Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and its subsequent applications
Authors: Subarkah Darwinaji , Nixon Angus Leslie , Jimenez Monica , Collins Alan Stephen , Blades Morgan Lee , Farkaš Juraj , Gilbert Sarah , Holford Simon 
Published: March 2022

Abstract. Recent developments in tandem laser ablation-mass spectrometer technology have been shown to be capable of 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 on Proterozoic shales that are dominated by authigenic clays. 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) using in situ Rb–Sr geochronology and constrained its thermal history 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 in this study yielded ages of 1470 ± 102 Ma, 1457 ± 29 Ma, and 1421 ± 152 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 stratigraphically deeper within the same borehole and succession were dated at 1318 ± 105 Ma and 1332 ± 67 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 coincide with the point 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 mobilise trace elements and resetting the Rb–Sr chronometer. Consequently, we propose that the Rb–Sr chronometer in shales may be sensitive to temperatures of ca. 110 °C in hydrothermal reactions but can withstand temperatures of more than 190 °C in thermal systems absent of fluid. 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.