Introduction to Radioisotope Geochronology/Part One - Introduction

Geologic Time

Geologic time can be measured in two very distinct ways: relative dating, which involves placing geologic events in sequential order, and absolute dating, which utilizes radioactive decay to quantify the number of years that have passed since a particular geologic event. When assessing geologic time, these two fundamentally different ways of measuring geologic time must work in tandem to reveal a more complete understanding of Earth processes.

Relative Dating

Before the development of radiometric dating, geologists had no reliable means of constraining an absolute age of geologic events. Relative dating was developed to place a series of geologic event in sequential. This method provides a great deal of important information as, even now in the age of geochronology it is not possible to constrain an absolute age for every geologic feature. Furthermore, even now in the age of geochronology, relative dating is key when selecting and understanding the context and field relationships of samples collected for the purpose of geochronology.

In the mid-17th century, Nicolas Steno observed that when a river flooded, it deposited horizontal layers of sediment and buried the flora and fauna living adjacent to the river. He posited that as the river flooded multiple times this would result in a series of horizontal deposits which would eventually turn into sedimentary rock. This sequential ordering of individual sediment layers is referred to as the Law of Superposition.

Coupled with these observations was that these layers were deposited horizontally, illustrating the principle of original horizontality. This implies that a series of rock layers that are steeply dipping must have originally been deposited horizontally. Steno also noted that sediment layers generally extend laterally and gradually pinches out near the boundaries of the basin in which it was deposited. This is termed the principle of lateral continuity. These three principles are known as Steno's three laws and for the foundation of relative dating in sedimentary successions.

Other primary principles of relative dating include:

1. cross cutting relationships
2. inclusions
3. faunal succession

Although that Steno's three laws made a significant impact to early geologic thought, when it came to issues of time and the rates of geologic processes, the naturalists of the day continued to rely on the Biblical chronology proposed by Ussher. Geologic features were explained by the concept of catastrophism in which the physical and biologic history of the Earth resulted from a series of catastrophes. It was not until naturalists such as James Hutton and Charles Lyell recognized that many field observations contradicted the concept of catastrophism. Hutton observed the erosive nature of the waves along the shoreline and flowing water in streams and rivers and posited that the erosion and redeposition of these sediments could explain the strata observed in Great Britain. Hutton claimed that "no powers to be employed that are not natural to the globe, no action to be admitted of except those of which we know the principle..." Elsewhere, Hutton is quoted as saying "the past history of our globe must be explained by what can be seen to be happening now." These concepts became collectively known as Uniformitarianism and forms the bulwark for modern geology.

Absolute Dating

History of radio-isotopic dating

After the establishment of Steno's three laws, the need became apparent to quantify the timing of geologic phenomena and the age of the Earth. There were several early attempts at constraining the timing of geologic events, all of them falling short and yet providing important insight into scientific progress. It was first Lord Kelvin who in 1862 calculated that the Earth was between 20 and 100 million years old based upon the assumption that the entire Earth was at one point completely molten. It was not until Marie Curie discovered that radioactivity produces heat and that radioactive elements deep in the Earth could sustain a higher temperature for much longer than Lord Kelvin had calculated that it was suggested the Earth was potentially much older than 100 million years. Continued work into radioactivity by Ernest Rutherford and Frederick Soddy revealed that radioactive elements such as uranium and thorium "transmutated" into other elements (in the case of uranium and thorium into lead) in a predictable sequence and in a measurable amount of time. This is known today as the decay chain. Acting on the suggestion of Rutherford, it was Bertram Boltwood who first measured the lead to uranium ratio and got results ranging from 410 to 2200 million years.

In regards to the age of the Earth, the first accurate estimate that is still broadly accepted today was proposed by Claire Patterson in 1953. Patterson's dissertation project was focused on using radiogenic lead (i.e. lead that was produced by the radioactive decay of uranium) to calculate the age of meteorites. It was assumed that meteorites and asteroids represented the left-over materials from the formation of the Solar System and had remained relatively undisturbed and therefore by measuring the lead isotopes of a meteorite on could assume the age of the Earth to be equivalent. Patterson found that the age of five meteorites from Canyon Diablo in Arizona, Nuevo Laredo in Mexico, and Henbury in Northern Australia was 4.55±0.07 billion years old. Amazingly, the current estimate for the age of the Earth is still within uncertainty of Patterson's first calculation at 4.54 ± 0.05 billion years (4.54 × 10⁹ years ± 1%).

Radio-isotopic dating in the 21 century

The past 100 years of radioisotope geochronology have seen major developments in our understanding of nuclear physics, isotope geochemistry, mass spectrometry, and microbeam sampling via laser ablation and ion guns, all of which have converged at augmenting our ability to constrain the timing and rates of geologic phenomena. In the sections that follow, we will explore the fundamentals of mass spectrometry and introduce the applications to understanding Earth processes.