Fourth Isotope

Since his days in Mozambique Holmes wanted to develop a geological timescale with dates defining the beginning of each period and epoch. The work of previous geologists allowed for the construction of a geological column organized by characteristics of the layers of rock and by the distinctive index fossils contained within the strata. These historical developments permitted relative ordering but not the assignment of specific dates for the time periods. Holmes needed a framework on which to build and asked chemistry professor Fritz Paneth in Berlin for assistance. In 1928 Paneth developed a precise assay for measuring very small amounts of helium and used it to analyze two famous rocks from known geological periods: the Whin Sill from the late Carboniferous period and the Cleveland Dyke from the middle to early Tertiary period. Paneth dated the Whin Sill at 182 million years and the Cleveland Dyke at 26 million years, ages that seemed to agree with the geological evidence. (Today the Whin Sill and Cleveland Dyke are believed to be approximately 295 and 60 million years old respectively.) These two rocks did not have enough lead to determine lead ages as controls, but Holmes was anxious to make progress and confident that a geological timescale was possible.

Geologists were now more accepting of the longer estimates for the age of the Earth, which Holmes reported in his second edition of The Age of the Earth (1927) to be between 1,600 and 3,000 million years based on the uranium and lead measurements. The book also contained a geological timescale based on lead ratios and helium ratios, but two decades brought little progress—Holmes summarized all the computed mineral ages in a single short table.

The invention of the first mass spectrograph by English chemist Francis William Aston enabled the identification of isotopes with different atomic weights. Aston used his mass spectrograph to discover no fewer than 212 naturally occurring isotopes and was awarded the Nobel Prize in chemistry for 1922. The mass spectrograph evolved into the more advanced modern mass spectrometer that separates isotopes by passing them through a magnetic field that deflects them to different degrees based on their mass and the charge of the field. In the late 1920s Aston clearly identified three known lead isotopes, a finding with major implications for radioactive dating. He also unexpectedly noted that the isotope believed to be ordinary lead was in fact an end product from the decay of another less abundant uranium isotope that Rutherford helped identify as uranium 235. Rutherford estimated the uranium 235 decay rate, assumed that at the time of the Earth's formation uranium 235 and uranium 238 were present in equal amounts, and calculated the time it would have taken for the equal amounts of the two isotopes to decay to their current ratios. He obtained an astounding value of 3,400 million years, but his results and similar results from a few other geologists were mostly ignored.

If uranium 235 decayed to lead 207, then did so-called ordinary lead exist at all? In 1937 a physicist from Harvard University, Alfred Nier, began exploring the questionable existence of ordinary lead using a new mass spectrometer kept ultraclean to ensure no contaminating lead was present. He easily identified the three known lead isotopes, but he also observed a tiny amount of a fourth lead isotope with an atomic mass of 204 not produced by radioactive decay.

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