Refining a geological timepiece
Lauren Gorojovsky (PhD Candidate) and Dr Olivier Alard have developed an important new age-dating technique at an AuScope enabled laboratory at Macquarie University. Image: supplied
Capturing the tempo of Earth processes, like critical mineral-carrying magma rising through the crust, is a key aim of geology. Recently, geochronology researchers at the AuScope enabled GeoAnalytical Facility at Macquarie University have developed a new in situ Rubidium-Strontium (Rb-Sr) age-dating technique, allowing geologists to obtain age data within minutes rather than weeks, and with the sample’s geological context intact. Here, Lauren Gorojovsky and Dr Olivier Alard explain their new science, which promises wide application and great benefit to minerals explorers.
Zoning in on our research
Geochronology is the science of determining the age of rocks, fossils, and sediments by examining the geochemical signatures (radioactive isotopes) inherent in the rocks themselves. Geologists have been dating rocks since the 19th Century to better understand the workings of our planet and the solar system.
But recently, geochronology has moved from time-consuming mineral-separation techniques, to in situ techniques, where a small section of the sample is burned off using a laser beam and then analysed using mass spectrometry. This in situ approach is rapid, cost-effective, has a high throughput and high spatial resolution, and most importantly, preserves crucial information required for an insightful interpretation of the geochronology data. Indeed “ages” need to be interpreted within a context.
As such, Zircon uranium-lead geochronology has been the flagstaff of this approach. Yet with the constant advance of mass spectrometry, other geochronometer minerals are becoming available and provide different but complementary information to zircon geochronology.
With the advancement of the tandem inductively plasma mass spectrometer (ICP-MS), it is now possible to skip the complex and time-consuming chemical process required to separate Rubidium (Rb) from Strontium (Sr). Our research represents one more step towards the use of the long-known Rb-Sr radiometric clock, which we have now adapted to the in situ approach.
An example of a sample used in the research to help advance the in situ Rb-Sr technique (left) and geochronological results derived from it (right). Image: supplied
It forms a milestone in advancing the Rb-Sr technique and laying the ground for a routine analysis and more widespread use of this in situ geochronometer, and comes at a time when the community is seizing opportunities to develop new geochronology approaches (2016; 2017) and several development projects in collaboration with worldwide and Australian research centres.
Why we need Rubidium-Strontium geochronometry
Both Rb and Sr are lithophile elements (bonding with abundant silicates in magmas) and are easily, at different degrees, mobilised by volatile-rich (magmatic) melts and fluids. The crystallisation and/or the percolation and reaction of these melts and fluids through Earth’s crust lead to the crystallisation of mica, amphibole or feldspars which are enriched in Rb and therefore become ideal chronometer capsules recording the timing of these events. These melts and fluids are either naturally endowed with critical minerals or concentrate them across the course of their journey upwards.
Rocks have a long history punctuated by multiple key events that could be separated by millions or even billions of years. The strength of the in situ approach is to recognise and characterise these events at the micrometric scale, by multiplying the radiometric system, we build up a richer, and more informed story.
Further, what it is mind-blowing is that it is now possible to access this information in only a few minutes, while it was taking weeks before, and that is at a spatial resolution of few tens of microns transforming thin sections in meter to kilometre-scale outcrops.
AUTHORS
Written by Lauren Gorojovsky and
Dr Olivier Alard of Macquarie University,
and edited by Jo Condon, AuScope
ACKNOWLEDGEMENTS
This research has been enabled by the NSW Government’s NSW Research Attraction and Acceleration Program (NSW RAAP) through the generous funding of Macquarie’s Q3-ICP Mass Spectrometer instrument.