Locked up for millions of years in the heart of the lead atom lies a geological secret which promises to aid scientists in extending the knowledge of earth's history back farther than ever before into the ages following this planet's birth from the sun, a Harvard physicist has found.
For a mineral timepiece scientists have used uranium-lead, which is one of the "dead" end-products remaining after the radio-active element uranium has spent its ray energy.
Because they know how long it takes for uranium to "die," scientists can tell how old a deposit is from the proportion of live uranium and inactive uranium lead found side by side within it. Mineral deposits have been dated back nearly two billion years by means of the uranium-lead timepiece.
Uranium's Older Brother
The new Harvard finding concerns ordinary, or common, lead, which may be called the "older brother" of uranium lead. Ordinary lead is as old as earth itself, and was included in the molten mass of the earth before it cooled down.
The valuable new secret about ordinary lead, which has been kept from scientists by a peculiar feature, is that the relative proportions of the isotopes of this metal vary from sample to sample. Isotopes are atoms of the same element which differ in weight.
Ordinary lead has four isotopes, weighing 204, 206, 207, and 208 atomic units. The Harvard finding shows that the relative abundance of these isotopes may vary as much as fifteen per cent. Scientists had always believed that the isotopes of ordinary lead had a certain fixed ratio.
Nier Makes Discovery
The discovery was made by Dr. Alfred O. Nier, National Research Fellow in the Harvard Physics laboratories, and resulted from his development of the most delicate atom "sifter" known to science.
Technically known as a mass spectrometer, Dr. Nier's instrument is designed to detect the presence of rare isotopic forms and also to give the most accurate measures even made of the relative abundance of different isotopes present in an element.
Ordinary lead's peculiar isotope distribution dates back millions of years and was probably caused by the early "contamination" of ordinary lead in its active elements uranium and thorium, Dr. Nier believes. This theory has yet to be thoroughly investigated.
More 204 In Older Deposits
This belief arose when Dr. Nier found that the oldest deposits of ordinary lead tend to have more of the isotope 204 in proportion to the isotopes 206, 207, and 208, than the younger deposits.
Ordinary lead, which has the isotopes 204, 206, 207, and 208, is the only lead to have the isotope 204; uranium lead is in two isotopes, 206, and 207; and thorium ends up in a lead of only one form, weighing 208 units.
By tracing back this ancient peculiarity, scientists may find that the ordinary lead atom carries within it at least a partial record of physical and chemical development when the earth was young. For one thing, a continuation of this study promises to reveal important clues as to the mechanism of the formation of lead ores.
Lead is one of seventeen elements which Dr. Nier has studied, beginning a comprehensive program of research which will eventually put every one of the ninety-odd known fundamental substances through his spectrometer.