Unraveling Parent Rock and Mineral Influences in Tropical Weathering Profiles: REE, Nd and Sr Isotopic Geochemistry

Round 1

Reviewer 1 Report

Please find below my review for the submitted manuscript. Several points need to be addressed to make the manuscript suitable for publication.

Section 2: Material and Methods

How did the authors calibrate the quartz abundance using the 101 calibration intensity? Was a standard utilized in this process? It's crucial to note that basing the calculation of illite content on MgO might introduce variability, especially considering the low bulk MgO content. This variability can consequently lead to significant uncertainties in determining other mineral abundances. I recommend considering iterative least square fitting techniques, such as employing the lsq_linear function in Python's scipy.optimize module, to compare the results. Additionally, it's essential to account for the H2O content of goethite in the calculations. Why was Rietveld refinement not performed to estimate mineral abundances, or at least verify the calculations?

Section 3.1: Profile Structure and Mineral Features

The parent rock, as depicted in Figures 2A and 3, appears to be a result of weathering (saprolite), evident from visual inspection and the presence of kalinite, goethite, and hematite. However, the origin of the interdigitated yellow, pink, and red zones in the saprolite remains unclear. Could these zones formed due to inherited heterogeneities in the parent rock, or element segregation (e.g. variations in weathering intensity, differences in pH-eH conditions)? Furthermore, if the authors assert that the ferruginous duricrust is residual, it should be referred to as lateritic residuum, contrasting with ferricrete, which relates to ferruginized transported material.

Section 3.2: Geochemistry

In Figure 6, it would be beneficial to include ratios such as Ti/Zr to better evaluate the residual versus transported portions of the profiles. Although this information is presented in Figure 12, it would enhance the clarity of the text if it were introduced earlier. Additionally, there is a typo in the figure numbering (7A instead of 1A) on line 272. The substantial amounts of kaolinite, gibbsite, and iron oxides in the upper lateritic duricrust of profile 2 are surprising, given the sandstone parent rock, which is predominantly composed of SiO2. This suggests extensive leaching of silica. Conducting mass balance calculations based on Zr immobility could assess the intensity of silica weathering and verify if Fe and Al enrichments are residual only.

What is the reason for phosphorus enrichment in the duricrust of both profiles? Phosphorus is typically considered a mobile element unless it is bound in minerals like monazite, which does not seem to be the case here.

General Comments:

The paper remains largely descriptive and general. The conclusions need to be more explicitly stated. Moreover, the existing literature, particularly works such as Innocent et al. (1997), Coimbra Horbe et al. (2022), and Wei et al. (2018), should be better integrated. These references can provide valuable insights, especially concerning isotopic evidence, REE investigations, and weathering processes in tropical laterites, enriching the discussion and analysis within the paper.

Innocent et al. ( 1997) Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Parana, Brazil. Chemical Geology 136 2 19-232

Coimbra Horbe et al. (2022) Nd and Sr Isotopes and REE Investigation in Tropical Weathering Profiles of Amazon Region

Wei X, Wang S, Ji H, Shi Z (2018) Strontium isotopes reveal weathering processes in lateritic covers in southern China with implications for paleogeographic reconstructions. PLoS ONE 13 (1)

The quality of English in the manuscript needs improvement.

Author Response

Dear Revisor

Thank you very much for the comments along the manuscript and sorry by our obvious mistakes in the numbers of figures and tables and in the spelled words. A very careful revision was done.

Answer to revisor comments.

Section 2: Material and Methods

How did the authors calibrate the quartz abundance using the 101 calibration intensity? Was a standard utilized in this process? It's crucial to note that basing the calculation of illite content on MgO might introduce variability, especially considering the low bulk MgO content. This variability can consequently lead to significant uncertainties in determining other mineral abundances. I recommend considering iterative least square fitting techniques, such as employing the lsq_linear function in Python's scipy.optimize module, to compare the results. Additionally, it's essential to account for the H2O content of goethite in the calculations. Why was Rietveld refinement not performed to estimate mineral abundances, or at least verify the calculations?

response: The minerals calculations always have some uncertainty. However, Negrão et al. (2018) indicate that Rietveld calculation and stoichiometric calculation have good agreement. They also point that the reflection overlap influence the Rietveld calculation. Thus, since stoichiometric calculation is an easy and quickly method, we decide used it. We also consider that some minerals uncertainties, and the low content of for example MgO, do not compromise the interpretation of the data. We calibrated the quartz content comparing the d =3.3 (101) reflection intensity obtained by XRD relative to the other minerals in the sample.

Negrão, L. B. A., Costa, M. L., Pöllmann, H. Horn, A. 2018. An application of the Rietveld refinement method to the mineralogy of a bauxite bearing regolith in the Lower Amazon. Mineralogical Magazine 82, 413 – 431.

Section 3.1: Profile Structure and Mineral Features

The parent rock, as depicted in Figures 2A and 3, appears to be a result of weathering (saprolite), evident from visual inspection and the presence of kalinite, goethite, and hematite. However, the origin of the interdigitated yellow, pink, and red zones in the saprolite remains unclear. Could these zones formed due to inherited heterogeneities in the parent rock, or element segregation (e.g. variations in weathering intensity, differences in pH-eH conditions)? Furthermore, if the authors assert that the ferruginous duricrust is residual, it should be referred to as lateritic residuum, contrasting with ferricrete, which relates to ferruginized transported material.

response: Yes, the parent rock is not 100% preserved in all region, only in bore hold and after several meters depth it will be possible to collect well preserved samples. There is no bore hole close to the study profiles. Thus, we choose the best parent rocks samples we found close the study profile. In the region only the quartzite is well preserved in outcrops. We improved figure 2 changing some photos.  

We interpreted the interdigitated yellow and red zones across profile 1 as most probably produced by percolation condition and differences in pH-Eh, this part of the manuscript was rewrite. Thank you for your suggestions. This process causing element segregation, are examples of remobilization process that form the lateritic duricrusts.

The duricrusts are in the top of a weathering profile, over a saprolite and mottled horizon sequence. This point to lateritic process. The ferricrete do not show this weathering horizons sequence and characteristics.

Section 3.2: Geochemistry

In Figure 6, it would be beneficial to include ratios such as Ti/Zr to better evaluate the residual versus transported portions of the profiles. Although this information is presented in Figure 12, it would enhance the clarity of the text if it were introduced earlier. Additionally, there is a typo in the figure numbering (7A instead of 1A) on line 272. The substantial amounts of kaolinite, gibbsite, and iron oxides in the upper lateritic duricrust of profile 2 are surprising, given the sandstone parent rock, which is predominantly composed of SiO2. This suggests extensive leaching of silica. Conducting mass balance calculations based on Zr immobility could assess the intensity of silica weathering and verify if Fe and Al enrichments are residual only.

response: We think that fig 6 is too complex to add Ti/Zr ratio. As well as we used the residual elements ratios to explain the weathering process in the discussion. We prefer to use Zr/Hf than Ti/Zr because both are in ppm and represent the same mineral, zircon, while Ti/Zr represent 2 distinct minerals rutile/anatase and zircon.

Yes, there is a substantial amount of kaolinite, gibbsite, and iron oxides in the soil of profile 2 like in several lateritic profiles in Amazonia also developed from sandstone parent rock. Furthermore, there is microcline in the sandstone that form kaolinite in the weathering. The high kaolinite and gibbsite content indicate the oxisol is produced by an intense and probably long weathering process that leached almost all mobile elements. The interdigitated clayey zones in the lateritic duricrust probably also help kaolinite and gibbsite accumulation forming the oxisol

We did several mass balance calculations using Ti, Zr and other elements, however the calculation do not give any new and relevant information about the evolution of the profiles, besides the residual elements are enriched and the mobile elements are leached.

We check all figures and table numbers.

What is the reason for phosphorus enrichment in the duricrust of both profiles? Phosphorus is typically considered a mobile element unless it is bound in minerals like monazite, which does not seem to be the case here.

response: Phosphorous can be link to residual minerals like monazite and xenotime, but also to apatite that is a weatherable mineral. When apatite is dissolved, P can be link to Al and Fe forming insoluble phosphates minerals that accumulate in the lateritic duricrust together with hematite and goethite. Enrichment of P in the top of lateritic profile is very common, especially when the amount of apatite is higher in the parent rock. Free P and Al can form crandallite that is an aluminous phosphate with Ca. When the P amount is high, P lateritic duricrust can have economic importance as fertilizer (Costa et al. 2016).

Costa, M.L., Leite, A.S., Pöllmann, H. 2016. A laterite-hosted APS deposit in the Amazon region, Brazil: The physical-chemical regime and environment of formation. Journal of Geochemical Exploration 170, 107–124

General Comments:

The paper remains largely descriptive and general. The conclusions need to be more explicitly stated. Moreover, the existing literature, particularly works such as Innocent et al. (1997), Coimbra Horbe et al. (2022), and Wei et al. (2018), should be better integrated. These references can provide valuable insights, especially concerning isotopic evidence, REE investigations, and weathering processes in tropical laterites, enriching the discussion and analysis within the paper.

Innocent et al. (1997) Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Parana, Brazil. Chemical Geology 136 2 19-232

Coimbra Horbe et al. (2022) Nd and Sr Isotopes and REE Investigation in Tropical Weathering Profiles of Amazon Region

Wei X, Wang S, Ji H, Shi Z (2018) Strontium isotopes reveal weathering processes in lateritic covers in southern China with implications for paleogeographic reconstructions. PLoS ONE 13 (1)

response: We rewrite most of the discussion part, we improved the uses of these references to better explain the REE and the isotopes behavior. We also rewrite the conclusion.

Reviewer 2 Report

Major revisions are required.

Lack of some data from petrography and mineralogy to support XRD data

The presentation of the data and the numbering of tables and figures need to be revised.

Please refer to the attached file for specific comments on the MS.

Comments for author File: Comments.pdf

Though I am not qualified enough to assess the English, I felt the article needs significant editing 

Author Response

Dear Revisor

Thank you very much for the comments along the manuscript and sorry by our obvious mistakes in the numbers of figures and tables and in the spelled words. A very careful revision was done.

Answer to revisor comments.

The bold letter in lines 57 to 60 was produced during pdf editing, in the original manuscript this do not happen. Unfortunately, we did not check this before submission.

Reference was add in the Figure 1

The organization of material and methods part was reformulated according revisor suggestions. It was add a geological setting part.

It was added information about the number of samples and detection limits.

Although revisor indicates should report only the trace elements in 3.2.2. Trace elements section, we decide to maintain the comments about the major elements showing the correlation among the trace elements, since it help the interpretation in the discussion section

Reviewer 3 Report

General comments:

 This manuscript presents valuable insights into the impact of parent rock and minerals on lateritic weathering in the Midwest region of Brazil. The study provides a detailed analysis of the influence of parent rock and minerals on lateritic weathering in the area, examining two regional tropical weathering profiles developed from metasedimentary and sedimentary rocks that exhibit different lateritic structures. One profile displays complex duricrust textures, while the other is characterized by oxisol overlying vermiform duricrust. The paper highlights the crucial role of Fe₂O₃ accumulation during weathering in controlling geochemical profiles, thus influencing trace element distribution and lateritic duricrust formation. The study also investigates REE fractionation and illustrates the association of light REE with Fe₂O₃ and heavy REE with either K₂O group minerals or Al₂O₃, depending on the profile. Despite REE fractionation, the Ɛ_Nd(0) ratios reflect the parent rock's signature. The weathering of muscovite and microcline affects the ⁸⁷Sr/⁸⁶Sr signatures, indicating geochemical reorganization under humid conditions and intense silica leaching, particularly in the profile with a gibbsite-kaolinite-dominated oxisol. Overall, the manuscript contains valuable information, but it could be improved in some areas.

 1. The manuscript contains several punctuation and spelling errors in professional terms. For example, there is an absence of a comma in 'etcAll' on Line 40. Additionally, the word 'Cretaceus' is spelled incorrectly instead of 'Cretaceous' in Figure 1.

2. To enhance the quality of the geological maps, it would be beneficial to improve the organization and highlight the key research findings. Restructuring Figure 1 would help achieve this goal.

3. It is unnecessary to include partial tables like Table 1 listing the mineral's theoretical chemical composition. It is enough to mention the source in the main text without presenting it in a table.

Author Response

Dear Revisors

Thank you very much for the comments along the manuscript and sorry by our obvious mistakes in the numbers of figures and tables and in the spelled words. A very careful revision was done.

Revisor comments: This manuscript presents valuable insights into the impact of parent rock and minerals on lateritic weathering in the Midwest region of Brazil. The study provides a detailed analysis of the influence of parent rock and minerals on lateritic weathering in the area, examining two regional tropical weathering profiles developed from metasedimentary and sedimentary rocks that exhibit different lateritic structures. One profile displays complex duricrust textures, while the other is characterized by oxisol overlying vermiform duricrust. The paper highlights the crucial role of Fe₂O₃ accumulation during weathering in controlling geochemical profiles, thus influencing trace element distribution and lateritic duricrust formation. The study also investigates REE fractionation and illustrates the association of light REE with Fe₂O₃ and heavy REE with either K₂O group minerals or Al₂O₃, depending on the profile. Despite REE fractionation, the Ɛ_Nd(0) ratios reflect the parent rock's signature. The weathering of muscovite and microcline affects the ⁸⁷Sr/⁸⁶Sr signatures, indicating geochemical reorganization under humid conditions and intense silica leaching, particularly in the profile with a gibbsite-kaolinite-dominated oxisol. Overall, the manuscript contains valuable information, but it could be improved in some areas.

2. The manuscript contains several punctuation and spelling errors in professional terms. For example, there is an absence of a comma in 'etcAll' on Line 40. Additionally, the word 'Cretaceus' is spelled incorrectly instead of 'Cretaceous' in Figure 1.

response: We revised all the manuscript, figures and table.

2. To enhance the quality of the geological maps, it would be beneficial to improve the organization and highlight the key research findings. Restructuring Figure 1 would help achieve this goal.

response: We improved figure 1

3. It is unnecessary to include partial tables like Table 1 listing the mineral's theoretical chemical composition. It is enough to mention the source in the main text without presenting it in a table

response: We understand that table 1 may be unnecessary, however these data in the manuscript help to better understand table 2. Thus, we decided maintaining table 1 in the manuscript.

Round 2

Reviewer 1 Report

The authors satisfactorily addressed the review.

Reviewer 2 Report

no further comments

no further comments