Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and their subsequent applications

Abstract

Abstract. Recent developments in tandem laser ablation mass spectrometer technology have demonstrated the capacity for separating parent and daughter isotopes of the same mass online. As a result, beta-decay chronometers can now be applied to the geological archive in situ as opposed to through traditional whole-rock digestions. One novel application of this technique is the in situ Rb–Sr dating of Proterozoic shales that are dominated by authigenic clays such as illite. This method can provide a depositional window for shales by differentiating signatures of early diagenetic processes versus late-stage secondary alteration. However, the hydrothermal sensitivity of the Rb–Sr isotopic system across geological timescales in shale-hosted clay minerals is not well understood. As such, we dated the Mesoproterozoic Velkerri Formation from the Altree 2 well in the Beetaloo Sub-basin (greater McArthur Basin), northern Australia, using this approach. We then constrained the thermal history of these units using common hydrocarbon maturity indicators and modelled effects of contact heating due to the intrusion of the Derim Derim Dolerite. In situ Rb–Sr dating of mature, oil-prone shales in the diagenetic zone from the Velkerri Formation yielded ages of 1448 ± 81, 1434 ± 19, and 1421 ± 139 Ma. These results agree with previous Re–Os dating of the unit and are interpreted as recording the timing of an early diagenetic event soon after deposition. Conversely, overmature, gas-prone shales in the anchizone sourced from deeper within the borehole were dated at 1322 ± 93 and 1336 ± 40 Ma. These ages are younger than the expected depositional window for the Velkerri Formation. Instead, they are consistent with the age of the Derim Derim Dolerite mafic intrusion intersected 800 m below the Velkerri Formation. Thermal modelling suggests that a single intrusion of 75 m thickness would have been capable of producing a significant hydrothermal perturbation radiating from the sill top. The intrusion width proposed by this model is consistent with similar Derim Derim Dolerite sill thicknesses found elsewhere in the McArthur Basin. The extent of the hydrothermal aureole induced by this intrusion coincides with the window in which kerogen from the Velkerri Formation becomes overmature. As a result, the mafic intrusion intersected here is interpreted to have caused kerogen in these shales to enter the gas window, induced fluids that mobilize trace elements, and reset the Rb–Sr chronometer. Consequently, we propose that the Rb–Sr chronometer in shales may be sensitive to temperatures of ca. 120 ∘C in hydrothermal reactions but can withstand temperatures of more than 190 ∘C in thermal systems not dominated by fluids. Importantly, this study demonstrates a framework for the combined use of in situ Rb–Sr dating and kerogen maturation indicators to help reveal the thermochronological history of Proterozoic sedimentary basins. As such, this approach can be a powerful tool for identifying the hydrocarbon potential of source rocks in similar geological settings.

Geochronology and formal stratigraphy of the Sturtian Glaciation in the Adelaide Superbasin

“…In-situ Rb–Sr geochronology is a rapidly developing technique that, when applied to shales, can provide information about depositional ages or early diagenetic illite formation (Subarkah et al 2021). In shales, the Rb–Sr isotopic system is susceptible to low–moderate temperature hydrothermal alteration and can be influenced by detrital input (Subarkah et al 2022) meaning that careful assessment must be done before assigning geological significance to obtained isochron ages. For the two samples analysed in this study, the obtained dates must be between 730 Ma and 663 Ma for them to be representative of depositional age.…”

“…Two siltstone/shale samples (3404236 and 3404235) were acquired for in-situ Rb-Sr geochronology from the Sturt Formation within drillhole SR13/2 located on the north-eastern margin of the Stuart Shelf, South Australia. Samples were mounted and polished as rock blocks in 25 mm round epoxy and then mapped for mineral composition and petrography using a Hitachi SU3800 SEM (Subarkah et al 2021(Subarkah et al , 2022. Samples were analysed using an Agilent 8900x ICP-MS/MS coupled to a RESOlution-LR 193 nM ArF excimer LA system.…”