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Chapter 8 : Radiogenic Isotope Geochemistry

By White, W. M.

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Book Id: WPLBN0000688294
Format Type: PDF eBook
File Size: 515.76 KB.
Reproduction Date: 2005
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Title: Chapter 8 : Radiogenic Isotope Geochemistry  
Author: White, W. M.
Volume:
Language: English
Subject: Science., Physics, Physics--Research
Collections: Physics Literature
Historic
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White, W. (n.d.). Chapter 8 : Radiogenic Isotope Geochemistry. Retrieved from http://hawaiilibrary.net/


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Physics Literature

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Introduction: Radiogenic isotope geochemistry has had an enormous influence on geologic thinking in the twentieth century. The story begins, however, in the latter half of the nineteenth century. At that time Lord Kelvin (who profoundly influenced the development of physics and thermodynamics in the 19th century), estimated the age of the solar system to be about 100 million years, based on the assumption that the Sun?s energy was derived from gravitational collapse. In 1897 he revised this estimate downward to the range of 20 to 40 million years. A year earlier, another Englishman, John Jolly, estimated the age of the Earth to be about 100 million years based on the assumption that salts in the ocean had built up through geologic time at a rate proportional their delivery by oceans. Geologists were particularly skeptical of Kelvin?s revised estimate, feeling the Earth must be older than this, but had no quantitative means of supporting their arguments. They did not realize it, but the key to the ultimate solution of the dilemma, radioactivity, had been discovered about the same time (1896) by Frenchman Henri Becquerel. Only eleven years elapsed before Bertram Boltwood, an American chemist, published the first ?radiometric age?. He determined the lead concentrations in three samples of pitchblende, a uranium ore, and concluded they ranged in age from 410 to 535 million years. In the meantime, Jolly also had been busy exploring the uses of radioactivity in geology and published what we might call the first book on isotope geochemistry in 1908. When the dust settled, the evidence favoring an older Earth was deemed conclusive. Satisfied though they might have been with this victory, geologists remained skeptical of radiometric age determinations. One exception was Arthur Holmes, who in 1913 estimated that the oldest rocks were at least 1600 million years old (Holmes was exceptional as well in his support for Alfred Wegener?s hypothesis of continental drift). Many geologists were as unhappy with Holmes?s age for the Earth as they had been with Kelvin?s.

 

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