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49
maximum age of the elements.
If some Pb^*-*7 was produced by the processes which produced
the elementsJ, the age is less than this. If the age is 5.2 x lO-'
years, that Is one half life of U^^5 earlier than the age of the
meteorites, the abundances of the lead Isotopes become 0,0101,
0.0153, and 0.0600 for Pb 207, pb^O^^ and Pb^^^, respectively.
The logarithms of these are 0.004-2, 0.177-2 and 0.778-2. Taking
the mean of the logarithms for Pb and Pb^^° and subtracting
that for Pb^'"^ gives 0.47 as the difference between the odd and
even mass curves. This difference as we have drawn the curves
is 0.20 and hence it would be necessai^y to lower Bi209 and the
thallium points by about 0.27 in order to secure a smooth curve.
The data of these elements is hardly sufficiently certain to
exclude this possibility. However, the difference of 0,47 between
the two curves is larger than the separation of our even and odd
mass curves at any point above mass 120, Since the odd and
even mass curves become closer together at high mass numberSj It
seems likely that the elements were synthesized more than
5,2 X 10^ years ago. This estimate does not depend on the
absolute values of the abundances of lead and uranium, but only on
their ratios which are determined from the lead isotope ratios.
If the Patterson data for Nuovo laredo meteorite were used the
calculated age of the elements would be less. On the other handp
from the absence of conspicuous amounts of Xe-^^^ and the known half
life of I^^9 in meteorites, Suess and Brown (I951) conclude that