One Hundred Years of Isotope Geochronology, and Counting

Abstract

In 1913, Frederick Soddy's research on the fundamentals of radioactivity led to the discovery of “isotopes.” Later that same year, Arthur Holmes published his now famous book The Age of the Earth, in which he applied this new science of radioactivity to the quantification of geologic time. Combined, these two landmark events did much to establish the field of “isotope geochronology” – the science that underpins our knowledge of the absolute age of most Earth (and extraterrestrial) materials. In celebrating the centenary, this issue brings together modern perspectives on the continually evolving field of isotope geochronology – a discipline that reflects and responds to the demands of studies ranging from the early evolution of the Solar System to our understanding of Quaternary climate change, and the 4.5 billion years in between. * Accuracy : The closeness of agreement between a measured quantity value and a true quantity value Decay constant (λ) : The reciprocal value of the average lifetime of a radionuclide, which is the time over which a population of parent radionuclides is reduced by 1/e times its initial value. The half-life of a radionuclide is equal to ln(2)/λ. Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) : A method of isotopic analysis in which an artificial or enriched isotopic tracer is added to a dissolved sample (e.g. zircon) to make a homogeneous isotopic mixture, the isotopic composition of which is analysed using TIMS. This technique is currently the form of isotopic measurement with the highest precision and accuracy, but it requires complete dissolution of the sample. LA–ICP–MS (laser ablation inductively coupled plasma mass spectrometry) : A microanalytical method that employs a focused laser beam to ablate material from samples. The ejected matter is ionised using a plasma before being passed through to a mass spectrometer. MSWD (mean squared weighted deviation) : A goodness of fit statistic that compares the sum of the squares of the deviations of a set of measurements from their mean value to the corresponding sum of the variances of each measurement Precision : The closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions Secondary ion mass spectrometry (SIMS) : Also referred to as an ion microprobe or microscope, SIMS measures the chemical or isotopic composition of small sample volumes by focusing a beam of high-energy primary ions onto a polished sample surface, ablating atoms and molecules, and generating secondary ions that are analysed by mass spectrometry. The high spatial resolution offered by SIMS (commonly <30 μm wide and <1 μm deep during analysis of geological materials) allows in situ analysis of geological materials. The method is relatively non-destructive, allowing multiple analyses to be performed within single grains or zones within grains, but has lower analytical precision than ID-TIMS. Radioactive decay : Nuclear reactions by which an atomic nucleus transforms via emission of ionising particles and electromagnetic radiation. Radioactive decay is a stochastic (i.e. random) process at the level of single atoms, in that it is impossible to predict when a given atom will decay. However, the probability that a given atom will decay is constant over time. Uncertainty (of measurements) : A parameter characterising the dispersion of the quantity values being attributed to the subject of a measurement, which can include components arising from both random and systematic measurement errors. Random errors are those that in replicate measurements vary in an unpredictable manner; systematic errors are those that remain constant or vary in a predictable manner across replicate measurements.