|The authors have streamlined their manuscript, and greatly improved it thereby. I also like the addition of the AFTINV modeling, as a demonstration that multiple approaches can extract the information contained in multikinetic AFT data. |
There remain a few things I’d say differently, which I go through below, but one issue on which the authors chose to make no changes continues to concern me. In my previous review, I claimed that the AFT data within kinetic populations are under-dispersed, and I gather I did not get my full meaning across. Even assuming everything is “ideal” with regard to measurement, kinetics, theory and calibration, etc., some degree of dispersion is unavoidable, due in this case to track counting statistics. Every single synthesized grain has an age well within one standard deviation of the true age, according to their individual uncertainties. That’s just not enough dispersion, mathematically speaking – 32% of grains should be more than one standard deviation away, assuming uncertainties are correctly estimated (or created). I’m guessing that the authors dispersed the single-grain ages based on the uncertainty of the central or pooled age (and even underdid that a little bit), not the uncertainties based on the single-grain track counts.
However, in the end, the issue may be mostly cosmetic, as the central age and its uncertainty are not sensitive to under-dispersion, and so it probably doesn’t affect their model fits (though I’m not 100% sure about QTQt). Still, it would still be good to show “data” with physically/statistically realistic scatter. For example, on a radial plot, ages from a single population should have a vertical distribution spanning the height of the +/- 2-sigma axis on the left. Correcting this would strengthen their case concerning what multi-kinetic AFT data are capable of.
[line 33] “however” used like this creates a run-on sentence.
[line 34] Consider inserting “or partially reheated” after “slowly cooled”
[line 84] Change “width” to “length” or “diameter parallel to the c axis”. With an anisotropic feature, “width” implies the short dimension.
[line 98] The tone of the first phrase in the paragraph reads as if you are carrying forward the argument from the peer review process, which is out of place here in text that should stand alone. It is bad form to start creating excuses for something before you even introduce it. Better just to state what you are doing: “We use an ideal synthetic sample…”
[line 100] Delete “We emphasize that”
[lines 103-105] Sentence is problematic – if information is “retained” it should be recoverable. I suggest changing “AFT samples” to “AFT data”, and deleting everything after that.
[line 111] Delete “those”
[line 129] Similar comment to line 98 concerning tone. Suggest just deleting sentence up to “many modern”.
[line 226] Should be “p(X2)”
[line 402-403] I wouldn't say enforcing the constraints is “detrimental” -- they just change one meaningless outcome into another. Heck, they at least make the models correct with respect to the geological constraints... It would be more accurate to say that the constraints do not provide a remedy if the kinetics are ignored.
[line 424-430] I’m not sure if this is the best fight to pick – Green and Duddy made their business by claiming to be the only ones doing multi-kinetics correctly (although I believe their only-Cl approach is flawed), so they are at least familiar with what can be done in practice. Their statement may be based on the experience they have from dealing with real data (interpreted with imperfect, over-simplified kinetics – hee hee) which usually don’t have the resolving power of the perfectly-behaved synthetic dataset and carefully selected thermal history employed here.
[line 457 – Figure 5 caption] Not sure I agree with the magenta line for population 3, insofar as the model seems to demonstrate that the data can be fit perfectly well without crossing into the demarked "sensitive" portion. In fact, I’d put the part of the history that population 3 is most sensitive to where the authors put the zone for population 2, since that’s what made the track lengths a little shorter. Plus, it's dicey to suggest that population 3 is sensitive to the part of the history above 200°C. The attempted implication that the different kinetic populations are sensitive to distinct, separate parts of the history is overdoing it, I think. A better approach might be to estimate a PAZ for each population, and plot that across the whole history for each with a couple of dashed lines, which would show not only the temperatures it will take to “matter” for each, but also their overlap.
[line 481-483] The authors are getting a little in the weeds when they discuss the tweaks they had to do to get AFTINV to converge, especially since there is no geological reason identified to impose those heating/cooling rate constraints – the only reason they used them here is because they knew the answer they were looking for, a dangerous example to set. Essentially, restricting the heating/cooling rates so much is another form of geologic constraint, and one that’s probably harder to base on real evidence than the sample being near the surface during some particular time interval (i.e., a box). Also, the excursion to low T (<0°C?) at ~600 Ma doesn’t look to be required by the data, so I’m wondering what it’s doing there; some other model-imposed constraint?
[line 488] In general, I never expect mean or weighted mean paths to fit; I’m not sure it’s really worth discussing.
[line 496-497] It might be of some concern if their model really suggests that population 3 tracks are retained from >290°C, depending on maximum cooling rate allowed. It looks to be maybe 1°C/m.y. initially, which implies a total annealing temperature of about ~237°C if rmr0=0. This may be due to HeFTy assuming a different cutoff for when a population becomes undetectable; it’s not clear if their 2 µm refers to the shortest track length of a retained population; if the latter, that might be the area of disagreement. Maybe just lose the final sentence?
[line 512] Maybe it would be better to call the conditions "favorable" instead of “ideal”; conditions are never ideal.