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The use of lead isotope ratios in estimating the age of the Earth

1952; American Geophysical Union; Volume: 33; Issue: 2 Linguagem: Inglês

10.1029/tr033i002p00156

2379-6723

Edward McCrady,

Isotope Analysis in Ecology

Since the publication of Nier's determinations of the lead isotope ratios in lead ores of different geologic horizons from Precambrian to Tertiary, four more or less independent efforts have been made to use these data as a basis for calculating the age of the Earth. In addition to the fact that more data are required to establish statistical validity for any of these methods, they all share a common fault of assuming that we already know with reasonable accuracy the age in years of one or more of these ores from their field associations. The author contends that the age of the enclosing rocks need have no correlation with the age of the ore if, by the latter, we mean the time of its extraction from source crustal rocks or magmas (not the time of its emplacement). A method is proposed for determining the age of granites in years measured backward from now instead of from the assumed age of any given rock body; and suggestions are made as to how the same data may provide a basis for determining the age of the original crust of the Earth. A least‐squares analysis of the data from isotope ratios in lead ores is used to show that the maximal possible age of matter is 5.07 × 10 9 years. Reasons are given for thinking the real age less than that. The fact that least‐squares equations, assuming alpha decay as the sole source of changes in the isotope ratios of uranium and lead in the past, can be extrapolated to the present ratio of U 235 /U 238 with a Lagrange multiplier not significantly different from zero, is interpreted to mean that other processes than alpha decay (such as natural fission, isotopic selectivity in crystal formation, and radon loss) have played no significant role in changing the composition of the ores.