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lead—lead dating A dating method based on the comparison of isotopes of lead (Pb). Throughout geologic time the isotopic composition of common lead in the Earth has evolved from that of primordial lead by the addition of radiogenic leads (206Pb, 207Pb, and 208Pb) derived from uranium and thorium decay (see DECAY SERIES). 204Pb is not derived from any radioactive parent and appears to be a standard against which the other lead values can be compared. It is normally assumed that in any small part of the Earth's crust and underlying mantle which, at the time of formation, contained primordial lead together with uranium and thorium, no radiogenic lead could have been present. With the passage of time atoms of radiogenic 206Pb, 207Pb, and 208Pb gradually replaced uranium and thorium atoms. If, at one instant in time, all the lead in the area under discussion was removed in solution and deposited as a lead ore, then this would preserve a record of the isotopic balance of lead at the time. Given that this ore mineral would not contain any uranium or thorium, it would be preserved as a unique point on the lead growth curve. Using the Holmes—Houtermans model, and plotting 207Pb:204Pb against 206Pb:204Pb, a series of growth curves would be obtained based on the different isotopic ratios. These curves can then be used to plot isochrons, the slope of which determines the age of the particular lead assemblage.