AGE OF THE EARTH—HOLMES 
lated from each of the three ratios 
Pb°5/U, Pb?°7/U and Pb?/Th, where 
these symbols here represent the per- 
centages of the parent elements and of 
the isotopes of radiogenic lead now 
present in the mineral under investiga- 
tion. The respective equations for 
tm, are: 
tm=15.15X10® logig (1+1.158 
Pb?°°/U) years 
ba=2.37X10" logig (1-+159.6 
Pb?°?/U) years 
tm=46.20X10® logy (141.116 
Pb8/Th) years 
A fourth value for ¢,, can be found 
from the ratio Pb”’/Pb?°°, 
If a mineral has remained unaltered, 
then all four values for t,, should be in 
close agreement. In practice this 
rarely happens, because even in the 
freshest-looking mineral migrations of 
the critical elements are likely to have 
occurred. Fortunately, even if the 
three values for tm based on Pb?°*/U, 
Pb?"’/U and Pb*’/Pb®* are widely dif- 
ferent, the relations between them pro- 
vide criteria for assessing the true age 
(fig. 2). If there has been leakage of 
radon (the gaseous member of the UI 
family), then the mineral is necessarily 
deficient in Pb*®*; in this case the most 
probable age is given by Pb?°’/U (see 
Wickman, 1942, and Holmes, 1947). 
If there has been loss of Pb or U or 
Percentages 
(R. C. Wells) 
eel co}iolaivef ul al ai(oi ise’ «| (e\ ilps) elie! jee. -e1/e) (6 Le (e)iene 
Isotopic proportions 
(Nier et aliter) 
In total lead 
Ou emelutie (elle Teltets 6 0 's11e1v (eave ere: oT 670) one) © 
COORD OO CO ORI CIS OC) CRG DIGG 6 ROR OG 
In radiogenic lead 
Percentages 
CIO ONO CHO OS DC GO SIG SIG OS cao ie SPO cS Oi] lic 
| 
Age ratios 
60-6) 6) S\¢; 0' [¢, eh. 0° 0) ees! o 6 4 se 4.0 © (ee «ele eee © 
233 
both, or gain of either or both, then 
the true age, or a close approximation 
to it, is given by Pb?°7/Pb?°%, 
Although a great many radioactive 
minerals have been chemically anal- 
yzed, relatively few isotopic analyses 
of the lead have been made as yet. 
Prof. A. O. Nier, following up Aston’s 
pioneer work, has been the most active 
and successful worker in this field. As 
an example of a fully investigated 
mineral, samarskite from Spinelli 
Quarry, Portland, Conn., may be 
taken. The mineral occurs in a peg- 
matite the geological age of which is 
not far from the end of the Devonian 
period. 
Loss of Loss of U 
Radon ; 
x e 
Loss of Pb | 
e “ eta ae 
b 
FicurE 2.—Diagram to illustrate the effects 
of various types of alteration on the ap- 
parent ages of a radioactive mineral cal- 
culated from Pb26/U (open circles), 
Pb 207/U_ (solid circles), and Pb 27/Pb 206 
(crosses). The horizontal line represents 
the true age of the mineral, points above 
and below being respectively too high and 
too low. 
‘* Apparent age” 
Pb U Th | from total Pb 
(million years) 
0. 314 6.91 3.05 295 
Pb204 Pb208 Pb207 Pb208 
0. 167 |100. 00 7. 60 21. 30 
SG7/ 3. 04 2. 61 6.35 
strane stata. 96. 96 4.99 14.95 
settee . 236 . 012 . 036 
Pb207 Ph208 Pb207 Pp208 
Pb208 U U Th 
0515 . 03415 . 00174 . 0118 
256 255 254 266 
eer eeeeerr eee ee seese 
Values of t, (million years) 
