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.

Detrital Zircon Age and Provenance of the Tonian-Cryogenian of the Adelaide Superbasin

Lloyd, Jarred C.1,2; Van Der Wolff, Erica1; Blades, Morgan L.1; Virgo, Georgina M.2,1; Collins, Alan S.1; Amos, Kathryn J.2

1Tectonics and Earth Systems Group, Mawson Centre for Geoscience; and MinEx CRC, Adelaide, Australia, 2Australian School of Petroleum and Energy Resources, Adelaide, Australia

The Adelaide Superbasin is a vast Neoproterozoic to middle Cambrian sedimentary basin in southern Australia that initiated due to the break-up of central Rodinia and, evolved into the Australian passive margin on edge of the Pacific Basin. We present over 2000 new detrital zircon analyses from more than 20 Tonian–Cryogenian formations of the central Adelaide Rift Complex of the Neoproterozoic–middle Cambrian Adelaide Superbasin. These new data focus on understudied formations from within the Burra and Umberatana Groups that were identified in Lloyd et al. (2020, Precambrian Research, 10.1016/j.precamres.2020.105849). Building on the >7,500 data previously published we now consider that we are getting an adequate idea of the spatial variation of detrital zircon populations for time-equivalent formations within this large basin. The same statistical method of Lloyd et al. (2020) is applied to this dataset. Samples of Burra and Umberatana Groups from the Mount Lofty Range region in the south of the superbasin, preserve local sources (Barossa Complex/Gawler Craton, ca. 2500–1560 Ma), suggesting local derivation. This contrasts time-equivalent formations from the north of the of basin (central and northern Flinders Ranges), where zircon sources include distal regions (Musgrave Orogen ca. 1550–1050 Ma) and suggest axial transport through the Willouran Trough. Samples in the south then show increasing zircon source diversity up sequence, similar to, although not as pronounced as, the progression seen in the north of the basin. All areas are then punctuated by the Sturtian Cryogenian ice-transported deposits. Post-Sturtian glacial deposits preserve younger zircon sources (ca. <1000 Ma), potentially from southern sources (Antarctica?). Notably ca. 980–950 Ma zircon populations are more common in the samples from the south of the basin than in the north of the basin. These observations and interpretations are suggestive of a progressive southward opening of the Superbasin, consistent with the current interpretation of lithostratigraphy.


Biography

Jarred Lloyd is a PhD student at the University of Adelaide with a passion for the evolving Earth system during the Neoproterozoic, South Australian geology,  education, and outreach.

Redefining the Basement Architecture of the Southern Mount Isa Inlier

Brown. D.D. 1 , Bultitude,R.J1., Simpson,J.M, Purdy,D.P1., Connors, K.A.2, Sanislav,I.V. 3.

1Geological Survey Of Queensland, 2University of Queensland, 3James Cook University

The Paleoproterozoic Kalkadoon-Leichhardt belt (KLB) forms the major basement block to a series of Paleoproterozoic Superbasin sequences of the Mount Isa Inlier. The basement sequences of the Mount Isa Inlier have a broad two-fold division comprising: 1) the Leichhardt Volcanics and the Kalkadoon Granodiorite (~1865Ma) formed during the Barramundi Orogeny and 2) a series of pre-1870Ma units which are restricted in distribution to the KLB and margins of the Western Succession (Kurbayia Metamorphic Complex, Yaringa Metamorphics, and Saint Ronans Metamorphics).

The eastern portion of the KLB is overlain by and complexly faulted with the Mary Kathleen Domain (MKD). The MKD is dominantly comprised of calcareous and siliciclastic metasedimentary units belonging to the Paleoproterozoic Leichhardt Superbasin (~1790 – 1740 Ma) and igneous rocks of the Wonga and Burstall suites (~1740 Ma). At the southern outcropping extent, several intrusive units had previously been included in either the Burstall Suite (Saint Mungo Granite), Wonga Suite (Birds Well Granite, Bushy Park Gneiss) or correlated with ~1740 Ma magmatism (Tick Hill Complex) and thus included in the Mary Kathleen Domain.

New SHRIMP and LA-ICP-MS U-Pb geochronology shows that some units in the southern MKD are much older and form part of the basement (KLB). The Bushy Park Gneiss and Birds Well Granite form part of the KLB with crystallisation ages consistent with the Barramundi portion of the KLB. Hafnium isotope studies indicate these units to have εHf which closer to CHUR whereas the previously analysed Barramundi portion of the KLB has uniform εHf values of ~ -4 which indicates some heterogeneity in the southern portion of the inlier.

This is confirmed by interpretation and reinterpretation deep seismic lines CF3 and M6, which reveal an extensive basement package in the subsurface of the Southern Mount Isa inlier and throughout the Eastern Sub province. This package appears to be a controlling factor for deformation and basin development from 1865 Ma.


Biography

Dominic Brown has worked on the geology, geochemistry, geochronology and geophysics of Queensland for 15 years at the Geeological Survey of Queensland across a diverse suite of projects.

Oxygenation of Mesoproterozoic Ocean and its Consequences for Eukaryotic Evolution

 Zhu, Dr Xiangkun1, Wang, Dr Xun1, Zhang, Dr Kan1, Sun, Dr Jian1, Poulton, Dr Simon2

1Institute Of Geology, Chinese Academy Of Geological Sciences, China, 2School of Earth and Environment, University of Leeds, UK

The Mesoproterozoic Era (1,600-1,000 million years ago; Ma) has long been considered a period of relative environmental stasis, with persistently low levels of atmospheric oxygen, and sluggish biological evolution. There remains much uncertainty, however, over the evolution of ocean chemistry during this time period, which may have been of profound significance for the early evolution of eukaryotic life. Here, we present multiple geochemical proxies, including rare earth element, iron speciation, Fe-, Ca- and Sr isotopes to investigate the redox evolution, and its relationship with tectonics, and effects on bio-essential elements,  of the 1,600-1,550 Ma Yanliao Basin, North China Craton. These data confirm that the ocean at the start of the Mesoproterozoic was dominantly anoxic and ferruginous. Significantly, we find evidence for a progressive oxygenation event starting at ~1,570 Ma, immediately prior to the occurrence of complex multicellular eukaryotes in shelf areas of the Yanliao Basin. Interestingly, the onset of the oxygenation is coincident with the transition from rift to drift of the North China Craton. Moreover, the oxygenation is most likely progressive rather than a short-lived pulse, which resulted in depletion of bio-essential elements such as P. Our study thus demonstrates that oxygenation of the Mesoproterozoic environment was far more dynamic and intense than previously envisaged, and establishes an important link between rising oxygen and the emerging record of diverse, multicellular eukaryotic life in the early Mesoproterozoic. Furthermore, It sheds significant light on the reason of the sluggish evolution of eukaryotes during the protracted period.


Biography

Dr Xiangkun Zhu obtained a Ph.D. from Cambridge University. His researches mainly focus on isotopes of transitional metals and their implications in geochemistry, cosmochemistry and biogeochemistry.

The Impact of Snowball Earth Glaciation on Ocean Water δ18O Values

Defliese, William F.1

 1School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD, Australia

It has been long recognized that glacial episodes can affect the δ18O value of ocean water, where preferential storage of 16O in ice changes the 18O/16O ratio of the ocean.  However, these effects are generally thought of as transient, as Cenozoic glaciation has had neither the magnitude nor duration to cause long-term change with ocean water buffered to values close to 0±2 ‰ VSMOW by tectonic processes.  The Snowball Earth glaciations of the Cryogenian have the potential to cause much larger changes in ocean water δ18O values due to their increased ice volume and long duration relative to Cenozoic glaciation, but these effects have not been previously investigated. 

Here, I use a numerical box model to investigate ocean water δ18O values over the Proterozoic and Phanerozoic.  The model simulates various temperature and tectonics dependant fluxes of 18O, while also incorporating a zero-dimensional climate model and ice volume component to model glacial cycles.  Monte Carlo simulations of the Sturtian and Marinoan glaciations reveal that these had the potential to alter ocean water δ18O values for hundreds of millions of years after the termination of glaciation, providing a mechanism for secular change in the δ18O value of ocean water.  This occurs as a very large volume of ice (presumably, but not necessarily 18O depleted) is sequestered from the ocean, causing the ocean to become enriched enough in 18O for exchange at mid-ocean ridges to remove 18O from the ocean and slowly change the overall ocean water δ18O value.  If Snowball Earth ice volumes were as large as proposed (~28-32% of ocean volume), present day values of ice δ18O would cause significant secular change in ocean water δ18O extending into the Phanerozoic.  An additional finding of this work is that the duration of the Sturtian glaciation required a very low CO2 degassing rate on the order of ~2 Tmol/year, significantly less than that estimated from most other mass balance approaches for the Phanerozoic.


Biography

Dr. William Defliese is Lecturer in Geochemistry in the School of Earth and Environmental Sciences at UQ.  His research interests include stable isotope geochemistry, clumped isotope geochemistry, carbonate sediments, basin analysis, and paleoclimatology.  He joined UQ in 2019 after postdocs at UCLA and Texas A&M University.

Extending Whole-Plate Tectonic Models into Deep Time: Linking the Neoproterozoic and the Phanerozoic

Merdith, Andrew1,*, Collins, Alan2, Williams, Simon3, Tetley, Michael1, Mulder, Jacob4, Blades, Morgan2, Young, Alexander5, Armistead, Sheree6, Cannon, John7, Zahirovic, Sabin7 and Müller, Dietmar7

1UnivLyon, Université Lyon 1, Ens de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France. 2Tectonics and Earth Systems Group (TES), Dept of Earth Sciences, The University of Adelaide, Adelaide, South Australia 5005, Australia. 3Northwest University, Xi’an, China. 4School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3168, Australia. 5GeoQuEST Research Centre, School of Earth, Atmospheric and Life Sciences, University of Wollongong, Northfields Avenue, NSW 2522, Australia. 6Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada & Metal Earth, Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada. 7Earthbyte Group, School of Geosciences, University of Sydney, Sydney, New South Wales, 2006, Australia

Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries through time. These advances are an essential step in in quantifying the role plate tectonics has had in the evolution of Earth-surface systems, including the biosphere, atmosphere and hydrosphere, as well as palaeogeographies and the evolving shape of the Earth surface (palaeobathymetry and topography). Previous work has resulted in a number of full-plate reconstructions spanning the last 1 Ga. However, so far these models cover discrete time periods, meaning that a complete model with a consistent set of plate motions and boundaries is not yet available to the Earth Science community. This is a particular problem for the Neoproterozoic and Cambrian, as it means that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. Here we present a continuous full-plate model spanning 1 Ga to the present-day, that is focused on a revised and improved model for the Neoproterozoic–Cambrian (1000–520 Ma), but connects with models of the Phanerozoic and opens up pre-Gondwana times for quantitative analysis and further regional refinements. The model is presented in a purely palaeomagnetic reference frame, and is otherwise geologically-derived, based on preserved data from past-plate boundaries. This is a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history.


Biography

Andrew completed his PhD at the University of Sydney in 2017, investigating the kinematic and tectonic history of the Neoproterozoic Earth. He has since been a Post-doctoral Fellow at Université Claude Bernard Lyon 1 since 2018, constructing tectonic models of serpentinisation in oceanic lithosphere.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.