In-situ Rb–Sr dating of Precambrian sediments

Subarkah, Darwinaji1, Blades, Morgan L1, Collins, Alan S1, Farkas, Juraj1, Gilbert, Sarah2, Lloyd, Jarred C1

1Tectonics and Earth Systems (TES) Group and Mineral Exploration CRC, Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia, 2Adelaide Microscopy, University of Adelaide, Adelaide, SA 5005,

Sedimentary rocks make up only 5% of the Earth’s crust and yet represents the primary archive of the planet’s biogeochemical cycles. As such, precise depositional age constraints of sedimentary sequences are critical to our understanding of how these systems have evolved through time. Here we present from a novel application of in-situ Rb‒Sr dating and elemental analyses using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS/MS) on a set of sedimentary rocks across the Precambrian under different geological settings. A reaction gas is introduced between two quadropoles in the system, allowing for the online separation of 87Sr and 87Rb. Three case studies were investigated using this method. Roper Group shale samples in proximity with the Derim Derim Dolerite intrusion were sampled from the UR5 drillhole in the Proterozoic McArthur Basin, northern Australia. These samples yielded ages extremely radiogenic initial 87Sr/86Sr values analogous to the dolerite sample taken from the same well. These time constraints are consistent with the crystallisation age of the igneous suite (ca. 1313 Ma) located elsewhere in the basin. We propose that these samples recorded an alteration event instigated by the intrusion that reset the Rb‒Sr chronometer and geochemistry of the surrounding sediments. On the other hand, a sample from the intrusion-absent UR6 borehole has been interpreted to reflect the depositional history of the Roper Group instead. Multiple analyses on glauconitic sediments from the Doomadgee Formation in the South Nicholson Basin gave two main outcomes. One set of results gave an age ca. 1300 Ma and retained a very high initial Sr isotopic ratio. On the other hand, another sample gave an age of 1607 ± 28 Ma, which overlaps with a tuff age interbedded within the same formation. Furthermore, this sample’s initial 87Sr/86Sr ratio was within error of contemporary seawater. Lastly, a calcareous shale from the Neoproterozoic Tapley Hill Formation in Arkaroola yielded an age of 664 ± 28 Ma with initial 87Sr/86Sr value overlapping with their coeval seawater during time of deposition. Together, our study demonstrates the capabilities of this technique to date Precambrian sediments and evaluate the nature of fluids that their isotopic system was in equilibrium with by coupling laser-derived Rb‒Sr and geochemical data. This approach thus allows for a rapid and accurate discrimination of depositional and alteration histories of sedimentary sequences and has potential to be a powerful dating tool for these archives through deep time.


Biography

I am an international PhD student from Indonesia in my second year of candidature at the University of Adelaide. My research interests focuses on using geochemical proxies from sedimentary rocks to reconstruct past palaeoenvironments and biogeochemical cycles through deep time.

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