Dear Veronica Peverelli,

Thank you for your detailed response to the comments by both reviewers. I believe the changes that you are proposing will address these comments adequately. In particular, more clarity regarding the reference materials (as argued for by reviewer 1) and the treatment of downhole fractionation (as raised by reviewer 2) will require careful attention.

In addition to the points already made by the reviewers, or echoing some of them, I have some additional specific questions and requests:

Line 80: “If the isotopic composition of initial Pb is unknown, 204Pb contents cannot be precisely determined...” The phrasing of this sentence is confusing (it can be read as if knowing initial – or common – Pb is a prerequisite to precisely measuring 204Pb). I therefore suggest: “This, however, is not always possible, either because the isotopic composition of initial Pb is unknown or 204Pb contents cannot be precisely determined (e.g. because of the analytical technique employed). Hence, if no other dating method is viable (e.g. too-low Th contents hampering Th–Pb dating), the best solution for U–Pb dating of low-μ phases is using a regression in a Tera–Wasserburg diagram (Tera and Wasserburg, 1972) that plots measured 207Pb/206Pb versus 238U/206Pb ratios.

Line 93: minerals

Line 97: second “may” not needed

Line 272: Where does n = 15 come from? This would need a citation of a justification.

Line 276: Omit the first “as possible”

Line 297: wrap = mantle (?)

Fig. 4: Legend needs to be improved. This should mention what individual lines are.

Line 372: present = stated

Fig. 6: Error envelopes should also be plotted here (see Fig. 7)

Line 389/397: poor/poorly appears to be judgmental; more neutral/descriptive wording would be appropriate. Use: “nearly invariable” or words like “limited” or “minor”.

Line 424: Same here with “meager”

Fig. 7: In agreement with reviewer 2, I wonder how the error envelope was calculated, and why the commonly used scheme of Ludwig (1980) was not used? (Ludwig, K. R., 1980, Calculation of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, 46-2, 212-220).

Line 493: high-enough = sufficient

Regards,

Axel Schmitt (Heidelberg University)

Dear Veronica Peverelli,

Thank you for the careful revisions that took care of all major comments and criticisms raised by the reviewers. A few minor technical issues remain that are listed below and require correction prior to ultimate acceptance.

Looking forward to your paper being published; I find it a very useful contribution.

Best regards,

Axel Schmitt

184 Difficult to understand; I would rephrase as: ...distance of ca. 100 m from each other, close to the “F100” and ”BK cavern” locations in Fig. 1.2 of Schneeberger et al. (2019), respectively.

206 (and 718) there are different spellings for the methods, but it should end with analysis or analyzer

239 planned = placed (?)

240 solution ICP-MS

475 each = constant

475 should not be combined and should be treated separately = one of these seems
redundant; better use positive phrasing: should be treated separately

582 lands = lends

719 KV = kV

Dear Veronica Peverelli,
Thank you for your revised manuscript. I support the technical corrections requested by the Associate Editor prior to ultimate acceptance.