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Iron Isotope Compositions of Coexisting Sulfide and Silicate Minerals in Sudbury-Type Ores from the Jinchuan Ni-Cu Sulfide Deposit: A Perspective on Possible Core-Mantle Iron Isotope Fractionation

Minerals 2021, 11(5), 464; https://doi.org/10.3390/min11050464

by Peiyao Wang^1,2,3,*, Yaoling Niu^1,2,4,5,*

, Pu Sun^1,2, Xiaohong Wang^1,2, Pengyuan Guo^1,2, Hongmei Gong^1,2, Meng Duan^1,2, Fangyu Shen^1,2,3, Yining Shi⁴, Song Xue⁴, Yanhong Chen⁴ and Li Shan^1,2,3

Reviewer 1: Anonymous

Reviewer 2:

Bleuenn Gueguen

Minerals 2021, 11(5), 464; https://doi.org/10.3390/min11050464

Submission received: 9 March 2021 / Revised: 23 April 2021 / Accepted: 24 April 2021 / Published: 28 April 2021

(This article belongs to the Special Issue Copper and Other Metallic Isotope Systems)

Round 1

Reviewer 1 Report

Just beginning by the above notes, I think I cannot decide about the overall merit of this work because I find an enormous difference between the excellent data the authors show and their insufficienty supported conclusions.

The authors' departure point, i.e., the display of iron isotope values and calculations in a Sudbury-type mineralization, is interesting by itself and should merit publication with minor modification -although after a thorough language revision. Regarding the study of iron isotopes, which constitutes the core of their work, I think that they show a lot of interesting, novel data and interpretations on iron isotope fractionation, both between contrasting magma types and beween different crystal structures.

However, and instead to focus their work on these (interesting!) issues, the authors enter a very different one: the possible analogy between the rocks and mineralization they have studied, and they try to address the possible analogy between the magma immiscibility processes involved in their genesis and the global process(es) leading to the Earth's mantle and core separation. They do so, in addition, very briefly and in little or no detail. To me, this has been an unfortunate choice.

In my opinion, the studied rocks and mineralization, all evolved at very low pressure (they contain plagioclase in most cases) simply cannot be compared with the rocks involved in the Earth's mantle and core separation: whatever the P-T-x conditions of this latter global event could have been, it is apparent that the process(es) occurred at higher T and, more importantly, much higher pressure. This must have resulted in the occurrence of mineral phases very different from those have been studied here.

This is not only that garnet (probably containing both Fe⁺² and Fe⁺³) must have been an important mineral phase at high-P conditions: the point is that any of the mineral phases involved in this global event surely had lattice, bond and cristal-chemical features that had nothing to do with the mineral phases analysed here, in such terms that make very difficult any extrapolation from these latter to the first ones, also in of iron isotope signature. Maybe the authors have arguments to ignore these differences; the point, however, is that they do not discuss them at all

I appreciate tha this major problem is recognized by the authors in their conclusions in several instances, as they finally argue that they do not imply that the Fe-isotope values they attribute to mantle -and more vaguely to the core- could be taken just as they stand. But if so, would not have been better them to focus on the real issue they describe so carefully, which is interesting enough by itself?

I strongly suggest to the authors a complete re-elaboration of their data, focusing in the frationation of iron isotopes in a very interesting type of rocks and mineralization. Also, they should cleary define the parental magma: apart from its low-P evolution, I think that its alkaline character (e.g., abundant phlogopite) is a feature to worth note.

After that, they could make a final comment on the possible, entirely speculative similarity between the processes they describe with those involved in other terrestrial events, maybe including the Earth's core and mantle separation; but not beyond this point -a step that to me remains uncertain, speculative and unnecessary.

I enclose an annotated file with a number of comments.

Apart from the above questions, I strongly suggest a thoruogh revision of laguage and style. This latter is as relevant as language in this case.

Comments for author File: Comments.pdf

Author Response

Comments and Reply:

Just beginning by the above notes, I think I cannot decide about the overall merit of this work because I find an enormous difference between the excellent data the authors show and their insufficient supported conclusions.

>> Thank you for your comments and affirmation of our high-quality data. We in fact agree on the comment in that we do not have concrete conclusion on Fe isotope fractionation between the silicate earth and the earth’s core, but the Sudbury-type net-textured ores are best materials available to discuss the issue. The Fe isotope community ignores almost 90% Earth’s Fe in the core to argue for chondritic Earth Fe isotopes. This is an essentially baseless assumption about one of the biggest earth problems. The latter is the motivation of this work. What we ensure is the highest data quality and what we try our best is to provide most logical interpretation we can. The value of this work is not about correct conclusion, but the VERY first audacious attempt with the plea for genuine attention of the scientific community. Scientific research is a self-correcting process, and it is our motivation to encourage the entire community to deal with globally important problem. There is a Chinese saying “cast a brick to attract jade”! This is the value of the paper. In fact, we are excited about this work for being the very first attempt for others to follow.

>> Thank you for your comments, iron isotope fractionation between coexisting minerals, calculations on mineral separates and discussions on iron isotope compositions have been studied to accumulate iron isotope observations (e.g., sulfide, silicate, oxide minerals, etc.) and understand possible fractionations between minerals phases in various geological processes (e.g., serpentinization, sulfide crystallization, silicate-sulfide melts segregation). Therefore, these iron isotopes data of coexisting minerals will play an important fundamental role in iron isotope research, and we hope to make some contributions to the future understanding on iron isotope fractionations.

In short, our purpose as stated above, is not to resolve the ore genesis, although we do discuss so with logical analysis, but to attempt to deal with one of the biggest earth problems.

>> We have indeed discussed all that been suggested here. What is considered “unfortunate choice” is in fact the most important scientific problem – Earth’s Fe isotope composition between the core and the silicate earth. So, given the data we have done three things: (1) Isotope fractionations between all the analyzed silicate and sulfide minerals; (2) Processes of sulfide-silicate liquid segregation and mineralization, and (3) Core-mantle Fe isotope redistribution.

As stated above, there are many unknowns about (3), but we are certain that many others will follow this first study by us published in Minerals.

we emphasize that iron isotope fractionation does take place between silicates and sulfides in the Sudbury-type magmatic mineralization. Importantly, the data and understanding of this work provide a fundamental first step for discussing possible Fe isotope fractionation between the Earth’s mantle and its core that hosts almost 90% of the Earth’s iron, ignoring the difference of P-T-x conditions that need further studies, because we can analyze Fe isotopes of naturally occurring sulfide-silicate liquids from these Sudbury-type magmatic deposit instead focus on the experiments and calculations on first principles.

This is not only that garnet (probably containing both Fe+2 and Fe+3) must have been an important mineral phase at high-P conditions: the point is that any of the mineral phases involved in this global event surely had lattice, bond and crystal-chemical features that had nothing to do with the mineral phases analyzed here, in such terms that make very difficult any extrapolation from these latter to the first ones, also in of iron isotope signature. Maybe the authors have arguments to ignore these differences; the point, however, is that they do not discuss them at all.

>> Thank you for kindly considering. We fully agree that this is low pressure and low temperature processes as clearly and emphatically stated in our manuscript. But we must start from somewhere – so the Sudbury-type ore materials are best material to start with. This is a concrete scientific strategy and methodology. We do not ignore the effects of pressure and temperature on potential Fe isotope fractionation, but we use the best natural materials to study this important problem as the strategically must-do first step. The results will form the foundation for further studies.

Different occurrence of mineral phases between Sudbury-type ores and core-mantle segregation may be exist, but we were trying to focus on the moment of segregation between silicate and sulfide melts (i.e., the iron isotope fractionation of these two melts before their crystallization). That is, the mineral assemblages crystallized from these two melts did not affect the previous iron isotope fractionation between these two melts.

Coexisting sulfide minerals and silicate minerals are used for iron isotope composition reconstruction of sulfide and silicate melts respectively, in what we already know sulfide crystallization in equilibrium, and iron isotope fractionation coefficient between silicate melt and crystals (i.e., Δ⁵⁶Fe_{silicate (e.g., Ol)-melt}) given by previous works.

I appreciate that this major problem is recognized by the authors in their conclusions in several instances, as they finally argue that they do not imply that the Fe-isotope values they attribute to mantle -and more vaguely to the core- could be taken just as they stand. But if so, would not have been better them to focus on the real issue they describe so carefully, which is interesting enough by itself?

>> Yes, further works would be needed to verify our conclusions, but it was important to clarify the fact that fractionation do occur in the process of silicate-sulfide liquids segregation, even it was under much lower pressure and temperature conditions than the core-mantle separation.

I strongly suggest to the authors a complete re-elaboration of their data, focusing in the fractionation of iron isotopes in a very interesting type of rocks and mineralization. Also, they should clearly define the parental magma: apart from its low-P evolution, I think that its alkaline character (e.g., abundant phlogopite) is a feature to worth note.

>> Thank you for your precious comments on the genesis of ore samples, we indeed discuss the fractionation of iron isotopes during processes (e.g., serpentinization, sulfide mineralization) that need further studies, but our study on possible iron isotope fractionation of core separation is the strategically must-do first step and the results will form the foundation for further studies.

We have added some statement on the content and origins of the altered minerals (e.g., phlogopite) as suggested. Sudbury-type ores in Jinchuan deposit are all cumulate origin including dunite and lherzolite dominated by olivine with some pyroxene and minor magmatic chromite (or chrome spinel). As for phlogopite and other altered minerals, they are barely <1% in the net-textured ores (JC-1, 2 and 3). While sample JC-4, the only disseminated ore obtained from metasomatic orebody at the contact of ore-bearing ultrabasic intrusion with the metamorphic rock series (dominated by dolomite marble undergone strong alteration, such as tremolitization, serpentinization), thus, JC-4 is dominated by dolomite, sulfide, serpentine and phlogopite. Since the iron isotope compositions of sulfide minerals (i.e., pyrrhotite, pentlandite and chalcopyrite) in JC-4 are within the range of other net-textured ores (JC-1, 2 and 3), we thus do not treat these sulfide minerals specifically or explain them separately.

>> Thank you for your sincere reminding. We have carefully revised the discussion and language style on the iron isotope fractionation during the core-mantle segregation.

I enclose an annotated file with a number of comments.

Apart from the above questions, I strongly suggest a thorough revision of language and style. This latter is as relevant as language in this case.

>> Thank you for all your helpful comments and suggestions on our manuscript. Please see below and revised version of manuscript for detailed revisions.

Reply to the critical comments as follows:

Line 18: “Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope compositions.”

Within a very narrow error interval, this seems also to be the conclusion of this work.

>> Thank you for kindly pointing this out, our iron isotope compositions of silicate minerals seem to be consistent with the chondrites but iron isotope fractionation coefficient between silicate melt and crystals (i.e., Δ⁵⁶Fe_{silicate (e.g., Ol)-melt}) given by previous works indicate the silicate melt has heavier iron isotope composition than chondrites.

Line 26: “silicate minerals (olivine [ol] > orthopyroxene [opx] > clinopyroxene [cpx])”

How about garnet? This (or maybe another Al-rich, Fe-bearing phase) is relevant at the pressures likely involved in mantle-core separation.

>> We appreciate for insightful suggestion and agree that it would be useful to demonstrate the relevant pressures compared with mantle-core separation. As we described in Table 1, the ore samples obtained from the mafic-ultramafic intrusion are of cumulate origin dominated by dunite (JC-1 and 2), with minor Al-rich Fe-bearing phase in the form of chromium spinel (~1%). Beyond that, plagioclase (~4%) can occur in lherzolite (JC-3). But unfortunately, no mineral separates of these Al-rich Fe-bearing phase can be selected for iron isotope analysis with such low content.

Line 73: “The hidden implicit assumption…”

>> Thank you, we have changed the word as suggested.

Lines 97-114: Caption of figure 1. “A schematic scenario of magmatic Ni-Cu sulfide mineralization…”.

Please rephrase for clarity. probably this legend can be shortened.

>> We have made the change. The new caption of figure 1 reads as follow:

A schematic scenario of magmatic Ni-Cu sulfide mineralization to illustrate the possible process of core-mantle separation through sulfide-silicate melt segregation. (a) Portion of a cross-section illustrating the silicate-sulfide melt segregation in a magma reservoir relevant to the Sudbury-type sulfide ore mineralization (modified from [8] (Naldrett, 2004), where the sulfide-saturated mafic magma segregates to immiscible sulfide liquid from the silicate liquid, and the dense sulfide liquid sinks to the bottom while olivine dominated mafic minerals are crystallizing from the silicate liquid [9] (Naldrett, 1969). (b) Continuation of the process in (a) which forms the peridotite of cumulate origin dominated by olivine and varying textured ore types downwards (disseminated, net-textured and massive ores) as illustrated (after [8,10] (Naldrett, 1973, 2004)). (c) While the core is largely metallic (Fe, Ni etc.), we reason that the core separation must have undergone sulfide-silicate segregation due to density difference, and the sulfide melt may have then developed into metallic core through sulfur removal. As the latter process have not caused iron isotope fractionation, the metallic iron will retain the sulfide liquid iron isotope characteristics. Hence, the net-textured ores showing equilibrium contact between silicates and sulfides provide ideal materials for understanding possible iron isotope fractionation between the metallic core and silicate earth during core formation neglect the P-T-x conditions effect.

Line 133: “coexisting with fresh or serpentinite silicate minerals”

>> Thank you, we have deleted those words as suggested.

Line 132: “silicate minerals (…plagioclase, etc.), minor oxide minerals (chromite…)”

So the mineralization is hosted by low-P, basic to ultrabasic rocks emplaced in a crustal segment, far from the P conditions of the Earth's upper mantle. In it, Al is successively hosted by spinel and garnet as pressure increases.

>> Thank you for kindly pointing this out. As the earliest product of mafic magmatic crystallization, chromite (or chrome spinel) is actually present in all ore samples (dunite (JC-1, 2) and lherzolite (JC-3), see Table 1 for details) and is mostly wrapped in olivine and other minerals generated later. After the crystallization of chromite and olivine, pyroxene and plagioclase began to crystallize roughly at the same time, so they only appeared in lherzolite (JC-3). We have removed the plagioclase here and rewritten this sentence to prevent misunderstanding.

Line 162: “…are minor phlogopite, …”

How abundant is phlogopite?

>> The content of phlogopite in dunite and lherzolite net-textured ores (JC-1, 2 and 3) is less than 1%, while in altered disseminated ore (JC-4) is ~16%.

Line 190: “JC-4 disseminated ore…dolomite (~50%), phlogopite (~16%), amphibole (~12%)”

Abundant dolomite indicates that this rock is not only intensely altered, but Ca-rich.

The nature and origin of this rock must be explained, at least tentatively: in ultrabasic rocks, Ca contents are usually low.

Accordingly, magnesite-rich rocks could be more easily understood by alteration related to CO₂ circulation, whereas dolomite-rich rocks should be rarer.

The abundance of Phlogopite (16%!) and amphibole abundance also deserves interpretation. In particular, abundant phlogopite implies a high K content, which is common only in some rare ultrabasic rocks.

>> Thank you for pointing out this problem. The metamorphic rock series, composed of migmatite, gneiss and dolomitic marble, is the main surrounding rock of the ore-hosting ultrabasic intrusion. The dolomite marble is the most abundant, dominated by dolomite, amphibole (tremolite and actinolite) and phlogopite. At the contact with the ultrabasic intrusion, the ores will undergo strong alteration, such as tremolitization, serpentinization, etc., resulting in the formation of metasomatic orebody. One of our ore samples (JC-4) was obtained from this orebody, with the only disseminated texture while other 3 samples are all net texture cumulate origin. For these reasons above, only sulfide separates of JC-4 were selected for the iron isotope analysis.

Line 366: “(1) … leeds to sulfide melt exsolution”

I guess it should be leads. In any case, please rephrase the paragraph.

>> Thank you, we have fixed the error.

Line 424: “(dominate dominated by olivine, orthopyroxenes...)”

>> Thank you, we have revised this sentence as suggested.

Line 438-458: Iron isotope compositional differences between minerals with different valence states

In the case of the igneous rocks they have studied, this is more or less as the authors argue. However, I think that these rocks are a poor analog of the conditions at which mantle-core separation occurred.

>> We are aware of what you have taken into account, so we discuss the mantle-core separation ignoring pressure effect. There have been few researches on this due to the inaccessible of the core, most of them are based on experiments and first-principles calculations.

The similar magma and physical processes of Sudbury-type ores as mantle-core separation make them ideal natural material for understanding this important problem, so this is the first-step that we have to do, and it will open up a new perspective for future research.

Line 460-480: Iron isotope compositional differences between coexisting sulfide minerals

These data constitute an important contribution by themselves.

>> Thank you for your affirmation of our data.

Line 516-517: “into serpentines is associated with light Fe loss, i.e., δ⁵⁶Fe[Ol, Opx, Cpx] < δ⁵⁶Fe[Serp] (or heavy Fe gain)”

What is“(or heavy Fe gain)”? Please rephrase for clarity.

>> We have revised this sentence avoiding misunderstanding, our original intention is to talk about the losses of light iron isotopes and the gains of heavy iron isotopes in serpentine. We have rewritten this sentence as follow:

“…with the losses of light Fe isotopes (e.g., ⁵⁴Fe), … (or gains of heavy Fe isotopes (e.g., ⁵⁶Fe))”

Line 517-519: “Where did the light Fe go to (or heavy Fe come from) without volumetrically significant secondary mineral formation associated with the serpentinization (including magnetite)?”

Rephrase for clarity.

>> We have rephrased as follow:

“Where did the light Fe isotope goes to (or heavy Fe isotopes come from) without volumetrically significant secondary mineral formation associated with the serpentinization (including magnetite)?”

Line 632-634: “Although the process of core-mantle differentiation is expected to differ from the formation of Jinchuan magmatic sulfide mineralization as we envision (Figures 1 and 6) in terms of temperature, pressure and other conditions”

Yes, of course. Please also remember that mineral phases involved must be completely different, so that all the calculations shown here could be insufficient in order to obtain a reliable analog.

>> Thank you for pointing this out, in fact, the formation of Sudbury-type ore can be used to simulate the process of core-mantle differentiation is that both of them are the equilibrium fractionation process between immiscible silicate melt and sulfide melt. The mineral phases crystallized after the separation of these two melts would not affect the iron isotope fractionation between silicate-sulfide melt segregation. Therefore, in order to obtain the possible iron isotope composition of these two melts in equilibrium with the various mineral phases, we calculated them by the weighted mean of bulk-mineral phase.

Line 636-637: “bulk earth may have δ⁵⁶Fe < -0.01±0.01 ‰.”

Please compare with the conclusion below.

>> Thank you, we have added it to the conclusion as suggested.

Line 660-669: “(3) By assuming that the coexisting sulfide and silicate minerals of …”

Any comparison with iron meteorites, including troilite data?

If you believe that comparison is not pertinent, please explain why.

>> Thank you for pointing this out, we added some experimental data of iron meteorites and high-temperature and high-pressure experimental results to compare with our conclusions. See the details below:

Previous studies suggest that iron meteorites (δ56Fe = 0.05±0.02‰) have heavier average iron isotope composition compared with chondrites (δ56Fe = -0.01±0.01‰) [2-6] (Zhu et al., 2001; Poitrasson et al., 2005; Poitrasson and Freydier, 2005; Schoenberg and von Blanckenburg, 2006). It is suspected that the specific nature of the bonding between Fe and S in iron meteorites may enhance the affinity for heavy Fe isotopes relative to other phases [70] (Poitrasson et al., 2005). The metal phases in the pallasites have a significant heavier iron isotope composition than the coexisting silicate phases [70] (Poitrasson et al., 2005). Experimental studies on the separation of me-tallic phase and silicate melt under 7.7 GPa with 2000 ℃ and 1 GPa with 1250-1300 ℃ conditions show no discernible fractionation of iron isotopes between metallic phase and silicate melt phase [12,13] (Poitrasson et al., 2009; Hin et al., 2012). Based on the crystal lattice parameters of the metallic phase and silicate phase in the diamond anvil experiment, previous researchers found that the calculated iron isotope fractionation factors between the metallic phase and silicate phase have obvious pressure effect: heavy Fe isotopes enriched in the metal phase compared with the silicate phase at low pressure. On the contrary, light Fe iso-topes enriched in the metal phase compared with the silicate phase under the condi-tions of temperature and pressure at the core mantle boundary (e.g., P=130 GPa) [16] (Polyakov et al., 2009).

Line 670-683: “(4) As the core formation is likely through silicate-sulfide liquid segregation followed by sulfur removal…”

No doubt very interesting, but most probably speculative: mineral phases involved in mantle-core segregation are not the same as those the authors have considered. I think that the fact that garnet is important in deep-seated basic and ultrabasic rocks can substantially alter all of the calculations shown along the work.

>> Thank you for all the useful comments, the difference of occurrence of mineral phases between Sudbury-type ores and core-mantle segregation may be exist, but we were trying to focus on the moment of segregation between silicate and sulfide melts (before their crystallization) as we discussed above.

Point-to-point the details of the revisions:

(In the order they were revised in the manuscript, revisions of typographical error and words were not given)

At title we have used “composition” to replace “characteristics”;
Unifies the use of words “Whole” and “Bulk”, such as “bulk earth”, “bulk-rock ore”, “bulk-silicate minerals (phase)”, “bulk-sulfide minerals (phase)”;

Introduction:

More clearly expressed what is being done in our study “For this reason, the isotope composition of the coexisting minerals…as indicated in the title of this paper and detail below”;
Some previous discussions on iron isotope composition of mantle and chondrite have been added, “However, some studies argued that peridotites are not necessarily…(or fractionation during earth’s core-mantle segregation).”;
Previous experiments (e.g., HTHP) and reported NRIXS data on iron isotopes during Earth’s core formation and how we come up with the Sudbury-type mineralization to solve the scientific problem have been added, “revious understanding of iron isotopes during…This approach has two assumptions:”;
Caption of figure 1 have been rewritten;

Sample and methods:

Sample descriptions have been rewritten mainly added the types of ore samples (e.g., dunite), dominated silicate minerals and the content and origin of altered minerals (e.g., phlogopite, dolomite, etc.);
GSB Fe standard relative to IRMM-014 and what is the alpha Fe standard have been added;
Data and other values have reported with 2 digits instead of 3 digits somewhere before revised;

Results:

The descriptions of the calculations have been rewritten, and removed to Appendix B (Appendix B from last version of manuscript have been changed in a corresponding into C);
We have added some notes that the iron isotope compositions of sulfide minerals (i.e., pyrrhotite, pentlandite and chalcopyrite) and magnetite in the only one disseminated ore (JC-4, from metasomatic orebody) were within the range of net-textured (dunite and lherzolite) ores, we thus have not treated these sulfide minerals specifically or explain them separately;

Discussion:

Previous works have been added on iron isotope compositions of iron meteorites and troilite, iron isotope fractionation between coexisting metal and silicate phases, possible pressure effect to iron isotope fractionation between metallic and silicate phases, “Previous studies suggested iron meteorites…(e.g., P=130 GPa, Polyakov et al., 2009).”;
We have added some sentences to claim that our results of Sudbury-type ores is the first step to introduce a useful basis for continued study towards better understanding iron isotope fractionation of the core-mantle segregation, which need more verification and further studies in the future;
We have noted that we suggested that Different occurrence of mineral phases between Sudbury-type ores and core-mantle segregation may be exist, but we thought the mineral assemblages crystallized from these two melts in different systems would not affect the previous iron isotope fractionation between these two melts;

Conclusions:

Some statements on how we come up with possible Ni isotope fractionation between earth’s core-mantle segregation as Fe isotope, “As metallic iron is primarily present…in the earth’s core-mantle segregation as Fe isotope.”;

Appendix:

We have added a new Appendix B, so the previous Appendix B have been changed into C;

References:

Some previous references have been added, the sequence number of references have been changed in revised version of our manuscript, and the list of references below have been remade.

Tables:

We have placed Table 7 at more appropriate position within the text.

Reference:

Poitrasson, F., Levasseur, S., & Teutsch, N. (2005). Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets. Earth and Planetary Science Letters, 234(1-2), 151-164.

Poitrasson, F., Roskosz, M., & Corgne, A. (2009). No iron isotope fractionation between molten alloys and silicate melt to 2000 C and 7.7 GPa: Experimental evidence and implications for planetary differentiation and accretion. Earth and Planetary Science Letters, 278(3-4), 376-385.

Hin, R. C., Schmidt, M. W., & Bourdon, B. (2012). Experimental evidence for the absence of iron isotope fractionation between metal and silicate liquids at 1 GPa and 1250–1300 C and its cosmochemical consequences. Geochimica et Cosmochimica Acta, 93, 164-181.

Polyakov, V. B. (2009). Equilibrium iron isotope fractionation at core-mantle boundary conditions. Science, 323(5916), 912-914.

Poitrasson F, Halliday AN, Lee D-C, Levasseur S, Teutsch N (2004). Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms. Earth Planet Sci Lett 223:253−266.

Dauphas, N., John, S. G., & Rouxel, O. (2017). Iron isotope systematics. Reviews in Mineralogy and Geochemistry, 82(1), 415-510.

Dauphas N, Roskosz M, Alp E, Golden D, Sio C, Tissot F, Hu M, Zhao J, Gao L, Morris R (2012). A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: application to goethite and jarosite. Geochim Cosmochim Acta 94:254−275

Dauphas N, Roskosz M, Alp E, Neuville D, Hu M, Sio C, Tissot F, Zhao J, Tissandier L, Médard E (2014). Magma redox and structural controls on iron isotope variations in Earth’s mantle and crust. Earth Planet Sci Lett 398:127−140.

Shahar A, Hillgren V, Horan M, Mesa-Garcia J, Kaufman L, Mock T (2015) Sulfur-controlled iron isotope fractionation experiments of core formation in planetary bodies. Geochim Cosmochim Acta 150:253−264.

Shahar A, Schauble EA, Caracas R, Gleason AE, Reagan MM, Xiao Y, Shu J, Mao W (2016) Pressure-dependent isotopic composition of iron alloys. Science 352:580−582.

Liu J, Dauphas N, Roskosz M, Hu MY, Yang H, Bi W, Zhao J, Alp EE, Hu JY, Lin J-F (2017) Iron isotopic fractionation between silicate mantle and metallic core at high pressure. Nat Commun 8, 14377.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper entitled “Iron isotope characteristics of coexisting sulfide and silicate minerals in Sudbury-type ores from the Jinchuan Ni-Cu sulfide deposit: a perspective on possible core-mantle iron isotope fractionation” by Wang and collaborators. The study presents the Fe isotope composition of sulfides and silicates from the Jinchuan Ni-Cu sulfide deposit. The authors measured the chemical composition and Fe isotope composition of several types of minerals, including silicates and sulfides from the deposit, and they modeled the Fe isotope composition of the sulfide melt and silicate melt. Their modeling allowed them to estimating the Fe isotope fractionation between a sulfide melt and a silicate melt that they applied for calculating the Fe isotope fractionation between the Earth’s core and the Earth’s mantle.

I am confident with the quality of the data but I have many remarks on the manuscript in general and description of the data. First the introduction has to be reworked. Many references are lacking to introduce the subject and very little references to previous work are described. The authors focused on introducing the samples and the geological setting but they did not clearly introduce the rationale of their study. Then, the discussion also needs much reworking. The text needs to be reworked some sentences are not easily understandable. In the title you mention Fe isotope fractionation between Earth’s core and mantle, but in fact it is barely discuss in your study. Implications of your work are not discussed and compared with previous studies on this topic. The most important part of your paper seems to have been overlooked.

I have added some detailed comments in my review report attached. I recommend major revisions before publication in Minerals.

Comments for author File: Comments.pdf

Author Response

Comments and Reply:

>> Thank you for your helpful comments and suggestions, and for your entirely accurate reviews of our manuscript with much precious time.

>> Thank you for your comments, we have rewritten the introduction, discussion and other parts of this manuscript as suggested, previous works on iron isotopes (including the iron isotope composition of iron meteorites, experiments and reported NRIXS data between silicate and metal phases) were added on the revised version of our manuscript, and detailed in the references below. We have also added some statements on how we came up with the idea to understand possible the iron isotope fractionation during silicate-sulfide segregation as well as the core-mantle separation by studying Sudbury-type ore samples.

I have added some detailed comments in my review report attached. I recommend major revisions before publication in Minerals.

>> Thank you for all your helpful and meticulous comments and suggestions on our manuscript. Please see detailed revisions and responses below.

Reply to the critical comments as follows:

Title: the title is very long, any suggestion to shorten it?

I would replace “iron isotope characteristics” by “iron isotope composition”.

>> Thank you, we have used the word “compositions” to replace “characteristics”, but we have finally decided not to shorten the title, as we have tried to make every word meaningful, so that readers can clearly understand the main content of our research as long as they see the title.

Line 33: Maybe “silicate melt” and “sulfide melt” are more appropriate words than “silicate liquid” and “sulfide liquid”.

>> Thank you for your sincere suggestion, but we finally decided to use the word “liquid” instead because we thought the meaning of “liquid” includes that of “melt”.

Line 39: Ni isotope fractionation are mentioned in the abstract but they are never mentioned in the manuscript. I suggest removing this sentence, it looks like it comes from nowhere.

>> After we have concluded that the fractionation of iron isotopes must occurred during the core formation, we have proposed reasonable predictions for the fractionation of nickel isotopes during the same process, including those mentioned in the abstract.

Although Ni isotopes are not the main content of this paper, as Ni and Fe are the most important two metallic elements in the earth's core, we must propose that Ni isotopes play an equally important role in this scientific problem, and thus the target of our future works.

In any case, we added a bit of statement on the Ni isotope in the conclusion to indicate its importance.

Lines 58-60: This sentence should be rewritten to clearly expressed what is being done in this study, and not just tell the reader to check the title of the study. Maybe you could shorten the title and develop the ideas in the introduction.

>> Thank you, we have rewritten this sentence as follows:

For this very reason, the iron isotope compositions of the coexisting minerals (e.g., silicates and sulfides) in Sudbury-type ores from the Jinchuan magmatic Ni-Cu sulfide deposit have been studied to accumulate observations and to evaluate the possible iron isotope fractionation between the silicate and sulfide liquids prior to the crystallization of these minerals. Importantly, the data and understanding of this work provide a fundamental first step for discussing possible Fe isotope fractionation between the Earth’s mantle and its core that hosts almost 90% of the Earth’s iron.

Line 80: why is the sentence quoted? Should there be a reference associated with this quotation?

>> Thank you for pointing this out, this sentence refers to the previous work in Line 71 by Wang and Zhu (2012), we think it is best not to quote it again, because the origin of this hypothesis has been clarified above, but we added some statement to make it clearer.

Line 82: “the inaccessibility of the Earth’s core ….”.

>> Thank you, we have changed the sentence as suggested.

Lines 82-87: why is previous work not cited here? Some studies have already investigated the iron isotope fractionation between mantle and core. This work should be cited here. A few examples of papers that could be cited to introduce your study (see also the review of Dauphas et al. in RIMG (2017).

For example, you could introduce your study by saying something like: previous studies (Shahar 2016; and Liu 2017) suggest that Fe isotope fractionation at high pressure is negligible. Thus, investigating Fe isotope fractionation in Sudbury-type ore to evaluate possible isotopic fractionation between the mantle and core is relevant.

It seems that the introduction lacks a review of previous work that has been published on that topic. There is too little information on previous studies and on what motivated you to investigate Fe isotopes in this Sudbury-type ore. After reading your introduction, it looks like this topic has not been addressed before, which is not true.

You only introduce the type of geological setting that you are investigating. It would have been good to have more explanations on why the Sudbury-type ore is relevant to address the Fe isotope fractionation between the Earth’s core and Earth’s mantle and thus why your study is relevant.

>> Thank you for the references about iron isotope fractionation between mantle and core, we have added some previous works on these issues as suggested:

Current research on core-mantle iron isotope fractionation is essentially absent because there is no core material available to study. Some experimental simulations suggest that iron isotope fractionation between silicate and metal phases at different conditions (including those relevant to core formation) is small and negligible (e.g., [10-17] (Dauphas et al., 2012, 2014; Poitrasson et al., 2009; Hin et al., 2012; Shahar et al., 2015, 2016; Polyakov, 2009; Liu et al., 2017)). The most credible approach would be to carry out Fe isotope analysis on sulfide-silicate liquids experimentally produced under the deep mantle conditions. The latter is challenging at present, but we can, as a logical first step, analyze Fe isotopes of naturally occurring sulfide-silicate liquids. The Sudbury-type Ni-Cu-Fe sulfide ores are such materials representing solidified sulfide-silicate liquids resulting from sulfur-rich and sulfide saturated mafic magma segregation. Therefore, we choose to study net-textured ore samples from the Jinchuan magmatic ore deposit, which is the third largest Sudbury-type magmatic Ni-Cu-Fe sulfide ore mineralization. The ore formation process is hypothesized as best approximating the magmatic and physical processes of the earth’s core formation because of the strong chemical Fe-S and Fe-O bunding be-fore S removal to convert Fe-S into the metallic Fe (Ni and other light elements and trace metals) core of the earth (detailed in Figure 1).

Line 87: What do you mean by “ideal samples”? This is too vague.

>> Thank you for pointing this out, we have clarified “ideal samples” as written above.

Lines 88-95: I do not really understand on what these two assumptions are based? Did you base your assumptions on previous work? These works should be cited.

>> These two assumptions are prerequisites for the approach (Sudbury-type ores) to investigate the iron isotope fractionation between core-mantle segregation we detailed below.

Lines 94-95: You say that you can neglect the pressure effect but still some studies are still necessary. This sentence is awkward, please rewrite.

>> Thank you, we have rewritten this sentence as follows:

The core-mantle separation took place under high pressure and temperature in the deep earth, but the magmatism takes place under upper mantle or deep crustal conditions. We do not ignore the effects of pressure and temperature on potential Fe iso-tope fractionation, but we use the best natural materials to study this important problem as the strategically must-do first step. The results will form the foundation for further studies.

Line 263: What is the Fe isotope composition of the GSB iron standard relative to IRMM-014.

>> We have added the iron isotope composition conversion between the GSB iron standard and IRMM-014:

δ⁵⁷Fe_IRMM-014 = δ⁵⁷Fe_GSB + 1.073; δ⁵⁶Fe_IRMM-014 = δ⁵⁶Fe _GSB + 0.729 (He et al., 2015).

Line 266: what is “an alpha iron standard”?

>> “Alfa Fe” is an ultrapure single elemental standard from the Alfa Aesar (China) Chemicals Co., Ltd, we have added it into our manuscript.

Line 266: The 2SD is reported with 2 digits while in Table 5 sample data are reported with 3 digits.

>> Thank you for your sincere reminding, we are aware of this problem, and we have now reported all data with two digits.

Line 267: Same comment as above. I would recommend to report the Fe isotope composition of your samples with only 2 digits. It would be more consistent with the methodology you employed (Ni-doping method for correcting mass bias) and with the results reported for standards and bracketing standards.

>> We have now reported all the results of standards and bracketing standards with two digits.

Line 271: how many times were the samples analysed? On what number of values is calculated the 2SD reported in table 5?

>> Each sample were analysed for 4 times, and these 4 values was used to calculate the 2SD reported in Table 5, as we have written in Line 264.

Line 310: Table 6: Some numbers have three digits others two digits, why?

>> We were trying to keep the table clean by keeping 4 digits (i.e., 0.xxx, x.xxx, xx.xx, xxx.x, xxxx) at that time, but since some values in Table 6 are related (adding up to 100%), we decided to report all numbers with 2 digits in Table 6.

Line 313: I would remove the word “correction” also for line 313 in Table 6b. Instead, I would write something like “Calculation of mineral separates composition” or something similar. This is rather a calculation than a correction that you are doing here.

>> Thank you, we have changed as suggested: “a. Mineral separates”, “b. pure sulfide end-members after calculation”

Lines 319-345: The description of the calculations is difficult to follow. Please review this section.

>> Thank you, we have revised the calculation description, and removed it to Appendix B (Previous Appendix B from last version of manuscript have been changed in a corresponding into C)

Line 322: I don’t understand, you said that bulk rock compositions are reported in Table 5, but I see “mineral separates” reported in Table 5. This is not clear what you call bulk rock.

>> Thank you for kindly pointing this out. We have deleted the “Table 5” here, because it was a mistake. In fact, here we just want to introduce a formula previous studies suggested by Ye et al. (2017, 2020) that can be used to calculate the iron isotope composition of the bulk-rock from iron isotope compositions of various iron minerals. The words “In turn” in Line 328 was that the formula can be calculated in reverse, so we let the impure mineral separates as “bulk-rock” in Line 334, then we obtained every pure sulfide end-members as “various iron minerals”. We have revised this section and tried to make our statement clearer.

Lines 321-331: It seems that you can only calculate the Fe isotope composition of mineral separates from the bulk rock and not the reverse, because the bulk rock is what you measured, correct?

>> We tried to introduce a formula that previous studies suggested in Lines 321-331, until Line 332 we started to calculate our own samples.

Line 334: Now I see mineral separates is what you called the “bulk rock” or “whole rock”. This should be clarified because at this stage it is very ambiguous. I would suggest to replace in this sentence “whole rock” by “bulk rock” and to remove the quotes. The quotes are very common in your manuscript, please remove them and find an appropriate word or use it very occasionally.

>> Thanks for all the comments, in fact, the questions from (18) to (21) come from the fact that we failed to make this calculation description clearly enough. We have revised with all these suggestions to made this calculation section more concise, and removed it to Appendix B.

Line 345: If I look at your figure 5c, I see that there is a significant variability within pyrrhotites, pentlandites and chalcopyrites. The variability can be more than one per mil.

Also, I do not see in your table 5 pyrrhotites with δ⁵⁶Fe values of ~-1.5 ‰. Do you mean δ⁵⁷Fe values in your plot? Please check and clarify, and correct your plot if necessary.

Maybe the δ⁵⁶Fe values do not vary with Fe content (except for magnetites), but there is some variability within mineral phases.

>> Actually, iron isotope data of minerals in figure 5c were all pure end-members that we calculated showed in Table 6b, while data in Table 5 were raw data including impure mineral separates. Thus, “pyrrhotites with δ⁵⁶Fe values of ~ -1.5 ‰” is actually -1.37 ‰ in Table 6b.

Line 348: I do not doubt that data presented in this study are of high quality but I would suggest removing the sentence “confirming high quality of the data”. I do not think the plot shown in figure 5a strictly shows the “high quality of the data”.

>> Thank you, we have revised this statement as you suggested.

Line 356: In your introduction you said that ~13% of Fe is in the mantle and ~87% in the core. Is the proportion of Fe in the sulfide and in the silicate is similar in the Jinchuan deposit than between the Earth’s core and mantle? If not, do you think it could have an impact on your model being representative of the Earth’s core and mantle?

>> Thank you for your helpful comments, we have calculated the proportion of Fe in our sulfide samples, take the sample JC-1 as example, with each sulfide content in sample (Po=13.43%, Cp=6.73%, Pn=6.82% in Table 8), the Fe content in each sulfide (Po=59.37%, Cp=30.23%, Pn=34.04% in Table 2) and Fe content in whole-rock (FeO_T=24.49%, thus, Fe=19.05%), we can obtain the proportion of Fe in the sulfide is ~64.72%, so the proportion of Fe in silicate is relatively ~35.28%. Compared with the core and mantle, there is a certain difference in the distribution of iron content between these two systems, but we think that the fractionation coefficient between the two systems should not be affect by the element content because both of them are under equilibrium fractionation.

Lines 384-385: rewrite the sentence, I don’t understand the meaning of “this logically sound conceptual understanding”. And the rest of the sentence is also not clear.

>> We have revised this sentence to make it clear enough as follows:

The above reasonable assumptions provide us with a new approach to investigate the difference (if any) of iron isotope compositions between silicate liquid represented by silicate mineral phases and sulfide liquid represented by sulfide mineral phases.

Line 388: why “iron isotope fractionation must take place between mineral phases”? This needs a reference. For example, Teng et al. 2008, but there are others.

>> Thank you for your helpful reference, we have added some references here.

Line 391: for a given magmatic system?

>> Thank you, we have revised the sentence as “for this magmatic system”.

Lines 380-394: this paragraph needs to be rewritten. It needs to be supported by literature work. Also, it is very vague.

>> Thank you and we have revised this section as suggested, and added some references.

Line 391: what is a “system”?

>> Thank you, we have revised the sentence as “for this magmatic system”.

Line 407: “Iron isotope composition compositional characteristics of different iron-bearing minerals”.

>> Thank you, we have revised as suggested.

Line 447: “heavy Fe isotopes are preferentially associated with prefers to be associated with high valent Fe³⁺.”

>> Thank you, we have revised as suggested.

Line 451: “the variably variable”.

>> Thank you, we have revised as suggested.

Line 454: why is “environments” quoted. What does it mean?

>> It means different ligand types “environments” for the metal elements.

Line 479: “more concentrated enriched”.

>> Thank you, we have revised as suggested.

Line 509: why is “hidden” quoted?

>> It means that the serpentine structures may have some Fe³⁺ that we can’t figure it out at present, so we used the personification as “hidden”.

Line 514: This sentence is useless or should be rewritten.

>> Thank you, we have deleted this sentence as suggested.

Line 519: “Consider Considering”.

>> Thank you, we have revised as suggested.

Line 522: “It is petrographically obvious from petrographical observations”.

>> Thank you, we have revised as suggested.

Lines 532-533: “In terms of understood scenario of “magma chamber” processes According to our hypothesized scenario of processes occurring in the magmatic chamber as illustrated in figure 6”.

>> Thank you, we have revised as suggested.

Line 530: This paragraph is long and it is difficult to follow the calculations for determination the Fe isotope composition of the sulfide melt and the silicate melt.

>> Thank you for helpful suggestions, we have shortened this paragraph and made it clearer as follows:

According to our hypothesized scenario of processes occurring in the magmatic chamber as illustrated in Figure 6 and discussion above, we can approximate iron isotope compositions of silicate liquid and sulfide liquid prior to crystallization using weighted mean iron isotope composition of the bulk-silicate minerals and bulk-sulfide minerals respectively. Hence, to understanding the iron isotope fractionation between the silicate liquid and sulfide liquid requires iron isotope composition reconstruction of these two phases in the net-textured ore samples (JC-1, JC-2 and JC-3). This will form the basis to discuss the possible iron isotope fractionation during earth’s core formation.

Line 537: “to understand understanding”.

>> Thank you, we have revised as suggested.

Line 530-542: This paragraph is clumsy and needs to be rewritten. It is difficult to understand what exactly you want to say here.

>> Thank you, we have shortened this paragraph and made it clearer as above.

Lines 560-561: “that the sulfide minerals are solidified as equilibrium crystallization are at equilibrium during crystallization”.

>> Thank you, we have revised as suggested.

Line 605: “its heavier Fe isotope composition”.

>> Thank you, we have revised as suggested.

Line 629: “that will not change involve iron isotope change”. This sentence should be rewritten, it is beyond understanding. Maybe you mean “will not involve Fe isotopic fractionation”?

>> Thank you, we have rewritten this sentence as follows:

“the final stage of core separation must…that will not involve iron isotope fractionation”

Lines 618-642: In your model you never consider the effect of pressure on Fe isotope fractionation. Whether the pressure has an impact on Fe isotope fractionation is unclear yet, but at least this should must be mentioned somewhere in the text. Processes that occurred in the Jinchuan Sudbury-type ore are not strictly an analog of processes occurring in the Earth’s core, in particular because the pressure and temperature must be very different.

>> We have added some references of pressure impact on Fe isotope fractionation.

Line 718: “That is, between mineral Fe isotope fractionation is largely independent”. What does this sentence mean?

>> Thank you, we have deleted this sentence to prevent misunderstanding.

Line 722: “from which they crystallized/solidified”.

>> Thank you, we have revised as suggested.

Line 725: “the reconstruction of the bulk-silicate is less demanding determination of the Fe isotope composition of the bulk-silicate is easier because…”.

>> Thank you, we have revised as suggested.

Point-to-point the details of the revisions:

(In the order they were revised in the manuscript, revisions of typographical error and words were not given)

At title we have used “composition” to replace “characteristics”;
Unifies the use of words “Whole” and “Bulk”, such as “bulk earth”, “bulk-rock ore”, “bulk-silicate minerals (phase)”, “bulk-sulfide minerals (phase)”;

Introduction:

More clearly expressed what is being done in our study “For this reason, the isotope composition of the coexisting minerals…as indicated in the title of this paper and detail below”;
Some previous discussions on iron isotope composition of mantle and chondrite have been added, “However, some studies argued that peridotites are not necessarily…(or fractionation during earth’s core-mantle segregation).”;
Previous experiments (e.g., HTHP) and reported NRIXS data on iron isotopes during Earth’s core formation and how we come up with the Sudbury-type mineralization to solve the scientific problem have been added, “revious understanding of iron isotopes during…This approach has two assumptions:”;
Caption of figure 1 have been rewritten;

Sample and methods:

Sample descriptions have been rewritten mainly added the types of ore samples (e.g., dunite), dominated silicate minerals and the content and origin of altered minerals (e.g., phlogopite, dolomite, etc.);
GSB Fe standard relative to IRMM-014 and what is the alpha Fe standard have been added;
Data and other values have reported with 2 digits instead of 3 digits somewhere before revised;

Results:

The descriptions of the calculations have been rewritten, and removed to Appendix B (Appendix B from last version of manuscript have been changed in a corresponding into C);
We have added some notes that the iron isotope compositions of sulfide minerals (i.e., pyrrhotite, pentlandite and chalcopyrite) and magnetite in the only one disseminated ore (JC-4, from metasomatic orebody) were within the range of net-textured (dunite and lherzolite) ores, we thus have not treated these sulfide minerals specifically or explain them separately;

Discussion:

Previous works have been added on iron isotope compositions of iron meteorites and troilite, iron isotope fractionation between coexisting metal and silicate phases, possible pressure effect to iron isotope fractionation between metallic and silicate phases, “Previous studies suggested iron meteorites…(e.g., P=130 GPa, Polyakov et al., 2009).”;
We have added some sentences to claim that our results of Sudbury-type ores is the first step to introduce a useful basis for continued study towards better understanding iron isotope fractionation of the core-mantle segregation, which need more verification and further studies in the future;
We have noted that we suggested that Different occurrence of mineral phases between Sudbury-type ores and core-mantle segregation may be exist, but we thought the mineral assemblages crystallized from these two melts in different systems would not affect the previous iron isotope fractionation between these two melts;

Conclusions:

Some statements on how we come up with possible Ni isotope fractionation between earth’s core-mantle segregation as Fe isotope, “As metallic iron is primarily present…in the earth’s core-mantle segregation as Fe isotope.”;

Appendix:

We have added a new Appendix B, so the previous Appendix B have been changed into C;

References:

Some previous references have been added, the sequence number of references have been changed in revised version of our manuscript, and the list of references below have been remade.

Tables:

We have placed Table 7 at more appropriate position within the text.

Reference:

Polyakov, V. B. (2009). Equilibrium iron isotope fractionation at core-mantle boundary conditions. Science, 323(5916), 912-914.

Dauphas, N., John, S. G., & Rouxel, O. (2017). Iron isotope systematics. Reviews in Mineralogy and Geochemistry, 82(1), 415-510.

Shahar A, Schauble EA, Caracas R, Gleason AE, Reagan MM, Xiao Y, Shu J, Mao W (2016) Pressure-dependent isotopic composition of iron alloys. Science 352:580−582.

Liu J, Dauphas N, Roskosz M, Hu MY, Yang H, Bi W, Zhao J, Alp EE, Hu JY, Lin J-F (2017) Iron isotopic fractionation between silicate mantle and metallic core at high pressure. Nat Commun 8, 14377.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have done a serious work both in answering my criticisms and in re-organize their work accordingly. This effort must be acknowledged with thanks.

Nevertheless, I cannot agree with them. I cannot be as enthusiastic as they are with their main idea which, as they repeatedly claim, continue to be the same from the beginning, i.e., that Fe isotope fractionation in the Sudbury-type mineralization they have studied constitute a valid analog to the Fe isotope fractionation during the global event of Earth's mantle-core separation.

Maybe studies as that of the authors propose are a must do, as they claim -or maybe not. But in any case, a much more detailed discussion is needed before accepting the authors' quantitative calculations, even as a rough, simple approach. I believe that this is so because the process(es) they have studied involve only low-pressure minerals and melts, plus secondary phases, in part related to serpentinization as the authors recognize.

Accordingly, I think that publication of this issue could only be acceptable if two pre.requisits are accepted by the authors: a) they should state very clearly the highly speculative character of their proposal, and b) They should explicitly accept that the significance of the calculations they offer is very limited, at least until a more detailed discussion of the possible (and probable!) effect of Fe⁺³-bearing, high P-T mineral phases could be pertinently evaluated.

To me, the publication of this paper should be possible only after satisfying these requisits -Just applying the benefit of doubt to very controversial issues.

Author Response

Comments and Reply:

The authors have done a serious work both in answering my criticisms and in re-organize their work accordingly. This effort must be acknowledged with thanks.

>> We thank you for your precious time and useful comments, and for your approval of our revision.

Nevertheless, I cannot agree with them. I cannot be as enthusiastic as they are with their main idea which, as they repeatedly claim, continue to be the same from the beginning, i.e., that Fe isotope fractionation in the Sudbury-type mineralization they have studied constitute a valid analog to the Fe isotope fractionation during the global event of Earth's mantle-core separation.

>> Thank you for candid expression of disagreement. As a serious scientific contributor, journal reviewer and editor, I (Yaoling Niu) do not think your disagreement is the basis for rejection. We do not claim that we have solved the biggest earth problem, but objectively and rigorously advocate that the Sudbury-type mineralization offers the best material available to approach the problem. If you disagree on this approach, we can openly debate, which will give you the opportunity to make comments and we would appreciate your alternative and better approach. This would be the best scientific practice. Science is a self-correcting process and we are happy to correct or improve in the future by ourselves, or more importantly, if this study can excite others to pay attention to this global problem, then we have achieved > 50% of our motivation and objective. Again, we used the words “hypothesize”, “may” and “must have” (inference), which are by no means “claiming”. Thank you (from Yaoling Niu).

Maybe studies as that of the authors propose are a must do, as they claim -or maybe not. But in any case, a much more detailed discussion is needed before accepting the authors' quantitative calculations, even as a rough, simple approach. I believe that this is so because the process(es) they have studied involve only low-pressure minerals and melts, plus secondary phases, in part related to serpentinization as the authors recognize.

>> Thank you. You have a valid point! Please avoid using the wording “claim” because we do not claim. Yes, we plan to do more on this by using the Sudbury-type ore samples from other places in China and overseas (Canada and Russia) with the hope to obtain 100% fresh samples, but that is beyond the scope of this current paper. (Yaoling Niu)

Accordingly, I think that publication of this issue could only be acceptable if two pre requisites are accepted by the authors: a) they should state very clearly the highly speculative character of their proposal, and b) They should explicitly accept that the significance of the calculations they offer is very limited, at least until a more detailed discussion of the possible (and probable!) effect of Fe⁺³-bearing, high P-T mineral phases could be pertinently evaluated.

To me, the publication of this paper should be possible only after satisfying these requisites -Just applying the benefit of doubt to very controversial issues.

>> Thank you. We think you will have found that our discussion of the data and interpretation of the processes with caveats are satisfactory already if you do not have pre-conditioned view of our “claim”, which we do not. Nevertheless, we have provided additional discussion in revision (Yaoling Niu) as follows:

Our discussions on the iron isotope fractionation between immiscible sulfide and silicate liquids are based on the containing mineral phases of the unique net-textured ores with iron with Fe²⁺. The P-T conditions during formation of these mineral phases are far lower than core-mantle separation conditions, which CANNOT be at the present-day core and deep mantle conditions, but throughout the entire earth – as shallow as the surface of the planet back > 4.5 Ga to as deep as the core simply because ~ 87% of the bulk chondritic earth would have to sink into the core depth! This cannot take place in the form of metallic Fe to sink into the deep earth, but in the form of sulfide-silicate liquid segregation as the very first stage. One must understand this basic principle of chemistry and physics because of the strong Fe-S and Fe-O bonding. (Yaoling Niu)

Thus, bulk-silicate phase in our net-textured ore dominated by spinel-facies peridotite can reflect the shallow separation less than 60 km of the early stage, but more probable phases from deeper earth must be further studied, such as garnet-facies peridotite (60-170 km), whose iron isotope fractionation between coexisting mineral phases may be quite different from the above results. Previous studies suggested that controlled by differences in the coordination environment of cations between coexisting minerals, heavier isotopes are concentrated in lattice positions with stronger bond energy (Bigeleisen and Mayer, 1947; Urey, 1947). This is maybe why garnet were reported having lighter iron isotope composition than olivine when under an iron isotope balance indicated by analyses on garnet-facies peridotite (Beard and Johnson, 2004; He et al., 2017b; Li et al., 2016a; Williams et al., 2009). Thus, higher P-T bulk-silicate phases may have lighter iron isotope compositions than those of net-textured ores. Besides, given the preference of heavier iron isotopes to the phases with higher Fe³⁺ or lower Fe²⁺ (Dauphas et al., 2017), it is expected that the involvement of Fe³⁺-bearing minerals (e.g., magnetite, hematite) may lead to heavier iron isotope compositions of these phases. (Peiyao Wang)

Author Response File: Author Response.pdf

Reviewer 2 Report

Please see my review attached.

Comments for author File: Comments.pdf

Author Response

Comments and Reply:

The authors have made significant efforts to improve their manuscript and I thank them for their detailed response to my comments. Nevertheless, I still have a few comments on the new version of the manuscript, which I detailed below.

>> We thank you for your precious time and useful comments, and for your approval of our revision.

**Line279: It is a little bit weird to report it like that. I would just say that the Fe isotope composition of GSB relative to IRMM-014 is δ⁵⁷Fe_IRMM-014 = 1.073 ‰ and δ⁵⁶Fe_IRMM-014 = 0.729 ‰.

>> Thank you! We have revised accordingly.

**Line 424: maybe you could say magmatic processes instead of “magma chambers” processes?

>> Thank you for your suggestion, we have reworded here and elsewhere if needed.

**Line 435: “because of the well-understood known fractionation factor”.

>> Thank you and we have revised accordingly.

**Line 462: Maybe you could say coordination environments instead of “environments”?

>> Thank you, we have changed as per your suggestion.

**Line 470: why do you give error bars for pentlandite and not chalcopyrite?

>> Thank you for this. Our pentlandite separates are all pure to give us actual analytical data with error bars from table 5 and 6b. However, the pure end-members of chalcopyrite and pyrrhotite (table 6b) are calculated from their impure mineral separates (table 5). Anyway, we have deleted all the error bars there to avoid misunderstanding.

**Line 584: “the difference of iron isotopes composition”.

>> Thank you, we have added the word in revision.

**Lines 589-590: “take the analytical value plus this kind of “lighter” to give…”. Rewrite the sentence, what do you mean by “this kind of lighter”? This is very approximate. And please reduce the use of quoted words (I think you reduced them compared to your version 2 but there are still many), use appropriate terms instead, try to find the appropriate word without having to use quotes.

>> Thank you. We have changed the sentence to make it clearer, and we have also reduced the quoted words.

**Line 622: This is more like a calculation than an “analysis”. I would write something like “Calculations yield Fe isotope composition for the silicate liquid…”.

>> Thank you, we have revised accordingly.

**Line 629: “sulfides tend to preferentially incorporate choose lighter Fe isotope”, or something similar but a mineral phase does not choose an isotope.

>> Thank you. We have revised accordingly.

**Line 645: “in a the diamond anvil experiment”.

>> Thank you, we have changed.

**Line 667: “prefers to host preferentially hosts”.

>> Thank you, we have changed.

**Lines 703-704: “heavy Fe isotope enrichment and light Fe isotope depletion” this sentence says twice the same thing, please remove one. Also for “while heavy Fe isotope depletion and light Fe isotope enrichment in the coexisting pyrrhotite”.

>> Thank you, we have removed one of them as you suggested.

**Line 723: “silicate Earth”.

>> Thank you, we have made the change.

**Line 725: “potential” instead of “probable”.

>> Thank you, we have made the change.

**Line 727: “Earth”.

>> Thank you, we have made the change.

**Lines 728-729: “We predict significant core-mantle Ni isotopic fractionation…” In fact, I do not see the purpose of this sentence here. Your study is about Fe isotopes, and you do not say anything useful in this sentence. This sentence is out of place. I would suggest to remove it or at least expand a little on why you predict significant Ni isotopic fractionation. This is one sentence in the middle of a paragraph about Fe isotopes.

>> Thank you. But we keep here because the Sudbury-type ores are Fe-Ni-Cu deposits and the Fe-Ni are very relevant to the earth’s core. But, yes, we reworded here.

**Lines 730-734: please rewrite this sentence, it is very long.

>> Thank you, and we have revised shortened it.

Author Response File: Author Response.pdf

Article Menu

Iron Isotope Compositions of Coexisting Sulfide and Silicate Minerals in Sudbury-Type Ores from the Jinchuan Ni-Cu Sulfide Deposit: A Perspective on Possible Core-Mantle Iron Isotope Fractionation

Point-to-point the details of the revisions:

Reference:

Point-to-point the details of the revisions:

Reference:

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