Carbonate U-Pb Geochronology and Clumped Isotope Constraints on the Origin of Hydrothermal Dolomites: A Case Study in the Middle Permian Qixia Formation, Sichuan Basin, South China
and Liyu Zhang 1,2
Round 1
Reviewer 1 Report
Reservoirs in the dolomites of the Middle Permian Qixia Formation in the Sichuan Basin are currently key oil-gas exploration objects in China. However, the questions concerning the sources for the dolomitized fluids and the control factors of the Qixia hydrothermal dolomites remain unclear.
The authors think over the carbonate U–Pb geochronology and clumped isotope constraints on the origin of hydrothermal dolomites, it is wise way to solve the question.
So after minor correction, it can be accepted.
(1) In the title, the Foramtion should be capitalized.
(2) Remove the letter a,b box in Figure 1.
(3) What is the differences in spatial distribution between porphyritic and saddle-shaped dolomites?
(4) Why not refer to the indication significance of rare earth elements?
Author Response
For Reviewer 1:
Comment 1: “In the title, the Foramtion should be capitalized.”
Answer:
In the revised manuscript, the Formation was capitalized.
Comment 2: “Remove the letter a,b box in Figure 1.”
Answer:
In the revised manuscript, the letter a and b boxes in Figure 1 were removed.
Comment 3: “What is the differences in spatial distribution between porphyritic and saddle-shaped dolomites?”
Answer:
Porphyritic dolomites (i.e., zebra-like hydrothermal dolomites) are mainly distributed in the lower part of the Qixia Formation, while saddle-shaped dolomites are widely distributed in the whole formation.
Thus, we added some explanations in the revised manuscript: “In general, abundant zebra-like hydrothermal dolomites that are distributed along deep and large faults can be found in the lower part of the formation, while saddle dolomites are widely distributed throughout the formation.” (See Line 131-134)
Comment 4: “Why not refer to the indication significance of rare earth elements?”
Answer:
Thank you for your reminder. Thus, we added some explanations in the revised manuscript: “In both studied sections, REEs show similarities with modern sea water composition (low REEs deficiency), whereas the 87Sr/86Sr values are comparable to or slightly higher than Middle Permian Sea water values [46]. Considering that REEs and strontium isotopes in carbonates have good inheritance, this indicates that the late hydrothermal fluids in both sections are mainly derived from a marine rather than a magmatic origin.” (See Line 497-502)
Reviewer 2 Report
Comments for author File: 
Comments.pdf
Author Response
For Reviewer 2:
Comment 1:
“Probably it was an oversight but it is strictly necessary that you attach the U-Pb data tables as supplementary material available online, also accompanied by the analyses carried out on the standards.”
Answer:
Thank you for your reminder. We attached the Table S1 (U-Pb dating data) as supplementary material. This supplementary table contains the sample and standard data.
In the revised manuscript, we added: “The complete isotope data sets are presented in Table S1.” (See Line 268)
Comments 2 and 3:
“Section 3: Sample and methods: The indication of the percentage of U-Pb discordance and the considered cut off used for the age filtering are recommended. Please, add it in the paragraph.”
“Section 4.2: In-situ U-Pb ages: In the U-Pb data table (the %U-Pb discordance and U-Pb data for the standards must be indicated following the recent paper published in Minerals and Terra Nova (see supplementary material files)”
Answer:
Thank you for these suggestions. The %U-Pb discordance and U-Pb data for the standards were added in Table 1 and the supplementary material (Table S1).
The percentage of U-Pb discordance are commonly used for zircon geochronology, such as the recent paper published in Minerals (Fornelli, A., et al. (2020). Minerals) you mentioned. However, the %U-Pb discordance and the considered cut off are not very suitable for carbonate geochronology. Hansman et al. (2018, Geology) argued that LA-ICP-MS U-Pb dating of carbonates is not fraught with closure temperature issues. Identifying the presence of open-system behavior within a given dataset is mainly through the isotopic data themselves (Roberts et al., 2020, Geochronology). The mean square of weighted deviates (MSWD) value is more applicable to the evaluation of data concordance (Su et al., 2022, Precambrian Research), as discussed in the manuscript: “Closed U-Pb isotopic systemics are the most important prerequisite for determining the validity of LA-ICP-MS U-Pb absolute isotopic ages. Carbonate rocks may suffer from potential post-sedimentary superposition during burial diagenesis or thermal events, which may interfere with or reset the U and Pb isotopic signal [35,36]. Here, the presence or absence of open system behavior in each dataset is evaluated mainly through the iso-topic data itself. First, if multiple diagenetic fluid sources are present, the mixed common Pb in dating dolomite cements will limit the derivation of a single age regression line. At the same time, an obviously open system behavior may lead to more scattered U-Pb data and higher mean square of weighted deviates (MSWD) values. Secondly, in the case of diagenetic superposition or fluid interaction with the original carbonate rocks, the Y-intercept (initial 207Pb/206Pb) of the mixing line of the two diagenetic fluid sources is usually lower than 0.8~0.9 because of the slope change in the regression two-component mixing line [37]. In this study, both U-Pb isotope data points defined a good single isochron with no significant dispersion and a low MSWD (2.4 and/ or 3.4). In addition, the initial 207Pb/206Pb in P-1-34 and P-1-42 were both higher than 0.8, and the overall isochron-line convergence was good. We thus conclude that the U-Pb isotopic systematics of P-1-34 and P-1-42 did not experience obvious post-deposition alteration and significant influence by mixed fluid. The obtained ages of 257.9±4.9 Ma and 251.0±5.5 Ma are thus interpreted as representing the absolute age of SD deposition.” (See Line 372-390)
But then, the %U-Pb discordance can provide a reference for further studies. Consequently, based on the calculation method in zircon geochronology, the %U-Pb discordance were added in Table S1. It should be noted that we have ignored the discordance filter, and so have previous studies (Roberts et al., 2018, Geochemistry, Geophysics, Geosystems). In the revised manuscript, we added: “The %U-Pb discordance was indicated following Roberts et al. [31].” (See Line 189-190)
Reviewer 3 Report
The authors present new O, C, U, and Pb isotope data in order to better understand the history and dolomitization of carbonate rocks in the Sichuan Basin in south China. The U and Pb isotope data are used to obtain discordia intercept ages for the growth of dolomite. O isotope data are obtained from dolomite are used to interpret that the fluids responsible for the dolomite are dominantly seawater; however, the anomalously low d18O values are explained to result from alteration at temperatures considerably higher than average diagenesis. D47 clumped isotope analysis is used to estimate a wide range of temperatures which are interpreted as those for hydrothermal alteration of the carbonate rocks to produce dolomite.
Overall, I give the manuscript a high rating for nature of data and interpretation.
However, there are several areas of the manuscript that could be improved.
For example, although the clumped isotopes provide T estimates and the authors indicate that the O isotopes require high diagenetic temperatures, they do not directly link these ideas and calculate the O isotope composition compatible with the estimated temperature.
All of the isotope sampling is from what the authors define as saddle dolomite; however, in spite of this restriction to a single textural occurrence there are wide ranges in temperature estimates and significant ranges is O isotope values.
The complete isotope data sets [e.g., all spot U-Pb] should be published as an appendix.
I have provided minor comments on the manuscript.
The manuscript would benefit from modest rewriting to improve the English and clarify the interpretations.
Author Response
For Reviewer 3:
Comment 1: “For example, although the clumped isotopes provide T estimates and the authors indicate that the O isotopes require high diagenetic temperatures, they do not directly link these ideas and calculate the O isotope composition compatible with the estimated temperature.”
Answer:
We are sorry that it is not clearly stated before. In the revised manuscript, we added: “This relationship can be expressed by an equation (103lnαdolomite-water = 3.14×106/T2-3.14) [33], and the O isotope composition (δ18OVSMOW-fluid) compatible with the estimated temperature is presented in Figure 8.” (See Line 518-521)
We also emphasized that: “… the calculated δ18OVSMOW-fluid values of diagenetic fluids (-2.92±0.60‰ to 0.81±0.74‰; Figure 8) in PR1 well are similar to those of coeval seawater (ca. -1.0 ‰) [48,49], thus indicating a marine origin. In contrast, the calculated δ18OVSMOW-fluid values for the Baoxing section are relatively high (1.99±0.52‰ to 7.02±0.56‰; Figure 8), indicating that the marine derived fluids at this locality were more salty and therefore more enriched in 18O. …” (See Line 529-534)
Furthermore, we added the trend of “high diagenetic temperature” in the Figure 8.
Comment 2: “All of the isotope sampling is from what the authors define as saddle dolomite; however, in spite of this restriction to a single textural occurrence there are wide ranges in temperature estimates and significant ranges is O isotope values.”
Answer:
We propose that the available geochemical evidence can only explain the main fluid source, but can not accurately determine the source. Therefore, wide ranges in temperature estimates and O isotope values could indicate a more complex fluid source. In the revised manuscript, we added: “Besides, the potential impact of various or different stages of fluids could lead to wide ranges in temperature estimates and O isotope values mentioned above.” (See Line 540-542)
Comment 3: “The complete isotope data sets [e.g., all spot U-Pb] should be published as an appendix.”
Answer:
Thank you for your reminder. We attached the Table S1 (U-Pb dating data) as supplementary material. This supplementary table contains the sample and standard data. In the revised manuscript, we added: “The complete isotope data sets are presented in Table S1.” (See Line 268)
Comment 4: “The manuscript would benefit from modest rewriting to improve the English and clarify the interpretations.”
Answer:
The English language in the revised manuscript has been polished and improved by a native English speaker.
