Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

Round 1

Reviewer 1 Report

This manuscript describes chromitites from two different environments forming two kinds of occurrences: podiform and stratiform. It shows that in spite of various formation setting both kinds of chromitites contain similar set of silicate solid inclusions. Author suggests that it may imply a common origin of chromite grains in Earth mantle. This suggestion is supported by convincing arguments including field, mineral and chemical observations. Author does not want to prejudge this scenario and calls it as a hypothesis, nevertheless his interpretation is very probable and may cause discussions regarding origin of chromitites in the future.

In my opinion, this manuscript is ready for publication. However, I would read there about an excess of chromium causing formation of chromitites. As we know, basaltic magmas responsible for chromitite formation are not very rich in chromium. Thus, through many years petrologists tried to find factors controlling precipitation of chromitites. One possibility is an interaction between melts and hydrated mantle (Borisova et al., 2012, JPetrol). Would their mechanism also be applicated to your hypothesis? If yes, what other evidence from layered intrusions would suggest their formation from hybridized melts?

Nevertheless, my question remains only one suggestion which does not diminish this manuscript.

Author Response

Thank you very much for your suggestions. I add one sentence according to your comment above at the bottom of the first paragraph of Discussion (lines 171-173).

Reviewer 2 Report

Review on the article by Shoji Arai

Genetic Link Between Podiform Chromities In The Mantle

And Stratiform Chromitites In The Crust: A Hypothesis

The author of the paper provided an interesting hypothesis that the chromites are generated at first at the mantle level by the reaction of the harzburgites with the H2O –Na2O bearing melt then the melt extracts chromites to the essentially dunite channels forming podiform bodies and after melts rise these chromitites to the upper layers and precipitated them as the layers in the stratified complexes. The base of this hypothesis is the present of the Na2O rich melt inclusion \s with the variable SiO2 content,

 I should say that the number of papers devoted to podiform chromitites only in Science direst is > 500 90 though the papers which refer to both stratified and podiform chromites are 10 time less ~ 45.

What is attractive in this idea it could explain relatively high concentration of the Cr2O3 in dunites. But these chromites in mantle level are different in composition and the author knew this – he co-author of the paper of (Habtoor et al., 2017). And cumulates in the layered complexes in ophiolites and of course in the crusts stratified intrusions are much more different both in major components being higher in Ti, Al and Fe and in PGE changing specialization from the Os-Ir to Pt-Pd .

In what I agree that H2O-bearing melts could produce chromitites both in the mantle and at the crust levels / H2O produces the effect of the decreasing pressure and thus it agees wiu th eth R Latypov (2020) theory with the expanding of the Chr crystallization field with the decreasing pressure. I should say that in case of the boinite- the most productive for the chromitites and oceanic MORB and intraplate plume magmas like Norilsk the mechanism of the chromite precipitation is different For the latter the effect of t e assimilation of the crust materials and abrupt increase of the oxygen fugacity is much higher. And the dynamics of the intrusion also is very important. For the Dovyren it is supposed the intrusion of the mush already compared the Ol – Chr crystals and the second intrusions of the highly oxidized melt produced the creation of the Chr –PGE rich horizon. For the Norilsk two major hypotheses are working - 1 – or several intrusions of plume basalts and mixing with the assimilated wall material and oxidation and precipitation. The other one suggests the pervasive jet stream of the deep material passing through the several magmatic chambers accompanied by the Ol precipitation and containing high concentration Cr and PGE (Pd) and also continuous and progressive reactions with the assimilated crust,

            For the Oceanic MORM magmas the difference is much less.

But author did not detailed his idea At first it was necessary to provide more information abou the melts inclusions- the bulk compositions including Ti, Al and Fe and other components. Will be interesting to have LA ICP analyses also. From my experiences Such analyses may give quantitative compositions of the PGE also. The second wat is necessary to provide the results of the estimation of zonation of different types of chromitites – stratified, reactional harzburgitic and podiform.

            As the specialist in the Oman ophiolites prof юShoji should provide the example of both three types of chromites and give their compositional variations. …

So is suggest that at first author should explain how the extracted from harburgites crystal had changed their compositions - was it reaction diffusion or something else. And have they changed their specializations from the Os-Is to Pt-Pd

The conclusion so major revision

Author Response

Thank you very much for your suggestions above. I deeply understand your points and the criticism on the paucity of detailed petrological and chemical data in this article. But please note that most of your points have been already refer to in the manuscript. This short article is only aiming to propose a genetic model linking the podiform and the stratiform chromitites. And detailed petrographical and chemical data can be obtained from the literature or will be published in separate papers elsewhere. The model is thus very speculative but may be much better to explain the origin of the chromite-hosted inclusions. Above all, this article will promote our discussions on the possible linkage between the two types of chromitite in the future.

I insert one sentence to stress this situation more in around the end of Introduction (lines 36-39). I also add one paragraph at the last chapter (Conclusions and implications; lines 223-230), referring to some more previous works on the layered intrusion.

Round 2

Reviewer 2 Report

I suggest that like an idea this paper could be published if the author does not like to give the detailed model in this publication. I'd like the author explains at least the behavior of PGE during chromite transformations.