Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and their subsequent applications
Authors: Subarkah Darwinaji , Nixon Angus L. , Jimenez Monica , Collins Alan S. , Blades Morgan L. , Farkaš Juraj , Gilbert Sarah E. , Holford Simon , Jarrett Amber 
Published: September 2022

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.