Fluid-Induced Inhomogeneous Cr-spinel in Dunite and Wehrlite from the Duke Island Complex, Southeastern Alaska
Round 1
Reviewer 1 Report
Review report of the MS# minerals-1699582 entitled «Fluid-induced inhomogeneous Cr-spinel in dunites and wehrlite from the Duke Island Complex, southeastern Alaska» by Yang Luo, Ben-Xun Su, Joyashish Thakurta, Yan Xiao and Yang Bai.
A brief summary
Chromspinels are the sensitive petrological indicators of partial melting, differentiation of mafic-ultramafic series, conditions of crystallization and tectonic settings. Usually chromspinels are represented by homogeneous or zonal grains. Inhomogeneous or unmixed Cr-spinel is rather rare and was described only in several papers. So, the new occurrence and investigation of such chromspinel is very interesting and may provide a new knowledge about the formation of these minerals and their meaning for the genesis of host rocks. Especially it is important for Uralian-Alaskan-type intrusions which were formed from the fluid rich ultramafic or mafic melts. The unmixed chromspinel has never been described before in the Duke Island complex, southeastern Alaska, which has been studied and well known in scientific literature. So, this study has a great interest for broad society of geologist and petrologist.
There are several comments and remarks for the authors of presented manuscript. Here it is only the most important. Others are presented in the PDF file of manuscript as comments.
Page 2. Chapter 2. Geological setting and sample descriptions
The chapter 2 has title "Geological setting and sample descriptions". But there are no descriptions of the studied samples in this chapter. There are only general descriptions of rock not related with the studied samples. It is not clear how many samples were studied, what kinds of rocks were investigated and from what places and real geological settings they were taken. No samples number on the map (Fig. 1). According to table 1, authors have studied 4 samples. It will be useful to present a true description of each samples and add the chemical compositions of these rock if they are.
Page 6. Table 1.
Reviewer: There are 26 chromspinel analyses from 4 samples in this table. But we can’t recognize from what type of rock this data was obtained. It will be useful to group type B chromspinel on B1, B2 and B3 subtypes and on Al-rich and Fe-rich varieties.
Page 7. Line 189-190. Figure 4. BSE images and multi-element mapping images of type A Cr-spinel in dunite. Type A spinel is subhedral in 189 shape and homogeneous in chemistry
Reviewer:There are a lot of small dots on Cr, Al and Fe maps. It is very likely, if you will use more magnification for type-A cromspinel, perhaps, you will find and see microscale (nanoscale) embryonal unmixing textures. We have such examples.
Page 7. Line 198-199. Type B1 Cr-spinel, showing complete separation of light and dark gray Cr-spinel with 198 well-defined boundaries, but irregular unmixing patterns (Fig. 5a-m).
Reviewer: Such textures can be explained by co-crystallization of two phases on the solvus surface. What are the evidences that B1 is a product of unmixing, but not a result of co-crystallization? For example, green spinel and magnetite are often coexisting in pyroxenites and form intergrowths.
Page 10. Line 262-263. Relict dark lamellae of Al-rich Cr-spinel are spotted in some part of the cataclastic Cr-spinel
Page 11. Line 275. Figure 8. BSE image of type C spinel with relict Al-rich spinel
Reviewer: In generally, the presented compositions of type C oxides correspond to chrom-titanomagnetite (table 1), typical for wehrlite or clinopyroxenite, but not for dunite. Al-rich spinel can be a product of unmixing of primary titanomagnetite solid-solution. Why these are relicts of Al-rich spinel?
Page 12. Line 290-292. In this study, type C Cr-spinel grains are cataclastic and completely altered into ferritchromite and Cr-bearing chlorite (Fig. 7), which suggests that hydrothermal alteration changed the composition of type C Cr-spinel.
Reviewer: This is a question. Usually, wehrlite contains chromagnetite, chrom-titanomagnetite or titanomagnetite, but not high-Cr chromspinel like in high-Mg dunite. The compositions of type-C cromspinel from table 1 are corresponded to Cr-titanomagnetite. Such Cr-titanomagnetite is in equilibrium with primary silicates of wehrlite (olivine and clinopyroxene) and crystallized together with them. It doesn't need to be formed from the Al-rich chromspinel during postmagmatic transformation. Spots of Al-rich spinel inside of such Cr-titanomagnetite can be the result of unmixing of solid-solution. The euhedral (roundish) shape of type C spinel argue that this is the shape of primary mineral.
Page 12. Table 2.
Reviewer: It will be useful to note what minerals are from dunite and what are from wehrlite.
Page 15. Line 348-351. The unmixed Cr-spinel was also reported from other Alaskan-type complexes (Garuti et al., 2003; Krause et al., 2007; Ahmed et al., 2008; Habtoor et al., 2016). All rocks containing the unmixed Cr-spinel show evidence of Fe3+-enrichment during metamorphism or crystallization.
Reviewer: Garuti et al. (2003) have noted that dunite bodies have zoning. High-Mg (forsterite) dunite containing high-Cr chromite occur in the central parts of bodies. These chromites don't have unmixing textures. Towards to marginal zone of dunite bodies the Mg-numbers of dunite and olivine decrease, and accessory Cr-spinel became more Fe and Al rich. Homogeneous chromite close to solvus and chromspinel with unmixing textures belong to the marginal dunite enriched in Fe. In Garuti's paper the whole trend of chromite crystallization up to approaching of solvus was shown. It was described as a magmatic trend. The high-Mg dunite of the Duke Island complex contains high-Cr chromite (40-46 wt.% Cr2O3, Thakurta et al., 2008) without unmixing textures. It will be interesting to know the relationships between high-Mg dunite with high-Cr chromite and studied samples presented in this manuscript.
Page 17. References
Reviewer: References must be numbered in order of appearance in the text. It is due to rules of Minerals.
Comments for author File: 
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Reviewer 2 Report
The authors of this study show that chemical and textural analyses of spl in mantle rocks can reveal interaction with oxidised fluids. This would have important implications for the application of oxy-themobarometers.
The selected samples are certainly very interesting, but the presentation and description is not always straightforward. The Abstract must be improved by citing how many samples were investigated, the different types of spl and providing a temporal order to the proposed mechanisms. The main text would benefit of: - A table to summarize the main features of the three type of spinel along with the chemical composition. Otherwise, the text is hard to follow. -It is hard to understand the difference of spl between dunite and wehrlite and within one sample. - textural and chemical features must be linked with the right panles in the Figures. - when proposing interaction with an oxidised fluid, this should be discussed in terms of origin (where and when), chemical composition and reaction mechanism. - the Fe3+ of spl is not accurately determined. But, most importantly, there is no answer, no application to the initial motivation of the study, i.e. effect on the P-T-fo2 whn using spl-bearing oxythermobarometry. However, the authors can calculate these variables since they know the composition of the coexisting phases. What is the equilibrium P-T of these rocks? How do P-T change after elements remobilization? Nice study, but it needs to be completed. Best wishesComments for author File: Comments.pdf
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Round 2
Reviewer 2 Report
The authors revised their manuscript accurately. All issues were adddressed, so I recommend publication in the present form.
Author Response
On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, we appreciate for your positive and constructive comments. We have tried our best to revise our manuscript according to the comments.
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