We would like to thank the reviewer for his positive, constructive and insightful comments.

Comment: “This is a neat study [...] and the data seem rigorous.”

Response: Thank you.

Comment: “Where this work could use some improvement is in the justification for the method and the exploration of its potential applications. [...] the publication would carry a lot more power if it dealt head on with the fundamental limitations of this type of measurement and presented a more robust set of potential applications to geological studies, if indeed these exist.”

Response: We are happy to add some more detail about potential applications, bearing in mind that our paper is a “Technical note” so its main focus should remain on the technical aspects of the work.

Comment: “Lines 7-8: [...] I suggest that the abstract explicitly state that the problem with current methods is that the required molar concentration calculations introduce an uncertainty that is far higher than the uncertainty of the conventional method, and that this uncertainty is itself difficult to accurately quantify. This line seems like a good place to make a tweak.”

Response: We will replace Lines 7-8 with the alternative formulation suggested by the reviewer.

Comment: “Lines 21-22 I question both parts of this assertion. If the authors disagree, this should be backed up with a lot more discussion.”

Response: Instead of adding “a lot more discussion”, we will replace Lines 21-24:

“In-situ U-Th-He laser microprobe analysis simplifies or removes steps 2–6 of this procedure, which increases sample throughput whilst potentially improving accuracy as well [...]. Furthermore, in-situ analysis of zircon essentially produces U-Pb double-dates as a byproduct, which opens new research opportunities in detrital geochronology.”

with

“In-situ U-Th-He laser microprobe analysis removes steps 2, 3 and 5 of this procedure, which potentially increases sample throughput whilst producing U-Pb double-dates as a byproduct. This opens new research opportunities in detrital geochronology.”

Comment: “Regarding throughput, [...] it requires laborious additional sample preparation procedures, proton irradiation, the use of a magnetic sector noble gas mass spectrometer, and mapping with an LA-ICP-MS [...]”

Response: It is true that in-situ U-Th-He currently does not increase throughput. This is why the revised lines 21-24 will specify that there is a ‘potential’ increase in throughput. We do believe that in-situ zircon U-Th-He dating is more scalable than conventional methods, especially when combined with U-Pb geochronology. We will add a paragraph to the discussion section of the revised manuscript to clarify this point. 

Comment: “Regarding accuracy, this is confusing here, [...] situ methods will not solve this problem. [...] this statement either needs to be hedged a lot more or needs to be backed up with a fair bit of discussion.”

Response: As shown above, any mention of accuracy will be removed from lines 21-22. This hopefully addresses the reviewer's concern.

Comment: “Lines 45-47: I am not sure I understand from this sentence why the requirement that standards be analyzed along with samples poses a serious problem[...]”

Response: We will flip the end of lines 45-47 around so that:

“Using Si as a proxy for pit depth works reasonably well for the samples of Vermeesch et al. (2012), but is imprecise and only works when samples and standards are analysed in the same analytical session, using identical laser settings.”

becomes:

“Using Si as a proxy for pit depth works reasonably well for the samples of Vermeesch et al. (2012), but is imprecise and only works when samples and standards are analysed using identical laser settings in the same analytical session”

This new formulation places the emphasis on the “identical laser settings”. Laser settings have a strong effect on elemental fractionation, which affects the Si-based drill rate proxy. So if a Si-based drill rate proxy is used, then the size, intensity and pulse rate of the laser should be the same for the sample and the standard. This is best done within a single analytical session because laser fluence can vary from day to day.

Comment: “Lines 55-57: Unnecessary. To the extent that any of this is needed, move it to the acknowledgements.”

Response: We admit that these two lines are unusual. However, we do think that they are necessary, so as to clarify why we did not experiment with smaller spot sizes, or why we did not apply the sandwich method to ‘real’ samples.

Comment: “Line 60: Recommend listing them briefly here by way of introduction.”

Response: Agreed.

Comment: “Line 65: This says "at first," implying that the authors changed their minds [...] it needs to be explicitly stated that testing this hypothesis led to the second experimental design.”

Response: Agreed.

Comment: “Line 70: [...] It is not clear to me why this procedure is expected to reduce or eliminate the effects of compositional zoning on the dispersion of the results [...]”

Response: Drilling along the c-axis reduces the effect of compositional zoning because (oscillatory) zoning tends to be concentric around the c-axis. This is clearly shown in Figure 3. The revised manuscript will explicitly use the word “concentric” to clarify this point.

Comment: “Line 105: Please provide some information about the actual signal sizes and concentrations being measured. Also, should this ratio (repeated throughout) be mV/Hz?”

Response: The actual signal size of the 3He beam (in Hz) is already included in Tables 1 and 2. Combining these values with the 4He/3He ratios allows the reader to infer the 4He signals. The reviewer is absolutely correct about the units of the isotopic ratio measurements. We apologise for this sloppy mistake.

Comment: “Lines 139-141: Given the centrality of this argument, I think it would be worthwhile to show the difference in the accuracy of the ages produced each way.”

Response: We will add the uncorrected ages as points to Figure 4d.

Comment: “Figure 4: If you are going to stick to "stage coordinates," please at least include some indication of the actual spatial dimensions of these figures”

Response: The stage coordinates are in microns. We will change the aspect ratio of panels a, b and c to unity and add a 1mm scale bar.

Comment: “Lines 151-152: It seems like you could do the math already using any number of typical ~300 micrometer-long zoned crystals from the literature, and it seems pretty clear that you would be in trouble.”

Response: Not necessarily. It would depend on the geometry of the zoning, and on whether the grains have been mounted vertically or not.

We will make all the technical corrections mentioned at the end of the review.

Comment: I do not have much to add except that section 2.1, which contains the results, comes before "Analytical methods and data processing" and before the "Results" section. This is not logical and you should consider grouping the results after the method description. 

Response: We understand the Associate Editor’s concern. It is conventional for scientific papers to make a clean separation between methods and data. However, such a clean separation is not possible in the manuscript under consideration. The problem is that the method discussed in Section 2.3 is motivated by the data discussed in Section 2.2, and the method discussed in Section 2.2 was motivated by the data provided in Section 2.1. The main focus of our paper is on method 3, whose description is cleanly separated from its data. We would request permission to retain the results in Section 2.1. However, we will add some sentences to the beginning of Section 2 to clarify the unconventional setup of the methods section.

Comment: The results plotted in Figure 2 should be presented in a table (there is no reference to a data table in the text).

Response: Agreed. We will add a table with the data for Figure 2 to the paper, as an online supplement.