How Do Scientists Determine the Age of a Rock?

A person’s age can be risky to determine without asking directly. But what about rocks and other materials on Earth? Geochronologists are real detectives able to unravel the age of minerals and rocks on Earth. One of the widespread methods within geochronology the radiometric dating technique based on the radioactive decay of Uranium (U) into Lead (Pb). With this technique, geochronologists can date rocks of 100 million to billions of years old.

How does it work?

It works like a clock that starts ticking as soon as the rock is formed. Rocks often contain traces of the element uranium and some of the uranium (²³⁸U) decays to lead (²⁰⁶Pb). The rate at which this happens is constant and reported as “half-life” (i.e., the time required for half of the uranium to decay to lead). During the life of a rock, the amount of uranium decreases and the amount of lead increases. Young rocks have very high amounts of uranium and low amounts of lead content, whereas very old rocks have very little uranium and high lead amounts. Since the half-life is known and one can measure the uranium and lead contents in the rock, one can calculate the age of a rock.

As rocks contain various minerals, geochronologists need to select the minerals that contain the most uranium. One of the most dated minerals is zircon (ZrSiO₄). In order to get the age of the rock with precisions better than 0.1%, one would need to measure the uranium and lead isotopes of the zircon crystals very precisely. It’s not an easy task, but with magnetic sector mass spectrometry this can be done.

Before being able to get analyzed on their isotope composition (²³⁸U/²⁰⁶Pb), uranium and lead must be separated from the zircon crystals. This is done by crushing the rock and separating the zircon crystals. Those get dissolved by chemical dissolution, followed by chemical separation procedure to separate the uranium from the lead. The final product is a solution containing the uranium and lead from the initial zircon crystals. This solution gets loaded onto a metal filament, heated and ionized in the mass spectrometer and separated on mass.

uranium

The Thermo Scientific™ Triton Plus™ TIMS is specially designed for these high precision isotope ratio analyses of zircons.

Features and Benefits for zircon analysis include:

- State-of-the-art ion detection system for the highest stability and precision
- New amplifier technology enabling high precision analysis for small samples
- Highly efficient and stable ion source optics for highest ion transmission
- Robust magnet for highest mass stability / high speed peak jumping
- Low abundance sensitivity for minimized tailing

- High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an i…
- High-precision zircon U/Pb geochronology by ID-TIMS using new 10¹³ ohm resistors by Von Quadt et al …

- ID-TIMS U–Pb geochronology at the 0.1‰ level using 10¹³ Ω resistors and simultaneous U and ¹⁸O/¹⁶O i…

For in-situ work, a laser ablation system coupled to an ICP-MS is ideal. With this, one can determine age variations within the zircon crystal. The Thermo Scientific™ Element XR™ is routinely coupled with laser ablation systems to provide U-Pb isotope data of zircons.

- Application Note on Laser Ablation Split Stream (LASS) Between Three Inductively Coupled Plasma Mass…
- Application Note on Simultaneous in situ Analysis of U-Pb Age and Hf Isotopes of Zircon by Laser Abl…

Next to the uranium-lead technique, geochronologists also use Ar-Ar radiometric dating to get age information, for example, sanidine crystals from volcanic tuff. This technique is based on the K-Ar dating technique, where the ratio ⁴⁰K/³⁹K is constant and ⁴⁰K is decaying to ⁴⁰Ca and ⁴⁰Ar. If ⁴⁰Ar is trapped in a crystal and one can measure the ratio ⁴⁰K/⁴⁰Ar, then the time evolved since the crystal was formed can be calculated. For such analysis, geoscientists use static vacuum mass spectrometers. Especially for high precise Ar-Ar dating, Thermo Fisher Scientific has developed the Thermo Scientific™ Argus VI™ Noble Gas Mass Spectrometer. This instrument enables simultaneous analysis of all five Ar isotopes on a mixed Faraday-Ion Counting detection system.