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the Creative Commons Attribution 4.0 License.
| 30 Mar 2023
A discovery of nanoscale sulfide droplets in MORB glasses: Implications for the immiscibility of sulfide melt and silicate melt
Abstract. Sulfur forms an immiscible liquid upon saturation in magma, and sulfide droplets were commonly found in fresh mid-ocean ridge basalt (MORB) magmas. In this paper, scanning electron microscopy (SEM) determined that MORB samples were primarily fine-grained and weakly phyric, with hypocrystalline to vitreous textures. A focused ion beam cut from the MORB glasses examined by transmission electron microscopy (TEM) revealed a range of nanoscale sulfide droplets (10–15 nm), featuring rounded shapes and smooth edges. Texturally, the droplets were crystalline and homogeneous in composition. Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are less in the droplets. Previous reports have elucidated the immiscibility between sulfide and silicate melts, and the structure of the silicate melt also affects the size distribution of sulfide droplets. This is the first report on nanoscale sulfide droplets within MORB glasses, and those results indicated that nanoscale sulfide droplets were the initial phase of sulfide saturation; such insight may prove useful in understanding how siderophile and chalcophile elements behaved during sulfide crystallization. In addition, this study determined the immiscibility of sulfides and silicate melts occurred in the early nanometer stage, the immiscibility of sulfides in magmatic Ni-Cu sulfide deposits was the key to the formation of magmatic Ni-Cu sulfide deposits. Therefore, all immiscibility phenomena may occur in the nanometer stage during magma evolution.
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RC1: 'Comment on egusphere-2023-213', Anonymous Referee #1, 11 May 2023
This paper aims at ”a deeper understanding of the initial solidification of sulfide-oxide liquids” (line 70-71) using an example from MORB-type glasses. Although the paper reports nanoscale sulphide globules two main conclusions that “nanoscale sulfide droplets were the initial phase of sulfide saturation” and “all immiscibility phenomena may occur in the nanometer stage during magma evolution” remain unsupported. Clearly, liquid immiscibility occurs on all scales, including nanoscale, which follows from results of numerous experimental and melt inclusion studies. Therefore, the results by Lei Zuo et al. are neither novel nor particularly significant. This study does not contribute to “understanding how siderophile and chalcophile elements behaved during sulfide crystallization” (line 35) and has nothing to do with identifying “the key to the formation of magmatic Ni-Cu sulphide deposits” through the study of sulphide unmixing in MORB magmas. The 4. RESULTS section is only 7 lines long!!!!!
The manuscript is sprinkled with the statements that are awkward and often wrong. Just a few examples are given below
lins 24 fresh mid-ocean ridge basalt (MORB) magmas
lines 29-31 Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are in the droplets.
lines 39-40 all immiscibility phenomena may occur in the nanometer stage during magma evolution.
lines 46 with liquid immiscibility the more important for magma evolution
lines 58 The sulfur in MORB magma is saturated before magma eruption…
lines 65-66 the sulfide droplet sizes influence the physical behavior of the separate sulfide phases
lines 66-67 the different of sulfide droplets 67 also plays an important role in partitioning the siderophile and cupolophile elements ….
lines 95-97 As determined by SEM, the MORB samples were primarily fine-grained and weakly phyric with hypocrystalline to vitreous textures (Fig. 2a). Some phenocrysts were 1-10 μm and relatively rich in Si and O (Figs. 2b-f, 3a-c).
Please note that surfaces shown on Figs. 2 and 3 are dusted, but reported as “weakly phyric”)))
lines 158-159 In this study, nanoscale sulfide droplets in natural MORB glass are reported for the first time, thus demonstrating S-saturated fractionation in another way.
lines 186-187 The experimental results show that when the composition of silicate melt becomes immiscible, two-phase nanoscale droplets will soon appear
lines 212-213 chalcophile elements appeared to enter the sulfide droplets and distribute evenly within the sulfide globule in the early stage
lines 218 5.4 Implications of sulfide droplets on magmatic evolution and formation of sulfide deposits
lines 222-223 The earlier immiscibility begins during magmatic evolution, the better its influence on magmatic evolution, and the better its geochemical and petrological significance
lines 226-227 As one of the immiscible two phases, the immiscibility of sulfide droplets and silicate melt plays an important role in magma evolution
lines 232-234 The study shows that the formation of magmatic Ni-Cu sulfide deposits is related to the separation and enrichment of sulfur saturated and immiscible sulfide liquids in mantle-derived basic and ultrabasic magmas (Arndt et al., 2005)
lines 245-247 this new understanding of sulfide and silicate melt immiscibility, which occurs during the early nanometer stage provides a new idea for further study of the immiscibility stage during magma evolution.
lines 72-82 the whole paragraph, and in particular, “Hawkings et al (2020) reported that Greenland Ice Sheet meltwaters may provide biolabile particulate Fe that fuels the large summer phytoplankton bloom in the Labrador Sea (Hawkings et al., 2018)” is irrelevant to this study.
Citation: https://doi.org/10.5194/egusphere-2023-213-RC1 -
AC1: 'Reply on RC1', Lei Zuo, 13 Jun 2023
- Although the paper reports nanoscale sulphide globules two main conclusions that “nanoscale sulfide droplets were the initial phase of sulfide saturation” and “all immiscibility phenomena may occur in the nanometer stage during magma evolution” remain unsupported. Clearly, liquid immiscibility occurs on all scales, including nanoscale, which follows from results of numerous experimental and melt inclusion studies. Therefore, the results by Lei Zuo et al. are neither novel nor particularly significant.
Re: Thank you for your comment, I agree with your point of view that immiscibility can indeed occur at any stage, but it is only based on the conclusions of previous inferences and experimental simulations, and the immiscible phenomenon has not been directly observed in natural samples. However, in our study, we did observe immiscibility, which is a very intuitive explanation for the immiscibility that occurs at the nano stage.
- This study does not contribute to “understanding how siderophile and chalcophile elements behaved during sulfide crystallization” (line 35) and has nothing to do with identifying “the key to the formation of magmatic Ni-Cu sulphide deposits” through the study of sulphide unmixing in MORB magmas.
Re: Thank you for your comment, regarding the behavior of the siderophile and chalcophile elements you mentioned, because the study is based on microscopic scale nanoparticles, which are in the initial stage of mineral formation, the study of the element enrichment mechanism in sulfide droplet is helpful to understand the siderophile and chalcophile elements behaved during sulfide crystallization. At the same time, I agree with your mention that it is not critical to explain the formation of sulfide deposits with the immiscibility of MORB sulfide droplet. But the basic principle of the formation of magmatic sulfide deposits origins stems from the sulfur saturation of magma. Therefore, the immiscibility of MORB sulfide droplet may have some research significance during the formation of sulfide deposits, but this requires further in-depth research to explain.
- The 4. RESULTS section is only 7 lines long!!!!!
Re：Thank you for the suggestion. The conclusion section I've changed to:
Nanoscale sulfide droplets were first identified in MORB glasses by FIB-cut and TEM analyses. These droplets might form rapidly before eruption and then undergo immediate supercooling, and these droplets have characteristics of smooth edges and crystalline features. The discovery of nanoscale sulfide droplets in natural MORB glass has demonstrated a new form of sulfur saturation exists. Therefore, the nanoscale sulfide droplets are more likely to be the initial stage of sulfide saturation. In addition, the sulfide droplets in this study are crystalline, and their lattices may have formed at this stage. As the initial phase of sulfide saturation, the chalcophile elements enter the sulfide droplet in the early stage and distribute uniformly in the droplet, rather than selectively enrich the siderophile and chalcophile elements. However, it is still unclear when the siderophile and chalcophile elements behave inconsistently in sulfide droplets, which warrants further investigation. Furthermore, according to the element distribution map of sulfide droplets, it can be seen that sulfide droplets are relatively enriched with Fe, Cu, Ni and Na, but lacked Si, O and Al, which further indicates that the immiscibility of sulfide and silicate melt occurs in the early nanometer stage, which provides new ideas for further study of the stage of immiscibility stage during magma evolution.
- lines 72-82 the whole paragraph, and in particular, “Hawkings et al (2020) reported that Greenland Ice Sheet meltwaters may provide biolabile particulate Fe that fuels the large summer phytoplankton bloom in the Labrador Sea (Hawkings et al., 2018)” is irrelevant to this study.”
Re：Thank you for the suggestion, I have removed irrelevant sentences.
- “The manuscript is sprinkled with the statements that are awkward and often wrong.” Below is the revised sentence.
lines 24 “fresh mid-ocean ridge basalt (MORB) magmas” charge to “fresh mid-ocean ridge basalts (MORB) samples”
lines 29-31 “Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are in the droplets.” charge to “Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while Si, Al and O are lacking.”
lines 39-40 “Therefore, all immiscibility phenomena may occur in the nanometer stage during magma evolution.” charge to “Therefore, it is speculated that all immiscibility phenomena may occur in the nanometer stage during magma evolution.”
lines 46 “with liquid immiscibility the more important for magma evolution.” charge to “among which liquid immiscibility the more important for magma evolution.”
lines 58 “The sulfur in MORB magma is saturated before magma eruption…” charge to “The MORB magma is S-saturated before eruption…”
lines 65-66 “the sulfide droplet sizes influence the physical behavior of the separate sulfide phases” charge to “the size of the sulfide droplets formed affects the physical behavior of the separated sulfide phases”
lines 66-67 “the different of sulfide droplets also plays an important role in partitioning the siderophile and cupolophile elements….” charge to “the separation of sulfide droplets also plays an important role in the distribution of siderophile and cupolophile elements…”
lines 95-97 “As determined by SEM, the MORB samples were primarily fine-grained and weakly phyric with hypocrystalline to vitreous textures (Fig. 2a). Some phenocrysts were 1-10 μm and relatively rich in Si and O (Figs. 2b-f, 3a-c).” charge to “The results of SEM analysis showed that the MORB samples were mainly fine-grained and weak porphyry, with hypocrystalline to vitreous textures. The size of some phenocrysts is 1 ~ 10 μm and relatively rich in Si and O.”
lines 158-159 “In this study, nanoscale sulfide droplets in natural MORB glass are reported for the first time, thus demonstrating S-saturated fractionation in another way.” charge to “In this study, the discovery of nanoscale sulfide droplets in natural MORB glasses demonstrates a new way of S-saturated fractionation.”
lines 186-187 “The experimental results show that when the composition of silicate melt becomes immiscible, two-phase nanoscale droplets will soon appear” charge to “The experimental results show that when the composition of silicate melt becomes immiscible, the phase separation of sulfide nanoscale droplets and silicate nanoscale droplets will soon appear.”
lines 212-213 “chalcophile elements appeared to enter the sulfide droplets and distribute evenly within the sulfide globule in the early stage” charge to “it can be inferred that chalcophile elements enter sulfide droplets in the early stage and are uniformly distributed in the range of sulfide droplets.”
lines 218 “5.4 Implications of sulfide droplets on magmatic evolution and formation of sulfide deposits” charge to “5.4 Significance of sulfide droplets on magmatic evolution and sulfide deposit formation”
lines 222-223 “The earlier immiscibility begins during magmatic evolution, the better its influence on magmatic evolution, and the better its geochemical and petrological significance” charge to “The earlier immiscibility begins in the magma evolution, the greater the impact on magma evolution and the greater its geochemical and petrological significance”
lines 226-227 “As one of the immiscible two phases, the immiscibility of sulfide droplets and silicate melt plays an important role in magma evolution” charge to “The immiscibility between sulfide droplets and silicate melts is also an important part of the magma evolution”
lines 232-234 “The study shows that the formation of magmatic Ni-Cu sulfide deposits is related to the separation and enrichment of sulfur saturated and immiscible sulfide liquids in mantle-derived basic and ultrabasic magmas (Arndt et al., 2005)” charge to “Studies have shown that the formation of magma-type Ni-Cu sulfide deposits is related to S-saturation in mantle-derived basic and ultramafic magmas and separation and enrichment of immiscible sulfide droplets”
lines 245-247 “this new understanding of sulfide and silicate melt immiscibility, which occurs during the early nanometer stage provides a new idea for further study of the immiscibility stage during magma evolution.” charge to “the new understanding that the immiscibility of sulfide and silicate melts occurs in the early nanometer stage provides new ideas for further study of immiscibility stage during magma evolution.”
Citation: https://doi.org/10.5194/egusphere-2023-213-AC1
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AC1: 'Reply on RC1', Lei Zuo, 13 Jun 2023
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CC1: 'Comment on egusphere-2023-213', Qiangtai Huang, 13 Jun 2023
The paper “A discovery of nanoscale sulfide droplets in MORB glasses: Implications for the immiscibility of sulfide melt and silicate melt” by Zuo et al. first reported nanoscale sulfide droplets in MORB glasses, and proposed that nanoscale sulfide droplets were formed in the sulfide segregation and had significant implications on the formation of sulfide deposits. I find the paper well-presented and a crucial addition in understanding the principles of the formation of sulfide droplets in MORB. However, I think that the formation of sulfide droplets in MORB and the sulfide deposits is not direct, and needs a careful reconsideration.
The main issues I concerned are the mechanism of S-saturation of MORB before eruption and the implications of the nanoscale sulfide droplets on the formation of sulfide deposits. First, the authors proposed that the sulfide deposits formed in the early stage of magma evolution, how the sulfur saturated in the MORB samples in this study? For the sulfide deposits occurred in the mafic-ultramafic intrusions, S-saturation of basaltic magma is generally ascribed to the crustal contamination and the abundant crystallization of mafic minerals. However, MORB is generally considered as a product of small-scale crystallization of basaltic magma, i.e., crystallization of mafic minerals in the early stage of MORB evolution is seldom. Besides, crustal contamination of MORB seems unlikely due to the very thin oceanic crust. So I think large scale of S-saturation is impossible for MORB due to mild crystallization and seldom crustal contamination.
Therefore, the second issue is whether the sulfide saturation is in local places or the bulk magma-scale? I think the S-saturation of MORB is in local places even if it occurs, based on the scarce sulfide droplets in samples in this study. If this is the case, I don’t think the discussions in section 5.4 on the formation of sulfide deposits are reasonable, because the local S saturation of MORB is not parallel to the bulk magma-scale sulfide segregation, i.e., the formation of sulfide deposits is beyond the scope of this study. So I suggest the authors focus on the process of formation of the sulfide droplets.
Please see more detailed comments:
- line 58-59: S-saturation of MORB is generally thought to be occurred in the source region due to the low-degree of partial melting (~10%), the S-undersaturated basaltic melt is thus impossible to reach saturation again during the ascending because of the increase of S solubility as the decrease of pressure, so S-saturation before eruption is impossible.
- line 155-156: the decrease of temperature of MORB would not be a trigger for the S saturation because the decrease of S solubility due to the temperature decrease is less than the increase of S solubility due to the pressure decrease during eruption, so the net effect is still undersaturation of sulfur.
- lines 158-159: as the authors proposed that the nanoscale sulfide droplets indicated a secondary S-saturated fractionation pathway, when and where the first S-saturation occurred?
- The petrography of the MORB samples in this study is suggested to be provided for a better understanding on the rock textures.
- Sparse sulfide would also be formed due to the hydrothermal alteration of MORB, how can the authors distinguish the magmatic origin and the hydrothermal origin of sulfide droplets in this study?
- lines 193-195: MSS is not a mineral.
- according to lines 203-204, the siderophile elements are preferred to partition into the MSS, and the chalcophile elements are preferred partition into the ISS. But the sulfide droplets in the MORB samples in this study are enriched in Cu and Ni, are the sulfide droplets MSS or ISS?
- Fig. 1: the sampling locations are absent here; what’s the relationship between zone A, zone B, zone C and the Melville FZ and the Gallieni FZ? Is there clear boundaries of zone A, zone B and zone C?
- Table 1: what’s meaning of “at.(%)” in this table.
Citation: https://doi.org/10.5194/egusphere-2023-213-CC1 -
AC2: 'Reply on CC1', Lei Zuo, 27 Jun 2023
- First, the authors proposed that the sulfide deposits formed in the early stage of magma evolution, how the sulfur saturated in the MORB samples in this study? For the sulfide deposits occurred in the mafic-ultramafic intrusions, S-saturation of basaltic magma is generally ascribed to the crustal contamination and the abundant crystallization of mafic minerals. However, MORB is generally considered as a product of small-scale crystallization of basaltic magma, i.e., crystallization of mafic minerals in the early stage of MORB evolution is seldom. Besides, crustal contamination of MORB seems unlikely due to the very thin oceanic crust. So I think large scale of S-saturation is impossible for MORB due to mild crystallization and seldom crustal contamination.
Re: Thank you for your comment. First, both falling temperature and incipient crystallization of silicate phases would lead to sulfide saturation. However, this is only to analyze the reasons for S-saturation from a macroscopic perspective, and we studied micro-scale nanodroplets, so the saturation of sulfur in the nano-stage has little correlation with the formation of mineral deposits. Second, we are studying the initial phase of sulfide droplets, and later magmatic evolution may show more sulfur saturation. Therefore, the research on sulfur saturation in the early nano-stage cannot be generalized with the conventional research on sulfur saturation.
- Therefore, the second issue is whether the sulfide saturation is in local places or the bulk magma-scale? I think the S-saturation of MORB is in local places even if it occurs, based on the scarce sulfide droplets in samples in this study. If this is the case, I don’t think the discussions in section 5.4 on the formation of sulfide deposits are reasonable, because the local S saturation of MORB is not parallel to the bulk magma-scale sulfide segregation, i.e., the formation of sulfide deposits is beyond the scope of this study. So I suggest the authors focus on the process of formation of the sulfide droplets.
Re: Thank you for your comment. The sulfide saturation is in local places. For the formation process of sulfide bead droplets, according to De Yoreo et al., 2015 mentioned a nonclassical mechanism of crystal growth, related to oriented attachment (OA). Repeated attachment events of crystalline particles on specific crystal faces matched by the lattice-matched can also be understood as particle attachment crystallization by oriented attachment.
De Yoreo, J.J., Gilbert, P.U., Sommerdijk, N.A., Penn, R.L., Whitelam, S., Joester, D., Zhang, H., Rimer, J.D., Navrotsky, A., Banfield, J.F., Wallace, A.F., Michel, F.M., Meldrum, F.C., Cölfen, H., Dove, P.M. 2015. Crystallization by particle attachment in synthetic, biogenic, and geologic environments. Science. 349(6247): aaa6760. Doi: 10.1126/science.aaa6760.
- line 58-59: S-saturation of MORB is generally thought to be occurred in the source region due to the low-degree of partial melting (~10%), the S-undersaturated basaltic melt is thus impossible to reach saturation again during the ascending because of the increase of S solubility as the decrease of pressure, so S-saturation before eruption is impossible.
Re: Thank you for the suggestion. I looked up some articles, such as Mathez, 1976; Czamanske and Moore, 1977; Wallace and Carmichael, 1992 and Yang et al., 2014. All of these articles say that the MORB magmas are S-saturated or nearly saturated before eruption.
Mathez, E., 1976. Sulfur solubility and magmatic sulfides in submarine basalt glass. Journal of Geophysical Research. 81 (3), 4269–4276. Doi: https://doi.org/10.1029/JB081i023p04269.
Czamanske, G.K., Moore, J.G., 1977. Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37 N lat. Geological Society of America Bulletin. 88(4), 587–599. Doi: https://doi.org/10.1130/0016-7606(1977)88<587:CAPCOS>2.0.CO;2.
Wallace, P., Carmichael, I.S.E., 1992. Sulfur in basaltic magmas. Geochim. Cosmochim. Acta 56 (5), 1863–1874. Doi: https://doi.org/10.1016/0016-7037(92)90316-B.
Yang, A.Y., Zhou, M.F., Zhao, T.P., Deng, X.G., Qi, L., Xu, J.F., 2014. Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation. Chemical Geology. 382, 1–13. Doi: https://doi.org/10.1016/j.chemgeo.2014.05.019.
- line 155-156: the decrease of temperature of MORB would not be a trigger for the S saturation because the decrease of S solubility due to the temperature decrease is less than the increase of S solubility due to the pressure decrease during eruption, so the net effect is still undersaturation of sulfur.
Re: Thank you for the suggestion. This sentence is really misexpressed, and I have changed the sentence to " The solubility of sulfur in basalt is related to changes in melt composition, temperature, and oxygen and sulfur fugacity.".
- lines 158-159: as the authors proposed that the nanoscale sulfide droplets indicated a secondary S-saturated fractionation pathway, when and where the first S-saturation occurred.
Re: Thank you for the suggestion. In this paper, we do not mention the secondary S-saturation fractionation pathway, but show that the discovery of sulfide droplets shows another way of S-saturation.
- The petrography of the MORB samples in this study is suggested to be provided for a better understanding on the rock textures.
Re: Thank you for the suggestion. The description of petrography has been described in more detail in Yang et al., 2014.
Yang, A.Y., Zhou, M.F., Zhao, T.P., Deng, X.G., Qi, L., Xu, J.F., 2014. Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation. Chemical Geology. 382, 1–13. Doi: https://doi.org/10.1016/j.chemgeo.2014.05.019.
- Sparse sulfide would also be formed due to the hydrothermal alteration of MORB, how can the authors distinguish the magmatic origin and the hydrothermal origin of sulfide droplets in this study?
Re: Thank you for the suggestion. Unfortunately, since we are studying nanoscale sulfide droplets, it is difficult to determine whether they are magmatic origin or hydrothermal origin.
- lines 193-195: MSS is not a mineral.
Re: Thank you for the suggestion. I have changed it to “solid”.
- according to lines 203-204, the siderophile elements are preferred to partition into the MSS, and the chalcophile elements are preferred partition into the ISS. But the sulfide droplets in the MORB samples in this study are enriched in Cu and Ni, are the sulfide droplets MSS or ISS?
Re: Thank you for the suggestion. The sulfide droplets are composed of Ni-Fe-rich MSS phase and Cu-Fe-rich ISS phase.
- Fig. 1: the sampling locations are absent here; what’s the relationship between zone A, zone B, zone C and the Melville FZ and the Gallieni FZ? Is there clear boundaries of zone A, zone B and zone C?
Re: The sampling locations are three major ridge segments divided by the Melville FZ and Gallieni FZ. Zones A, B, and C have clear boundaries, as shown in the figure.
- Table 1: what’s meaning of “at.(%)” in this table.
Re: It is atomic percent.
Citation: https://doi.org/10.5194/egusphere-2023-213-AC2
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RC2: 'Comment on egusphere-2023-213', Anonymous Referee #2, 20 Jul 2023
This paper provides a very interesting TEM observation of the nanoscale sulfide droplets found in the MORB from the Southwest Indian Ridge, which may shed a new light on the initial sulfide segregation process during magma evolution. I am very interested in this question, and confident with the TEM observation and results provided by the authors. But, the discussion section here seems to be so weak, and the authors may not get underneath the skin of TEM observation, and really explore the core of sulfide segregation. More information can be found in the following main comments. My recommendation as of this reading is, narrowly, major revision, but would understand if the suggested revisions cannot be done in the time required for a revision and thus a rejection with resubmission is required, and will leave that to the judgment of the editor.
Major comments:
- ----About the sulfide compositions:
The EDS analytical measurement of sulfide droplets indicates that the nanoscale sulfide contains 20.68 wt.% O, 7.17 wt.% Si, 1.82 wt.% Na, 24.57 wt.% Cu and only 13.15 wt.% S, which is very strange and far away from the composition of sulfide droplet segregated from S-saturated silicate melt. The normal endogenous sulfide droplets usually contain 2-5 wt.% O and >~35-40 wt.% S. Here, the nanoscale sulfide droplet contains so much O, but the authors declared that “the elements Si, O and Al are depleted at the same time…”, which seems to be not right. On the other hand, the S content of nanoscale sulfide droplet is really low, in conjunction with the high O content, which convinces me that the nanoscale particle found here is not a pour sulfide droplet, and must contain some oxides and/or alloys. This will strongly shake the very foundations of this manuscript. If the nanoscale particle is just sulfide droplet, as suggested by the authors, its composition is far away from the thermodynamic equilibrium with the silicate melt, which should be addressed in the ‘discussion’ section. Hence, the authors should provide so much more evidence and discussion about the composition of nano particle, not just show the enrichment or depletion of elements relative to the surrounding silicate melt.
- Line 58-59: “the sulfur in MORB magma is saturated before magma eruption, and may also be saturated in the source region…”
The interpretation here is too simple. Mantle-derived mafic-ultramafic magmas are generally undersaturated in sulfide until immediately prior to or during emplacement due to the negative effect of decreasing pressure on the sulfur content at sulfide saturation. Hence, we may not take the sulfur saturation in MORB for granted, and the possible trigger mechanism of the sulfide saturation should be mentioned. For the source region, the sulfur saturation condition can only occur when the mantle source has just experienced a limited partial melting degree, and the sulfide in restite has not been completely dissolved or exhausted.
- Line 179: “the sulfide droplet sizes varied at different stages of sulfur saturation”
This sentence is ambiguous and should be explained in more details.
- Line187-188: “Small droplets then disappear, which enlarges the larger droplets, and this process is generally accompanied by a reduction in interface free energy…”
The interpretation here may come from the classical Ostwald Ripening model, but is not applied to the nano scale particle. While, intense research on coarsening behaviour of nano particles has led to the formulation of a new mechanism of crystal growth, which is called as the “oriented attachment” that describes the spontaneous self-organization of adjacent particles, so that they share a common crystallogrphaic orientation, followed by the joining of these particles at a planar interface. The authors should find more information about the “oriented attachment” mechanism when discussing the crystallization and growth of nano particles.
- About the Section 5.4.
In this discussion section 5.4, the authors just repeated the current understanding of the sulfide segregation and accumulation in forming the magmatic sulfide deposits in the first paragraph, but I can not find a close relationship between the TEM observations here and the magmatic sulfide deposit. The applications of this work on the magmatic sulfide deposits are weak and ambiguous. There is an obvious separation between the literature reviews and the interpretation of TEM observations adopted here, which is a universal feature of the whole discussion section. This makes the whole manuscript seem to be too simple without a deep consideration, which should be strongly improved in the new revision.
- The figure captions are too simple and contain less information.
Minor comments:
Line 89: the abbreviation “SWIR” should be explained when it firstly occurs
Line 150: add “which” after “glasses”
Line 157: add “of sulfide” after “partial dissolution”
Line 171: replace “correspond” by “refer”
Line 199: rewrite this sentence
Line 213: add “can really” before “help understand…”
Citation: https://doi.org/10.5194/egusphere-2023-213-RC2 -
AC3: 'Reply on RC2', Lei Zuo, 08 Aug 2023
- About the sulfide compositions:
The EDS analytical measurement of sulfide droplets indicates that the nanoscale sulfide contains 20.68 wt.% O, 7.17 wt.% Si, 1.82 wt.% Na, 24.57 wt.% Cu and only 13.15 wt.% S, which is very strange and far away from the composition of sulfide droplet segregated from S-saturated silicate melt. The normal endogenous sulfide droplets usually contain 2-5 wt.% O and >~35-40 wt.% S. Here, the nanoscale sulfide droplet contains so much O, but the authors declared that “the elements Si, O and Al are depleted at the same time…”, which seems to be not right. On the other hand, the S content of nanoscale sulfide droplet is really low, in conjunction with the high O content, which convinces me that the nanoscale particle found here is not a pour sulfide droplet, and must contain some oxides and/or alloys. This will strongly shake the very foundations of this manuscript. If the nanoscale particle is just sulfide droplet, as suggested by the authors, its composition is far away from the thermodynamic equilibrium with the silicate melt, which should be addressed in the ‘discussion’ section. Hence, the authors should provide so much more evidence and discussion about the composition of nano particle, not just show the enrichment or depletion of elements relative to the surrounding silicate melt.
Re: Thank you for your comment. ESD testing can only be semi-quantitative, and some light elements are difficult to test, so their content will be classified as O elements, resulting in high O content. At the same time, because we analyze nanoscale particles, we cannot accurately judge the composition of their particles, and can only judge the types and contents of elements they contain according to the EDS test results.
- Line 58-59: “the sulfur in MORB magma is saturated before magma eruption, and may also be saturated in the source region…”
The interpretation here is too simple. Mantle-derived mafic-ultramafic magmas are generally undersaturated in sulfide until immediately prior to or during emplacement due to the negative effect of decreasing pressure on the sulfur content at sulfide saturation. Hence, we may not take the sulfur saturation in MORB for granted, and the possible trigger mechanism of the sulfide saturation should be mentioned. For the source region, the sulfur saturation condition can only occur when the mantle source has just experienced a limited partial melting degree, and the sulfide in restite has not been completely dissolved or exhausted.
Re: Thank you for your comment. Mathez and Yeats (1976) have concluded from paragenetic relations in the submarine basalts they studied that an immiscible sulfide liquid phase exists in the magma prior to eruption. We concur and suggest that this is the typical situation attending eruption of submarine basalt. Czamanske and Moore (1977) found between the S and FeO content in basalt glass (As shown in the Figure 1), strongly suggest that the original famous basalt glass was sulfur saturated and remained sulfur saturated during differentiation. At the same time, the articles of Mathez (1976) and Czamanske and Moore (1977) indicate that MORB magma is S-saturated before eruption, and may also be S-saturated in its source region.
Czamanske, G. K. and Moore, J. G.: Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat, Bull. Geol. Soc. Am., 1977, 88, 587–599, https://doi.org/10.1130/0016-7606(1977)88<587:CAPCOS>2.0.CO;2.
Mathez, E. A.: Sulfur Solubility and Magmatic Sulfides in Submarine Basalt Glass., J Geophys Res, 1976, 81, 4269–4276, https://doi.org/10.1029/JB081i023p04269.
Mathez E A, Yeats R S. Magmatic sulfides in basalt glass from DSDP Hole 319A and Site 320, Nazca Plate[J]. Initial reports of the deep sea drilling project, 1976, 34: 363-373.
- Line 179: “the sulfide droplet sizes varied at different stages of sulfur saturation”
This sentence is ambiguous and should be explained in more details.
Re: Thank you for the suggestion. This sentence has been amended to: “On the other hand, in different stages of S-saturation, the size of sulfide droplet is different, and the structural difference of sulfide droplet is also controlled by their size.”
- Line187-188: “Small droplets then disappear, which enlarges the larger droplets, and this process is generally accompanied by a reduction in interface free energy…”
The interpretation here may come from the classical Ostwald Ripening model, but is not applied to the nano scale particle. While, intense research on coarsening behaviour of nano particles has led to the formulation of a new mechanism of crystal growth, which is called as the “oriented attachment” that describes the spontaneous self-organization of adjacent particles, so that they share a common crystallogrphaic orientation, followed by the joining of these particles at a planar interface. The authors should find more information about the “oriented attachment” mechanism when discussing the crystallization and growth of nano particles.
Re: Thank you for the suggestion. This model is not applied to the nano scale particle, which has now been removed. For the proposed "oriented attachment" mechanism, since there is no definitive evidence in the observation of sulfide droplets in this study as nanoparticles formed by oriented attachment. Therefore, it is impossible to investigate the crystallization and growth of nanoparticles with the oriented attachment mechanism.
- About the Section 5.4.
In this discussion section 5.4, the authors just repeated the current understanding of the sulfide segregation and accumulation in forming the magmatic sulfide deposits in the first paragraph, but I can not find a close relationship between the TEM observations here and the magmatic sulfide deposit. The applications of this work on the magmatic sulfide deposits are weak and ambiguous. There is an obvious separation between the literature reviews and the interpretation of TEM observations adopted here, which is a universal feature of the whole discussion section. This makes the whole manuscript seem to be too simple without a deep consideration, which should be strongly improved in the new revision.
Re: Thank you for the suggestion. In this study, nanoscale sulfide droplets are mainly studied, which can only be considered as a pre-enrichment mechanism, and cannot explain the close relationship with magmatic sulfide deposits. At the same time, nanoscale sulfide droplets, as products in the early stage of magma, are indeed weak and ambiguous for the application of magmatic sulfide deposits. However, it may also be innovative to start with nanoscale sulfide droplets to analyze the causes and applications of later ore deposits. The problems with the explanation of some of the discussions have been revised in the original text.
- The figure captions are too simple and contain less information.
Re: Thank you for the suggestion.
Figure 1. “Regional location of the study area” was changed to “Topographic map of the Southwest Indian Ridge (SWIR), showing sample locations used in this study (red circles).”
Figure 2. “SEM image (a,) and elemental maps (b-f) of MORB samples” was changed to “SEM image (a,) and O, Si, Na, Al and Fe EDS mappings (b-f) of selected particles of MORB samples.”
- Line 89: the abbreviation “SWIR” should be explained when it firstly occurs.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 150: add “which” after “glasses”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 157: add “of sulfide” after “partial dissolution”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 171: replace “correspond” by “refer”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 199: rewrite this sentence.
Re: Thank you for the suggestion. This sentence has been amended to “Therefore, the distribution of different chalcophile elements in MSS and ISS usually behaves differently.”
- Line 213: add “can really” before “help understand…”.
Re: Thank you for the suggestion. The sentence has been revised.
Citation: https://doi.org/10.5194/egusphere-2023-213-AC3
Status: closed
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RC1: 'Comment on egusphere-2023-213', Anonymous Referee #1, 11 May 2023
This paper aims at ”a deeper understanding of the initial solidification of sulfide-oxide liquids” (line 70-71) using an example from MORB-type glasses. Although the paper reports nanoscale sulphide globules two main conclusions that “nanoscale sulfide droplets were the initial phase of sulfide saturation” and “all immiscibility phenomena may occur in the nanometer stage during magma evolution” remain unsupported. Clearly, liquid immiscibility occurs on all scales, including nanoscale, which follows from results of numerous experimental and melt inclusion studies. Therefore, the results by Lei Zuo et al. are neither novel nor particularly significant. This study does not contribute to “understanding how siderophile and chalcophile elements behaved during sulfide crystallization” (line 35) and has nothing to do with identifying “the key to the formation of magmatic Ni-Cu sulphide deposits” through the study of sulphide unmixing in MORB magmas. The 4. RESULTS section is only 7 lines long!!!!!
The manuscript is sprinkled with the statements that are awkward and often wrong. Just a few examples are given below
lins 24 fresh mid-ocean ridge basalt (MORB) magmas
lines 29-31 Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are in the droplets.
lines 39-40 all immiscibility phenomena may occur in the nanometer stage during magma evolution.
lines 46 with liquid immiscibility the more important for magma evolution
lines 58 The sulfur in MORB magma is saturated before magma eruption…
lines 65-66 the sulfide droplet sizes influence the physical behavior of the separate sulfide phases
lines 66-67 the different of sulfide droplets 67 also plays an important role in partitioning the siderophile and cupolophile elements ….
lines 95-97 As determined by SEM, the MORB samples were primarily fine-grained and weakly phyric with hypocrystalline to vitreous textures (Fig. 2a). Some phenocrysts were 1-10 μm and relatively rich in Si and O (Figs. 2b-f, 3a-c).
Please note that surfaces shown on Figs. 2 and 3 are dusted, but reported as “weakly phyric”)))
lines 158-159 In this study, nanoscale sulfide droplets in natural MORB glass are reported for the first time, thus demonstrating S-saturated fractionation in another way.
lines 186-187 The experimental results show that when the composition of silicate melt becomes immiscible, two-phase nanoscale droplets will soon appear
lines 212-213 chalcophile elements appeared to enter the sulfide droplets and distribute evenly within the sulfide globule in the early stage
lines 218 5.4 Implications of sulfide droplets on magmatic evolution and formation of sulfide deposits
lines 222-223 The earlier immiscibility begins during magmatic evolution, the better its influence on magmatic evolution, and the better its geochemical and petrological significance
lines 226-227 As one of the immiscible two phases, the immiscibility of sulfide droplets and silicate melt plays an important role in magma evolution
lines 232-234 The study shows that the formation of magmatic Ni-Cu sulfide deposits is related to the separation and enrichment of sulfur saturated and immiscible sulfide liquids in mantle-derived basic and ultrabasic magmas (Arndt et al., 2005)
lines 245-247 this new understanding of sulfide and silicate melt immiscibility, which occurs during the early nanometer stage provides a new idea for further study of the immiscibility stage during magma evolution.
lines 72-82 the whole paragraph, and in particular, “Hawkings et al (2020) reported that Greenland Ice Sheet meltwaters may provide biolabile particulate Fe that fuels the large summer phytoplankton bloom in the Labrador Sea (Hawkings et al., 2018)” is irrelevant to this study.
Citation: https://doi.org/10.5194/egusphere-2023-213-RC1 -
AC1: 'Reply on RC1', Lei Zuo, 13 Jun 2023
- Although the paper reports nanoscale sulphide globules two main conclusions that “nanoscale sulfide droplets were the initial phase of sulfide saturation” and “all immiscibility phenomena may occur in the nanometer stage during magma evolution” remain unsupported. Clearly, liquid immiscibility occurs on all scales, including nanoscale, which follows from results of numerous experimental and melt inclusion studies. Therefore, the results by Lei Zuo et al. are neither novel nor particularly significant.
Re: Thank you for your comment, I agree with your point of view that immiscibility can indeed occur at any stage, but it is only based on the conclusions of previous inferences and experimental simulations, and the immiscible phenomenon has not been directly observed in natural samples. However, in our study, we did observe immiscibility, which is a very intuitive explanation for the immiscibility that occurs at the nano stage.
- This study does not contribute to “understanding how siderophile and chalcophile elements behaved during sulfide crystallization” (line 35) and has nothing to do with identifying “the key to the formation of magmatic Ni-Cu sulphide deposits” through the study of sulphide unmixing in MORB magmas.
Re: Thank you for your comment, regarding the behavior of the siderophile and chalcophile elements you mentioned, because the study is based on microscopic scale nanoparticles, which are in the initial stage of mineral formation, the study of the element enrichment mechanism in sulfide droplet is helpful to understand the siderophile and chalcophile elements behaved during sulfide crystallization. At the same time, I agree with your mention that it is not critical to explain the formation of sulfide deposits with the immiscibility of MORB sulfide droplet. But the basic principle of the formation of magmatic sulfide deposits origins stems from the sulfur saturation of magma. Therefore, the immiscibility of MORB sulfide droplet may have some research significance during the formation of sulfide deposits, but this requires further in-depth research to explain.
- The 4. RESULTS section is only 7 lines long!!!!!
Re：Thank you for the suggestion. The conclusion section I've changed to:
Nanoscale sulfide droplets were first identified in MORB glasses by FIB-cut and TEM analyses. These droplets might form rapidly before eruption and then undergo immediate supercooling, and these droplets have characteristics of smooth edges and crystalline features. The discovery of nanoscale sulfide droplets in natural MORB glass has demonstrated a new form of sulfur saturation exists. Therefore, the nanoscale sulfide droplets are more likely to be the initial stage of sulfide saturation. In addition, the sulfide droplets in this study are crystalline, and their lattices may have formed at this stage. As the initial phase of sulfide saturation, the chalcophile elements enter the sulfide droplet in the early stage and distribute uniformly in the droplet, rather than selectively enrich the siderophile and chalcophile elements. However, it is still unclear when the siderophile and chalcophile elements behave inconsistently in sulfide droplets, which warrants further investigation. Furthermore, according to the element distribution map of sulfide droplets, it can be seen that sulfide droplets are relatively enriched with Fe, Cu, Ni and Na, but lacked Si, O and Al, which further indicates that the immiscibility of sulfide and silicate melt occurs in the early nanometer stage, which provides new ideas for further study of the stage of immiscibility stage during magma evolution.
- lines 72-82 the whole paragraph, and in particular, “Hawkings et al (2020) reported that Greenland Ice Sheet meltwaters may provide biolabile particulate Fe that fuels the large summer phytoplankton bloom in the Labrador Sea (Hawkings et al., 2018)” is irrelevant to this study.”
Re：Thank you for the suggestion, I have removed irrelevant sentences.
- “The manuscript is sprinkled with the statements that are awkward and often wrong.” Below is the revised sentence.
lines 24 “fresh mid-ocean ridge basalt (MORB) magmas” charge to “fresh mid-ocean ridge basalts (MORB) samples”
lines 29-31 “Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are in the droplets.” charge to “Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while Si, Al and O are lacking.”
lines 39-40 “Therefore, all immiscibility phenomena may occur in the nanometer stage during magma evolution.” charge to “Therefore, it is speculated that all immiscibility phenomena may occur in the nanometer stage during magma evolution.”
lines 46 “with liquid immiscibility the more important for magma evolution.” charge to “among which liquid immiscibility the more important for magma evolution.”
lines 58 “The sulfur in MORB magma is saturated before magma eruption…” charge to “The MORB magma is S-saturated before eruption…”
lines 65-66 “the sulfide droplet sizes influence the physical behavior of the separate sulfide phases” charge to “the size of the sulfide droplets formed affects the physical behavior of the separated sulfide phases”
lines 66-67 “the different of sulfide droplets also plays an important role in partitioning the siderophile and cupolophile elements….” charge to “the separation of sulfide droplets also plays an important role in the distribution of siderophile and cupolophile elements…”
lines 95-97 “As determined by SEM, the MORB samples were primarily fine-grained and weakly phyric with hypocrystalline to vitreous textures (Fig. 2a). Some phenocrysts were 1-10 μm and relatively rich in Si and O (Figs. 2b-f, 3a-c).” charge to “The results of SEM analysis showed that the MORB samples were mainly fine-grained and weak porphyry, with hypocrystalline to vitreous textures. The size of some phenocrysts is 1 ~ 10 μm and relatively rich in Si and O.”
lines 158-159 “In this study, nanoscale sulfide droplets in natural MORB glass are reported for the first time, thus demonstrating S-saturated fractionation in another way.” charge to “In this study, the discovery of nanoscale sulfide droplets in natural MORB glasses demonstrates a new way of S-saturated fractionation.”
lines 186-187 “The experimental results show that when the composition of silicate melt becomes immiscible, two-phase nanoscale droplets will soon appear” charge to “The experimental results show that when the composition of silicate melt becomes immiscible, the phase separation of sulfide nanoscale droplets and silicate nanoscale droplets will soon appear.”
lines 212-213 “chalcophile elements appeared to enter the sulfide droplets and distribute evenly within the sulfide globule in the early stage” charge to “it can be inferred that chalcophile elements enter sulfide droplets in the early stage and are uniformly distributed in the range of sulfide droplets.”
lines 218 “5.4 Implications of sulfide droplets on magmatic evolution and formation of sulfide deposits” charge to “5.4 Significance of sulfide droplets on magmatic evolution and sulfide deposit formation”
lines 222-223 “The earlier immiscibility begins during magmatic evolution, the better its influence on magmatic evolution, and the better its geochemical and petrological significance” charge to “The earlier immiscibility begins in the magma evolution, the greater the impact on magma evolution and the greater its geochemical and petrological significance”
lines 226-227 “As one of the immiscible two phases, the immiscibility of sulfide droplets and silicate melt plays an important role in magma evolution” charge to “The immiscibility between sulfide droplets and silicate melts is also an important part of the magma evolution”
lines 232-234 “The study shows that the formation of magmatic Ni-Cu sulfide deposits is related to the separation and enrichment of sulfur saturated and immiscible sulfide liquids in mantle-derived basic and ultrabasic magmas (Arndt et al., 2005)” charge to “Studies have shown that the formation of magma-type Ni-Cu sulfide deposits is related to S-saturation in mantle-derived basic and ultramafic magmas and separation and enrichment of immiscible sulfide droplets”
lines 245-247 “this new understanding of sulfide and silicate melt immiscibility, which occurs during the early nanometer stage provides a new idea for further study of the immiscibility stage during magma evolution.” charge to “the new understanding that the immiscibility of sulfide and silicate melts occurs in the early nanometer stage provides new ideas for further study of immiscibility stage during magma evolution.”
Citation: https://doi.org/10.5194/egusphere-2023-213-AC1
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AC1: 'Reply on RC1', Lei Zuo, 13 Jun 2023
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CC1: 'Comment on egusphere-2023-213', Qiangtai Huang, 13 Jun 2023
The paper “A discovery of nanoscale sulfide droplets in MORB glasses: Implications for the immiscibility of sulfide melt and silicate melt” by Zuo et al. first reported nanoscale sulfide droplets in MORB glasses, and proposed that nanoscale sulfide droplets were formed in the sulfide segregation and had significant implications on the formation of sulfide deposits. I find the paper well-presented and a crucial addition in understanding the principles of the formation of sulfide droplets in MORB. However, I think that the formation of sulfide droplets in MORB and the sulfide deposits is not direct, and needs a careful reconsideration.
The main issues I concerned are the mechanism of S-saturation of MORB before eruption and the implications of the nanoscale sulfide droplets on the formation of sulfide deposits. First, the authors proposed that the sulfide deposits formed in the early stage of magma evolution, how the sulfur saturated in the MORB samples in this study? For the sulfide deposits occurred in the mafic-ultramafic intrusions, S-saturation of basaltic magma is generally ascribed to the crustal contamination and the abundant crystallization of mafic minerals. However, MORB is generally considered as a product of small-scale crystallization of basaltic magma, i.e., crystallization of mafic minerals in the early stage of MORB evolution is seldom. Besides, crustal contamination of MORB seems unlikely due to the very thin oceanic crust. So I think large scale of S-saturation is impossible for MORB due to mild crystallization and seldom crustal contamination.
Therefore, the second issue is whether the sulfide saturation is in local places or the bulk magma-scale? I think the S-saturation of MORB is in local places even if it occurs, based on the scarce sulfide droplets in samples in this study. If this is the case, I don’t think the discussions in section 5.4 on the formation of sulfide deposits are reasonable, because the local S saturation of MORB is not parallel to the bulk magma-scale sulfide segregation, i.e., the formation of sulfide deposits is beyond the scope of this study. So I suggest the authors focus on the process of formation of the sulfide droplets.
Please see more detailed comments:
- line 58-59: S-saturation of MORB is generally thought to be occurred in the source region due to the low-degree of partial melting (~10%), the S-undersaturated basaltic melt is thus impossible to reach saturation again during the ascending because of the increase of S solubility as the decrease of pressure, so S-saturation before eruption is impossible.
- line 155-156: the decrease of temperature of MORB would not be a trigger for the S saturation because the decrease of S solubility due to the temperature decrease is less than the increase of S solubility due to the pressure decrease during eruption, so the net effect is still undersaturation of sulfur.
- lines 158-159: as the authors proposed that the nanoscale sulfide droplets indicated a secondary S-saturated fractionation pathway, when and where the first S-saturation occurred?
- The petrography of the MORB samples in this study is suggested to be provided for a better understanding on the rock textures.
- Sparse sulfide would also be formed due to the hydrothermal alteration of MORB, how can the authors distinguish the magmatic origin and the hydrothermal origin of sulfide droplets in this study?
- lines 193-195: MSS is not a mineral.
- according to lines 203-204, the siderophile elements are preferred to partition into the MSS, and the chalcophile elements are preferred partition into the ISS. But the sulfide droplets in the MORB samples in this study are enriched in Cu and Ni, are the sulfide droplets MSS or ISS?
- Fig. 1: the sampling locations are absent here; what’s the relationship between zone A, zone B, zone C and the Melville FZ and the Gallieni FZ? Is there clear boundaries of zone A, zone B and zone C?
- Table 1: what’s meaning of “at.(%)” in this table.
Citation: https://doi.org/10.5194/egusphere-2023-213-CC1 -
AC2: 'Reply on CC1', Lei Zuo, 27 Jun 2023
- First, the authors proposed that the sulfide deposits formed in the early stage of magma evolution, how the sulfur saturated in the MORB samples in this study? For the sulfide deposits occurred in the mafic-ultramafic intrusions, S-saturation of basaltic magma is generally ascribed to the crustal contamination and the abundant crystallization of mafic minerals. However, MORB is generally considered as a product of small-scale crystallization of basaltic magma, i.e., crystallization of mafic minerals in the early stage of MORB evolution is seldom. Besides, crustal contamination of MORB seems unlikely due to the very thin oceanic crust. So I think large scale of S-saturation is impossible for MORB due to mild crystallization and seldom crustal contamination.
Re: Thank you for your comment. First, both falling temperature and incipient crystallization of silicate phases would lead to sulfide saturation. However, this is only to analyze the reasons for S-saturation from a macroscopic perspective, and we studied micro-scale nanodroplets, so the saturation of sulfur in the nano-stage has little correlation with the formation of mineral deposits. Second, we are studying the initial phase of sulfide droplets, and later magmatic evolution may show more sulfur saturation. Therefore, the research on sulfur saturation in the early nano-stage cannot be generalized with the conventional research on sulfur saturation.
- Therefore, the second issue is whether the sulfide saturation is in local places or the bulk magma-scale? I think the S-saturation of MORB is in local places even if it occurs, based on the scarce sulfide droplets in samples in this study. If this is the case, I don’t think the discussions in section 5.4 on the formation of sulfide deposits are reasonable, because the local S saturation of MORB is not parallel to the bulk magma-scale sulfide segregation, i.e., the formation of sulfide deposits is beyond the scope of this study. So I suggest the authors focus on the process of formation of the sulfide droplets.
Re: Thank you for your comment. The sulfide saturation is in local places. For the formation process of sulfide bead droplets, according to De Yoreo et al., 2015 mentioned a nonclassical mechanism of crystal growth, related to oriented attachment (OA). Repeated attachment events of crystalline particles on specific crystal faces matched by the lattice-matched can also be understood as particle attachment crystallization by oriented attachment.
De Yoreo, J.J., Gilbert, P.U., Sommerdijk, N.A., Penn, R.L., Whitelam, S., Joester, D., Zhang, H., Rimer, J.D., Navrotsky, A., Banfield, J.F., Wallace, A.F., Michel, F.M., Meldrum, F.C., Cölfen, H., Dove, P.M. 2015. Crystallization by particle attachment in synthetic, biogenic, and geologic environments. Science. 349(6247): aaa6760. Doi: 10.1126/science.aaa6760.
- line 58-59: S-saturation of MORB is generally thought to be occurred in the source region due to the low-degree of partial melting (~10%), the S-undersaturated basaltic melt is thus impossible to reach saturation again during the ascending because of the increase of S solubility as the decrease of pressure, so S-saturation before eruption is impossible.
Re: Thank you for the suggestion. I looked up some articles, such as Mathez, 1976; Czamanske and Moore, 1977; Wallace and Carmichael, 1992 and Yang et al., 2014. All of these articles say that the MORB magmas are S-saturated or nearly saturated before eruption.
Mathez, E., 1976. Sulfur solubility and magmatic sulfides in submarine basalt glass. Journal of Geophysical Research. 81 (3), 4269–4276. Doi: https://doi.org/10.1029/JB081i023p04269.
Czamanske, G.K., Moore, J.G., 1977. Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37 N lat. Geological Society of America Bulletin. 88(4), 587–599. Doi: https://doi.org/10.1130/0016-7606(1977)88<587:CAPCOS>2.0.CO;2.
Wallace, P., Carmichael, I.S.E., 1992. Sulfur in basaltic magmas. Geochim. Cosmochim. Acta 56 (5), 1863–1874. Doi: https://doi.org/10.1016/0016-7037(92)90316-B.
Yang, A.Y., Zhou, M.F., Zhao, T.P., Deng, X.G., Qi, L., Xu, J.F., 2014. Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation. Chemical Geology. 382, 1–13. Doi: https://doi.org/10.1016/j.chemgeo.2014.05.019.
- line 155-156: the decrease of temperature of MORB would not be a trigger for the S saturation because the decrease of S solubility due to the temperature decrease is less than the increase of S solubility due to the pressure decrease during eruption, so the net effect is still undersaturation of sulfur.
Re: Thank you for the suggestion. This sentence is really misexpressed, and I have changed the sentence to " The solubility of sulfur in basalt is related to changes in melt composition, temperature, and oxygen and sulfur fugacity.".
- lines 158-159: as the authors proposed that the nanoscale sulfide droplets indicated a secondary S-saturated fractionation pathway, when and where the first S-saturation occurred.
Re: Thank you for the suggestion. In this paper, we do not mention the secondary S-saturation fractionation pathway, but show that the discovery of sulfide droplets shows another way of S-saturation.
- The petrography of the MORB samples in this study is suggested to be provided for a better understanding on the rock textures.
Re: Thank you for the suggestion. The description of petrography has been described in more detail in Yang et al., 2014.
Yang, A.Y., Zhou, M.F., Zhao, T.P., Deng, X.G., Qi, L., Xu, J.F., 2014. Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation. Chemical Geology. 382, 1–13. Doi: https://doi.org/10.1016/j.chemgeo.2014.05.019.
- Sparse sulfide would also be formed due to the hydrothermal alteration of MORB, how can the authors distinguish the magmatic origin and the hydrothermal origin of sulfide droplets in this study?
Re: Thank you for the suggestion. Unfortunately, since we are studying nanoscale sulfide droplets, it is difficult to determine whether they are magmatic origin or hydrothermal origin.
- lines 193-195: MSS is not a mineral.
Re: Thank you for the suggestion. I have changed it to “solid”.
- according to lines 203-204, the siderophile elements are preferred to partition into the MSS, and the chalcophile elements are preferred partition into the ISS. But the sulfide droplets in the MORB samples in this study are enriched in Cu and Ni, are the sulfide droplets MSS or ISS?
Re: Thank you for the suggestion. The sulfide droplets are composed of Ni-Fe-rich MSS phase and Cu-Fe-rich ISS phase.
- Fig. 1: the sampling locations are absent here; what’s the relationship between zone A, zone B, zone C and the Melville FZ and the Gallieni FZ? Is there clear boundaries of zone A, zone B and zone C?
Re: The sampling locations are three major ridge segments divided by the Melville FZ and Gallieni FZ. Zones A, B, and C have clear boundaries, as shown in the figure.
- Table 1: what’s meaning of “at.(%)” in this table.
Re: It is atomic percent.
Citation: https://doi.org/10.5194/egusphere-2023-213-AC2
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RC2: 'Comment on egusphere-2023-213', Anonymous Referee #2, 20 Jul 2023
This paper provides a very interesting TEM observation of the nanoscale sulfide droplets found in the MORB from the Southwest Indian Ridge, which may shed a new light on the initial sulfide segregation process during magma evolution. I am very interested in this question, and confident with the TEM observation and results provided by the authors. But, the discussion section here seems to be so weak, and the authors may not get underneath the skin of TEM observation, and really explore the core of sulfide segregation. More information can be found in the following main comments. My recommendation as of this reading is, narrowly, major revision, but would understand if the suggested revisions cannot be done in the time required for a revision and thus a rejection with resubmission is required, and will leave that to the judgment of the editor.
Major comments:
- ----About the sulfide compositions:
The EDS analytical measurement of sulfide droplets indicates that the nanoscale sulfide contains 20.68 wt.% O, 7.17 wt.% Si, 1.82 wt.% Na, 24.57 wt.% Cu and only 13.15 wt.% S, which is very strange and far away from the composition of sulfide droplet segregated from S-saturated silicate melt. The normal endogenous sulfide droplets usually contain 2-5 wt.% O and >~35-40 wt.% S. Here, the nanoscale sulfide droplet contains so much O, but the authors declared that “the elements Si, O and Al are depleted at the same time…”, which seems to be not right. On the other hand, the S content of nanoscale sulfide droplet is really low, in conjunction with the high O content, which convinces me that the nanoscale particle found here is not a pour sulfide droplet, and must contain some oxides and/or alloys. This will strongly shake the very foundations of this manuscript. If the nanoscale particle is just sulfide droplet, as suggested by the authors, its composition is far away from the thermodynamic equilibrium with the silicate melt, which should be addressed in the ‘discussion’ section. Hence, the authors should provide so much more evidence and discussion about the composition of nano particle, not just show the enrichment or depletion of elements relative to the surrounding silicate melt.
- Line 58-59: “the sulfur in MORB magma is saturated before magma eruption, and may also be saturated in the source region…”
The interpretation here is too simple. Mantle-derived mafic-ultramafic magmas are generally undersaturated in sulfide until immediately prior to or during emplacement due to the negative effect of decreasing pressure on the sulfur content at sulfide saturation. Hence, we may not take the sulfur saturation in MORB for granted, and the possible trigger mechanism of the sulfide saturation should be mentioned. For the source region, the sulfur saturation condition can only occur when the mantle source has just experienced a limited partial melting degree, and the sulfide in restite has not been completely dissolved or exhausted.
- Line 179: “the sulfide droplet sizes varied at different stages of sulfur saturation”
This sentence is ambiguous and should be explained in more details.
- Line187-188: “Small droplets then disappear, which enlarges the larger droplets, and this process is generally accompanied by a reduction in interface free energy…”
The interpretation here may come from the classical Ostwald Ripening model, but is not applied to the nano scale particle. While, intense research on coarsening behaviour of nano particles has led to the formulation of a new mechanism of crystal growth, which is called as the “oriented attachment” that describes the spontaneous self-organization of adjacent particles, so that they share a common crystallogrphaic orientation, followed by the joining of these particles at a planar interface. The authors should find more information about the “oriented attachment” mechanism when discussing the crystallization and growth of nano particles.
- About the Section 5.4.
In this discussion section 5.4, the authors just repeated the current understanding of the sulfide segregation and accumulation in forming the magmatic sulfide deposits in the first paragraph, but I can not find a close relationship between the TEM observations here and the magmatic sulfide deposit. The applications of this work on the magmatic sulfide deposits are weak and ambiguous. There is an obvious separation between the literature reviews and the interpretation of TEM observations adopted here, which is a universal feature of the whole discussion section. This makes the whole manuscript seem to be too simple without a deep consideration, which should be strongly improved in the new revision.
- The figure captions are too simple and contain less information.
Minor comments:
Line 89: the abbreviation “SWIR” should be explained when it firstly occurs
Line 150: add “which” after “glasses”
Line 157: add “of sulfide” after “partial dissolution”
Line 171: replace “correspond” by “refer”
Line 199: rewrite this sentence
Line 213: add “can really” before “help understand…”
Citation: https://doi.org/10.5194/egusphere-2023-213-RC2 -
AC3: 'Reply on RC2', Lei Zuo, 08 Aug 2023
- About the sulfide compositions:
The EDS analytical measurement of sulfide droplets indicates that the nanoscale sulfide contains 20.68 wt.% O, 7.17 wt.% Si, 1.82 wt.% Na, 24.57 wt.% Cu and only 13.15 wt.% S, which is very strange and far away from the composition of sulfide droplet segregated from S-saturated silicate melt. The normal endogenous sulfide droplets usually contain 2-5 wt.% O and >~35-40 wt.% S. Here, the nanoscale sulfide droplet contains so much O, but the authors declared that “the elements Si, O and Al are depleted at the same time…”, which seems to be not right. On the other hand, the S content of nanoscale sulfide droplet is really low, in conjunction with the high O content, which convinces me that the nanoscale particle found here is not a pour sulfide droplet, and must contain some oxides and/or alloys. This will strongly shake the very foundations of this manuscript. If the nanoscale particle is just sulfide droplet, as suggested by the authors, its composition is far away from the thermodynamic equilibrium with the silicate melt, which should be addressed in the ‘discussion’ section. Hence, the authors should provide so much more evidence and discussion about the composition of nano particle, not just show the enrichment or depletion of elements relative to the surrounding silicate melt.
Re: Thank you for your comment. ESD testing can only be semi-quantitative, and some light elements are difficult to test, so their content will be classified as O elements, resulting in high O content. At the same time, because we analyze nanoscale particles, we cannot accurately judge the composition of their particles, and can only judge the types and contents of elements they contain according to the EDS test results.
- Line 58-59: “the sulfur in MORB magma is saturated before magma eruption, and may also be saturated in the source region…”
The interpretation here is too simple. Mantle-derived mafic-ultramafic magmas are generally undersaturated in sulfide until immediately prior to or during emplacement due to the negative effect of decreasing pressure on the sulfur content at sulfide saturation. Hence, we may not take the sulfur saturation in MORB for granted, and the possible trigger mechanism of the sulfide saturation should be mentioned. For the source region, the sulfur saturation condition can only occur when the mantle source has just experienced a limited partial melting degree, and the sulfide in restite has not been completely dissolved or exhausted.
Re: Thank you for your comment. Mathez and Yeats (1976) have concluded from paragenetic relations in the submarine basalts they studied that an immiscible sulfide liquid phase exists in the magma prior to eruption. We concur and suggest that this is the typical situation attending eruption of submarine basalt. Czamanske and Moore (1977) found between the S and FeO content in basalt glass (As shown in the Figure 1), strongly suggest that the original famous basalt glass was sulfur saturated and remained sulfur saturated during differentiation. At the same time, the articles of Mathez (1976) and Czamanske and Moore (1977) indicate that MORB magma is S-saturated before eruption, and may also be S-saturated in its source region.
Czamanske, G. K. and Moore, J. G.: Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat, Bull. Geol. Soc. Am., 1977, 88, 587–599, https://doi.org/10.1130/0016-7606(1977)88<587:CAPCOS>2.0.CO;2.
Mathez, E. A.: Sulfur Solubility and Magmatic Sulfides in Submarine Basalt Glass., J Geophys Res, 1976, 81, 4269–4276, https://doi.org/10.1029/JB081i023p04269.
Mathez E A, Yeats R S. Magmatic sulfides in basalt glass from DSDP Hole 319A and Site 320, Nazca Plate[J]. Initial reports of the deep sea drilling project, 1976, 34: 363-373.
- Line 179: “the sulfide droplet sizes varied at different stages of sulfur saturation”
This sentence is ambiguous and should be explained in more details.
Re: Thank you for the suggestion. This sentence has been amended to: “On the other hand, in different stages of S-saturation, the size of sulfide droplet is different, and the structural difference of sulfide droplet is also controlled by their size.”
- Line187-188: “Small droplets then disappear, which enlarges the larger droplets, and this process is generally accompanied by a reduction in interface free energy…”
The interpretation here may come from the classical Ostwald Ripening model, but is not applied to the nano scale particle. While, intense research on coarsening behaviour of nano particles has led to the formulation of a new mechanism of crystal growth, which is called as the “oriented attachment” that describes the spontaneous self-organization of adjacent particles, so that they share a common crystallogrphaic orientation, followed by the joining of these particles at a planar interface. The authors should find more information about the “oriented attachment” mechanism when discussing the crystallization and growth of nano particles.
Re: Thank you for the suggestion. This model is not applied to the nano scale particle, which has now been removed. For the proposed "oriented attachment" mechanism, since there is no definitive evidence in the observation of sulfide droplets in this study as nanoparticles formed by oriented attachment. Therefore, it is impossible to investigate the crystallization and growth of nanoparticles with the oriented attachment mechanism.
- About the Section 5.4.
In this discussion section 5.4, the authors just repeated the current understanding of the sulfide segregation and accumulation in forming the magmatic sulfide deposits in the first paragraph, but I can not find a close relationship between the TEM observations here and the magmatic sulfide deposit. The applications of this work on the magmatic sulfide deposits are weak and ambiguous. There is an obvious separation between the literature reviews and the interpretation of TEM observations adopted here, which is a universal feature of the whole discussion section. This makes the whole manuscript seem to be too simple without a deep consideration, which should be strongly improved in the new revision.
Re: Thank you for the suggestion. In this study, nanoscale sulfide droplets are mainly studied, which can only be considered as a pre-enrichment mechanism, and cannot explain the close relationship with magmatic sulfide deposits. At the same time, nanoscale sulfide droplets, as products in the early stage of magma, are indeed weak and ambiguous for the application of magmatic sulfide deposits. However, it may also be innovative to start with nanoscale sulfide droplets to analyze the causes and applications of later ore deposits. The problems with the explanation of some of the discussions have been revised in the original text.
- The figure captions are too simple and contain less information.
Re: Thank you for the suggestion.
Figure 1. “Regional location of the study area” was changed to “Topographic map of the Southwest Indian Ridge (SWIR), showing sample locations used in this study (red circles).”
Figure 2. “SEM image (a,) and elemental maps (b-f) of MORB samples” was changed to “SEM image (a,) and O, Si, Na, Al and Fe EDS mappings (b-f) of selected particles of MORB samples.”
- Line 89: the abbreviation “SWIR” should be explained when it firstly occurs.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 150: add “which” after “glasses”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 157: add “of sulfide” after “partial dissolution”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 171: replace “correspond” by “refer”.
Re: Thank you for the suggestion. The sentence has been revised.
- Line 199: rewrite this sentence.
Re: Thank you for the suggestion. This sentence has been amended to “Therefore, the distribution of different chalcophile elements in MSS and ISS usually behaves differently.”
- Line 213: add “can really” before “help understand…”.
Re: Thank you for the suggestion. The sentence has been revised.
Citation: https://doi.org/10.5194/egusphere-2023-213-AC3
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