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

Recent developments in tandem laser ablation-mass spectrometer technology have demonstrated 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 20 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 thermal history of these units using common hydrocarbon


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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 thermal history of these units using common hydrocarbon maturity indicators, and modelled effects of contact heating due to the intrusion of the Derim Derim 25 Dolerite.
In situ Rb-Sr dating of mature, oil-prone shales in the diagenetic zone from the Velkerri Formation yielded ages of 1448 ± 81 Ma, 1434 ± 19 Ma, and1421 ± 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 Ma and 1336 ± 30 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 2 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 35 extent of the hydrothermal aureole induced by this intrusion coincide 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 mobilise 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 40 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.

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The Rb-Sr isotopic system has historically been one of the most powerful dating tools in Earth science. Rb is abundant in K-rich minerals such as micas, clays, and K-feldspar, and these minerals are commonly found in a wide range of geological settings (Simmons, 1998). Therefore it is an effective technique to date processes such as igneous emplacement, metamorphism, sedimentation, clay authigenesis, and hydrothermal alteration when these phases can be differentiated (Nebel, 2014). Its long half-life also makes it applicable to date events as early as the 50 infant stages of our solar system (Minster et al., 1979;Nebel et al., 2011;Papanastassiou and Wasserburg, 1970).
Traditionally, the application of this method required an arduous process of column chromatography to chemically separate the parent ( 87 Rb) and daughter ( 87 Sr) isotopes and avoid isobaric interference between the two elements (Charlier et al., 2006;Dickin, 2018;Faure, 1977;Hahn et al., 1943;Hahn and Walling, 1938;Yang et al., 2010).
Alas, this approach has historically been expensive, time consuming, and results in the loss of the genetic 55 relationships between the minerals analysed, causing the technique to lose its popularity in recent years (Nebel, 2014).
Recent advancements in tandem laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS/MS) and similar instruments have revitalised the use of Rb-Sr by allowing them to be applied in situ (Bevan et al., 2021b;Gorojovsky and Alard, 2020;Hogmalm et al., 2017;Redaa et al., 2021a;Yim et al., 2021;Zack and 60 3 Hogmalm, 2016). Reactive gasses such as N2O and SF6 can be introduced into a reaction cell between quadrupoles in an LA-ICP-MS/MS system, which permits the online separation of 87 Sr from 87 Rb through the measurement of the mass shifted Sr reaction product (Hogmalm et al., 2017;Redaa et al., 2021a;Zack and Hogmalm, 2016). This allows for a more rapid and economic analysis time, as well as the ability to preserve petrographic relationships during these analyses. Consequently, secondary input of Rb or Sr from inclusions, zonation, alteration, and detritus 65 can be isolated, resulting in a better understanding of the geochronological results. However, it should be noted that nm or μm sized mineral intergrowths of different origins still provide challenges when large spot sizes are used. The application of similar setups with other beta-decay dating systems have also yielded promising results (Bevan et al., 2021a;Brown et al., 2022;Gorojovsky and Alard, 2020b;Harrison et al., 2010;Hogmalm et al., 2019;Ribeiro et al., 2021;Rösel and Zack, 2022;Scheiblhofer et al., 2022;Simpson et al., 2021;Simpson et al., 2022;Tamblyn et al., 70 2021).
Hence, the in situ Rb-Sr dating method can now be used very similarly to laser ablation U-Pb dating, where age information can be obtained reliably, rapidly, and cheaply. In addition, the initial 87 Sr/ 86 Sr ratio from the calculated isochron and the elemental data concurrently collected with the Rb and Sr isotopes can fingerprint the geochemical nature of the samples analysed (Li et al., 2020;Redaa et al., 2021b;Subarkah et al., 2021;Tamblyn et al., 2020).

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Another novel use of this technique is to date Proterozoic shales in order to constrain their depositional window (Subarkah et al., 2021). Evidence suggests that clay minerals in Proterozoic shales are dominated by authigenic products from reverse weathering processes during reactions in equilibrium with the formation waters (Deepak et al., 2022;Isson and Planavsky, 2018;Kennedy et al., 2006;Mackenzie and Kump, 1995;Rafiei and Kennedy, 2019;Rafiei et al., 2020). Conversely, clay assemblages in late Ediacaran and Phanerozoic shales are commonly 85 dominated by detrital products from continental weathering and erosion of soils and unstable parent rocks Chamley, 1989;Galán, 2006;Hillier, 1995;Kennedy et al., 2006;Rafiei et al., 2020; 4 Singer, 1980;Wilson, 1999). Simple multicellular organisms such as fungi and lichen have been shown to dramatically influence the rate of chemical weathering in continental rocks (Chen et al., 2000;Cuadros, 2017;Kennedy et al., 2006;Lee and Parsons, 1999;McMahon and Davies, 2018;Mergelov et al., 2018;Rafiei et al., 90 2020). As such, the surge in abundance of detrital clays in shales in the Ediacaran and Phanerozoic has been attributed to the production of soils driven by emergence of these microorganisms (Kennedy et al., 2006;Lee and Parsons, 1999;McMahon and Davies, 2018;Mergelov et al., 2018;Rafiei and Kennedy, 2019;Zambell et al., 2012).
Thus, the primarily authigenic nature of clay minerals in Proterozoic shales make them ideal targets for in situ Rb-Sr dating (Subarkah et al., 2021).

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Despite the promising potential of the Rb-Sr isotopic system, the chronometer still holds some limitations. Rb and Sr are large ion lithophile elements that can sit in well-bound interstitial sites within a mineral lattice as well as adsorbed onto the surface where they are more susceptible to fluid mobilisation (Li et al., 2019;Nebel, 2014;Villa, 1998). In these environments, fluid-induced recrystallisation and alteration can drive element and isotopic exchange at lower effective closure temperatures than those empirically determined for classic thermal volume diffusion 100 reactions (Dodson, 1973;Field and Råheim, 1979;Jenkin et al., 1995;Villa, 1998). Nevertheless, these complications can in turn be used advantageously to date secondary events such as episodes of hydrothermal fluidflow (Dodson, 1973;Li et al., 2020;Redaa et al., 2021b;Shepherd and Darbyshire, 1981;Subarkah et al., 2021;Tamblyn et al., 2020). However, it should be noted that the Rb-Sr system in shale-hosted illite may also be affected during diagenesis via the transformation of smectite to illite-smectite mixed layer minerals.

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In this study, we dated the Mesoproterozoic Velkerri Formation from the Roper Group of the McArthur Basin in northern Australia by in situ Rb-Sr geochronology and show that clay-mineral recrystallization in these shales occur at similar temperatures to kerogen catagenesis. The Roper Group is a good case study for in situ Rb-Sr shale dating, as it has been shown to be dominated by authigenic clays (Rafiei and Kennedy, 2019;Subarkah et al., 2021) and is chronologically well-constrained (Ahmad and Munson, 2013;Bodorkos et al., 2022;Cox et al., 2022;Kendall et al., 110 2009;Southgate et al., 2000;Subarkah et al., 2021;. Furthermore, the resurgence of interest in the resource potential of the organic-rich Velkerri Formation has also yielded a framework of palaeotemperature data that aim to discern the maturation history of hydrocarbons within the unit (Ahmad and Munson, 2013;Capogreco, 5 2017;Cox et al., 2016;Crick et al., 1988;George and Ahmed, 2002a;Jarrett et al., 2019a;Lemiux, 2011;Revie, 2014;Summons et al., 1994;Taylor et al., 1994;Volk et al., 2005).
We show that common hydrocarbon maturation proxies such as Tmax data from Rock-Eval pyrolysis, aromatic hydrocarbons, bitumen reflectance, and illite crystallinity can help define the temperature sensitivity of the Rb-Sr 120 isotopic system in organic-rich shales. In addition, we have also modelled the geothermal aureole of a mafic intrusion that may have matured the kerogen into the gas window, altered trace elemental signatures and reset the Rb-Sr isotopic system within the unit. As a result, we demonstrate that combining this novel dating method with traditional kerogen maturation proxies can be a powerful tool for reconstructing the thermochronological evolution of Proterozoic basin systems. This approach can then be applied to aid in hydrocarbon exploration for similar 125 settings.

Geological Background
The Palaeo-to-Mesoproterozoic greater McArthur Basin is an intra-cratonic sedimentary system exposed across 180,000 km 2 of northern Australia (Ahmad and Munson, 2013). The basin is sub-divided into five unconformitybounded sedimentary packages characterized by similarities in age, lithology, and stratigraphic position (Jackson et 130 al., 1999;Rawlings, 1999). The Roper Group is part of the Wilton Package which is the youngest of these subdivisions (Jackson et al., 1999;Jackson et al., 1987;Munson, 2016;Rawlings, 1999). The thickness of the Roper Group varies around 1 to 5 km across several different fault zones (Abbott and Sweet, 2000;Abbott et al., 2001;Ahmad and Munson, 2013;Jackson et al., 1987;Rawlings, 1999). The Beetaloo Sub-basin ( Figure 1) is interpreted to be the main depocentre of the sedimentary system and preserves the thickest Roper Group sequences (Abbott and 135 Sweet, 2000;Ahmad and Munson, 2013;Jackson et al., 1987;Plumb and Wellman, 1987). Lithologically, the Roper Group comprises of a series of coarsening-up sequences dominated by marine mudstone and interbedded sandstone with minor successions of intraclastic limestone (Abbott and Sweet, 2000;Jackson et al., 1987;Munson, 2016;Yang et al., 2018). Records of water-column euxinia and redox stratification, as well as fluctuating salinity levels, suggests that the Roper Group formed in an intermittently restricted marine basin within an epicontinental setting 140 6 similar to the modern Black Sea, or Baltic Sea (Ahmad and Munson, 2013;Cox et al., 2022;Cox et al., 2016;Mukherjee and Large, 2016;Revie and MacDonald, 2017;Yang et al., 2018).
Age constraints of the Roper Group have been established through several geochronological methods (Ahmad and Munson, 2013;Jackson et al., 1999;Kendall et al., 2009;Nixon et al., 2021;Page et al., 2000;Southgate et al., 2000;Subarkah et al., 2021;Yang et al., 2019;Yang et al., 2018). The beginning of the group's 145 genesis is bracketed by a SHRIMP U-Pb zircon study from a tuff within the unconformably underlying Nathan Group as well as minimum depositional age from an in situ Rb-Sr analysis in the lower Roper Group that yielded ages of 1589 ± 3 Ma and 1577 ± 56 Ma, respectively Subarkah et al., 2021). The unconformity between the Roper Group and the immediately underlying Nathan Group is likely related to the Isan Orogeny ca.
1.58 Ga (Ahmad and Munson, 2013;Jackson et al., 1999). Absolute dating of the Roper Group has been obtained 150 through two SHRIMP U-Pb zircon studies from tuff layers in the Mainoru Formation resulting in ages of 1492 ± 4 Ma and 1493 ± 4 Ma (Jackson et al., 1999;Southgate et al., 2000). On the other hand, the Kyalla Formation at the top of the Roper Group is constrained to being deposited between the U-Pb age of its youngest detrital zircon at 1313 ± 47 Ma  and the age of crosscutting Derim Derim dolerite intrusions at 1313 ± 1 Ma, 1324 ± 4 Ma, and 1327.5 ± 0.6 Ma (Bodorkos et al., 2022;.

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Mature organic-rich shales from the Velkerri Formation have been dated by Re-Os analysis at 1417 ± 29 Ma and 1361 ± 21 Ma (Kendall et al., 2009). These ages have been interpreted to be the depositional age of the formation.
The geochronological constraints of the Roper Group are summarized in Figure 1. The Velkerri Formation is dominated by deep-basinal lithologies such as mudstones and siltsones that coarsens-up into the cross-bedded Moroak Sandstone and Sherwin Ironstone (Abbott et al., 2001). The Velkerri Formation is interpreted to represent a 160 deep-water, high-stand systems tract within a marine environment (Abbott et al., 2001;Warren et al., 1998). The Velkerri Formation is commonly sub-divided into three distinct members (from bottom to top, the Kalala, Amungee, and Wyworrie Members) based on variations in total organic carbon (TOC) content, gamma ray response, geochemistry, sedimentology and mineralogy (Ahmad and Munson, 2013;Cox et al., 2016;Cox et al., 2019;Jarrett et al., 2019b;Munson and Revie, 2018;Revie, 2016;Warren et al., 1998). with the oldest intrusions likely contemporaneous with the end of sedimentation in the basin (Ahmad and Munson, 2013;Bodorkos et al., 2022;Nixon et al., 2021;Subarkah et al., 2021;. Little evidence of subsequent tectono-thermal perturbation is present within the basin until much of the region was overlain by 170 subaerial basaltic lavas of the Kalkarindji Large Igneous Province (LIP) extruded at ca. 510 Ma (Evins et al., 2009;Glass and Phillips, 2006;Jourdan et al., 2014;Nixon et al., 2022). Following the Cambrian eruption of the Kalkarindji lavas, no significant (> 110°C) heating has been detected within the shallow parts of the basin (Duddy et al., 2004;Nixon et al., 2022).
The Altree 2 well drilled in the Beetaloo Sub-basin was chosen for this study as it intersects the entirety of the 175 Velkerri Formation (Figure 2). In addition, the well also intersected lavas of the Kalkarindji LIP directly overlying the Proterozoic sedimentary rocks and terminated at an intrusion of the Derim Derim Dolerite. Importantly, this well has also been the focus of numerous geochronological, geochemical and geobiological investigations from academia, private explorers as well as the Northern Territory Geological Survey (NTGS) which provide important complementary data to supplement this study (Bodorkos et al., 2022;Cox et al., 2022;Cox et al., 2016;Cox et al., 180 2019;George and Ahmed, 2002a;Jarrett et al., 2019a;Lemiux, 2011;Nguyen et al., 2019;Nixon et al., 2021;NTGS, 2009NTGS, , 2010NTGS, , 2012Sander et al., 2018;Warren et al., 1998;Yang et al., 2018).
The lithology of the Velkerri Formation was interpreted in detail ( Figure 2) using the electrical logs Gamma-Ray (GR), Neutron (NRS) and Density (RHOB) of the Altree-2 well (NTGS, 1989). Four lithologies were defined after applying cut-offs at each electrical log. They are then correlated along depth. Sandstone units corresponds to a GR < 190 130 API, NRS < 0.20 % and RHOB of around 2.5 g/cm 3 . This relates to a cross over between the RHOB and NPRS logs and competent material at the GR. Interbedded shale and sands are defined by a GR > 130 and < 250 API, NRS > 0.20 and < 0.25 m 3 /m 3 and RHOB between 2.5 and 2.53 g/cm 3 . This lithology reflected a smaller breach between the density and neutron logs in comparison to the previous sandstone lithology. Shale units were constrained by a 8 GR > 250 API, Neutron > 0.25 m 3 /m 3 and RHOB > 2.53 g/cm 3 and a minimum to no separation between the 195 porosity logs. On the other hand, dolomitic siltstones have a GR response similar to the sandstone, with NRS ranging between 0.25 to 0.27 and RHOB > 2.62 g/cm 3 . This indicates a competent lithology in the GR with a gap between the neutron and density curves. In addition, Tmax data werealso collated to discriminate the hydrocarbon maturation levels downhole. From this, a shift in hydrocarbon potential and Tmax gradients were identified at around 900 m (Figure 2), where kerogen enters the gas window and becomes overmature. Five shale chips were then 200 sampled from the Velkerri Formation in Altree 2 at depths of 415 m, 520 m, 696 m, 938 m, and 1220 m for further characterisation.
Samples were first imaged for their mineral composition and petrographic relationships. Back Scatter Electron (BSE) imaging and Mineral Liberation Analysis (MLA) maps of samples were collected using a Hitachi SU3800 Automated Mineralogy Scanning Electron Microscope at Adelaide Microscopy. BSE image tiles were done at 10 205 mm working distance and 20kV acceleration voltage with MLA maps were completed using a raster analysis with spectra collected at 0.35 µm/pixel resolution. Minerals previously categorised by bulk XRD analysis of the Velkerri Formation from Cox et al. (2016) were used to develop a 'library' to help identify phases found by spectral reflectance MLA mapping. In situ Rb-Sr geochronology and trace element analysis were undertaken at Adelaide Microscopy using a laser ablation (RESOlution-LR ArF 193nm excimer laser) inductively coupled plasma tandem 210 mass spectrometer (Agilent 8900x ICP-MS/MS) with the analytical parameters and tuning conditions following Redaa et al. (2021a). The laser set-up used in this study is provided in the Supplementary Material. Laser ablation data and error correlations were processed using the LADR software package (Norris and Danyushevsky, 2018;Schmitz and Schoene, 2007). During the data-processing step, Zr, Si, Ti, and rare earth element signatures were monitored to filter the detrital component of each analysis. Non-stable isotopic and elemental signatures were also 215 culled or cropped during the processing of each analysis to aid in ensuring spot homogeneity. The 87 Rb decay constant used was 0.000013972 ± 4.5e-7 Myr -1 following Villa et al. (2015). Isochron and single-spot ages were calculated with ISOPLOTR (Vermeesch, 2018). Single-spot ages were calculated using the isochron intercept as their initial 87 Sr/ 86 Sr ratios (Nebel, 2014;Rösel and Zack, 2022;Vermeesch, 2018). In addition, kernel density estimation (KDE) graphs (Vermeesch, 2012), cumulative age distributions (CAD) plots (Vermeesch, 2007), and 220 multidimensional scaling (MDS) graphs (Vermeesch, 2013) were also constructed using ISOPLOTR (Vermeesch, 2018) to differentiate between the population of single-spot ages from each sample.

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The phlogopite nano-powder Mica-Mg (Govindaraju et al., 1994) was used as the primary reference material and its natural mineral equivalent MDC from the Ampandrandava Mine in Madagascar (Armistead et al., 2020;Hogmalm et al., 2017;Li et al., 2020;Redaa et al., 2021a) as well as glauconite grain reference material GL-O (Charbit et al., 225 1998;Derkowski et al., 2009) were used secondary age standards. As previously discussed in Subarkah et al. (2021), nano-powdered reference materials have similar ablation characteristics to fine-grained shales, with analogous matrix effects. As such, they are ideal standards for in situ analyses of these samples.
When anchored to a 87 Sr/ 86 Sr initial ratio = 0.72607 ± 0.00363 as reported by Hogmalm et al. (2017), MDC yielded an age of 524 ± 7 Ma. This is within error of the published mean age of Mica-Mg at 519 ± 7 Ma (Hogmalm et al., 230 2017). In addition, the independent reference material GL-O gave an age of 96 ± 4 Ma, accurate to its published K-Ar age of 95 ± 1.5 Ma (Charbit et al., 1998;Derkowski et al., 2009). It should be noted that this age is younger than the tuff-horizon age of the GL-O host rock, dated at 113 ± 0.3 Ma (Selby, 2009). Consequently, the ages yielded from GL-O have instead been proposed to be indicative of the formation of glauconite occurring early after the deposition of the host rock (Selby, 2009).

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Glass standard NIST SRM 610 was used as a primary standard for elemental quantification in this study. Analysis of secondary standard BCR-2G yielded major, trace and rare earth element composition that were in good agreement (Pearson R > 0.999, Pearson R 2 > 0.999, and P Value < 0.0001) with their published values as compiled in the GeoREM database (Jochum and Stoll, 2008;Jochum et al., 2011;Jochum et al., 2005;Pearce et al., 1997).
One-dimensional thermal modelling of the Altree 2 well was conducted using the SILLi 1.0 numerical model, which 240 is designed for simulating thermal perturbation associated with sill emplacement within sedimentary basins (Iyer et al., 2018). First, palaeotemperatures were estimated from the compiled thermal maturity data (Disnar, 1986(Disnar, , 1994Waliczek et al., 2021) following equations based on similar sedimentary systems that experienced a heating event due to burial and a subsequent igneous intrusion (Piedad-Sánchez et al., 2004). Forward modelling was then conducted to replicate maximum thermal conditions calculated in the well from the thermal maturity data, where 245 palaeotemperatures suggest that the Wyworrie and Amungee Members experienced significant additional sedimentary cover in the Mesoproterozoic. During modelling, an additional 1.5 km of sedimentary rocks were added above the erosional unconformity now present above the McArthur Basin fill (Hall et al., 2021), while all post-Mesoproterozoic units were excluded. The upper contact of a sill with an initial temperature of 1150°C (Wang et al., 10 2012) ) was set at 2868 m, in accordance with adjusted burial depths during the Mesoproterozoic. As sill thickness is 250 unconstrained within the Altree 2 well, multiple iterations were run with different thicknesses in order to establish the scenario able to best satisfy the thermal aureole extent observed in this well. From this, a sill thickness of 75 m was considered most appropriate, and is consistent with Derim Derim sill thicknesses of ~10-100 m commonly observed across the basin (Lanigan and Ledlie, 1990;Lanigan and Torkington, 1991;Ledlie and Maim, 1989;NTGS, 2014NTGS, , 2015NTGS, , 2016. Full modelling parameters and petrophysical properties are provided in the 255 Supplementary Material Table S2 and Table S3.

Compilation of legacy data
All legacy data are compiled in the Supplementary Material and were checked for quality before interpretation as several factors such as contamination of cuttings due to drilling fluid or poor organic content can make results 260 unreliable (Carvajal-Ortiz and Gentzis, 2015;Dembicki Jr, 2009;Peters, 1986). Rock-Eval pyrolysis values were screened using the thresholds described by Hall et al. (2016). Data were subsequently excluded from interpretation if these criteria were not met. More than 90% of the data yielded S2 > 0.1 mg HC/g, indicating that they were sufficiently abundant in organic content to provide well-defined peaks for characterising Tmax and Hydrogen Index.
Importantly, compilation of Rock-Eval pyrolysis values were all internally consistent (e.g., Hydrogen Index = 265 S2/TOC x 100). Next, there was no evidence of anomalously low Tmax values (< 380°C) present. Extremely low Tmax values are commonly a product of incorrect selection of the S2 peak by the program or the widening of the S1 peak from non-indigenous free hydrocarbons. Tmax results compiled in this study ranges between 384°C to 502°C with a mean of 433°C (st. dev. = 17). TOC content in the Velkerri Formation varies from 0.07% to 8.07%, averaging at 2.25% (st. dev. = 2.26). Clay mineral crystallinity and size data sourced for this compilation were standardised for 270 interlaboratory comparisons (Warr and Mählmann, 2015;Warr and Rice, 1994). Full width at half maximum values from Altree 2 shale samples were computationally remeasured as a secondary check (Capogreco, 2017;NTGS, 2010NTGS, , 2012. Thirteen samples from the Velkerri Formation were analysed for their illite crystallinity. The Kübler Index for these shales range between 0.88 to 0.36, with decreasing values at depth and the lowest data originating from the Kalala Member. Methylphenanthrene distribution factor (MPDF), methylphenanthrene ratios (MPR), and 275 11 bitumen reflectance data collated from Jarrett et al. (2019a) and Revie et al. (2022) also displays an increasing trend down-hole.

Mineralogy of the Velkerri Formation
Eleven mineral phases were identified by bulk XRD analysis of the Velkerri Formation from Cox et al. (2016). The major mineral phases were quartz, kaolinite, illite, and orthoclase, which on average make up 90% of the total 280 composition of the samples. Trace minerals include glauconite, montmorillonite, pyrite, magnetite, siderite, dolomite, and plagioclase. Our MLA mapping also identified these assemblages. Importantly, the two different methods categorised these minerals at similar abundances. However, results from MLA mapping also found other mineral assemblages not identified by bulk XRD analysis, including biotite, chlorite, clinochlore, apatite, and zircon.
These differences could be due to the slightly different sub-intervals from which samples were analysed. Bulk XRD 285 is a destructive procedure and therefore, the same section cannot be reused for in situ analysis. As a result, samples spaced 1 -2 cm apart may yield different results. In addition, the targeted areas for MLA are often spatially localised and only based on 2D information. As such, they may not be representative for the bulk rock, making the comparison with XRD datasets difficult. The complete mineralogical abundance and correlations between results bulk XRD and MLA mapping are summarised in Table 1

Laser ablation data
Geochronological results yielded by samples from the Wyworrie and Amungee Members gave ages within error of each other. The sample from 415 m depth was dated at 1448 ± 81 Ma. Next, the mudstone analysed from 520 m depth yielded an age of 1434 ± 19 Ma. Thirdly, the shale sample studied from 696 m depth resulted in an age of 295 1411 ± 139 Ma. A Kalala Member shale chip from 938 m at depth was dated at 1322 ± 93 Ma. Another sample from this member, towards the boundary with the underlying Bessie Creek Sandstone at depth 1220 m resulted in an age of 1336 ± 40 Ma. The range of precision from these Rb-Sr ages are primarily constrained by a substantial spread in 87 Rb/ 86 Sr ratios, the number of data points defining the regression line as well as errors on each individual analysis (Nebel, 2014). The most precisely dated samples, extracted from 520 m and 1220 m depth, had the widest range of 300 87 Rb/ 86 Sr ratios (0 -50) whilst the other two samples preserved a range of 87 Rb/ 86 Sr values less than 10 ( Figure 6).
The variability in these values could be a subject of future studies in order to improve the success of this dating 12 method. Single-spot ages were calculated for all spot analyses in each sample, and their populations categorically differ (Figure 8).
Elemental concentrations of each sample were concurrently collected during the in situ Rb-Sr laser ablation 305 investigation and they are in good agreement with data collected by bulk geochemical analysis from Cox et al. (2016). Samples from depth 415 m, 520 m, and 696 m do not show any covariation between their total REEY concentrations and Sm/Nd ratios (Figure 7). On the other hand, sample 938 m showed a statistically significant relationship between these two parameters (Pearson R = 0.58, Pearson R 2 = 0.336, P Value < 0.0001). In addition, Velkerri Formation shale sourced from depth 1220 m also showed a strong covariation between total REEY values 310 and Sm/Nd (Pearson R = -0.545, Pearson R 2 = 0.297, P Value < 0.0001). These associations were also identified in the whole-rock geochemical data collected from Cox et al. (2016). Figure 7B shows that samples between 390 -900 m depth hold no statistically significant relationships between the two factors. However, samples from deeper than 900 m display a strong relationship between the two variables (Pearson R = -0.559, Pearson R 2 = 0.312, P Value = 0.003). The full geochronological and inorganic geochemical dataset for samples in this study can be found in the 315 Supplementary Material.

Thermal Modelling
One-dimensional thermal modelling of the emplacement of a 75 m thick Derim Derim Dolerite sill at the base of the Altree 2 well is sufficient to produce a thermal aureole reaching temperatures > 110°C, ca. 800 m above the top contact of the sill ( Figure 9A). Maximum palaeotemperatures recorded in the Wyworrie Members exceed those 320 predicted in this simulation, however, this may be attributed to elevated temperatures in the shallow basin during eruption of the Kalkarindji LIP in the Cambrian (Nixon et al., 2022). The resultant maximum thermal profile is consistent with Tmax derived palaeotemperature estimates and is thus considered a plausible model for the observed data from the well. Post-intrusion temperatures at depths that match the samples with reset Rb-Sr ages are much lower than observed in comparable isotopic systems for thermally induced diffusion (Dodson, 1973;Tillberg et al., 325 2020;Torgersen et al., 2015;Yoder and Eugster, 1955), with the shallowest reset sample peaking at ca. 120°C.
Furthermore, elevated temperatures predicted by the modelling are geologically short lived, with temperatures returning to steady-state conditions by approximately half a million years following sill intrusion ( Figure 9B).

Geochemical and mineralogical-based thermal maturity indicators collected via Rock-Eval studies and bulk XRD
analyses were compiled in this study in order to establish a vertical profile of the Velkerri Formation and assess the 335 local palaeo-thermal structure. The Tmax parameter is the temperature at which the maximum rate of hydrocarbon generation occurs during pyrolysis analysis and is a common method used to reconstruct thermal histories of basin systems (Espitalié, 1986;Espitalié et al., 1977;Peters and Cassa, 1994;Welte and Tissot, 1984). Additionally, the Kübler index (KI) is determined by the 001-reflection of illite and is also a popular maturation proxy used to classify low-grade metamorphism in pelitic rocks (Blenkinsop, 1988;Guggenheim et al., 2002;Kubler, 1967). However, 340 both of these thermal indicators can be influenced by multiple factors other than burial-related heating and therefore struggle to resolve absolute quantitative palaeotemperatures. Changes in heating rate, abundance in hydrogen, sulphur, and uranium content or the organic richness of samples can result in inaccurate Tmax values (Dembicki Jr, 2009;Espitalié et al., 1977;Peters, 1986;Yang and Horsfield, 2020). Similarly, the KI has also been shown to be sensitive to several parameters such as changes heating rate and geochemical variability in the sample's initial 345 mineralogy (Abad and Nieto, 2007;Eberl and Velde, 1989;Mählmann et al., 2012;Warr and Mählmann, 2015). In addition, variations in procedures between laboratories can further complicate the direct comparison of these values (Cornford et al., 1998;Jarvie, 1991;Peters and Cassa, 1994;Tissot et al., 1987). Consequently, these thermal indicators need to be treated with caution when applied independently and are more suitable as qualitative  (Burtner and Warner, 1986;Dellisanti et al., 2010;Ola et al., 2018;Velde and Espitalié, 1989;Waliczek et al., 2021). Ultimately, both organic and inorganic indicators are essential for a robust understanding of the thermal histories of sedimentary sequences through time.
In this study, we examine the covariation between the Tmax values and KI to reconstruct the thermal history of the 355 Velkerri Formation in the Altree 2 well (Figure 3). In our compilation, samples with immature kerogen (Tmax < 435°C) correspond to rocks in the diagenetic zone (KI > 0.45°Δ2θ). This relationship is true in similar studies and generally translates to palaeotemperatures of ca. 100°C (Abad and Nieto, 2007;Dellisanti et al., 2010;Espitalié et al., 1977;Kosakowski et al., 1999;Kubler, 1967).
Interestingly, the samples within the mature oil window (435°C < Tmax < 465°C) show a wide range of KI values 360 between 0.39 and 0.65°Δ2θ (Figure 3). This is possibly due to the delay between thermal reactions in clay minerals as opposed to organic matter (Ola et al., 2018). Although the maturation of organic matter and the morphology of clay minerals both largely depend on temperature, other processes such as the kinetics of the thermal reaction and geochemical composition of the sample can make these relationships non-linear (Meunier et al., 2004;Ola et al., 2018;Pollastro, 1993;Varajao and Meunier, 1995;Velde and Vasseur, 1992). The disparity between kerogen 365 evolution and the equilibrium stage of illitisation at these temperatures may also play a role in this variability (Dellisanti et al., 2010). Nevertheless, an increase in Tmax pyrolysis results from these samples still appears to correlate with a decrease in KI values. These thermometers would approximately equate to palaeotemperatures between 100 and 150°C (Árkai et al., 2002;Frey and Merriman, 1999;Kosakowski et al., 1999;Welte and Tissot, 1984).

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Lastly, the thermal parameters for the Velkerri Formation can be further examined by inspecting the changes in MPDF Kvalheim et al., 1987), MPR (Radke et al., 1982;Wilhelms et al., 1998), and bitumen reflectance (Riediger, 1993). Previous studies have shown that aromatic hydrocarbons were effective in providing thermal constraints for the Velkerri Formation (George and Ahmed, 2002b;Jarrett et al., 2019a). These proxies were similarly sensitive to maturity variations from thermally immature to late oil window. As such, we normalise the 380 thermal indicators used in this study by converting them all (Jarrett et al., 2019a;Jarvie et al., 2001;Revie et al., 2022) to calculated vitrinite reflectance values (VRCALC; Figure 4). The VRCALC from four different thermal indicators show that the Velkerri Formation quickly elevated in maturity and enters the gas window at ca. 900 m depths (Figure 4). The agreement of all proxies add further confidence in the temperature constrains used in this study.

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Multiple geochemical and mineralogical thermal parameters from our compiled data-set demonstrate strong correlation between them, suggesting that the proxies used in this study primarily recorded changes in palaeotemperature as opposed to other possible variables. Notably, five different, independent, source-rock maturation proxies statistically agree and recorded similar step-wise increase in thermal history down-hole. As such, we investigated five samples approaching the geothermal anomaly in the Kalala Member for in situ Rb-Sr and trace 390 element analysis. The changes in thermal maturation indexes throughout the well are used to help constrain the parameters of the Rb-Sr isotopic system in Proterozoic shales.

Thermochronological History of the Velkerri Formation
Although some of the geochronological results of these samples may overlap due to their errors ( Figure 6), they are still categorically different down-hole (Figure 8). We display these differences by plotting the population of single-395 spot ages from each sample against each other. Kernel Distribution Estimate (KDE) plots of these results show that the distribution of single-spot ages from samples shallower than 900 m largely overlaps with the Re-Os age constraint ( Figure 8A; light pink) for the Velkerri Formation (Kendall et al., 2009). On the other hand, the population of single-spot ages from shales deeper than 900 m instead agree with the age for the Derim Derim Dolerite ( Figure 8A; dark pink) intrusion ca. 1330-1300 Ma (Ahmad and Munson, 2013;Bodorkos et al., 2022; 400 Nixon et al., 2021;.
Importantly, these sample-sets are statistically different from each other. This is graphically shown by their cumulative age distribution (CAD, Figure 8B) and Multidimensional Scaling (MDS, Figure 8C) plots. The second of these techniques statistically measures the dissimilarity between different age distributions through the Kolmogorov-Smirnov test (Vermeesch, 2013). In short, similar age distributions will plot closely to each other whilst distributions 405 that are increasingly dissimilar will plot further away (Vermeesch, 2012(Vermeesch, , 2013. Figure 8B and 8C show that samples shallower than 900 m had age distributions that are similar to each other ( Figure 8). Overall, these ages are statistically similar to the Re-Os constraint of the Velkerri Formation (Kendall et al., 2009), suggesting that they likely represent an early diagenetic/burial age soon after deposition. On the other hand, the single-age distributions of samples deeper than 900 m are statistically different to the previous sample set. They form their own cluster, 410 which in turn coincides with the age of the Derim Derim Dolerites (Ahmad and Munson, 2013;Bodorkos et al., 2022;Nixon et al., 2021;. Consequently, the Rb-Sr shales ages from this section are unlikely to date the deposition of the Velkerri Formation, but instead reflect a late-stage hydrothermal resetting induced by the intrusion. The petrographic characteristics of assemblages in these samples are further evidence that the shales in the Velkerri 415 Formation recorded two distinct thermochronological events. The abundant clays in samples from depth 415 m, 520 m, and 696 m are predominantly illite, with trace amounts of chlorite, kaolinite, and montmorillonite (Table 1).
However, they do not show typical irregular, angular detrital morphologies ( Figure 5, Figure S2A-C). Instead, clay minerals in these samples form a matrix cement, filling in porous spaces, wrapping around detrital grains and suggesting that they formed within the sediment during burial diagenesis (Rafiei and Kennedy, 2019;Rafiei et al., 420 2020;Subarkah et al., 2021). Primary sedimentary structures such compaction of clays along the bedding plane can also still be identified in these samples ( Figure 5, Figure S2A-C and Supplementary Material). These petrographic relationships are further discussed in the Supplementary Material and were similarly found in Roper Group shales elsewhere, indicating an early-diagenetic origin (Rafiei and Kennedy, 2019;Subarkah et al., 2021). Moreover, the ages from these samples are all in agreement with the deposition of the Velkerri Formation dated at 1417 ± 29 Ma 425 by Re-Os geochronology (Kendall et al., 2009), suggesting that the majority of illite formed relatively soon after sediment deposition.
Nevertheless, we also sought to identify any potential secondary alteration of these shales by analysing their geochemical signatures. Sm/Nd ratios are common geochemical proxies for screening alteration in shales as Nd is preferentially lost relative to Sm during post-depositional processes (Awwiller and Mack, 1989;Awwiller and 430 Mack, 1991). In addition, fluid-rock reactions also have a significant impact on rare earth element and yttrium solubility and transportation during hydrothermal events (Lev et al., 1999;Williams-Jones et al., 2012). Therefore, these parameters can be an effective tool for highlighting fingerprints of post-depositional geochemical mobilisation ( Figure 7).
Samples from depths shallower than 900 m show no significant relationships between their total rare earth elements 435 and yttrium (REEY) concentrations and Sm/Nd ratios collected by laser ablation analysis or through traditional bulk trace element geochemistry (Cox et al., 2016). These support an interpretation that these ages form a minimum depositional age for the unit, recording an early diagenetic event as opposed to a late-stage secondary overprint.
Furthermore, temperature constrains for the Velkerri Formation at depths 390 -900 m suggest that they are wellwithin the oil window (Figure 3 and 4). As a result, we propose that this temperature window is not sufficient to 440 disturb the Rb-Sr and trace element systems in these shales.
Conversely, shales collected from depths >900 m showed petrographic evidence of post-depositional alteration  Cox et al. (2016). In the compiled data, shales from deeper than 900 m demonstrate a strong affinity between these controls (Pearson r: -0.559, R-squared: 0.312). These 19 alteration indicators are further evidence that the Kalala Member at depths below 900 m experienced a late-stage 455 secondary heating event, as trace elements are more readily mobilised in hydrothermal reactions (Awwiller and Mack, 1989;Awwiller and Mack, 1991;Condie, 1991;Lev et al., 1999;Poitrasson et al., 1995;Williams-Jones et al., 2012).
Importantly, thermal indicators from this interval suggest that kerogen in these shales are thermally overmature (Figure 3 and 4). Previous studies have shown that the source rocks in the Velkerri Formation became overmature 460 only when affected by magmatic events George and Ahmed, 2002a). As such, it is plausible that the Derim Derim Dolerite intersected in this well has imposed a hydrothermal alteration footprint onto the surrounding sediments via conductive heat loss and/or heat-transfer fluids. This magmatic pulse would have recrystallised the former mineral assemblages or induced a second mineralisation of clays, mobilised trace elements, and heated the kerogen within the Kalala Member to overmaturity. Thermal indicators (Figure 4 and 9) suggest that 465 source-rocks within this interval may have experienced palaeotemperatures of at least 150°C (Dellisanti et al., 2010;Frey and Merriman, 1999;Hunt, 1995;Welte and Tissot, 1984). This is in good agreement with evidence from aqueous fluid inclusions in quartz veins within the Derim Derim Dolerite elsewhere, which have suggested that hydrocarbons from the Velkerri Formation migrated in the cooling sill at similar temperatures (Dutkiewicz et al., 2004). Importantly, such hydrothermal systems seem to be sufficient for disturbing the Rb-Sr isotopic system of 470 these samples.

Modelled Predictions of the Geothermal Aureole Induced by the Derim Derim Dolerite
Resetting of Rb-Sr geochronology and overmaturation of hydrocarbons in the Kalala Member within the Altree 2 well implies the presence of a secondary hydrothermal aureole extending ca. 800 m away from the Derim Derim Dolerite sill, which is intersected at present day depth 1696 m. One-dimensional thermal modelling for a sill 475 thickness of 75 m in the Mesoproterozoic suggests temperatures exceeding the oil window over 120°C (Tissot et al., 1974;Waples, 1980) only extended ca. 700 m from the intrusion ( Figure 9A).
Samples at present day depths of 938 m and 1220 m yield Rb-Sr ages corresponding to emplacement timing of the Derim Derim Dolerite , which suggests that the intrusion caused the chronometer to reset or induced a second mineralisation of clay phases. Predicted temperatures experienced by the 480 shallowest reset sample, however, are lower than the inferred closure temperatures for observed K-Ar and Rb-Sr in 20 sheet silicates (Dodson, 1973;Tillberg et al., 2020;Torgersen et al., 2015;Yoder and Eugster, 1955). In a scenario in which a sill of thickness 75 m was intruded below samples, rocks from present day depth of 938 m are only predicted to have experienced maximum heating to ca. 110°C ( Figure 9C), with temperatures exceeding 100°C for a duration of only ca. 150 ka ( Figure 9B).

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Additionally, the eruption of lavas from the Kalkarindji LIP (Evins et al., 2009;Glass and Phillips, 2006;Jourdan et al., 2014), within the same vertical profile offer an intriguing opportunity to evaluate thermal resistance of the Rb-Sr system in shale-hosted clays in different conditions. Basaltic lavas of the ca. 510 Ma Cambrian Kalkarindji LIP (Evins et al., 2009;Glass and Phillips, 2006;Jourdan et al., 2014) are preserved above Proterozoic sedimentary rocks in the Altree 2 well. Furthermore, regional apatite fission track data suggest that the thermal pulse induced 490 during this LIP extrusion were short-lived but sufficient (> 190°C) to anneal tracks in the upper ~500 m of the basin (Nixon et al., 2022). However, the shallowest samples taken in this study (at depths 415 m, 520 m, and 696 m) did not have their Rb-Sr isotopic system disturbed despite experiencing such temperatures from this reheating event.
Consequently, the thermal profile for the sample at 415 m depth provides a minimum closure temperature constraint for short-lived conditions which have not reset the Rb-Sr chronometer in these (presumably) dry shales over 800 495 million years after the Derim Derim dolerite intrusion. Interestingly, the Cambrian palaeotemperatures imposed by the Kalkarindji lavas (Nixon et al., 2022) are notably higher (> 190°C) than Mesoproterozoic palaeotemperatures reached by samples with Rb-Sr ages reset by the Derim Derim Dolerite (ca. 120°C; Figure 9A and 9B).
Such disparity suggests that the presence of fluid (either connate, or sourced from the intrusion), rather than just temperature, is likely to play a critical role in determining whether the Rb-Sr record in a shale is reset. As such, the 500 geochemical system in shales within the aureole may be disturbed at lower temperatures, as trace and rare earth elements are more easily mobilised in hydrothermal fluid systems (Li et al., 2019;Nebel, 2014;Poitrasson et al., 1995;Villa, 1998;Williams-Jones et al., 2012).

Conclusion
We show that the Velkerri Formation shales intersected by the Altree 2 well preserve evidence of an elevated 505 Mesoproterozoic thermal gradient through an ~800 m thick section away from the intrusion of a Derim Derim Dolerite sill (Figure 3, 4, and 9). In situ Rb-Sr isotopic ages from the Wyworrie and Amungee Members above this hydrothermal aureole yielded ages (Figure 6 and 8) within error of their depositional age (Kendall et al., 2009). In 21 addition, unaltered trace element compositions ( Figure 7) and petrographic relationships indicate that the shales preserve an early-diagenetic origin ( Figure 5 and Supplementary Material). However, the older Kalala Member that 510 lies within the hydrothermal aureole yielded younger Rb-Sr ages (Figure 6 and 8) consistent with the age of the Derim Derim Dolerite (Ahmad and Munson, 2013;Bodorkos et al., 2022;Nixon et al., 2021;. Samples from this subset also recorded perturbed trace element signatures (Figure 7) as well as fissile, foliated, and crystalline illite morphologies ( Figure 5 and Supplementary Material). This interval corresponds with disturbed thermal maturity indicators (Figure 2, 3, and 4), suggesting that the Rb-Sr system is stable up to the maturation oil 515 window and reset when the kerogen is overmature. Thermal modelling of the Derim Derim Dolerite suggests that a 75 m thick intrusion at the base of the Altree 2 well would have significantly elevated temperatures within 800 m of the sill, driving kerogen into the gas window, mobilising trace elements and resetting the Rb-Sr isotopic system in the Kalala Member.
In conclusion, we show that the in situ Rb-Sr dating of the Velkerri Formation combined with common hydrocarbon 520 maturity proxies can help reveal the thermochronological history of Proterozoic argillaceous rocks. When used in tandem, these methods can constrain the age of deposition as well as subsequent secondary, late-stage geological events. Importantly, we demonstrate that this technique can aid sedimentary-hosted resource exploration, as hydrothermal overprints can be identified and dated as previously demonstrated in Subarkah et al. (2021).
Specifically, for hydrocarbon exploration, we show that the thermo-kinetic parameters of shale-hosted Rb-Sr 525 isotopic system in hydrothermal settings can coincide with the maturation of kerogen into the gas window (Dodson, 1973;Espitalié, 1986;Kubler, 1967).