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A problem arises because some206Pb occurs naturally in Earth materials, without radioactive decay, so that measured238U/206Pb ratios include both daughter and nondaughter206Pb. This must be corrected if an accurate age is to be determined. The correction can be accomplished because yet another lead isotope,204Pb, is not produced by radioactive decay. The ratio of204Pb/206Pb in meteorites, formed at the time Earth formed, is known and is taken to be the original ratio in the rock whose age is being determined. The correction is then accomplished by measuring the amount of204Pb in the sample and subtracting the meteoritic proportion of206Pb from total206Pb to arrive at the amount of206Pb that is the daughter of238U.
concordia plot
ssss1 Uranium–lead concordia plot (blue) showing sample ages as a function206Pb/238U and207Pb/235U ratios and samples with lower206Pb/238U and207Pb/235U ratios (red), reset during a 2.5 Ga metamorphic event.
concordant
A significant problem arises from the fact that subsequent events can alter the geochemistry of rocks, so that age determinations are no longer concordant. For example, when rocks undergo metamorphism, lead is commonly mobilized and lost from the minerals or rocks in question. This results in lower than expected amounts of daughter lead isotopes, lower Pb/U ratios and points that fall below the concordant age line on206Pb/238U versus207Pb/235U diagrams. When samples of similar real ages, affected differently during a single metamorphic event, are plotted on a concordia diagram, they fall along a straight line below the concordant age line. One interpretation of such data is that the right end of the dashed straight line intersects the concordant age line at the original age of the sample (4.0 Ga in ssss1) and that the left end intersects the concordant age line at the time of metamorphism (2.5 Ga in ssss1). Wherever possible, such interpretations should be tested against other data so that a robust conclusion can be drawn. One such independent radiometric decay series, with widespread application, is described in the section that follows.