HISTORICAL DEVELOPMENT OF ZIRCON GEOCHRONOLOGY

Abstract

An exact knowledge of rock formation ages is perhaps the single most important tool needed for assembling the geologic record into a coherent history. Moreover, the age of Earth and the time scale of pre-human events are central to a civilization’s sense of origin and purpose. Therefore, the quest for precise and reliable geochronometers has had a scientific and a cultural importance that few other enterprises can match. Since the beginning of the last century it has been recognized that long-lived radioactive decay systems provide the only valid means of quantifying geologic time. The uranium-lead decay system has always played a central role for several reasons. Minerals that contain very high U concentrations, although rare, are well known and easily obtained. The half lives of the natural U isotopes 238U and 235U are long enough to span all of Earth’s history but short enough that both parent and radiogenic daughter elements could be measured in such minerals even with the methods of a century ago. After the discovery that the U decay system is paired \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[^{238}U\ {\rightarrow}\ ^{206}Pb\ +\ 8\ {\alpha}\ +\ 6\ {\beta}^{{-}}\ (T_{1/2}\ =\ 4468\ Myr)\] \end{document} \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[^{235}U\ {\rightarrow}\ ^{207}Pb\ +\ 7\ {\alpha}\ +\ 4\ {\beta}^{{-}}\ (T_{1/2}\ =\ 704\ Myr)\] \end{document} it was realized that two age determinations could be made on the same sample using the same two elements. If the system has been closed to mobility of parent or daughter these two ages should agree, thus furnishing an internal test on the accuracy of the age. Further, the chemical coupling of the decays allows the age of the radiogenic daughter to be determined solely from its isotopic composition without knowing the parent-daughter ratio, a more difficult and less reliable parameter to measure. The wide utilization of the U-Pb geochronometer would not have been possible without the mineral zircon (ZrSiO4). Zircon normally contains U in concentrations well above its host rock average but discriminates strongly against the daughter element Pb during crystallization. …