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Magnetic Fabrics and Petrography of Rocksalts Reveal Preferred Orientation of Anhydrites within a Halite Matrix

Minerals 2022, 12(2), 192; https://doi.org/10.3390/min12020192

by Ran Issachar^1,2,*, Ram Weinberger^1,3, Tsafrir Levi¹, Jessica Barabasch⁴ and Janos L. Urai⁴

Reviewer 1:

Jyotirmoy Mallik

Reviewer 2:

Cláudia Cruz

Reviewer 3:

Michael Petronis

Minerals 2022, 12(2), 192; https://doi.org/10.3390/min12020192

Submission received: 16 December 2021 / Revised: 25 January 2022 / Accepted: 28 January 2022 / Published: 31 January 2022

(This article belongs to the Special Issue Petrophysical Characteristics of Naturally Deformed Rocks)

Round 1

Reviewer 1 Report

This is an interesting paper where the magnetic fabrics of rock salts are analyzed. This is also interesting because the analysis is done on diamagnetic material whose susceptibility is low and negative, where AMS analysis is not common.

The major criticism that I have is on the number of cores drilled out from each sample. It seems only one !!!! I would like the authors to consider increasing the sample/core numbers so that the clustering of susceptibility axes is better tested.

Please provide a general table where all the AMS data are listed.

The other criticism is if quick microscopy can provide a clear idea on the direction of salt flow during deformation, what is the necessity of carrying out AMS at all? The point is if rock fabric can be figured out for direct observation, what is the necessity of any proxy like AMS at all?

The representation and resolution of some of the plots like Fig. 2b can be improved.

In fig.2f, please mention which one is the heating and which one is the cooling curve in the caption, also mark the major susceptibility drops. A higher resolution plot can be used.

In fig.6 the bottommost scale is unreadable.

Generally, the K1-K2 plane that is the magnetic foliation is parallel to the bedding plane, in this case, it is the K1-K3 plane, please provide your thoughts.

I gather VSM is used for rock magnetic analysis, did you figure out the domain state and whether it has any effect on magnetic fabric?

Author Response

Reply: We thank the reviewer for his comments and positive response to the use of diamagnetic fabrics for studying rock deformation.

I agree with the general finding where shape anisotropy of mineral grains is actually controlling the AMS fabric rather than the crystallographic anisotropy. The same is observed for magnetite grains (refer: Establishment of correlation between anisotropy of magnetic susceptibility and magma flow fabric: an insight from Nandurbar–Dhule dyke swarm of Deccan Volcanic Province, Das and Mallik, 2019, current science) where needle-shaped magnetite provides well defined magnetic fabric than fully grown euhedral magnetite crystals.
The major criticism that I have is on the number of cores drilled out from each sample. It seems only one !!!! I would like the authors to consider increasing the sample/core numbers so that the clustering of susceptibility axes is better tested.

Reply: We think there was a misunderstanding. We collected 12 specimens from locality LS1 and 9 specimens from locality LS6. Initially we used the term samples for cores, which might be misleading, so in the revised version we edited the text to avoid confusion (lines 107-108).

Please provide a general table where all the AMS data are listed.

Done: we added Table 1.

Reply: We agree that direct observations are superior, but inferring the 3D orientation using optical/SEM methods is very challenging and time consuming. We used the orientation of the AMS to cut coaxial thin-sections and schematically compared the microscopy images. The relations between AMS and microstructures in rocksalts advances our understanding on the nature of deformation. In order to strengthen this point, in the revised version we added photomicrographs of gamma-irradiated salt samples that highlight the origin of the AMS signals and their relation to the salt micro-deformation mechanisms.

The representation and resolution of some of the plots like Fig. 2b can be improved.

Done: High resolutions images are provided in the re-submission.

In fig.2f, please mention which one is the heating and which one is the cooling curve in the caption, also mark the major susceptibility drops. A higher resolution plot can be used.

Done: we added heating/cooling marks in the figure and in the caption.

In fig.6 the bottommost scale is unreadable.

Done.

Generally, the K1-K2 plane that is the magnetic foliation is parallel to the bedding plane, in this case, it is the K1-K3 plane, please provide your thoughts.

Done: we agree and added a discussion in lines 240-246. In LS1 site we recognize a magnetic lineation (clustered K1 axes and K2-K3 foliation) parallel to bedding strike. In a previous work we recognized this fabric in intra-salt clasts (Issachar et al., 2019) and interpreted it an indication for N-S folding at the subsurface. In LS6 site we recognize horizontal foliation oblique to bedding, and its relation to the rocksalt deformation is yet not so obvious. To better understand the deformation recorded by the AMS, and to correlate it with the salt wall kinematics, the fabric needs further study.

I gather VSM is used for rock magnetic analysis, did you figure out the domain state and whether it has any effect on magnetic fabric?

Reply: We couldn’t figure domain states as the hysteresis is very week, on the limit of the noise level, indicating that ferromagnetic minerals are very rare in the sample.

Reviewer 2 Report

This manuscript reports AMS results on diamagnetic rocksalts samples from the Sedom salt wall (Dead Sea basin). Several techniques were applied to study the magnetic properties, the magnetic fabric, and also the petrographic, chemical, and mineralogical characteristics of the samples. Several authors have applied AMS to rocksalts with (few) impurities, but this paper describes the application of AMS to “pure” diamagnetic rocksalt samples. The article presents valuable, original, and interesting data and shows that the studied rocksalts samples consisted mainly of halite crystals and needle-like anhydrite crystals. The ferro- and paramagnetic minerals are scarce and their contribution to the bulk magnetic properties was found to be negligible. The AMS reflects the preferred alignment of needle-like anhydrite crystals with their long axes parallel to K1 axes. Some appointments about the relation between AMS and regional deformation/ flow direction were also made. The authors suggest that the AMS of the rocksalts provides a textural proxy that reflects the deformation and flow processes of the rocksalts, despite their very low magnetic susceptibility.

In my opinion, the article can be accepted after minor revision.

General comments:

Add some Keywords. Please provide 3 to 10 pertinent keywords (see “Instructions for Authors” on the Minerals website).
References must be numbered in order of appearance in the text (including table captions and figure legends) and listed individually at the end of the manuscript. So, in the legend of Figure 1, you should have the [47] and [48]. Please, change the reference numbers (in the text and at the end) according to the new sequence (see “Instructions for Authors” on the Minerals website).
Improve/ clarify the topic “4.2 Magnetic fabric”.
Some typo/ formatting errors were identified throughout the manuscript.
All sources of funding of the study should be mentioned in a specific topic "Funding" (see “Instructions for Authors” on the Minerals website).

Detailed comments:

After Line 23 - Insert keywords:

- You forgot to add some keywords (see “Instructions for Authors” on the Minerals website).

Line 46 - Typo/ formatting error:
- You forgot a dot before the comma.
- “…[e.g., 21-23].”
Line 56 - Typo/ formatting error:
- Add a *space* between x and 10^-6; and between 10^-6 and SI.
Line 61 - Typo/ formatting error:
- Add a *space* between x and 10^-6; and between 10^-6 and SI
Line 86 - Please specify which are the five members:
- You mention that Sedom salt wall is divided into five members, but you do not specify which they are. In figure 1, you identify 11 geological sections but do not mention the five members. Can you please add that information in the text or in a new figure?
Line 86 - Typo/ formatting error:
- Add *space* between [37] and (Figure...)
- I think you mean figure 1 (not figure 2)
Line 107 - Alteration in Figure 1:
- I can not see the small dots of the “Intrasalt bed sites” in the figure 1b… Can you please make the symbology of the "Intrasalt bed sites" (more) visible?
- Put “G” of “Geological sections” in capital letters.
Lines 109 + 110 - References in the legend:
- References must be numbered in order of appearance in the text (including table captions and figure legends) and listed individually at the end of the manuscript. So, here we have the [47] and [48].
- Cut off the year in the references.
Line 118 - Typo/ formatting error:
- Please, give a *space* between 77 and K.
- “…77 K.”
Line 129 - Typo/ formatting error:
- Numbering the topic “Hysteresis Loops”
- “3.2.3. Hysteresis Loops”
Line 140 - Typo/ formatting error:
- Numbering the topics according to the new sequence.
- “3.2.4. Low-temperature….”
Lines 141-146 - Typo/ formatting error:
- Please format the text
Line 147 - Typo/ formatting error:
- Numbering the topics according to the new sequence
- “3.2.5. Temperature-dependent…..”
Line 149 - Typo/ formatting error:
- Put “C” of “Curie” in capital letters.
- “…typical Curie temperatures…”
Line 154 - RT-AMS:
- Please add the meaning of RT-AMS
- For example: “The room-temperature AMS (RT-AMS) was measured…”
Line 161 - LT-AMS:
- Please add the meaning of LT-AMS
- For example: “…we measured the low-temperature AMS (LT-AMS)…”
Line 163 - Typo/ formatting error:
- Please, give a *space* between 77 and K.
- “…77 K.”
Line 174 - 6 samples:
- In order to uniformize, change “6 samples” to "…six samples".
Lines 205 + 206 - Different number of decimal units (d.u.):
- Have a different number of decimal units (d.u.) in: km ^RT= -13.4 (1 d.u.); km ^LT = 13.28 (2 d.u); km ^RT/ km ^LT = 1.005 (3 d.u.)... Why? Can you uniformize and use the same decimal units...?
Line 206 - Typo/ formatting error:
- I think you mean figure 2a (not figure 2b)
- “… respectively (Figure 2a).”
Line 206 - Typo/ formatting error:
- Add a *space* between ± and the number
- “…are 1.005 ± 0.018.”
Line 231 - Figure captions:
- For someone who is not familiar with temperature-dependent susceptibility curves, it helps to mention which curve corresponds to the heating and cooling cycles.
- Suggestion: “…during heating (red) and cooling (blue)…”
Lines 232-251 - The topic “Magnetic fabrics” is confusing:
- You mention the Figure 4 before the Figure 3 (see “Instructions for Authors” on the Minerals website: “All Figures, Schemes and Tables should be inserted into the main text close to their first citation and must be numbered following their number of appearance”);
- Is not clear that the results between lines 245 and 247 are related to the RT-AMS;
- Line 246 – I think you mean figure 3 (not figure 4) – “… to bedding (Figure 3).”;
- Line 247 – you can add a reference to figures 4b and 4c – “…have no correlation to km (Figure 4b and Figure 4c).”;
- Line 249 – add a *space* between < and 1.5 and between < and 2.6
- After line 251 – Consider adding a table with the RT-AMS, LT-AMS, and AARM data. Adding a table can help the reader follow the text.
Line 253 - Figure caption:

- You mention in the text K1, K2, K3, but the stereograms indicate Max, Int, Min. For someone who is not familiar with AMS/AARM, it is difficult to follow. I suggested adding in the figure caption some information, for example: “…salt Member (sites LS1 and LS6). Squares represent the maximum susceptibility (K1) or maximum remanence; triangles are intermediate susceptibility (K2) or intermediate remanence; circles are minimum susceptibility (K3) or minimum remanence. Lower hemisphere…”
Line 257 - Figure caption:
- This data is about which technique? RT-AMS?
- I suggested: “Figure 4. Anisotropy parameters (RT-AMS) of Lot rocksalt….”
Line 298 – Typo/ formatting error:
- Add a *space* between < and 50%
Line 319 - References:
- Cut off the year in the reference - “Heinrich et al. [31] measured…”
Line 331 - References:
- Cut off the year in the reference - “Heinrich et al. [31] were…”
Line 335 - References:
- Cut off the year in the reference - “Biedermann et al. [53] increases…”
Line 344 - Typo/ formatting error:
- Correct the 10-6 to 10^-6 (superscript)
- Cut off the comma before 32
Line 346 - Typo/ formatting error:
- Add a *space* between < and 1.0002
Line 348 - Typo/ formatting error:
- Deleted the *space* before the comma and add a *space* after the comma
- “…principle, e.g., 25].”
Line 351 - Typo/ formatting error:
- Add a *space* before 10-6 SI –
- “… to -13.4 x 10-6 SI…”
Line 385 - Acknowledgments versus Funding:
- See the “Instructions for Authors” on the Minerals website.
- All sources of funding of the study should be mentioned in a specific topic "Funding".
- In the section “Acknowledgments” you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind.
After line 392 - References:
- DOI numbers (Digital Object Identifier) are not mandatory but highly encouraged by the Minerals. You are free to add or not the DOI, however, you should uniformize the references (see “Instructions for Authors - Free Format Submission”. Do you want to refer the DOI numbers? If yes, you forgot the DOI in the lines: 409, 429, 443, 455, 459, 471, 474, 490, 495, 500.
- Some *years* are not in bold: lines 393, 395, 403, 439, 457, 493, 500.
- In line 500 correct: “…, anhydrite and gypsum…”

Comments for author File: Comments.zip

Author Response

We thank the reviewer for the support and review. We agree with all the comments and corrected the manuscript accordingly. We had problems with bolding the year in some of the references.

Reviewer 3 Report

See attached review and annotated manuscript.

Magnetic fabrics and petrography of rocksalts reveal preferred orientation of anhydrites within a halite matrix

Ran Issachar1,, Ram Weinberger and Tsafrir Levi

Summary

This manuscript presents results that bear on the interpretation of the AMS fabrics in diamagnetic materials with and without shape anisotropy. They conclude that despite the weak diamagnetic response (-13.3 × 10-6 SI) and the low degree of anisotropy the AMS results are reliable and reproducible, reflecting an intrinsic anisotropy within the rocksalts. This is a significant finding for the AMS community and should be highlighted by the editor.

General Comments

The #'s are keyed to the attached annotated manuscript.

Line 27: "Salt structures (diapirs) are generally associated with large amounts of deformation as the rheological properties of rocksalts enable them to flow under gravitational and tectonic forces [1,2]"

Change force to stress. The salt diapirs are flowing due to an applied stress such as tectonic, buoyancy, sediment facies distribution, and other processes. All these process impart or react to forces over an area; that is a stress.

#1 Line # 51: "Anhydrite and carnallite forms orthorhombic crystals, and are expected to have AMS co-axial to the crystallographic axes."

This statement bears further explanation. By analogy, in the Fe-Ti oxide magnetite phases; the body diagonal of the cubic crystal serves as the easy axis of magnetization. Please explain "co-axial to the crystallographic axes". Is this parallel to the crystal axis, i.e., a, b, c axis of the orthorhombic crystal? If so, all axis are of different lengths. Which axis does the AMS fabric align with; K1-max, K2-int, K3-min? Why??? Please elaborate.

#2 Line # 113: : This is an instrumentation settings question. The authors say "magnetic field of 750 A/m (peak field) and frequency of 1220 Hz". This seems to be a bit high. Standard settings are 200 A/m, 976 Hz. Please explain.

#3 Line # 147: : Why so few samples (n=2)? These experiments are quick and diagnostic of the mineral phases present in the sample. Given the level of effort on other aspects of the study, I am curious why the authors did not run a full suite of experiments on multiple samples.

#4 Line # 207: "Nevertheless, the AF curves indicate that the samples were fully demagnetized at 60 mT, suggesting the presence of low-coercivity ferromagnetic minerals that carry the remanence."

I recommend that you state here "The composition and domain state of the magnetic material is likely a multidomain, Fe-Ti oxide titanomagnetite grains of restricted composition". Also mention that the AF demagnetization data reveals, based on the MDF (medium destructive field) of the NRM at about 20 mT is consistent with MD grain size (see also comment #7 below).

#5 . Line # 238: The only comment is that the authors did a good job here demonstrating the repeatability of the AMS data from the diamagnetic materials. Good job.

#6 Line # 261: The optical properties of the halite grains are consistent with strain i.e., deformation of the crystal structure of the grains. The anhydrate grains are elongate essentially mimicking the fabric of the halite matrix of the salt deposit. The anhydrate grains likely grew under an applied stress and mimic the fabric. I recommend that the authors explore this possibility.

In addition, the authors state "In many cases these crystals appear along the edges of the halite crystals and mimic their original shapes, suggesting that the anhydrite corroded the halite crystals. Most importantly, there are clusters of needle-like anhydrite with a strong preferred alignment (Figure 5b)."

The statement that the "anhydrite corroded the halite crystals" is only one possible explanation. The orientation of the grains is more likely a response to stress (flowing salt) with grain growth occurring in low-stress orientations. I recommend that this section be expanded to discuss alternative interpretations.

#7 Line # 300: Origin of the magnetite grains - a likely source is aeolian dust grains of multidomain size (20 microns or larger ) blown into the deposit. The authors should include a brief discussion of the a likely source of the oxide grains. My guess is that the grains are aeolian in origin. Also see #4; Magnetic experiments show a MD grain size. Optically do you see any oxide crystal?

#8 Line # 319: I recommend that the authors make a clear statement that the AMS fabrics are carried by the CaSO4 elongate grains. You might want to reiterate the AMS consistency argument from #5 above. As this section reads, the authors hint at CaSO4 as the fabric origin but never actually say it clearly here and again on Line 358 (were it is stated).

#9 Line # 360: It might be interesting to do an experiment or at least recommend an experiment that measures the AMS fabric and rock magnetic properties of synthetic anhydrite mush. The material is deformed and you asses the fabrics.

Overall, this manuscript presents an excellent discussion of the caveats of using AMS fabrics in diamagnetic materials. The reviewer suggests that this manuscript should be published after only very minor revision. The suggestions attached are provided in the hopes that they will improve the overall quality of the manuscript and clarify a few points throughout the text. I have annotated the manuscript with numerous comments throughout. Please see the attached PDF files.

Comments for author File: Comments.pdf

Author Response

Summary

We thank the reviewer for the support and review

General Comments

The #'s are keyed to the attached annotated manuscript.

Done: we changed to stress.

#1 Line # 51: "Anhydrite and carnallite forms orthorhombic crystals, and are expected to have AMS co-axial to the crystallographic axes."

Done: we edited the text. For orthorhombic and hexagonal crystals, the easy magnetization direction is generally sub-parallel to the crystal symmetry axis, i.e. c-axis.

Reply: We conducted several tests and choose these settings because (1) the results were more significant with a lower signal to noise ratio. (2) because we inferred that ferro/ferromagnetic contribution to the AMS is negligible we could increase the induced field without worrying on the non-linear effects.

Reply: We choose a representative sample from each sampling site. As the susceptibility values were pretty similar between the samples we thought it is enough, aiming not to destroy many samples.

#5 . Line # 238: The only comment is that the authors did a good job here demonstrating the repeatability of the AMS data from the diamagnetic materials. Good job.

Thanks.

Reply: We agree with the comment. In order to have some more robust insights regarding micro deformation in the rock salts we added results of gamma irradiated photomicrographs (new Figure 8). We accordingly revised this chapter and deleted the mentioned sentence.

Reply: We didn’t find any Fe-oxides in the SEM. Although the presence of ferromagnetic phase is evident by AF demagnetization experiments, the samples has no NRM and the contribution of these phase to the AMS in negligible. For this we see no point to add a deep discussion regarding the origin of the ferromagnetic phase. Nevertheless, these rocks are lake sediments (the Sedom lagoon in the Dead Sea region), which sample a large region and also trap Saharan dust and thus can be the source of magnetic minerals.

Reply: Here we just discuss the reliability of the measurements. In the Discussion section we state clearly that the AMS is carried by the anhydrite (lines 331-332).

Done: We appreciate this idea and include this suggestion in lines 333-334 for further study.

Thanks

Reviewer: Petronis

Round 2

Reviewer 1 Report

The authors have addressed all the concerns raised.

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Magnetic Fabrics and Petrography of Rocksalts Reveal Preferred Orientation of Anhydrites within a Halite Matrix

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