How to date your favourite rock!

Darwinaji Subarkah - PhD student

Featured author - Darwinaji Subarkah
PhD student (Geology), Department of Earth Sciences

Geology is a science that studies the materials of the Earth, its history and how tectonic plays shaped them into the planet we live in today. These stories are written in stone!

Rocks can record the different environments it formed in as well as the natural processes it has experienced. In particular, sedimentary rocks like mudstone and limestone can archive the chemistries of ancient waters and atmosphere.

How much oxygen was in the air? How abundant was life in our oceans? They provide a glimpse into what the surface of our planet was like deep through time.

However, these records are meaningless unless it is placed within a context. Therefore, dating these rocks and determining their age is necessary for us to constrain how different Earth systems have evolved since their inception.

This necessity is further complicated when you look deeper into the planet’s history, more than half a billion years ago, when diverse fossil records essentially became absent.

Figure 1 - Diagram of the mass spectrometer

Figure 1: Diagram of the mass spectrometer used (Agilent 8900 QQQ) showing the reaction between N2O and Sr (modified from Woods, 2016). 

For rocks this old, we rely on radiometric dating. This involves understanding how a radioactive parent isotope can decay into a stable daughter product. For example, radioactive 87Rb is incorporated into clay minerals which makes up a lot of mudstones. This then decays into stable 87Sr by emitting an electron, with a half-life of roughly 49 billion years. By applying the known rate of decay for that radioactive element and measuring the amount of parent and daughter isotopes, we can calculate the age in which the mineral has formed. Historically, this dating method can be very difficult. 87Rb has the same mass as 87Sr, which makes them hard to measure with a traditional mass spectrometer.

To get around this dilemma, previous techniques require your samples to be dissolved in solution, where the different elements can then be chemically separated. However, there is a number of issues with this approach. Firstly, it is expensive and quite time-consuming. Second and more alarmingly, this technique destroys your samples in the process. Therefore, if something goes wrong, there is not much you can do.

Figure 2 - TES group members in the field in McArthur Basin, Northern Territory

Figure 2: TES group members out in the field, exploring and sampling the McArthur Basin, Northern Territory.

As such, the Tectonics and Earth Systems (TES) research group in the Department of Earth Sciences has developed a new method to overcome these obstacles.This method involves the use of a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA–ICP–MS). Repeated pulses of micron-scale laser blasts are fired at our rocks, vaporising bits of the samples. This material is then ionised by a plasma.

Afterwards, a device is used to filter everything except isotopes of mass 87 (i.e. Rb and Sr). The leftover ions then enter a reaction cell, where a N2O gas is introduced (figure 1). Sr reacts with this gas but Rb does not. Therefore, the Sr reaction product will now have a different mass compared to the Rb, allowing them to be separated and measured individually in a few milliseconds!

Figure 3 - Summary of Rb-Sr isotopic data

Figure 3 (A): Summary of Rb-Sr isotopic data and 87Sr/86Sr initial ratios of the samples (see inset). 87Sr/86Sr initial value of sample UR6_269 overlaps with that of seawater 1.5 billion years ago.

In our study, we showcase the utility of this method by targeting mudstones from the McArthur Basin in northern Australia (figure 2). The basin hosts the oldest petroleum play in the world. These rocks are enriched in dead bacteria that inhabited ancient oceans and have been widely used as a notable archive for reconstructing the planet’s environments 1.5 billion years ago. Our work is the first of its kind and has attracted large interests since its publication in the internationally acclaimed journal Geology.

Recently, similar techniques for other parent and daughter isotopic systems with the same mass (i.e. Lu-Hf dating) have been developed in our department. The University of Adelaide is a world-wide pioneer for these techniques, that have massive implications for global energy and critical minerals exploration.

Tagged in Faculty of Sciences, School of Physical Sciences

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