Comment on gchron-2021-41

Metamorphic rocks are an integral product of a series of complex atomic to orogen scale processes that operate over geological times sequentially and parallel at variable but mostly unknown physico-chemical and thermodynamic conditions. Deciphering the timing of these processes (i.e. discrete and geologically significant age signals) from them is a highly challenging task in geochronology. The manuscript by its title intends to contribute to this issue and takes efforts to demonstrate that step heating of low-grade metamorphic rocks (and in part mineral fractions) can provide valuable information on the age of detrital components as well as on the timing of the low-grade metamorphic processes.

documentation and presentation, which are insufficient. The data further miss a thorough and careful in-depth evaluation (inverse isochron ages and intercepts of sub-datasets used to derive ages, evaluation of contributions from Ca-, K-and Cl-derived Ar-isotopes), which in particular is required given the uncommon approach of doing step-heating experiments on whole rock low-grade metamorphic rocks that contain high amounts of chlorite. Such complex samples require a more critical and sophisticated data evaluation and discussion.
The discussion is weak, not very well written, and far away from being acceptable for an international high-quality journal. It mostly remains vague, is often hard to follow, not to the point, full of unsupported and confusing statements, and contains passages that are rather empty phrases than robust scientific arguments (example: 'Complex age spectra and multiple gas domains are present in our samples from the low metamorphic grade rocks in the Delamerian Orogen. The presence of upper age asymptotes that also have distinct isotopic compositions, is consistent with those ages being from distinct gas reservoirs. Taken together, the complex age spectra in this study clearly represent the mixing between older detrital components and younger age components).
The study hampers another fundamental but rather common problem, namely that the geologists among us often desire to have age information from rocks that these rocks and the available methods can not provide in an easy, fast, cheap and simple way due to fundamental physical limitations. Working on such (mineralogically) complex rocks with such a complex history (by far too many degrees of freedom) requires tremendous methodical efforts and an in-depth evaluation of the gained data by considering physicochemical principles. Just plotting 'easy to read' age spectra plots and deriving obvious age information is not sufficient.
In summary, the manuscript needs substantial and very comprehensive revisions. A more comprehensive and systematic evaluation and presentation of all data as well as a clear and concise discussion of the results is required, along with a much better consideration of existing (methodical) literature. Doing so, and comparing the results with existing literature data, there might be a valuable scientific outcome with some importance for the community working on the evolution of such rocks and the controlling geological processes.

Chapter related comments
The abstract is in parts too vague. For example, low grade rocks also contain detrital minerals that have been overgrown during metamorphism. Then it becomes not clear that whole-rocks have been analyzed, as likely most readers will assume that a common approach has been applied where mineral separation precedes dating.
Complex age spectra are interpreted as consisting of several age components. This is misleading, as it implies that discrete and real age events are present. An 'age component', however, might simply result from mixing of gas fractions from different (disturbed) grains with different ages, size and/or Ar release properties, and thus is not an age but an artifact. Would suggest to term them 'virtual age components'. This then might question the subsequent interpretation, where neither the problem of mixing of different gas fractions, nor the potential presence of excess argon or argon loss during reheating or recrystallization have been obviously addressed. This needs to be done.
Line 12. 'Low metamorphic grade rocks' need to be 'Low grade metamorphic rocks'.
The introduction is too short and misses a brief review of methods and case studies that have addressed dating of low-grade metamorphic rocks. Such a brief review should follow the first paragraph, which ends with a statement that it is difficult to separate different age signals. This is true, but several approaches likely exist and these should be briefly mentioned (K-Ar dating of shales, clay-and siltstones, etc.; Ar-Ar dating approaches such as in-situ, single-grain total fusion, etc.).
Line 29-30. Sentence does not make sense.
Line 29-30. This sentence seems somehow to end strangely.
Line 40. … provide information on the age of possible ….
Line 43. What do you mean by in-situ laser microprobe methods? Laser ablation in-situ Ar-Ar dating? Then this should be outlined more clearly.
Line 46. 'Age populations' seems being misleading. Such commonly result from a large number of samples with a single age, or from single-grain total fusion experiments on a single or of multiple samples, and I doubt that sufficient ages can be derived from five whole-rock samples by a step heating approach to resolve statistically significant age populations.
The geological setting chapter should be shortened by at least 15-30%, it seems a bit too comprehensive given that the goal of the paper is mostly a methodical one. Reduce the text to what is needed to understand where the samples are from and how they evolved. Rock types should be mentioned from time to time instead of only unit names (line 40-41).
Line 66. Is there a word missing after metamorphism?
Line 73. Do you mean a thermal laser? I.e. laser induced step heating?
The sample descriptions chapter is not straightforward to read, as it is organized in form of a sample by sample description (and in part contains useless information: e.g. line 130: 14 km east-north-east of Adeleide). The chapter should be condensed by summarizing the major features present in all samples (including the constituting minerals as these are mostly the same, just note if some untypical features/minerals are present) and by outsourcing information in a table (as in part has been already done in Table 1, why describing places in the text when coordinates are given?). The images in Fig. 4 are not very helpful, thin-section images would be much better. Either replace the field images by thin-section images, or complement them by the latter.
As the sample description chapter, the Ar-Ar results chapter describes the data and/or plots in a quite monotonous manner sample by sample. As said already, this is rather annoying to read. The authors should reorganize it, and instead should work out the general features and describe them once, and then, supported by examples, should describe the important and significant aspects of the data.
All ages that were derived from the age spectrum plots and which are assumed to be geologically meaningful need to be reported in a table along with all associated data (errors, 40Ar/36Ar intercepts, MSWD, etc.; see Schaen et al., 2020;Renne et al., 2009).
The term 'York plot' is fully uncommon. Either use inverse isochron plot or isotope ratio plot, as common in the Ar-Ar community. Ironically, the original York publications in 1968/1969 deal with linear best fit isochrons and correlated errors, a procedure that is unfortunately not applied in this manuscript, which needs to be done in a revised version.
Use the term 'virtual age' instead of 'age', as it remains finally unclear whether the derived 'ages' are of geological significance or represent averages of different events with different ages. . What do you mean by a more or a less retentive domain? How does the physicochemical process of retention (i.e. diffusion) affect an isotope ratio? Not significantly to my opinion if we ignore diffusive isotope fractionation. You may tap different domains with different isotope abundances within a mineral, or different minerals at different temperatures, but just using the term 'retention' is insufficient to cover the complexity of degassing a multi-phase system. Line 201. The term asymptote needs to be defined. What is the 'concept of asymptotes' as mentioned already in the abstract (line 17-18)? What you mean is simply an upper or lower age limit. So the term asymptote is misleading here as in a strict mathematical sense an asymptote is a limit to which a variable converges.
Line 209. What do you mean by the statement '36Ar/40Ar values have no atmospheric component'? This sentence is fully meaningless, if not wrong. Any gas fraction analyzed during a step heating experiment that provides detectable 36Ar potentially can contain a fraction of atmospheric argon (i.e. an atmospheric component). In principle, any datapoint in an inverse isochron diagram that does not plot on the x-or y-axis can be interpreted as representing a three-component gas mixture between atmospheric argon (plots on the yaxis at 0.0033), radiogenic argon (plots on the x-axis at its 'true' age 39Ar/40Ar) and a third component ('initial' Ar) that plots on any point on the y-axis except at 0.0033. Data scatter then simply reflects changing mixing proportions between these three 'components' during step heating.
Line 212. What do you mean by 'two domains'? Does not make sense to me.
The Discussion is not appropriate to an international journal. There is no but some vague geological outcome from this discussion. Although the overall approach of this study (chloritized whole-rocks) in principle is questionable, it might be worth doing such an investigation in an effort to circumvent the problem of recoil loss of 39Ar in fine grained mineral separates during irradiation. The way it has been done here, however, is insufficient as the data have not been evaluated in depth.
Line 239-242. Why just different mineralogy? Mixing of the same mineral type with different ages will also provide complex age spectra. Please clarify.
Line 243. The term crystallization in this context is misleading. A low grade metamorphic rock can't be described by a discrete 'crystallization age'. Be more clear what the maximum could be (an old detrital component).
Line 244-245. What is a geological resetting event in this context? Be more clear. And note that a 'less retentive domain' is a physical representation of a thermal 'resetting' event. So using 'or' in this context is misleading.
Line 254-255. What is the 'main mineral gas reservoir'? What you mean is the other endmember, the purely radiogenic component (see above). Be more clear. Why should this mixing be the result of 'deformation-induced recrystallization'? This is a fully wrong statement, as this mixing is simply due to different argon 'components' having different diffusivities (or retentivities), or different bonding energies, or whatever we want to term it. The presence of atmospheric argon and radiogenic argon has not necessarily to do with 'deformation'. The latter may trigger partial argon loss (and also uptake of argon), but this is another issue. In an inverse isochron plot, any Ar loss and any excess component force the data deviating from plotting along a straight line. Line 260. The statement 'The presence of upper age asymptotes that also have distinct isotopic compositions' is circular. The whole sentence is not understandable.
Line 285-287. Hard to understand what is meant here.
Line 289. State from where the deposition age is known.
Line 292. What are similar ages? How similar? Provide data please.
Line 303. What is the depositional age and from where is it known?
Line 317-324. These setting inferences from very questionable 'ages' derived solely from age spectra plots obtained on very complex samples are very daring.
Line 353-354. Or simply a stronger resetting of smaller grains. I do not like the term 'retentivity domains'. It makes things mystic in some way, and obscures that we were dealing with a complex mixture of crystalline material. What do we physically have? We have rock fragments. Each fragment consists of a number of different mineral types, where each type has its own grain size distribution (affecting diffusivity, or retentivity). Each mineral, in the simplest case, may have its own age, but may be compromised by either Ar loss or the uptake of excess argon (or both), possibly at different times (multiple events). Clearly, this complex material and history will provide complex age spectra, but interpreting them solely by considering 'mystic' domains does definitely not do justice to the complexity of the problem. Therefore, data interpretation needs to be done in the light of what is physically present.
Line 349-350. This is completely at odds. If 'ages' out of a simple sample are younger than other ages from other samples, how can this be indicative for 'a complex history of mica growth and cooling'? Such a statement is fully unsupported.
Line 372. … low metamorphic grade … Tables   Table 1. First line: 515 Ma: Provide the exact datum including error (line 149) as in the second line. Table 2 provides too little information. The power of Ar-Ar is the fact that a dataset provides internal control on consistency. Just reporting ages and errors is insufficient.   3 should be deleted. It provides nothing necessary to support the goal of this study, and contains information that in part can be found in the text (and is mostly speculative). Three large images with geological maps and profiles in a paper with a methodical goal are too much. Fig. 5. The way the data is presented is mostly insufficient. First, the y-axis scale in some of the age spectra plots is too large, making it difficult to resolve individual steps and differences between them. Adopt this even at the expense of having the same scale in any of these plots. In addition, provide in the figure caption or in the figures what initial 40Ar/36Ar has been used to correct the isotope composition of each step for initial argon in the procedure of calculating individual ages for the age spectrum plots (I guess that an atmospheric value has been used, but which one?). Second, if an age is resolved from an age spectrum, an inverse isochron through the respective steps needs to be plotted along with the corresponding inverse isochron age and inverse y-axis intercept value (i.e. the 40Ar/36Ar of the initial argon). If the latter deviates from the atmospheric value, this needs to be considered in age spectrum age-value calculation and the potential causes need to be considered (except in cases where the correction can be neglected due to near-100% 40Ar*-values). Denote which steps in an age spectrum have been used to calculate which virtual age. And mark the temperature steps with numbers in both the age spectra as well as the inverse isochron plots so that one can see which step belongs to which in each plot. Mark the position of each of the derived 'ages' from the age spectrum at its corresponding x-axis position in the inverse isochron plot. And include the datapoint errors in the inverse isochron plots.

Figures and figure captions
Most of the upper 'age values' in the age spectra plots are hardly of any geological significance. For example, in Fig. 5c the age of 371.1 +/-7.8 Ma. From which steps has it been derived? There is hardly anything that looks like a plateau. Do you really believe this age is of any geological meaning? Hard to believe, the steps are more or less continuously increasing. And as far as I can see, hardly any (sub-)dataset in the inverse isochron plots will provide an atmospheric intercept. Something as for example in Fig. 5d needs to be interpreted, to my opinion the plot suggests Ar-loss for the very low temperature steps, and lot's of excess Ar in the steps that deviate to the lower left side (same for Fig. 5h). Fig. 6. The whole explanation is insufficient and not comprehensible. What, for example, is meant by 'Model age spectrum of a simple age spectrum with two component system, mixing between more and less retentive argon gas reservoir.'?