4 which shows the variation in d206Pb measured in published dispensed gasoline data versus the ALAS model curve (solid black). This is an illustration of the lead paradox described above.

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