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190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system

A. Luguet1,

1Institute of Geosciences, University of Bonn, Germany

G.M. Nowell2,

2Earth Sciences Department, University of Durham, UK

E. Pushkarev3,

3Russian Academy of Sciences, Institute of Geology and Geochemistry Ural Division, Moscow, Russia

C. Ballhaus1,

1Institute of Geosciences, University of Bonn, Germany

R. Wirth4,

4Helmholtz Centre Potsdam, GeoForschungs Zentrum, Section 3.5 Surface Geochemistry, Potsdam, Germany

A. Schreiber4,

4Helmholtz Centre Potsdam, GeoForschungs Zentrum, Section 3.5 Surface Geochemistry, Potsdam, Germany

I. Gottman3

3Russian Academy of Sciences, Institute of Geology and Geochemistry Ural Division, Moscow, Russia

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Luguet, A., Nowell, G.M., Pushkarev, E., Ballhaus, C., Wirth, R., Screiber, A., Gottmann, I. (2019) 190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system. Geochem. Persp. Let. 11, 44–48.

Deutsche Forschungsgemeinschaft (LU 1603/5-1, CB 964/36-1) for Luguet and Ballhaus, respectively.
Russian State Scientific programme № АААА-А18-118051190022-4 for Pushkarev.

Geochemical Perspectives Letters v11  |  doi: 10.7185/geochemlet.1924
Received 19 May 2019  |  Accepted 28 August 2019  |  Published 22 October 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0


Keywords: Pt-Os isotope system, Pt-He isotope system, Re-Os isotope system, platinum group minerals, Pt-alloys, Kondyor



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Abstract


Platinum Group Minerals are typically dated using the 187Re-187Os and 190Pt-186Os isotope systems and more recently using the 190Pt-4He geochronometer. The 187Re-187Os and 190Pt-186Os compositions of Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC) analysed here reveal overprinting for both geochronometers except in one alloy exhibiting the most unradiogenic 187Os/188Os and most radiogenic 186Os/188Os signatures. These signatures argue for an Early Triassic mineralisation, when silicate melts/fluids derived from the partial melting of an Archean mantle crystallised to form the Kondyor ZUC while the 190Pt-4He chronometer supports an Early Cretaceous mineralisation. We propose that Kondyor ZUC represents the root of an alkaline picritic volcano that constitutes the remnants of an Early Triassic island arc formed during the subduction of the Mongol-Okhotsk ocean seafloor under the Siberia craton. After the Early Cretaceous collision of Siberia with the Mongolia-North China continent, the exhumation of deep-seated structures - such as the Kondyor ZUC - allowed these massifs to cool down below the closure temperatures of the Pt-He and K-Ar, Rb-Sr isotope systems, explaining their Early to Late Cretaceous ages for the Kondyor ZUC.

Figures and Tables

Figure 1 Bright field image and EDS spectra of Kondyor Pt-Fe alloys and their pure Os exsolution lamellae (FIB-TEM image, GFZ, Potsdam, Germany). The Cu peaks on the EDS spectra are due to the Cu grid that carries the FIB section.

Figure 2 (a) Variations of 187Os/188Os vs. 187Re/188Os, (b) of 186Os/188Os vs. 190Pt/188Os and (c) of 190Os/188Os vs. 187Re/188Os. The primitive mantle (PM) 186Os/188Os and 187Os/188Os values are respectively from Day et al. (2017) and Meisel et al. (2001). If the positive correlation between 186Os/188Os vs. 190Pt/188Os is considered to be an isochronous relationship, it yields an age of 249.8 ± 12 Ma and an intercept of 0.119821 ± 0.000024 (2 sigma) (MSWD = 0.81).

Figure 3 (a) 186Os/188Os and (b) 187Os/188Os variations with the Os concentrations (1/188Os beam). Red dotted line represents the overprinting of the most pristine Pt-alloy D-S2 by an Os-rich contaminant characterised by 187Os/188Os and 186Os/188Os signatures of Pt-alloy E-S2 (0.12457 and 0.119851, respectively).

Figure 1 Figure 2 Figure 3

View all figures and tables  


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Introduction


Platinum group minerals (PGM, e.g., Os-alloys, Pt-alloys, Pt-arsenides) are critical host phases of the Highly Siderophile Elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re) in the Earth’s mantle and crust. They are typically dated with the 187Re-187Os and/or 190Pt-186Os isotope systems (e.g., Walker et al., 1997

Walker, R.J., Morgan, J.W., Beary, E.S., Smoliar, M.I., Czamanske, G.K., Horan, M.F. (1997) Applications of the 190Pt-186Os isotope system to geochemistry and cosmochemistry. Geochimica et Cosmochimica Acta 61, 4799-4807.

; Meibom and Frei, 2002

Meibom, A., Frei, R. (2002) Evidence for an Ancient Osmium Isotopic Reservoir in Earth. Science 296, 516-518.

; Pearson et al., 2007

Pearson, D.G., Parman, S.W., Nowell, G.M. (2007) A link between large mantle melting events and continent growth seen in osmium isotopes. Nature 449, 202-205.

; Coggon et al., 2012

Coggon, J.A., Nowell, G.M., Pearson, D.G., Oberthür, T., Lorand, J.-P., Melcher, F., Parman, S.W. (2012) The 190Pt-186Os decay system applied to dating platinum-group element mineralization of the Bushveld Complex, South Africa. Chemical Geology 302-303, 48-60.

).

Recently, the 190Pt-4He isotopic system has emerged as an alternative geochronometer for Pt-rich PGM. The 190Pt-4He and 190Pt-186Os geochronometers are both measuring the alpha decay of 190Pt, with the only difference being that one measures the accumulation of the daughter product 186Os and the other the accumulation of the decay particle 4He. The Pt-He geochronometer was so far used to date the Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC), which is located in the Aldan Shield on the South-East margin of the Siberian Craton (Fig. S-1 and Supplementary Information) (Shukolyukov et al., 2012a

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

; Mochalov et al., 2016

Mochalov, A.G., Yakubovich, O.V., Bortnikov, N.S. (2016) 190Pt-4He Age of PGE Ores in the Alkaline-Ultramafic Kondyor Massif (Khabarovsk District, Russia). Doklady Earth Sciences 469, 846–850.

, 2018

Mochalov, A.G., Yakubovich, O.V., Zolotarev, A.A. (2018) Structural Transformations and Retention of Radiogenic 4He in Platinum Minerals under Mechanical Deformations. Doklady Earth Sciences 480, 591–594.

). The Early Cretaceous Pt-He isochron ages (112 ± 7 Ma and 129 ± 6 Ma, calculated using a 190Pt half-life of 469 Gyr: Begemann et al., 2001

Begemann, F., Ludwig, K.R., Lugmair, G.W., Min, K., Nyquist, L.E., Patchett, P.J., Renne, P.R., Shih, C.-Y., Villa, I.M., Walker, R.J. (2001) Call for an improved set of decay constants for geochronological use. Geochimica et Cosmochimica Acta 65, 111-121.

) agree well with the Rb-Sr, Sm-Nd and K-Ar ages obtained on the main lithologies (whole rock and mineral phases) but conflict with the Re-Os TRD model ages obtained on erlichmanite (OsS2), sperrylite (PtAs2), Os-alloys and Pt-alloys (Cabri et al., 1998

Cabri, L.J., Stern, R.A., Czamanske, G.K. (1998) Osmium isotope measurements of the Pt-Fe alloy placer nuggets from the Konder Intrusion using a Shrimp II Ion Microprobe. 8th International Platinum Symposium Abstracts, 55-58.

; Malitch and Thalhammer, 2002

Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.

) that vary from Neoproterozoic (658-603 Ma) to future ages, when back calculated to the present-day primitive mantle (PM) 187Os/188Os estimate (Meisel et al., 2001

Meisel, T., Walker, R.J., Irving, A.J., Lorand, J.-P. (2001) Osmium isotopic compositions of mantle xenoliths: A global perspective. Geochimica et Cosmochimica Acta 65, 1311-1323.

).

The combination of multiple isotope systems for dating single mineral phases offers the opportunity to resolve “open system behaviour” and to assess which isotopic signatures provide geologically meaningful information on the age and origin of minerals. Here we report the coupled 190Pt-186Os and 187Re-187Os signatures obtained by Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) (Supplementary Information) on 13 sub-millimetric Pt-alloys separated from a chromitite schlieren (sample 1265; Pushkarev et al., 2015

Pushkarev, E.V., Kamenetsky, V.S., Morozova, A.V., Khiller, V.V., Glavatskykh, S.P., Rodemann, T. (2015) Ontogeny of Ore Cr-spinel and composition of inclusions as indicators of the Pneumatolytic-Hydrothermal Origin of PGM-bearing Chromitites from Kondyor Massif, The Aldan Shield. Geology of Ore Deposits 57, 352-380.

) hosted in the dunitic core of the Kondyor ZUC. Our Pt-alloys are a different subset than those investigated for the 190Pt-4He isotope system. Shukolyukov et al. (2012a)

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

and Mochalov et al. (2016

Mochalov, A.G., Yakubovich, O.V., Bortnikov, N.S. (2016) 190Pt-4He Age of PGE Ores in the Alkaline-Ultramafic Kondyor Massif (Khabarovsk District, Russia). Doklady Earth Sciences 469, 846–850.

, 2018

Mochalov, A.G., Yakubovich, O.V., Zolotarev, A.A. (2018) Structural Transformations and Retention of Radiogenic 4He in Platinum Minerals under Mechanical Deformations. Doklady Earth Sciences 480, 591–594.

) dated (i) Pt-alloys from different lithologies of the Kondyor ZUC, including the chromitite lenses of the dunitic core and (ii) alluvial Pt-Pd PGM. The FIB-TEM investigations on a few of our Pt-alloys revealed a very complex nanoscale exsolution pattern consisting of spinodal exsolutions of Pt-Fe alloys (e.g., Pt3Fe, PtFe) and pure Os exsolution lamellae (Fig. 1).


Figure 1 Bright field image and EDS spectra of Kondyor Pt-Fe alloys and their pure Os exsolution lamellae (FIB-TEM image, GFZ, Potsdam, Germany). The Cu peaks on the EDS spectra are due to the Cu grid that carries the FIB section.
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Results


The Kondyor Pt-alloys display radiogenic 186Os/188Os and unradiogenic 187Os/188Os compositions (Fig. 2a,b ). The most radiogenic 187Os/188Os signatures (0.1246; alloys L-S2 and E-S2, Table S-2) agree well with those previously obtained on five Kondyor Os-rich alloys (0.1248-0.1252; Malitch and Thalhammer, 2002

Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.

). Conversely, the least radiogenic 187Os/188Os (0.110096 ± 2136; alloy D-S2) is close to the composition of Re-free, least metasomatised peridotite xenoliths of the Tok locality (0.109; estimated for Al2O3 = 0 wt. % on the 187Os/188Os vs. Al2O3 “aluminochron”; Ionov et al., 2006

Ionov, D.A., Shirey, S.B. Weiss, D., Brügmann G. (2006) Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle. Earth and Planetary Science Letters 241, 47-64.

), which like the Kondyor ZUC is located in the East Aldan Shield (Fig. S-1). Overall, the 187Os/188Os compositions are decoupled from the 187Re/188Os ratios (Fig. 2a). In contrast, the 186Os/188Os compositions define a positive trend with 190Pt/188Os, which - if considered to represent an isochronous relationship - yields an age of 249.8 ± 12 Ma (Fig. 2b). The 187Os/188Os and 186Os/188Os signatures are negatively correlated despite the sympathetic variation of both parent/daughter elemental ratios (Fig. 2c).


Figure 2 (a) Variations of 187Os/188Os vs. 187Re/188Os, (b) of 186Os/188Os vs. 190Pt/188Os and (c) of 190Os/188Os vs. 187Re/188Os. The primitive mantle (PM) 186Os/188Os and 187Os/188Os values are respectively from Day et al. (2017)

Day, J.M.D., Walker, R.J., Warren, J.M. (2017) 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys. Geochimica et Cosmochimica Acta 200, 232-254.

and Meisel et al. (2001)

Meisel, T., Walker, R.J., Irving, A.J., Lorand, J.-P. (2001) Osmium isotopic compositions of mantle xenoliths: A global perspective. Geochimica et Cosmochimica Acta 65, 1311-1323.

. If the positive correlation between 186Os/188Os vs. 190Pt/188Os is considered to be an isochronous relationship, it yields an age of 249.8 ± 12 Ma and an intercept of 0.119821 ± 0.000024 (2 sigma) (MSWD = 0.81).
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Robustness of the Re-Os and Pt-Os Isotope Systematics


The decoupling of the 187Os/188Os from both 187Re/188Os and 186Os/188Os signatures demonstrate the open system behaviour of the Re-Os isotope system in the Kondyor Pt-alloys. This is best explained by the overprinting of the Os-poor, least radiogenic 187Os/188Os of the Pt-alloy D-S2 by an Os-rich (ca. 700 times richer) contaminant with a 187Os/188Os of 0.1246 (Fig. 3a), similar to the most radiogenic 187Os/188Os of our Kondyor alloys (e.g., points E-S2) and very close to the least radiogenic 187Os/188Os compositions previously reported by Malitch and Thalhammer (2002)

Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.

and Cabri et al. (1998)

Cabri, L.J., Stern, R.A., Czamanske, G.K. (1998) Osmium isotope measurements of the Pt-Fe alloy placer nuggets from the Konder Intrusion using a Shrimp II Ion Microprobe. 8th International Platinum Symposium Abstracts, 55-58.

for Kondyor PGM (Fig. 2a). Both the 186Os/188Os vs. 187Os/188Os and 186Os/188Os vs. 1/Os relationships (Fig. 3b) can be reproduced with such a mixing scenario. Importantly, the negative 187Os/188Os vs. 187Re/188Os and the relationships between the 187Os/188Os and the abundance of Os exsolution lamellae (monitored by the 188Os signal) in the Pt-alloys likely suggest that this mixing scenario reflects a gradual overprinting of the mantle source of the Kondyor mineralisation by subduction-related fluids (Supplementary Information).

The Pt-alloy D-S2 is then the least overprinted of our Kondyor subset (Fig. 3a,b). This view is further supported by the closeness of its 187Os/188Os and 187Re/188Os ratios (0.001196 and 0.00541; Table S-2) to those of the Re-free, least metasomatised Tok peridotite xenoliths (0.109 and 0; Ionov et al., 2006

Ionov, D.A., Shirey, S.B. Weiss, D., Brügmann G. (2006) Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle. Earth and Planetary Science Letters 241, 47-64.

), implying that the 187Os/188Os composition of alloy D-S2 may still hold geologically meaningful constraints. Its Re-Os TRD model age points at a 2630 Ma old PUM-like mantle source for the Kondyor Pt-mineralisation (the Re-Os TMA model age is 2664 Ma). Occurrence of Archean mantle underlying the Aldan Shield is also supported by the TRD model ages of the Tok peridotites (2770 Ma) and by Pb-Pb isotope systematics of the Mesozoic lamproitic magmatism (~3 Ga; Davies et al., 2006

Davies, G.R., Stolz, A.J., Mahotkin, I.L., Nowell, G.M., Pearson, D.G. (2006) Trace element and Sr-Pb-Nd-Hf isotope evidence for ancient, fluid-dominated enrichment of the source of Aldan shield lamproites. Journal of Petrology 47, 1119-1146.

). Considering that the present-day PM has a 186Os/188Os of 0.1198388 and a 190Pt/186Os of 0.0022 (Day et al., 2017

Day, J.M.D., Walker, R.J., Warren, J.M. (2017) 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys. Geochimica et Cosmochimica Acta 200, 232-254.

), the 2630 Ma PUM-like mantle source of the Kondyor Pt-mineralisation then had a maximum 186Os/188Os of 0.1198303. If we consider such an initial 186Os/188Os composition, the D-S2 Pt-alloy would require 242.6 Myr to evolve to its present day 186Os/188Os signature. This age is similar within error to that extrapolated from the multi-grain Pt-Os isochron-like trend defined by our Kondyor Pt-alloys (249.8 ± 12 Ma; Fig. 2b).


Figure 3 (a) 186Os/188Os and (b) 187Os/188Os variations with the Os concentrations (1/188Os beam). Red dotted line represents the overprinting of the most pristine Pt-alloy D-S2 by an Os-rich contaminant characterised by 187Os/188Os and 186Os/188Os signatures of Pt-alloy E-S2 (0.12457 and 0.119851, respectively).
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Ages of ~250-240 Ma are recognised regionally within the Aldan Shield (Lena and Aldan (Palaeo) Rivers: Wang et al., 2011

Wang, C.Y., Campbell, I.H., Stepanov, A.S., Allen, C.M., Burtsev, I.N. (2011) Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers. Geochimica et Cosmochimica Acta 75, 1308-1345.

; Miller et al., 2013

Miller, E.L., Soloviev, A.V., Prokopiev, A.V., Toro, J., Harris, D., Kuzmichev, A.B., Gehrels, G.E., (2013) Triassic river systems and the paleo-Pacific margin of northwestern Pangea. Gondwana Research 23, 1631-1645.

), the Baikal Lake Region (e.g., Gladkochub et al., 2010

Gladkochub, D.P., Donskaya, T.V., Ivanov, A.V., Ernst, R., Mazukabzov, A.M., Pisarevsky, S.A., Ukhova, N.A. (2010) Phanerozoic mafic magmatism in the southern Siberian craton: geodynamic implications. Russian Geology and Geophysics 51, 952–964.

) and within basins (e.g., Onon and Mohe-Upper Amur), located South of the Aldan Shield and adjacent to the Mongol-Okhotsk Fold belt (Guo et al., 2017

Guo, Z-X., Yang, Y-T., Zyabrev, S., Hou, Z-H. (2017) Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic-earliest Cretaceous. Journal of Asian Earth Sciences 145, 494-511.

). The Mongol-Okhotsk fold belt (Fig. S-1), which rims the Siberian Craton on its South Margin over ca. 3000 km, represents the suture zone left after the closure of the Mongol-Okhotsk Ocean - as its seafloor was subducted under the Siberia craton and under the Mongolia-North China continent (Amur plate) -, and the subsequent collision of the Siberian craton with the Mongolia-North China continent (e.g., Zorin, 1999

Zorin, Y.A. (1999) Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia. Tectonophysics 306, 33-56.

; Guo et al., 2017

Guo, Z-X., Yang, Y-T., Zyabrev, S., Hou, Z-H. (2017) Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic-earliest Cretaceous. Journal of Asian Earth Sciences 145, 494-511.

). The age distribution along the Mongol-Okhotsk fold belt demonstrates an eastward zip-like closure of the Mongol-Okhotsk ocean (Zorin, 1999

Zorin, Y.A. (1999) Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia. Tectonophysics 306, 33-56.

) initiated in the Late Palaeozoic in NE Mongolia (Zhao et al., 2017

Zhao, P., Xu, B., Jahn, B.-M. (2017) The Mongol-Okhotsk Ocean subduction-related Permian peraluminous granites in northeastern Mongolia: constraints from zircon U-Pb ages, whole-rock elemental and Sr-Nd-Hf isotopic compositions. Journal of Asian Earth Sciences 144, 225-240.

) and in the Early Triassic in the eastern part of the Mongol-Okhotsk belt, south of Aldan Shield Region (Guo et al., 2017

Guo, Z-X., Yang, Y-T., Zyabrev, S., Hou, Z-H. (2017) Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic-earliest Cretaceous. Journal of Asian Earth Sciences 145, 494-511.

). The age of the subsequent collision between the Mongolia-North China continent and Siberia craton also evolves eastwards from Middle Jurassic to Early Cretaceous (Zorin, 1999

Zorin, Y.A. (1999) Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia. Tectonophysics 306, 33-56.

).

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Why are the 190Pt-186Os and the 190Pt-4He “Ages” of the Kondyor Pt-alloys Different?


Both the 190Pt-4He and 190Pt-186Os isotopic systems are based on the radioactive alpha decay of the 190Pt so they should yield identical ages. However, for the Kondyor Pt-alloys, the Pt-He isochronal ages (Shukolyukov et al., 2012a

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

; Mochalov et al., 2016

Mochalov, A.G., Yakubovich, O.V., Bortnikov, N.S. (2016) 190Pt-4He Age of PGE Ores in the Alkaline-Ultramafic Kondyor Massif (Khabarovsk District, Russia). Doklady Earth Sciences 469, 846–850.

, 2018

Mochalov, A.G., Yakubovich, O.V., Zolotarev, A.A. (2018) Structural Transformations and Retention of Radiogenic 4He in Platinum Minerals under Mechanical Deformations. Doklady Earth Sciences 480, 591–594.

) are ~110-140 Myr younger than the Pt-Os ages.

Several lines of evidence suggest that the age inconsistency may reflect an open system behaviour of the Pt-He isotopic system. First, Shukolyukov et al. (2012a

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

,b

Shukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012b) Geothermochronology based on noble gases: III. Migration of radiogenic He in the crystal structure of native metals with applications to their isotopic dating. Petrology 20, 1-20.

) and Mochalov et al. (2016)

Mochalov, A.G., Yakubovich, O.V., Bortnikov, N.S. (2016) 190Pt-4He Age of PGE Ores in the Alkaline-Ultramafic Kondyor Massif (Khabarovsk District, Russia). Doklady Earth Sciences 469, 846–850.

argued that radiogenic 4He is retained in the structure of native metals as vesicles that are only released upon melting of the native metals (>1000 °C). However, the only 4He thermal desorption experiment conducted on Pt-alloys by Shukolyukov et al. (2012a)

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

revealed 4He loss ([4He] ≠ 0) for temperatures as low as ~700 °C (see Fig. 4 in Shukolyukov et al., 2012a

Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.

). While the 4He loss appears marginal during their experiment, it will be significant if Pt-alloys reside in the lithospheric mantle (with an equilibration temperature >700 °C) for 10s-100s of Myr. It is thus possible that the 4He is not fully trapped in the structure of the Pt-alloys until the 4He closure temperature in these minerals is attained. One can additionally consider how the nanoscale exsolution patterns within the Kondyor Pt-alloys will affect the 4He loss/retention. The grain boundaries proposed as a preferential sink for 4He (Shukolyukov et al., 2012b

Shukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012b) Geothermochronology based on noble gases: III. Migration of radiogenic He in the crystal structure of native metals with applications to their isotopic dating. Petrology 20, 1-20.

) may turn out to be preferential 4He loss sites when Pt-alloys are intensely exsolved (Fig. 1). The Pt-free nature of the Os exsolution lamellae combined with the extremely Os-poor composition of their Pt-alloy hosts (Fig. 1; Malitch and Thalhammer, 2002

Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.

; Nekrasov et al., 2005

Nekrasov, I.Y., Lennikov, A.M., Zalishchak, B.L., Oktyabrsky, R.A., Ivanov, V.V., Sapin, V.I., Taskaev, V.I. (2005) Composition variations in platinum-group minerals and gold, Konder alkaline-ultrabasic massif, Aldan Shield, Russia. The Canadian Mineralogist 43, 637-654.

) argues for an equilibration temperature below 500 ºC (see Pt-Os phase diagram in Okrugin, 2002

Okrugin, A.V. (2002) Phase transformations and genesis of platinum-group minerals in various types of platinum-bearing deposits. In: Boudreau, A.E. (Ed.) 9th International Platinum Symposium (Billings), Extended Abstracts. Duke University Press, Durham, North Carolina, 349-353.

), thus well below the 700 °C temperature mark of 4He loss onset observed for Pt alloys (see above). The last evidence suggesting a low closure temperature (<600 °C) of the Pt-He isotopic system comes from the similarity of the Pt-He isochronal ages with the Rb-Sr, Sm-Nd and K-Ar obtained on whole-rock and single minerals (biotite, feldspar) of the dunitic core, pyroxenites and late metasomatic dikes of Kondyor ZUC (149-83 Ma: e.g., Orlova, 1992

Orlova, M.P. (1992) Geology and genesis of the Konder Massif. Geology of the Pacific Ocean 8, 120-132.

; Cabri et al., 1998

Cabri, L.J., Stern, R.A., Czamanske, G.K. (1998) Osmium isotope measurements of the Pt-Fe alloy placer nuggets from the Konder Intrusion using a Shrimp II Ion Microprobe. 8th International Platinum Symposium Abstracts, 55-58.

; Pushkarev et al., 2002

Pushkarev, Yu.D., Kostoyanov, A.I., Orlova, M.P., Bogomolov, E.S. (2002) Peculiarities of the Rb-Sr, Sm-Nd, Re-Os and K-Ar isotope systems in the Kondyor massif: mantle substratum, enriched by PGE. Regional Geology and Metallogeny 16, 80-91 (in Russian).

).

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Implication for the Origin and Evolution of the Kondyor ZUC


The combined LA-MC-ICPMS investigation of the Re-Os and Pt-Os isotope signatures demonstrates that the Pt-mineralisation, contemporaneous to the formation of the Kondyor ZUC, originates ~250-240 Myr ago from the melts and fluids produced by partial melting of possibly an Archean PUM-like mantle source, which could be the Siberian cratonic mantle. Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992

Orlova, M.P. (1992) Geology and genesis of the Konder Massif. Geology of the Pacific Ocean 8, 120-132.

; Malitch and Thalhammer, 2002

Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.

) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009

Burg, J.-P., Bodinier, J.-L., Gerya, T., Bedini, R.-M., Boudier, F., Dautria, J.-M., Prikhodko, V., Efimov, A., Pupier, E., Balanec, J.-L. (2009) Translithospheric Mantle Diapirism: Geological Evidence and Numerical Modelling of the Kondyor Zoned Ultramafic Complex (Russian Far-East). Journal of Petrology 50, 289-321.

), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011

Simonov, V.A., Prikhod’ko, V.S., Kovyazin, S.V. (2011) Genesis of Platiniferous Massifs in the Southeastern Siberian Platform. Petrology 19, 549-567.

), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999

Zorin, Y.A. (1999) Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia. Tectonophysics 306, 33-56.

; Guo et al., 2017

), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017

Guo, Z-X., Yang, Y-T., Zyabrev, S., Hou, Z-H. (2017) Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic-earliest Cretaceous. Journal of Asian Earth Sciences 145, 494-511.

). The uplift associated with the Early Cretaceous collision of the Siberian craton with the Mongolia-North China continent (after the closure of the Mongol-Okhotsk ocean) combined with the subsequent major extensional phase evidenced by the development of Early Cretaceous rift systems may have contributed to the unroofing and exhumation of deep-seated structures such as metamorphic core complexes (Zorin, 1999). In such an unroofing and exhumation scenario, the Kondyor ZUC would attain sub-surface conditions and cool down below the closure temperatures of the K-Ar, Rb-Sr and Pt-He isotope systems, explaining why these geochronometers yield almost exclusively Early to Late Cretaceous ages for the Kondyor ZUC.

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Acknowledgements


AL and CB thank the Deutsche Forschungsgemeinschaft for supporting this project (LU 1603/5-1, CB 964/36-1) and EP acknowledges the Russian State Scientific programme № АААА-А18-118051190022-4. We gratefully thank two anonymous reviewers and our editor, Cin-Ty Lee, for their insightful comments and suggestions.

Editor: Cin-Ty Lee

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References



Begemann, F., Ludwig, K.R., Lugmair, G.W., Min, K., Nyquist, L.E., Patchett, P.J., Renne, P.R., Shih, C.-Y., Villa, I.M., Walker, R.J. (2001) Call for an improved set of decay constants for geochronological use. Geochimica et Cosmochimica Acta 65, 111-121.
Show in context

The Early Cretaceous Pt-He isochron ages (112 ± 7 Ma and 129 ± 6 Ma, calculated using a 190Pt half-life of 469 Gyr: Begemann et al., 2001) agree well with the Rb-Sr, Sm-Nd and K-Ar ages obtained on the main lithologies (whole rock and mineral phases) but conflict with the Re-Os TRD model ages obtained on erlichmanite (OsS2), sperrylite (PtAs2), Os-alloys and Pt-alloys (Cabri et al., 1998; Malitch and Thalhammer, 2002) that vary from Neoproterozoic (658-603 Ma) to future ages, when back calculated to the present-day primitive mantle (PM) 187Os/188Os estimate (Meisel et al., 2001).
View in article


Burg, J.-P., Bodinier, J.-L., Gerya, T., Bedini, R.-M., Boudier, F., Dautria, J.-M., Prikhodko, V., Efimov, A., Pupier, E., Balanec, J.-L. (2009) Translithospheric Mantle Diapirism: Geological Evidence and Numerical Modelling of the Kondyor Zoned Ultramafic Complex (Russian Far-East). Journal of Petrology 50, 289-321.
Show in context

Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
View in article


Cabri, L.J., Stern, R.A., Czamanske, G.K. (1998) Osmium isotope measurements of the Pt-Fe alloy placer nuggets from the Konder Intrusion using a Shrimp II Ion Microprobe. 8th International Platinum Symposium Abstracts, 55-58.
Show in context

The Early Cretaceous Pt-He isochron ages (112 ± 7 Ma and 129 ± 6 Ma, calculated using a 190Pt half-life of 469 Gyr: Begemann et al., 2001) agree well with the Rb-Sr, Sm-Nd and K-Ar ages obtained on the main lithologies (whole rock and mineral phases) but conflict with the Re-Os TRD model ages obtained on erlichmanite (OsS2), sperrylite (PtAs2), Os-alloys and Pt-alloys (Cabri et al., 1998; Malitch and Thalhammer, 2002) that vary from Neoproterozoic (658-603 Ma) to future ages, when back calculated to the present-day primitive mantle (PM) 187Os/188Os estimate (Meisel et al., 2001).
View in article
This is best explained by the overprinting of the Os-poor, least radiogenic 187Os/188Os of the Pt-alloy D-S2 by an Os-rich (ca. 700 times richer) contaminant with a 187Os/188Os of 0.1246 (Fig. 3a), similar to the most radiogenic 187Os/188Os of our Kondyor alloys (e.g., points E-S2) and very close to the least radiogenic 187Os/188Os compositions previously reported by Malitch and Thalhammer (2002) and Cabri et al. (1998) for Kondyor PGM (Fig. 2a).
View in article
The last evidence suggesting a low closure temperature (<600 °C) of the Pt-He isotopic system comes from the similarity of the Pt-He isochronal ages with the Rb-Sr, Sm-Nd and K-Ar obtained on whole-rock and single minerals (biotite, feldspar) of the dunitic core, pyroxenites and late metasomatic dikes of Kondyor ZUC (149-83 Ma: e.g., Orlova, 1992; Cabri et al., 1998; Pushkarev et al., 2002).
View in article


Coggon, J.A., Nowell, G.M., Pearson, D.G., Oberthür, T., Lorand, J.-P., Melcher, F., Parman, S.W. (2012) The 190Pt-186Os decay system applied to dating platinum-group element mineralization of the Bushveld Complex, South Africa. Chemical Geology 302-303, 48-60.
Show in context

They are typically dated with the 187Re-187Os and/or 190Pt-186Os isotope systems (e.g., Walker et al., 1997; Meibom and Frei, 2002; Pearson et al., 2007; Coggon et al., 2012).
View in article


Davies, G.R., Stolz, A.J., Mahotkin, I.L., Nowell, G.M., Pearson, D.G. (2006) Trace element and Sr-Pb-Nd-Hf isotope evidence for ancient, fluid-dominated enrichment of the source of Aldan shield lamproites. Journal of Petrology 47, 1119-1146.
Show in context

Occurrence of Archean mantle underlying the Aldan Shield is also supported by the TRD model ages of the Tok peridotites (2770 Ma) and by Pb-Pb isotope systematics of the Mesozoic lamproitic magmatism (~3 Ga; Davies et al., 2006).
View in article


Day, J.M.D., Walker, R.J., Warren, J.M. (2017) 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys. Geochimica et Cosmochimica Acta 200, 232-254.
Show in context

Figure 2 [...] The primitive mantle (PM) 186Os/188Os and 187Os/188Os values are respectively from Day et al. (2017) and Meisel et al. (2001).
View in article
Considering that the present-day PM has a 186Os/188Os of 0.1198388 and a 190Pt/186Os of 0.0022 (Day et al., 2017), the 2630 Ma PUM-like mantle source of the Kondyor Pt-mineralisation then had a maximum 186Os/188Os of 0.1198303.
View in article


Gladkochub, D.P., Donskaya, T.V., Ivanov, A.V., Ernst, R., Mazukabzov, A.M., Pisarevsky, S.A., Ukhova, N.A. (2010) Phanerozoic mafic magmatism in the southern Siberian craton: geodynamic implications. Russian Geology and Geophysics 51, 952–964.
Show in context

Ages of ~250-240 Ma are recognised regionally within the Aldan Shield (Lena and Aldan (Palaeo) Rivers: Wang et al., 2011; Miller et al., 2013), the Baikal Lake Region (e.g., Gladkochub et al., 2010) and within basins (e.g., Onon and Mohe-Upper Amur), located South of the Aldan Shield and adjacent to the Mongol-Okhotsk Fold belt (Guo et al., 2017).
View in article


Guo, Z-X., Yang, Y-T., Zyabrev, S., Hou, Z-H. (2017) Tectonostratigraphic evolution of the Mohe-Upper Amur Basin reflects the final closure of the Mongol-Okhotsk Ocean in the latest Jurassic-earliest Cretaceous. Journal of Asian Earth Sciences 145, 494-511.
Show in context

Ages of ~250-240 Ma are recognised regionally within the Aldan Shield (Lena and Aldan (Palaeo) Rivers: Wang et al., 2011; Miller et al., 2013), the Baikal Lake Region (e.g., Gladkochub et al., 2010) and within basins (e.g., Onon and Mohe-Upper Amur), located South of the Aldan Shield and adjacent to the Mongol-Okhotsk Fold belt (Guo et al., 2017).
View in article
The Mongol-Okhotsk fold belt (Fig. S-1), which rims the Siberian Craton on its South Margin over ca. 3000 km, represents the suture zone left after the closure of the Mongol-Okhotsk Ocean - as its seafloor was subducted under the Siberia craton and under the Mongolia-North China continent (Amur plate) -, and the subsequent collision of the Siberian craton with the Mongolia-North China continent (e.g., Zorin, 1999; Guo et al., 2017).
View in article
The age distribution along the Mongol-Okhotsk fold belt demonstrates an eastward zip-like closure of the Mongol-Okhotsk ocean (Zorin, 1999) initiated in the Late Palaeozoic in NE Mongolia (Zhao et al., 2017) and in the Early Triassic in the eastern part of the Mongol-Okhotsk belt, south of Aldan Shield Region (Guo et al., 2017).
View in article
Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
View in article


Ionov, D.A., Shirey, S.B. Weiss, D., Brügmann G. (2006) Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle. Earth and Planetary Science Letters 241, 47-64.
Show in context

Conversely, the least radiogenic 187Os/188Os (0.110096 ± 2136; alloy D-S2) is close to the composition of Re-free, least metasomatised peridotite xenoliths of the Tok locality (0.109; estimated for Al2O3 = 0 wt. % on the 187Os/188Os vs. Al2O3 “aluminochron”; Ionov et al., 2006), which like the Kondyor ZUC is located in the East Aldan Shield (Fig. S-1).
View in article
This view is further supported by the closeness of its 187Os/188Os and 187Re/188Os ratios (0.001196 and 0.00541; Table S-2) to those of the Re-free, least metasomatised Tok peridotite xenoliths (0.109 and 0; Ionov et al., 2006), implying that the 187Os/188Os composition of alloy D-S2 may still hold geologically meaningful constraints.
View in article


Malitch, K.N., Thalhammer, O.A.R. (2002) Pt–Fe nuggets derived from clinopyroxenite–dunite massifs, Russia: a structural, compositional and osmium-isotope study. The Canadian Mineralogist 40, 395-418.
Show in context

The Early Cretaceous Pt-He isochron ages (112 ± 7 Ma and 129 ± 6 Ma, calculated using a 190Pt half-life of 469 Gyr: Begemann et al., 2001) agree well with the Rb-Sr, Sm-Nd and K-Ar ages obtained on the main lithologies (whole rock and mineral phases) but conflict with the Re-Os TRD model ages obtained on erlichmanite (OsS2), sperrylite (PtAs2), Os-alloys and Pt-alloys (Cabri et al., 1998; Malitch and Thalhammer, 2002) that vary from Neoproterozoic (658-603 Ma) to future ages, when back calculated to the present-day primitive mantle (PM) 187Os/188Os estimate (Meisel et al., 2001).
View in article
The most radiogenic 187Os/188Os signatures (0.1246; alloys L-S2 and E-S2, Table S-2) agree well with those previously obtained on five Kondyor Os-rich alloys (0.1248-0.1252; Malitch and Thalhammer, 2002).
View in article
This is best explained by the overprinting of the Os-poor, least radiogenic 187Os/188Os of the Pt-alloy D-S2 by an Os-rich (ca. 700 times richer) contaminant with a 187Os/188Os of 0.1246 (Fig. 3a), similar to the most radiogenic 187Os/188Os of our Kondyor alloys (e.g., points E-S2) and very close to the least radiogenic 187Os/188Os compositions previously reported by Malitch and Thalhammer (2002) and Cabri et al. (1998) for Kondyor PGM (Fig. 2a).
View in article
The Pt-free nature of the Os exsolution lamellae combined with the extremely Os-poor composition of their Pt-alloy hosts (Fig. 1; Malitch and Thalhammer, 2002; Nekrasov et al., 2005) argues for an equilibration temperature below 500 ºC (see Pt-Os phase diagram in Okrugin, 2002), thus well below the 700 °C temperature mark of 4He loss onset observed for Pt alloys (see above).
View in article
Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
View in article


Meibom, A., Frei, R. (2002) Evidence for an Ancient Osmium Isotopic Reservoir in Earth. Science 296, 516-518.
Show in context

They are typically dated with the 187Re-187Os and/or 190Pt-186Os isotope systems (e.g., Walker et al., 1997; Meibom and Frei, 2002; Pearson et al., 2007; Coggon et al., 2012).
View in article


Meisel, T., Walker, R.J., Irving, A.J., Lorand, J.-P. (2001) Osmium isotopic compositions of mantle xenoliths: A global perspective. Geochimica et Cosmochimica Acta 65, 1311-1323.
Show in context

The Early Cretaceous Pt-He isochron ages (112 ± 7 Ma and 129 ± 6 Ma, calculated using a 190Pt half-life of 469 Gyr: Begemann et al., 2001) agree well with the Rb-Sr, Sm-Nd and K-Ar ages obtained on the main lithologies (whole rock and mineral phases) but conflict with the Re-Os TRD model ages obtained on erlichmanite (OsS2), sperrylite (PtAs2), Os-alloys and Pt-alloys (Cabri et al., 1998; Malitch and Thalhammer, 2002) that vary from Neoproterozoic (658-603 Ma) to future ages, when back calculated to the present-day primitive mantle (PM) 187Os/188Os estimate (Meisel et al., 2001).
View in article
Figure 2 [...] The primitive mantle (PM) 186Os/188Os and 187Os/188Os values are respectively from Day et al. (2017) and Meisel et al. (2001).
View in article


Miller, E.L., Soloviev, A.V., Prokopiev, A.V., Toro, J., Harris, D., Kuzmichev, A.B., Gehrels, G.E., (2013) Triassic river systems and the paleo-Pacific margin of northwestern Pangea. Gondwana Research 23, 1631-1645.
Show in context

Ages of ~250-240 Ma are recognised regionally within the Aldan Shield (Lena and Aldan (Palaeo) Rivers: Wang et al., 2011; Miller et al., 2013), the Baikal Lake Region (e.g., Gladkochub et al., 2010) and within basins (e.g., Onon and Mohe-Upper Amur), located South of the Aldan Shield and adjacent to the Mongol-Okhotsk Fold belt (Guo et al., 2017).
View in article


Mochalov, A.G., Yakubovich, O.V., Bortnikov, N.S. (2016) 190Pt-4He Age of PGE Ores in the Alkaline-Ultramafic Kondyor Massif (Khabarovsk District, Russia). Doklady Earth Sciences 469, 846–850.
Show in context

The Pt-He geochronometer was so far used to date the Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC), which is located in the Aldan Shield on the South-East margin of the Siberian Craton (Fig. S-1 and Supplementary Information) (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018).
View in article
Shukolyukov et al. (2012a) and Mochalov et al. (2016, 2018) dated (i) Pt-alloys from different lithologies of the Kondyor ZUC, including the chromitite lenses of the dunitic core and (ii) alluvial Pt-Pd PGM.
View in article
However, for the Kondyor Pt-alloys, the Pt-He isochronal ages (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018) are ~110-140 Myr younger than the Pt-Os ages.
View in article
First, Shukolyukov et al. (2012a,b) and Mochalov et al. (2016) argued that radiogenic 4He is retained in the structure of native metals as vesicles that are only released upon melting of the native metals (>1000 °C).
View in article


Mochalov, A.G., Yakubovich, O.V., Zolotarev, A.A. (2018) Structural Transformations and Retention of Radiogenic 4He in Platinum Minerals under Mechanical Deformations. Doklady Earth Sciences 480, 591–594.
Show in context

The Pt-He geochronometer was so far used to date the Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC), which is located in the Aldan Shield on the South-East margin of the Siberian Craton (Fig. S-1 and Supplementary Information) (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018).
View in article
Shukolyukov et al. (2012a) and Mochalov et al. (2016, 2018) dated (i) Pt-alloys from different lithologies of the Kondyor ZUC, including the chromitite lenses of the dunitic core and (ii) alluvial Pt-Pd PGM.
View in article
However, for the Kondyor Pt-alloys, the Pt-He isochronal ages (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018) are ~110-140 Myr younger than the Pt-Os ages.
View in article


Nekrasov, I.Y., Lennikov, A.M., Zalishchak, B.L., Oktyabrsky, R.A., Ivanov, V.V., Sapin, V.I., Taskaev, V.I. (2005) Composition variations in platinum-group minerals and gold, Konder alkaline-ultrabasic massif, Aldan Shield, Russia. The Canadian Mineralogist 43, 637-654.
Show in context

The Pt-free nature of the Os exsolution lamellae combined with the extremely Os-poor composition of their Pt-alloy hosts (Fig. 1; Malitch and Thalhammer, 2002; Nekrasov et al., 2005) argues for an equilibration temperature below 500 ºC (see Pt-Os phase diagram in Okrugin, 2002), thus well below the 700 °C temperature mark of 4He loss onset observed for Pt alloys (see above).
View in article


Okrugin, A.V. (2002) Phase transformations and genesis of platinum-group minerals in various types of platinum-bearing deposits. In: Boudreau, A.E. (Ed.) 9th International Platinum Symposium (Billings), Extended Abstracts. Duke University Press, Durham, North Carolina, 349-353.
Show in context

The Pt-free nature of the Os exsolution lamellae combined with the extremely Os-poor composition of their Pt-alloy hosts (Fig. 1; Malitch and Thalhammer, 2002; Nekrasov et al., 2005) argues for an equilibration temperature below 500 ºC (see Pt-Os phase diagram in Okrugin, 2002), thus well below the 700 °C temperature mark of 4He loss onset observed for Pt alloys (see above).
View in article


Orlova, M.P. (1992) Geology and genesis of the Konder Massif. Geology of the Pacific Ocean 8, 120-132.
Show in context

The last evidence suggesting a low closure temperature (<600 °C) of the Pt-He isotopic system comes from the similarity of the Pt-He isochronal ages with the Rb-Sr, Sm-Nd and K-Ar obtained on whole-rock and single minerals (biotite, feldspar) of the dunitic core, pyroxenites and late metasomatic dikes of Kondyor ZUC (149-83 Ma: e.g., Orlova, 1992; Cabri et al., 1998; Pushkarev et al., 2002).
View in article
Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
View in article


Pearson, D.G., Parman, S.W., Nowell, G.M. (2007) A link between large mantle melting events and continent growth seen in osmium isotopes. Nature 449, 202-205.
Show in context

They are typically dated with the 187Re-187Os and/or 190Pt-186Os isotope systems (e.g., Walker et al., 1997; Meibom and Frei, 2002; Pearson et al., 2007; Coggon et al., 2012).
View in article


Pushkarev, Yu.D., Kostoyanov, A.I., Orlova, M.P., Bogomolov, E.S. (2002) Peculiarities of the Rb-Sr, Sm-Nd, Re-Os and K-Ar isotope systems in the Kondyor massif: mantle substratum, enriched by PGE. Regional Geology and Metallogeny 16, 80-91 (in Russian).
Show in context

The last evidence suggesting a low closure temperature (<600 °C) of the Pt-He isotopic system comes from the similarity of the Pt-He isochronal ages with the Rb-Sr, Sm-Nd and K-Ar obtained on whole-rock and single minerals (biotite, feldspar) of the dunitic core, pyroxenites and late metasomatic dikes of Kondyor ZUC (149-83 Ma: e.g., Orlova, 1992; Cabri et al., 1998; Pushkarev et al., 2002).
View in article


Pushkarev, E.V., Kamenetsky, V.S., Morozova, A.V., Khiller, V.V., Glavatskykh, S.P., Rodemann, T. (2015) Ontogeny of Ore Cr-spinel and composition of inclusions as indicators of the Pneumatolytic-Hydrothermal Origin of PGM-bearing Chromitites from Kondyor Massif, The Aldan Shield. Geology of Ore Deposits 57, 352-380.
Show in context

Here we report the coupled 190Pt-186Os and 187Re-187Os signatures obtained by Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) (Supplementary Information) on 13 sub-millimetric Pt-alloys separated from a chromitite schlieren (sample 1265; Pushkarev et al., 2015) hosted in the dunitic core of the Kondyor ZUC.
View in article


Shukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B., Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012a) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491-507.
Show in context

The Pt-He geochronometer was so far used to date the Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC), which is located in the Aldan Shield on the South-East margin of the Siberian Craton (Fig. S-1 and Supplementary Information) (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018).
View in article
Shukolyukov et al. (2012a) and Mochalov et al. (2016, 2018) dated (i) Pt-alloys from different lithologies of the Kondyor ZUC, including the chromitite lenses of the dunitic core and (ii) alluvial Pt-Pd PGM.
View in article
However, for the Kondyor Pt-alloys, the Pt-He isochronal ages (Shukolyukov et al., 2012a; Mochalov et al., 2016, 2018) are ~110-140 Myr younger than the Pt-Os ages.
View in article
First, Shukolyukov et al. (2012a,b) and Mochalov et al. (2016) argued that radiogenic 4He is retained in the structure of native metals as vesicles that are only released upon melting of the native metals (>1000 °C).
View in article
However, the only 4He thermal desorption experiment conducted on Pt-alloys by Shukolyukov et al. (2012a) revealed 4He loss ([4He] ≠ 0) for temperatures as low as ~700 °C (see Fig. 4 in Shukolyukov et al., 2012a).
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Shukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012b) Geothermochronology based on noble gases: III. Migration of radiogenic He in the crystal structure of native metals with applications to their isotopic dating. Petrology 20, 1-20.
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First, Shukolyukov et al. (2012a,b) and Mochalov et al. (2016) argued that radiogenic 4He is retained in the structure of native metals as vesicles that are only released upon melting of the native metals (>1000 °C).
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The grain boundaries proposed as a preferential sink for 4He (Shukolyukov et al., 2012b) may turn out to be preferential 4He loss sites when Pt-alloys are intensely exsolved (Fig. 1).
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Simonov, V.A., Prikhod’ko, V.S., Kovyazin, S.V. (2011) Genesis of Platiniferous Massifs in the Southeastern Siberian Platform. Petrology 19, 549-567.
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Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
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Walker, R.J., Morgan, J.W., Beary, E.S., Smoliar, M.I., Czamanske, G.K., Horan, M.F. (1997) Applications of the 190Pt-186Os isotope system to geochemistry and cosmochemistry. Geochimica et Cosmochimica Acta 61, 4799-4807.
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They are typically dated with the 187Re-187Os and/or 190Pt-186Os isotope systems (e.g., Walker et al., 1997; Meibom and Frei, 2002; Pearson et al., 2007; Coggon et al., 2012).
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Wang, C.Y., Campbell, I.H., Stepanov, A.S., Allen, C.M., Burtsev, I.N. (2011) Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers. Geochimica et Cosmochimica Acta 75, 1308-1345.
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Ages of ~250-240 Ma are recognised regionally within the Aldan Shield (Lena and Aldan (Palaeo) Rivers: Wang et al., 2011; Miller et al., 2013), the Baikal Lake Region (e.g., Gladkochub et al., 2010) and within basins (e.g., Onon and Mohe-Upper Amur), located South of the Aldan Shield and adjacent to the Mongol-Okhotsk Fold belt (Guo et al., 2017).
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Zhao, P., Xu, B., Jahn, B.-M. (2017) The Mongol-Okhotsk Ocean subduction-related Permian peraluminous granites in northeastern Mongolia: constraints from zircon U-Pb ages, whole-rock elemental and Sr-Nd-Hf isotopic compositions. Journal of Asian Earth Sciences 144, 225-240.
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The age distribution along the Mongol-Okhotsk fold belt demonstrates an eastward zip-like closure of the Mongol-Okhotsk ocean (Zorin, 1999) initiated in the Late Palaeozoic in NE Mongolia (Zhao et al., 2017) and in the Early Triassic in the eastern part of the Mongol-Okhotsk belt, south of Aldan Shield Region (Guo et al., 2017).
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Zorin, Y.A. (1999) Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia. Tectonophysics 306, 33-56.
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The Mongol-Okhotsk fold belt (Fig. S-1), which rims the Siberian Craton on its South Margin over ca. 3000 km, represents the suture zone left after the closure of the Mongol-Okhotsk Ocean - as its seafloor was subducted under the Siberia craton and under the Mongolia-North China continent (Amur plate) -, and the subsequent collision of the Siberian craton with the Mongolia-North China continent (e.g., Zorin, 1999; Guo et al., 2017).
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The age distribution along the Mongol-Okhotsk fold belt demonstrates an eastward zip-like closure of the Mongol-Okhotsk ocean (Zorin, 1999) initiated in the Late Palaeozoic in NE Mongolia (Zhao et al., 2017) and in the Early Triassic in the eastern part of the Mongol-Okhotsk belt, south of Aldan Shield Region (Guo et al., 2017).
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The age of the subsequent collision between the Mongolia-North China continent and Siberia craton also evolves eastwards from Middle Jurassic to Early Cretaceous (Zorin, 1999).
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Considering the orthopyroxene-poor, olivine- and clinopyroxene-rich nature of Kondyor ZUC (Orlova, 1992; Malitch and Thalhammer, 2002) and its extreme Pt-mineralisation, we argue that, rather than being a metasomatised mantle diapir (Burg et al., 2009), Kondyor ZUC represents the root of a ~250-240 Ma old alkaline picritic volcano (Simonov et al., 2011), which together with other Aldan ZUC (e.g., Chad) likely formed part of an Early Triassic island arc at the southeast margin of the Aldan shield due to the subduction of the Mongol-Okhotsk ocean seafloor northwards under the Siberian Craton (see Zorin, 1999; Guo et al., 2017).
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Supplementary Information


The Supplementary Information includes:
  • 1. The Kondyor Zoned Ultramafic Complex (ZUC)
  • 2. The Pt-alloys of the Kondyor ZUC and the Origin of their Pure Os Exsolution Lamellae
  • 3. Methods
  • Figure S-1
  • Tables S-1 and S-2
  • Supplementary Information References

Download the Supplementary Information (PDF).

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