Subarkah et al. have investigated the links between burial history, palaeo-temperature evolution, illite formation age, illite crystallinity and organic matter maturation in Proterozoic shales from the McArthur Basin, Australia, by combining a literature study with novel petrographic analysis, thermal modelling of the effect of a dolerite intrusion and in-situ Rb–Sr dating of authigenic illitic clays via LA-ICP-MS/MS. The authors argue that the illite Rb-Sr system is stable at temperatures around the oil window but tends to be reset at higher temperatures around or above the gas window. The manuscript is well structured and the outcome is of potential interest for the geochemical society. Nonetheless, I have identified numerous (mostly minor) problems and inaccuracies that must be included before the manuscript can be accepted for publication. The present work would greatly benefit, if the authors:

1) can provide vitrinite reflectance data for a direct temperature assessment?

2) could provide in-situ Rb-Sr glauconite ages (arguably the earliest diagenetic product) and compare with burial diagenetic illite formation ages?

3) consider and discuss the possibility of a second pulse of illlite growth associated with the dolerite intrusion rather than a simple reset of the pristine signature of an inherited illite phase (pedogenic or early burial diagenetic), which would explain the decreasing Kübler indices with depth and the different illite morphologies.

4) can identify (if possible) a relation between illite type (pore growth vs. lamellar), host rock lithology (shale vs. sandstone) and illite age (pristine vs. reset vs. newly formed). It looks like the sandstone-associated illite ages are reset (or represent illite neo-formation) while the shale-associated illite ages are true depositional or burial diagenetic ages. Is there a correlation between rock porosity, permeability, fluid transport and illite mineralization vs. resetting events?

5) can provide XRD patterns of illite and clay assemblages) to prove the absence of interstratified I-S in their samples?

Taking all these aspects together, I recommend publication of the manuscript after some moderate revision with the aforementioned points in mind.

Other minor and major issues:

L20: “Hydrothermal” implies interaction of rocks with hot fluids generated from a cooling magma – better change to “thermal sensitivity”.

L22: Worthy to say that the study site is located in Australia…

L27-29: There is something wrong with the sentence structure, please check.

L39: “thermal systems absent of fluids” – this is extremely simplified; fluids are always present in shales given that hydrous phyllosilicates and organic matter are present.

L46-47: Worthy to say that distinctions between detrital and authigenic (plus resetting events) mineral phases must be made.

L56: instruments // techniques

L58: Define “reactive gas”. // a LA-ICP-MS/MS system

L62: I agree, but its worthy to mention that LA spot sizes (typically 50 or 75 µm) limit the area of investigation and that nm or µm sized mineral intergrowths of different origin are problematic, as they give mixed ages that need to be deconvoluted.

L68: Clay minerals present in shales are barely visible with naked eyes, please re-phrase.

L71: …the samples analysed and of the fluids involved.

L72: has been

L78: Most clay mineral reactions take place under far-from-equilibrium conditions, especially in diagenetic settings, where, for instance, smectite matures to illite through mixed-layer I-S. This process can proceed over millions of years depending on T, K content, subsidence rate, fluid composition etc. (see Hower et al., 1976), and is never ever in equilibrium. Also, the term “water-column” is misleading here, as the seawater-derived fluids are always modified during diagenesis; better change to “burial fluids” or “formation waters” (or a similar phrase).

L81: …and erosion of soils and unstable parent rocks”.

L90-98: I agree. However, it is worthy to say that illite maturation in shales via I-S formation is not an event-driven mineral reaction. Indeed, this process takes time (~Myr) and it is, until now, unclear how the Rb-Sr system in illite and I-S is affected during this continuous alteration process.

L99-101: Hard to follow, please simplify.

L102: Which type of clays is involved, illite, chlorite?

L133: The saline Black Sea is characterized by a strong water-column redox stratification, while the Baltic Sea is brackish and mostly oxygenated (except for very deep basin parts, like Landsort deep). Perhaps the modern Black Sea is a better analogue for the Roper Group?

L145: have been deposited? The U-Pb age of a detrital zircon cannot be used to establish a minimum age of a formation, because the zircons can record any age. However, I agree that the intrusion must be younger that the depositional age of the shale, so better say that the Kyalla Formation is somewhat older than 1313 Ma.

L149: Depositional age or diagenetic age, given that we look at authigenic clays?

L171: … which provide important complementary data to supplement this study, such as…

L182: Use of the word “sands”. Isn`t it sandstone or weakly consolidated sandstone, given that metamorphic rocks (shale) are involved?

L188: collated // collected

L190: were // where

L191: at depths of

L195: Adelaide Microscopy – is this correct?

L196: …acceleration voltage. MLA maps were collected

L199: Which chemical criteria were chosen to distinguish between smectite, illiite and I-S, or between different chlorite minerals? How good are the MLA quantifications compared to the bulk XRD data? Note here that Rafiei et al. (2020) report good data comparability for fine sized samples.

L199-205: Were the Sr initials calculated from the isochrons or assumed to reflect Proterozoic seawater?

L214: The GLO age should be somewhat older (~100 Ma), because the host rock is of lower Cenomanian age. Please comment on this.

L229: Is there any mineralogical evidence for the assumed temperature of the sill, such as high-T mineral assemblages?

L247: Tmax results compiled in this study range

L253: The Kübler Index is extremely high for true illite. Provide XRD patterns at EG-solvated state to determine the potential presence of I-S intermediates. If chlorite is present in the samples it is worthy to report the Archai Index as well, and cross-check with the Kübler Index data.

L257: If the temperature is > 120 °C kaolinite will change into dickite, please verify. Also, Mnt is not stable under these burial conditions. The only possibility of having Mnt in the samples is more recent sub-surface weathering (of for example feldspar).

L260: Clinochlore is a Mg-rich chlorite – what is the difference between chlorite and clinochlore here?

L263: XRD is not a destructive method by definition, but samples need to be crushed to obtain fine powders so that fabric information are lost, please revise.

L264: Worthy to say that 1) MLA is often localized on small areas or layers that are not always representative for the bulk rock, 2) MLA assumes ideal mineral compositions and densities for mineral quantification and 3) MLA is based on 2D information. These aspects can make comparison with XRD datasets enigmatic.

L265: bulk XRD and MLA mapping results are summarized…

L271: Why is the age uncertainty so high in case of the shale samples investigated?

Table 1: The correlation between XRD and MLA datasets is OK (good) but far away from being consistent or excellent, as indicated by the authors. For example, kaolinite, montmorillonite and quartz are off by >10 wt.% in many cases, please clarify. Clinochlore is the Mg member of the chlorite family.

L313: Kübler index (KI) is determined by the 001-reflection of illite ….

L316: Vitrinite reflectance data can provide absolute formation temperatures.

L319: such as changes in heating rate

L335: …or possibly due to the presence of I-S in the samples? Provide XRD patterns for confirmation.

L376: formed within the host sediment during burial diagenesis

L377: A minor component of illite also replaces micas and feldspars…

L383: suggesting that the majority of illite formed relatively soon after sediment deposition.

L403: Unclear meaning of “more crystalline morphologies”, re-phrase

L429: …recrystallized the former mineral assemblage or induced a second mineralization of clays.

L431-449: This is largely repetition, delete.

L456: Re-phrase: “which as intersected at present day depth 1696 m.”.

L463: or induced a second mineralization event?

L496: crystalline illite morphologies

Figure 2: Why has TOC a negative value?

Figure 5: Mineral coding is difficult to read, i.e. change mineral colors on the maps.

Figure 6: Can the authors explain the differences in the initial 87Sr/86Sr values among the sample set, i.e. ranging from radiogenic to seawater-type?

Best regards

General comments

The study uses in situ Rb-Sr dating of shale unit fragments and thermal modelling of literature data to constrain the thermochronological evolution with depth of an Australian Proterozoic basin site. It represents an important contribution towards increased understanding of the effects of heating and diffusion from secondary events on isotopic Rb-Sr systematics of basin rock units since few natural studies occur on the matter. The study also highlights the use for coupling knowledge of the thermal evolution in multiple dimensions when interpreting the significance and meaning of geochronological data. The utilized methodology offers a route to achieve that. However, the strength of each specific method and the combination of them is not demonstrated in much detail, as outlined by the following points.

1) The modelling of literature thermometry data with time is considerate and useful with vast amounts of input data, but since secondary mechanisms causing isotopic disturbance also can move laterally, the limit of a one-well model increases the interpretation uncertainty. In the absence of a horizontal modelling dimension, the following considerations should be discussed or clarified regarding the central issue of estimating the boundary conditions isotopic disturbance:

a) How may any lateral variations in the geological setting and the processes and events affecting the rock sequence impact the conclusions drawn on the timing, spatial occurrence and sheer cause of isotopic disturbance given that conclusions are based on a modelled vertical line?

b) Migrating fluids are inferred as cause for isotopic resetting beneath 900m depth. Can these fluids be traced by veins, mineralizations, crystal zonations or else? If so, can direct thermometry or other geochemical characterization be applicable of such? Has this been observed and considered in any previous study of the site? Indifferent of negative or positive answers to these questions, the matter should be mentioned in the manuscript.

2) The in situ Rb-Sr dating is the only new data collected in the study, and the technique is indeed promising and applicable for dating of diverse processes affecting shale units. In order for this analytical campaign to demonstrate the utility of the method, improvements in sample selection, presentation and discussion are due as outlined in the following points:  

a) Given the spot size used and the fine-grained nature of illite, each isochron point represent a mixture of grains that may have stabilized isotopically at different times. The authors mention that each sample is predominantly composed of either unaffected or reset authigenic illite as observed by mineralogy alone. If XRD has been used to identify these in this or previous studies of the site, please provide and explain more explicitly the basis of the illite generation identifications. Is it verified that no mixtures of clay mineral phases or multiple clay growth generations (including detrital) are present in any of the samples? If so, how was this verified? If not, please comment on the implications for the age results and the interpretation of its meaning that multiple generations may exist. Please also clarify reasons for excluding grain size separation and Illite Age Analysis (Pevear, 1999) from the study.

b) Continuing on the illite mixing topic, estimates of the illite homogeneity can also be provided by dissecting isotopic ratios in each LA spot signal in the absence of grain size separation. Please provide a detailed account on how the procedure of analysing spot homogeneity was carried out, on the outcomes and conclusions drawn from the observations, and mention any implications for the age results going from single downspot time frames to the combination of spots in the isochron diagram.

c) Relating to the above points, inclusion of other minerals than illite in the LA signals is mentioned and disregarded as merely quartz in the Supplementary Material and therefore irrelevant for the Rb-Sr contents. Relating to the multimineral mineralogy maps, have the signals and spot locations been checked for mineral occurrence? If so, how was this performed? Were any spots rejected for the sake and if so, for what reasons? It was made sure that no K-bearing minerals interfered down-hole in each spot? Clarifying notes of these procedures and results should be contained at least in the supplementary information.

d) the illite crystal textures and intermineralic textural relationships are qualitatively described without detailed petrographic images or accounts on variations within samples. Can such be added for the specific samples in this study and compared to previous studies describing these features at the site?

e) The in situ Rb-Sr dating sample set consists of five samples over a ca. 800m depth interval. Given the discrepancy of several hundreds of meters (most shallow effect from the sill is interpreted at 600m or 800m) in the different thermal modelling predictions, please comment on how the sample interval larger than 200m below 696m depth affects the interpretation and uncertainty of the results regarding potential isotopic disturbance of fluid migration.

f) The initial Sr values are not anchored to actual data but rather inferred from the isochrones and comes with large error ranges. Since the importance of initial Sr values for tracing crustal fluids and their sources is  indeed stated in the manuscript, have any previous data source been considered for narrowing down on them in the stratigraphic sequence, or may new data collection on this be advisable? Given the spread in initial Sr values and their inference from large-error and low-Rb isochrons, it should be explicitly stated that

g) The isochrons produce age errors ranging up to 300 Ma. Many of them overlap each other and several other dating results in the area, and yet their interpretation and meaning is provided without any note or disclaimer. The age errors and their implications for the conclusions based on the dating should be discussed. In addition, the concluding reasoning of the method as a useful discriminator of geological events in sedimentary units should regard the large age error ranges.

3) The combination of the methods have been shown to provide thermochronological constraints, but since the authors repeatedly emphasize its utility, may it be described what actually makes this particular combination so powerful and how it distinguishes from other thermochronological methodological schemes?

Specific comments

L161 Are there any tectono-thermal perturbation that would be expected to have affected the area, and if so, when and of what type? Any orogeny that may have disturbed radiogenic isotopic system?

L167 Word missing after terminated? Otherwise the sentence does not make sense.

L280, L365 Avoid subjective adjectives such as good, here and on later places in the text.

L382-384 How can such a specific statement be motivated considering the large age errors?

L456-458 Generally, chapter 5-8 contains multiple repetitions which can be slimmed. The sentence starting with “This event..” is one of those that includes statements already appearing repeatedly up to this point in the manuscript.

L488-501 Contains statements repeated from previous sections, but if this has the function of a concluding section it should work.

L493 Ages are in the text not seldom referred as comparative and relative, e.g. here in mentioning “younger ages”. Precise age ranges would have made the text more concise and apt to follow in instances such as this.

Figure text Figure 6. Avoid the use of “better” and possibly the whole last sentence that can be deemed obvious and irrelevant.

Figure 5. The color scheme indeed needs adjustment, too many undistinguishable green colours.

Supplementary Material

L21 What is the last sentence supposed to mean? The signal interval selected in the data reduction procedure? Please clarify, and it is not helpful to put an explanatory word in apostrophes and then not explain it.

L45 Ideal is an interesting word of choice, would not more ideal for in situ spot-based LA dating at least be that individual grains can be targeted?

L56 Expressing that the textures do not look detrital should be replaced with a description and/or a detailed, high magnification petrographic photograph forming the basis of these genetic interpretations, which is also a general remark for the mineral texture descriptions (see comment 2d above).

References

Pevear, D.R., 1999. Illite and hydrocarbon exploration. Proc. Natl. Acad. Sci. USA 96, 3440–3446. http://dx.doi.org/10.1073/pnas.96.7.3440.