A Combined Re-Os and Pt-Os Isotope and HSE Abundance Study of Ru-Os-Ir Alloys from the Kunar and Unga Placer Deposits, the Taimyr Peninsula, Polar Siberia
Round 1
Reviewer 1 Report
The manuscript is well organized and structured.
It describes in great detail the methods of isotopic analysis of Re-Os and Pt-Os systems. Of particular interest is the comparison of the results of LA and TIMS analysis. It should be noted the high quality of the data obtained, an interesting object, which are also the merit of the work.
The discussion of the results provides an overview of a large number of published data, which will certainly arouse the interest of readers.
Author Response
Response: We would like to thank the reviewer for reading our manuscript and for noting the high quality of the data presented.
Reviewer 2 Report
The study provides new insights into the origin of Ru-Os-Ir alloys from a placer source, presenting 12 new highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances and 187Re187Os and 13 190Pt186Os isotope data for detrital grains of native Ru-Os-Ir alloys in placer deposits of the Kunar 14 and Unga Rivers. The authors compare their bulk and isotopic data with the potential ultramafic source rocks from the Kunar dunite-harzburgite Taimyr Peninsula in the Polar Siberi. The paper is well structured and the data are of high quality. The Discussion provides an adequate review and a sound explanation of the data is provided. I would suggest to introduce a separate paragraph for the conclusions for the better reading of the paper, instead of a common Discussion and conclusions.
Overall the paper deserves to be published.
Comments - revisions
Figure 1 and Figure 2. Lack of scale in the map, this should be added.
Figure 6. The estimated error of the analyses should also be plotted on the Figure in order to evaluate the significance of the range in the values of the isotopic ratios measured.
Line 381: “our data imply that the Re-Os-Pt system in the Ru-Os-Ir minerals has remained closed since the time of their formation”. Based on what evidence do you draw this implication? Please explain in the text and provide evidence.
Author Response
Point 1. I would suggest to introduce a separate paragraph for the conclusions for the better reading of the paper, instead of a common Discussion and conclusions.
Response 1 [Conclusions were added separately after Discussion part and are reproduced below].
1. A multi-technique approach, including the use of electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation multiple-collector inductively-coupled plasma mass-spectrometry (LA MC-ICP-MS) provided a set of HSE abundance and Re-Os and Pt-Os isotope constraints on the origin of detrital Ru-Os-Ir alloys from placer deposits of the Kunar and Unga Rivers in the northern part of the Taimyr Peninsula in the Polar Siberia.
2. The Ru-Os-Ir alloys from both localities show similar compositional signatures dominated by osmium and iridium over ruthenium and rutheniridosmine. The common occurrence of euhedral inclusions of ferroan platinum in Os-Ir-(Ru) alloys is indicative of their high-temperature origin. The primary nature of PGM studied is further supported by the presence of a ruthenium trend in the mineral compositions of Ru-Os-Ir alloys, and occurrence of euhedral inclusions of high-Mg olivine (Fo92–93) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93).
3. The LA MC-ICP-MS data from this study show similar average initial 187Os/188Os values for both PGM assemblages at Kunar and Unga (0.1218±0.0010, γOs(740 Ma) = –0.18±0.84, and 0.1222±0.0025, γOs(740 Ma) = +0.10±2.1, respectively). These values are identical, within uncertainty, to the initial 187Os/188Os value for the Ru-Os-Ir alloy obtained by N-TIMS (0.1218463±0.0000015, γOs(740 Ma) = −0.1500±0.0012). The average initial 187Os/188Os value of the Ru-Os-Ir alloys at Kunar and Unga are indicative of derivation from a source that evolved with a long-term chondritic Re/Os ratio; this source is within the range of those for the majority of komatiite and abyssal peridotite sources and chondritic meteorites.
4. In contrast to the 187Os/188Os data, the initial 186Os/188Os value of 0.1198409±0.0000012 obtained by N-TIMS for the same Ru-Os-Ir alloy sample T-2 at Kunar is 34±10 ppm higher than this value in the chondritic reference of Brandon et al. [15] at that time, but is similar to the µ186Os value of +29±2 in the source of the 2.05 Ga Lapland komatiite system [65]. This implies evolution of the Kunar mantle source with time-integrated suprachondritic Pt/Os ratio. The reason for such long-term Pt/Os enrichment in the Kunar mantle source is not yet clear and would require further investigation.
Point 2: Figure 1 and Figure 2. Lack of scale in the map, this should be added.
Response 2 [Scale bars were added in both sketch maps].
Point 3: Figure 6. The estimated error of the analyses should also be plotted on the Figure in order to evaluate the significance of the range in the values of the isotopic ratios measured.
Response 3 [Error bars will not be seen, as they are much smaller than the size of the symbols].
Point 4: Line 381: “our data imply that the Re-Os-Pt system in the Ru-Os-Ir minerals has remained closed since the time of their formation”. Based on what evidence do you draw this implication? Please explain in the text and provide evidence.
Response 4 [We provided evidence for primary origin of Ru-Os-Ir alloys studied, implying that the Os-isotopic composition of these PGMs reflects that of their source region at the time of their formation, and is consistent with closed-system behavior of both isotopic systems. Having this in mind, however, we decided to delete this sentence].
Reviewer 3 Report
The paper is interesting but it needs some work before publications.
Authors need to check the page setup of the paper because in some cases figure captions are in a different page respect to the figure ( for example figure 3) and text and figures are not aligned. Pay also attention to the quality of the figure 6. As regards tables, it can be better if each table is reported in one page. Authors need also to to check if all the references are cited in the text and integrate in some cases.
In the attached file I write some specific notes on the text.
Comments for author File: 
Comments.pdf
Author Response
TEXT
Point 1: Line 81-82 “More detailed information on the geology of the Chelyuskin ophiolite belt has been summarized by Zalyaleev, Bezzubtsev [35], Bezzubtsev et al. [51], and Vernikovsky and Vernikovskaya [56].” Please rewrite “The geology of the the Chelyuskin ophiolite belt has been described in previous studies ( ref.)”
Response 1 [Fixed].
Point 2: Line 100-102 “The elevated content of chromite in the heavy fraction of the production concentrates and the spatial proximity of the Kunar complex imply that chromitites of the Kunar complex are the most probable source of the PGM studied.”Please clarify this point and, if is the case, add some references.
Response 2 [Data clarifying elevated content of chromite and the reference to work by Gavrish (2000) are added. It is well known that chromitites are the main contributing source of PGM for placer deposits associated with ophiolitic rocks. Therefore, close proximity of ultramafic rocks of the Kunar dunite-harzburgite complex indicates that chromitites of this complex are the most probable source for detrital PGM studied].
Point 3: Line 109 “CCFS/GEMOC” Is this an acronym of what? Please write it.
Response 3 [Explanation of the abbreviation is specified, i.e. Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (ARC CoE CCFS)/ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC ARC National Key Centre)].
Point 4: Line 246 “Al2O3, CaO, TiO2, MnO were below the detction limit, Fo=100*Mg/(Mg+Fe)2 “Please rewrite with detection
Response 4 [Fixed].
Point 5: Line 262 “data after [17]” Please rewrite as “data from”
Response 5 [Fixed].
Point 6: Line 335 “Since the studied alloy grain contains very little Re” Please rewrite as “Since the studied alloy grain has a very low Re content ( value)”
Response 6 [Since the studied alloy grain has a very low Re content (187Re/188Os=0.00024±10)….].
Point 7: Line 338-340 “This calculated TRD model age is 760 Ma, which is consistent with the estimate for the age of the Kunar 339 mafic-ultramafic complex at 740 Ma.”Please if it is possible write also the error of the model age 760 Ma, and a reference for the age 740 Ma.
Response 7 [The error for the calculated TRD model age would be 764±2 Ma; references for the age 740 Ma also added].
Point 8: Line 362-634 “The calculated initial γ187Os (740 Ma) of the Kunar mantle source is within the range of those for the majority of komatiite and abyssal peridotite sources and chondritic meteorites (Figure 8).” Please indicate the γ187Os value and the corresponding standard deviation reported in figure 8.
Response 8 [The γ187Os value and the corresponding standard deviation as shown in Figure 8 are added in the text as follows: “The calculated initial γ187Os (740 Ma) = -0.02±1.6 (2SD) of the Kunar mantle source is within the range of those for the majority of komatiite and abyssal peridotite sources and chondritic meteorites (Figure 8)”].
Point 9: Line 38-383“Finally, our data imply that the Re-Os-Pt system in the Ru-Os-Ir minerals has remained closed since the time of their formation, despite later thermal events that affected the Kunar ophiolite-type complex.” Please clarify this point
Response 9 [We provided evidence for primary origin of Ru-Os-Ir alloys studied, implying that the Os-isotopic composition of these PGMs reflects that of their source region at the time of their formation, and is consistent with closed-system behavior of both isotopic systems. Having this in mind, however, we decided to delete this sentence].
Point 10: References: Verify that all the listed papers are mentioned in the text.
Response 10 [We confirm that all the listed papers are mentioned in the text].
FIGURES
Point 11: Figure 1. Mamont-Shrenk(1) – Faddey (2) …. not indicated in figure numebrs 1 and 2 associated to these terranes.
Response 11 [Numbers 1 and 2 were added].
Point 12: Figure 3. In panel a relative to the sample 231 are reported Fig. 4, RIO and OI . Please describe them in the caption.
Response 12 [Fixed].
Point 13: Figure 6. Kunar complex and Unga?
Response 13 [We put on the Figure 6 “Kunar and Unga placer deposits”].
TABLES
Point 14: Table 1. In Analysis 3 relative to the sample 233 it has been indicated “b.d.l.”for Rh and Pd, while for the other elements it is used the notation “-“. Please use the same notation for all elements
Response 14 [Fixed].
