Proterozoic sedimentary rocks, altered finely crystalline mafic rocks, subtle/distal hydrothermal alteration halos—these form essential components of many mineral systems, yet are challenging to get accurate age from due to either a) their lack of distinctive fossils, or extensive authigenic datable phases, b) the altered nature and chemistry that precludes the growth of robust geochronologically useful minerals, or c) the subtle and mineralogically invisible overprint.

There are radioactive decay systems that have been developed over the years that can date these systems, but many of them have the handicap of having parent and daughter isotopes that are the same mass. This includes Rb–Sr, Re–Os, Lu–Hf, La–Ba, K–Ca.

That has restricted these techniques to be wet-chemistry based where the elements containing the parent and daughter isotopes have been separated in a lab, then individual elements have been isotopically characterised to get at the chronological information. This is quite difficult, long, and expensive.

This is just about to change as the new generation of ICP-Ms/Ms instruments have a reaction cell within the instrument that allows a reactant gas to be used to selectively change the mass of more reactive elements while others are unaffected. It means that we can now use laser-based techniques to introduce multi-element plasmas into the ICP and separated elements such as Sr from Rb, Hf from Lu, Os from Re.

In this presentation I will discuss this technique and present the results of laser Rb-Sr analyses undertaken by members of the Tectonics and Earth Systems Group at The University of Adelaide where we have collaborated with Dr Sarah Gilbert from Adelaide Microscopy to a) date altered mafic volcanic rocks in core from the Stuart Shelf (SA) and from the McArthur Basin (NT) where alteration is syn-emplacement; b) date the deposition age of shales from the Mesoproterozoic Roper Group of the McArthur Basin (NT) and the Tapley Hill Formation, Adelaide Superbasin (SA); c) date subtle hydrothermal alteration and identify when it involved the ingress of external fluids or simply the heating of formation waters.

Finally, I will present the results of Sam Rasch’s University of Adelaide Honours thesis from 2021 where he used these techniques to date an unknown Proterozoic sedimentary sequence encountered in the MinEx CRC’s National Drilling initiative core NDIBK10 in the East Tennant Region (Brunette Downs Rift Corridor - NT).