Reply to Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Yakubovich et al. (2022)
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Keywords: Pt-Os isotopic system, Pt-He isotopic system, Pt alloy, Kondyor, zoned ultramafic complex
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 Figure 1 (a) Normal probability plot of the 190Pt/188Os composition of the Kondyor Pt alloys analysed by Luguet et al. (2019) and revealing compositional clusters/groups (a to d). (b) 190Pt-188Os isochrons of the Kondyor Pt alloys (global and compositional clusters) determined using IsoplotR. Isochronal ages (±1 sigma uncertainty) obtained on the compositional clusters overlap with each other as well as with the isochron calculated on the 28 analyses of Luguet et al. (2019). |
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Reply
Luguet et al. (2019)
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. Geochemical Perspectives Letters 11, 44–48.
applied the Re-Os and Pt-Os chronometers to date the Pt mineralisation of the Kondyor Zoned Ultramafic Complex (ZUC) and suggested this to be ∼250-240 Ma and related to the subduction of the Mongol-Okhotsk (MO) ocean seafloor under the Siberian craton. The discrepancy with the Early Cretaceous 190Pt-4He isochronal ages of 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.
led Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
to conclude that the 190Pt-4He chronometer was not robust due to the complex history of the Kondyor Pt mineralisation, thus not providing a meaningful insight into the mineralisation age. Yakubovich et al. (2022)Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
challenge these interpretations. There is however little controversy when the current Pt-Os (Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
) and Pt-He (Mochalov et al., 2016Mochalov, 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.
) isotopic systems are considered together with regional geological history.(1) 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, Canadian Mineralogist 43, 637–654.
identified, within the chromitites of the Kondyor ZUC dunitic core, an early high temperature (HT) Pt alloy suite associated with chromites containing up to 64 wt. % of Cr2O3, and a late suite of Pt alloys associated with chromites containing up to 54 wt. % Cr2O3. The chromites from the schlieren 1265 (Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
) yield 55.4 to 56.4 wt. % Cr2O3 (Pushkarev et al., 2002Garuti, G., Pushkarev, E., Zaccarini, F. (2002) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskantype Kytlym and Uktus complexes, northern and central Urals, Russia. Canadian Mineralogist, 40, 357–376.
), supporting the Pt mineralisation to be akin to the early HT Pt alloys. Furthermore, the 190Pt/188Os ratios of the Pt alloys range from 0.06 to 19.89 (Table S-2 of Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
), encompassing those of the Pt−, Pt > Ir, Pt > Os and Pt > Pd Pt-alloys (0.24 to 9, Table S-2 of Yakubovich et al., 2022Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
). The claim of Yakubovich et al. (2022)Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
that Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
analysed exclusively the latest generations of Pt alloys (i.e. Pt > Ir and Pt > Pd type grains) is thus flawed.(2) Figure 1 of Yakubovich et al. (2022)
Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
only demonstrates that the 190Pt/188Os ratio does not show a normal distribution, defining compositional clusters between Os-poor Pt alloys and increasingly Os-rich Pt alloys. The subsequent “mineralogical classification” undertaken by Yakubovich et al. (2022)Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
is erroneous and arbitrary. These authors argue that the Os-richest Pt alloys of Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
are native osmium. The Os-richest Pt alloy analysed by Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
has a 190Pt/188Os of 0.064, corresponding to Pt/Os ∼60 - clearly not native osmium. It is also puzzling that two grains classified as “Os” by Yakubovich et al. (2022)Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
have 190Pt/188Os overlapping with their own “Pt + Os” group. These “Os” grains appear to be classified as such to fortuitously yield a 116 Ma Pt-Os isochron, similar to the Pt-He age. In contrast, the internal isochrons of the 190Pt/188Os compositional clusters yield ages (see Fig. 1, this reply) in agreement with the model age of the Os-poorest Pt alloy and the isochronal age calculated on the whole dataset (Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
).
Figure 1 (a) Normal probability plot of the 190Pt/188Os composition of the Kondyor Pt alloys analysed by Luguet et al. (2019)
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. Geochemical Perspectives Letters 11, 44–48.
and revealing compositional clusters/groups (a to d). (b) 190Pt-188Os isochrons of the Kondyor Pt alloys (global and compositional clusters) determined using IsoplotR. Isochronal ages (±1 sigma uncertainty) obtained on the compositional clusters overlap with each other as well as with the isochron calculated on the 28 analyses of Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
.(3) According to Yakubovich et al. (2022)
Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
, native osmium, which we did not analyse, formed prior to the Pt alloys, implying a primary heterogeneity in 186Os/188Os and systematic Pt/Os variations among the Pt alloys. The sole observation that native osmium occurs as nanometric exsolution lamellae within the Pt alloys (Fig. 1 of Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
) rules out native osmium being primary, and undeniably argues for a sub-solidus origin. Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002Garuti, G., Pushkarev, E., Zaccarini, F. (2002) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskantype Kytlym and Uktus complexes, northern and central Urals, Russia. Canadian Mineralogist, 40, 357–376.
; Zaccarini et al., 2018Zaccarini, F., Garuti, G., Pushkarev, E., Thalhammer, O. (2018) Origin of Platinum Group Minerals (PGM) Inclusions in Chromite Deposits of the Urals. Minerals 8, 379. https://doi.org/10.3390/min8090379.
) and from the Kondyor ZUC (Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
; Malitch et al., 2020Malitch K.N., Puchtel, I.S., Belousova, E.A., Badanina I.Y. (2020) Contrasting platinum-group mineral assemblages of the Kondyor massif (Russia): Implications for the sources of HSE in zoned type ultramafic massifs. Lithos 376–377. https://doi.org/10.1016/j.lithos.2020.105800.
) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002Garuti, G., Pushkarev, E., Zaccarini, F. (2002) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskantype Kytlym and Uktus complexes, northern and central Urals, Russia. Canadian Mineralogist, 40, 357–376.
). The large range of Pt/Os of the Pt alloys reflect variable proportions of Os exsolution lamellae (Luguet et al., 2019Luguet, 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. Geochemical Perspectives Letters 11, 44–48.
) and the “heterogeneous” 186Os/188Os is simply due to the ingrowth of 186Os over ca. 250 Myr. This is the basic implication of an isochron and a model age!(4) Magmatic ca. 250 Ma ages are reported all along the MO suture, from Mongolia to the Okhotsk Ocean, resulting from the bivergent subduction of the MO ocean under Siberia in the north and Amuria in the south. Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005
Sotnikov, V.I., Ponomarchuk, V.A., Shevchenko, D.O., Berzina, A.P. (2005) The Erdenetiyn-Ovoo porphyry Cu–Mo deposit, Northern Mongolia: Ar-40/Ar-39 geochronology and factors of large-scale mineralization. Russian Geology and Geophysics 46, 620–631.
; Gladkochub et al., 2010Gladkochub, 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.
; Donskaya et al., 2013Donskaya, T.V., Gladkochub D.P., Mazukabzov, A.M. Ivanov, A.V. (2013) Late Paleozoic – Mesozoic subduction-related magmatism at the southern margin of the Siberian continent and the 150 million-year history of the Mongol-Okhotsk Ocean. Journal of Asian Earth Sciences 62, 79–97.
), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006Sal’nikova, E. B., Larin, A. M., Kotov, A. B., Sorokin, A. P., Sorokin, A. A., Velikoslavinsky, S. D., Yakovleva, S. Z., Fedoseenko, A. M., Plotkina, Y.V. (2006) The Toksko–Algomin Igneous Complex of the Dzhugdzhur-Stanovoi Folded Region:Age and Geodynamic Setting. Doklady Earth Sciences 409A, 888–892.
) and the Dzhagdy Transect (Sorokin et al., 2020Sorokin, A.A., Zaika, V.A., Kovach, V.P., Kotov, A.B., Xu, W., Yang, H. (2020) Timing of closure of the Eastern Mongol-Okhotsk Ocean Constraints from U-Pb and Hf isotopic data of detrital zircons from metasediments along the Dzhagdy Transect. Gondwana Research 81, 58–78.
), located on the far east of the MO suture, regionally close to the Kondyor ZUC.(5) Contrary to the proposal of Yakubovich et al. (2022)
Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
, it is impossible to disturb and age the Pt-Os ages during or consequent to a younger event having overprinted the Kondyor ZUC, due to the very low abundance of 190Pt (0.0136 %; Böhlke et al., 2001Böhlke, J.K., de Laeter J.R., De Bièvre, P., Hikada, H., Peiser, H.S., Osman, K.J.R., Taylor, P.D.P. (2001) Isotopic Compositions of the Elements, 2001. Journal of Physical and Chemical Reference Data 34, 57–67.
) and the long half-life of the 190Pt decay (∼469 Gyr; Begemann et al., 2001Begemann, 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 Cosmochimica Acta 65, 111–121.
). We maintain that the Pt-He ages record a younger event than the Pt-Os ages, possibly reflecting poor He retention in Kondyor Pt alloys. Shukolyukov et al. (2012aShukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012a) 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.
,bShukolyukov, Yu.A., Yakubovich, O.V., Mochalov, A.G., Kotov, A.B., Sal’nikova, E.B.,Yakovleva, S.Z., Korneev, S.I., Gorokhovskii, B.M. (2012b) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491–507.
) estimated in short (30 min) heating experiments the He loss in native Au (n = 10) and Pt (n = 1) alloys. To understand the He behaviour in native alloys, possibly stored at T > 600 °C in the mantle for millions of years, proper tests would have been to expose alloys to given temperature (600 to 1200 °C) for weeks or months and measure their Pt-He isotopic signatures before and after the experiments. Still, Au alloys clearly show He loss as low as 600 °C, while they are expected to efficiently retain He due to their high crystal lattice packing density (higher than Pt alloys; Shukolyukov et al., 2012aShukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012a) 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.
). Alternatively, as highlighted by Malitch et al. (2020)Malitch K.N., Puchtel, I.S., Belousova, E.A., Badanina I.Y. (2020) Contrasting platinum-group mineral assemblages of the Kondyor massif (Russia): Implications for the sources of HSE in zoned type ultramafic massifs. Lithos 376–377. https://doi.org/10.1016/j.lithos.2020.105800.
, the Pt-He ages are concomitant to the Mesozoic lamproitic magmatism, which affected the south margin of the Siberia craton and is suggested to have overprinted part of the Kondyor ZUC (Burg et al., 2009Burg, 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.
). The age discrepancy points to open system behaviour of Pt-He system, making it unreliable for dating the timing of mineralisation in complex igneous systems.(6) Finally, we highlight the conclusion of Yakubovich et al. (2022)
Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
[“190Pt-4He age reflects the mineralisation itself, while the 190Pt-186Os isotopic system fingerprints an earlier redistribution of PGE”], which ultimately asserts that the Pt-Os records an earlier event (i.e. the Pt mineralisation of Kondyor ZUC) than the Pt-He isotopic system. By agreeing with Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
, Yakubovich et al. (2022)Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18.
signal that they do not understand the implication of their own assertion, rendering their comment to Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
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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 Cosmochimica Acta 65, 111–121. https://doi.org/10.1016/S0016-7037(00)00512-3
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(5) Contrary to the proposal of Yakubovich et al. (2022), it is impossible to disturb and age the Pt-Os ages during or consequent to a younger event having overprinted the Kondyor ZUC, due to the very low abundance of 190Pt (0.0136 %; Böhlke et al., 2001) and the long half-life of the 190Pt decay (∼469 Gyr; Begemann et al., 2001).
View in article
Böhlke, J.K., de Laeter J.R., De Bièvre, P., Hikada, H., Peiser, H.S., Osman, K.J.R., Taylor, P.D.P. (2001) Isotopic Compositions of the Elements, 2001. Journal of Physical and Chemical Reference Data 34, 57–67. https://doi.org/10.1063/1.1836764
Show in context
(5) Contrary to the proposal of Yakubovich et al. (2022), it is impossible to disturb and age the Pt-Os ages during or consequent to a younger event having overprinted the Kondyor ZUC, due to the very low abundance of 190Pt (0.0136 %; Böhlke et al., 2001) and the long half-life of the 190Pt decay (∼469 Gyr; Begemann 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. https://doi.org/10.1093/petrology/egn083
Show in context
Alternatively, as highlighted by Malitch et al. (2020), the Pt-He ages are concomitant to the Mesozoic lamproitic magmatism, which affected the south margin of the Siberia craton and is suggested to have overprinted part of the Kondyor ZUC (Burg et al., 2009).
View in article
Donskaya, T.V., Gladkochub D.P., Mazukabzov, A.M. Ivanov, A.V. (2013) Late Paleozoic – Mesozoic subduction-related magmatism at the southern margin of the Siberian continent and the 150 million-year history of the Mongol-Okhotsk Ocean. Journal of Asian Earth Sciences 62, 79–97. https://doi.org/10.1016/j.jseaes.2012.07.023
Show in context
Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005; Gladkochub et al., 2010; Donskaya et al., 2013), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006) and the Dzhagdy Transect (Sorokin et al., 2020), located on the far east of the MO suture, regionally close to the Kondyor ZUC.
View in article
Garuti, G., Pushkarev, E., Zaccarini, F. (2002) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskantype Kytlym and Uktus complexes, northern and central Urals, Russia. Canadian Mineralogist, 40, 357–376. https://doi.org/10.2113/gscanmin.40.2.357
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Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002; Zaccarini et al., 2018) and from the Kondyor ZUC (Luguet et al., 2019; Malitch et al., 2020) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002).
View in article
The chromites from the schlieren 1265 (Luguet et al., 2019) yield 55.4 to 56.4 wt. % Cr2O3 (Pushkarev et al., 2002), supporting the Pt mineralisation to be akin to the early HT Pt alloys.
View in article
Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002; Zaccarini et al., 2018) and from the Kondyor ZUC (Luguet et al., 2019; Malitch et al., 2020) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002).
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. https://doi.org/10.1016/j.rgg.2010.08.005
Show in context
Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005; Gladkochub et al., 2010; Donskaya et al., 2013), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006) and the Dzhagdy Transect (Sorokin et al., 2020), located on the far east of the MO suture, regionally close to the Kondyor ZUC.
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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. Geochemical Perspectives Letters 11, 44–48. https://doi.org/10.7185/geochemlet.1924
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Luguet et al. (2019) applied the Re-Os and Pt-Os chronometers to date the Pt mineralisation of the Kondyor Zoned Ultramafic Complex (ZUC) and suggested this to be ∼250-240 Ma and related to the subduction of the Mongol-Okhotsk (MO) ocean seafloor under the Siberian craton.
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There is however little controversy when the current Pt-Os (Luguet et al., 2019) and Pt-He (Mochalov et al., 2016) isotopic systems are considered together with regional geological history.
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The chromites from the schlieren 1265 (Luguet et al., 2019) yield 55.4 to 56.4 wt. % Cr2O3 (Pushkarev et al., 2002), supporting the Pt mineralisation to be akin to the early HT Pt alloys.
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Furthermore, the 190Pt/188Os ratios of the Pt alloys range from 0.06 to 19.89 (Table S-2 of Luguet et al., 2019), encompassing those of the Pt−, Pt > Ir, Pt > Os and Pt > Pd Pt-alloys (0.24 to 9, Table S-2 of Yakubovich et al., 2022).
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The discrepancy with the Early Cretaceous 190Pt-4He isochronal ages of Mochalov et al. (2016) led Luguet et al. (2019) to conclude that the 190Pt-4He chronometer was not robust due to the complex history of the Kondyor Pt mineralisation, thus not providing a meaningful insight into the mineralisation age. Yakubovich et al. (2022) challenge these interpretations.
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Furthermore, the 190Pt/188Os ratios of the Pt alloys range from 0.06 to 19.89 (Table S-2 of Luguet et al., 2019), encompassing those of the Pt−, Pt > Ir, Pt > Os and Pt > Pd Pt-alloys (0.24 to 9, Table S-2 of Yakubovich et al., 2022).
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The claim of Yakubovich et al. (2022) that Luguet et al. (2019) analysed exclusively the latest generations of Pt alloys (i.e. Pt > Ir and Pt > Pd type grains) is thus flawed.
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The Os-richest Pt alloy analysed by Luguet et al. (2019) has a 190Pt/188Os of 0.064, corresponding to Pt/Os ∼60 - clearly not native osmium.
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In contrast, the internal isochrons of the 190Pt/188Os compositional clusters yield ages (see Fig. 1, this reply) in agreement with the model age of the Os-poorest Pt alloy and the isochronal age calculated on the whole dataset (Luguet et al., 2019).
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(a) Normal probability plot of the 190Pt/188Os composition of the Kondyor Pt alloys analysed by Luguet et al. (2019) and revealing compositional clusters/groups (a to d).
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Isochronal ages (±1 sigma uncertainty) obtained on the compositional clusters overlap with each other as well as with the isochron calculated on the 28 analyses of Luguet et al. (2019).
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The sole observation that native osmium occurs as nanometric exsolution lamellae within the Pt alloys (Fig. 1 of Luguet et al., 2019) rules out native osmium being primary, and undeniably argues for a sub-solidus origin.
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The subsequent “mineralogical classification” undertaken by Yakubovich et al. (2022) is erroneous and arbitrary. These authors argue that the Os-richest Pt alloys of Luguet et al. (2019) are native osmium.
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The large range of Pt/Os of the Pt alloys reflect variable proportions of Os exsolution lamellae (Luguet et al., 2019) and the “heterogeneous” 186Os/188Os is simply due to the ingrowth of 186Os over ca. 250 Myr.
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By agreeing with Luguet et al. (2019), Yakubovich et al. (2022) signal that they do not understand the implication of their own assertion, rendering their comment to Luguet et al. (2019) obsolete.
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Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002; Zaccarini et al., 2018) and from the Kondyor ZUC (Luguet et al., 2019; Malitch et al., 2020) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002).
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By agreeing with Luguet et al. (2019), Yakubovich et al. (2022) signal that they do not understand the implication of their own assertion, rendering their comment to Luguet et al. (2019) obsolete.
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Malitch K.N., Puchtel, I.S., Belousova, E.A., Badanina I.Y. (2020) Contrasting platinum-group mineral assemblages of the Kondyor massif (Russia): Implications for the sources of HSE in zoned type ultramafic massifs. Lithos 376–377. https://doi.org/10.1016/j.lithos.2020.105800
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Still, Au alloys clearly show He loss as low as 600 °C, while they are expected to efficiently retain He due to their high crystal lattice packing density (higher than Pt alloys; Shukolyukov et al., 2012a). Alternatively, as highlighted by Malitch et al. (2020), the Pt-He ages are concomitant to the Mesozoic lamproitic magmatism, which affected the south margin of the Siberia craton and is suggested to have overprinted part of the Kondyor ZUC (Burg et al., 2009).
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Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002; Zaccarini et al., 2018) and from the Kondyor ZUC (Luguet et al., 2019; Malitch et al., 2020) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002).
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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.
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The discrepancy with the Early Cretaceous 190Pt-4He isochronal ages of Mochalov et al. (2016) led Luguet et al. (2019) to conclude that the 190Pt-4He chronometer was not robust due to the complex history of the Kondyor Pt mineralisation, thus not providing a meaningful insight into the mineralisation age. Yakubovich et al. (2022) challenge these interpretations.
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There is however little controversy when the current Pt-Os (Luguet et al., 2019) and Pt-He (Mochalov et al., 2016) isotopic systems are considered together with regional geological history.
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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, Canadian Mineralogist 43, 637–654. https://doi.org/10.2113/gscanmin.43.2.637
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(1) Nekrasov et al. (2005) identified, within the chromitites of the Kondyor ZUC dunitic core, an early high temperature (HT) Pt alloy suite associated with chromites containing up to 64 wt. % of Cr2O3, and a late suite of Pt alloys associated with chromites containing up to 54 wt. % Cr2O3.
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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. https://doi.org/10.1134/S1075701515050049
Sal’nikova, E. B., Larin, A. M., Kotov, A. B., Sorokin, A. P., Sorokin, A. A., Velikoslavinsky, S. D., Yakovleva, S. Z., Fedoseenko, A. M., Plotkina, Y.V. (2006) The Toksko–Algomin Igneous Complex of the Dzhugdzhur-Stanovoi Folded Region:Age and Geodynamic Setting. Doklady Earth Sciences 409A, 888–892. https://doi.org/10.1134/S1028334X06060110
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Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005; Gladkochub et al., 2010; Donskaya et al., 2013), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006) and the Dzhagdy Transect (Sorokin et al., 2020), located on the far east of the MO suture, regionally close to the Kondyor ZUC.
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Shukolyukov, Yu.A., Yakubovich, O.V., Yakovleva, S.Z., Sal’nikova, E.B., Kotov, A.B., Rytsk, E.Yu (2012a) 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. https://doi.org/10.1134/S0869591112010043
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We maintain that the Pt-He ages record a younger event than the Pt-Os ages, possibly reflecting poor He retention in Kondyor Pt alloys. Shukolyukov et al. (2012a,b) estimated in short (30 min) heating experiments the He loss in native Au (n = 10) and Pt (n = 1) alloys.
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Still, Au alloys clearly show He loss as low as 600 °C, while they are expected to efficiently retain He due to their high crystal lattice packing density (higher than Pt alloys; Shukolyukov et al., 2012a). Alternatively, as highlighted by Malitch et al. (2020), the Pt-He ages are concomitant to the Mesozoic lamproitic magmatism, which affected the south margin of the Siberia craton and is suggested to have overprinted part of the Kondyor ZUC (Burg et al., 2009).
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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. (2012b) New Geochronometer for the Direct Isotopic Dating of Native Platinum Minerals (190Pt-4He Method). Petrology 20, 491–507. https://doi.org/10.1134/S0869591112060033
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We maintain that the Pt-He ages record a younger event than the Pt-Os ages, possibly reflecting poor He retention in Kondyor Pt alloys. Shukolyukov et al. (2012a,b) estimated in short (30 min) heating experiments the He loss in native Au (n = 10) and Pt (n = 1) alloys.
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Sorokin, A.A., Zaika, V.A., Kovach, V.P., Kotov, A.B., Xu, W., Yang, H. (2020) Timing of closure of the Eastern Mongol-Okhotsk Ocean Constraints from U-Pb and Hf isotopic data of detrital zircons from metasediments along the Dzhagdy Transect. Gondwana Research 81, 58–78. https://doi.org/10.1016/j.gr.2019.11.009
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Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005; Gladkochub et al., 2010; Donskaya et al., 2013), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006) and the Dzhagdy Transect (Sorokin et al., 2020), located on the far east of the MO suture, regionally close to the Kondyor ZUC.
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Sotnikov, V.I., Ponomarchuk, V.A., Shevchenko, D.O., Berzina, A.P. (2005) The Erdenetiyn-Ovoo porphyry Cu–Mo deposit, Northern Mongolia: Ar-40/Ar-39 geochronology and factors of large-scale mineralization. Russian Geology and Geophysics 46, 620–631.
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Of particular interest are those obtained in plutonic and volcanic rocks from the northern side of the MO suture (Sotnikov et al., 2005; Gladkochub et al., 2010; Donskaya et al., 2013), especially from the Dzhugdzhur-Stanovoi magmatic belt (Sal’nikova et al., 2006) and the Dzhagdy Transect (Sorokin et al., 2020), located on the far east of the MO suture, regionally close to the Kondyor ZUC.
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Yakubovich, O.V., Mochalov, A., Savatenkov, V., Stuart, F. (2022) Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019). Geochemical Perspectives Letters 20, 16–18. https://doi.org/10.7185/geochemlet.2201
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The discrepancy with the Early Cretaceous 190Pt-4He isochronal ages of Mochalov et al. (2016) led Luguet et al. (2019) to conclude that the 190Pt-4He chronometer was not robust due to the complex history of the Kondyor Pt mineralisation, thus not providing a meaningful insight into the mineralisation age. Yakubovich et al. (2022) challenge these interpretations.
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The claim of Yakubovich et al. (2022) that Luguet et al. (2019) analysed exclusively the latest generations of Pt alloys (i.e. Pt > Ir and Pt > Pd type grains) is thus flawed.
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(2) Figure 1 of Yakubovich et al. (2022) only demonstrates that the 190Pt/188Os ratio does not show a normal distribution, defining compositional clusters between Os-poor Pt alloys and increasingly Os-rich Pt alloys.
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The subsequent “mineralogical classification” undertaken by Yakubovich et al. (2022) is erroneous and arbitrary. These authors argue that the Os-richest Pt alloys of Luguet et al. (2019) are native osmium.
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It is also puzzling that two grains classified as “Os” by Yakubovich et al. (2022) have 190Pt/188Os overlapping with their own “Pt + Os” group.
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(3) According to Yakubovich et al. (2022), native osmium, which we did not analyse, formed prior to the Pt alloys, implying a primary heterogeneity in 186Os/188Os and systematic Pt/Os variations among the Pt alloys.
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(5) Contrary to the proposal of Yakubovich et al. (2022), it is impossible to disturb and age the Pt-Os ages during or consequent to a younger event having overprinted the Kondyor ZUC, due to the very low abundance of 190Pt (0.0136 %; Böhlke et al., 2001) and the long half-life of the 190Pt decay (∼469 Gyr; Begemann et al., 2001).
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(6) Finally, we highlight the conclusion of Yakubovich et al. (2022) [“190Pt-4He age reflects the mineralisation itself, while the 190Pt-186Os isotopic system fingerprints an earlier redistribution of PGE”], which ultimately asserts that the Pt-Os records an earlier event (i.e. the Pt mineralisation of Kondyor ZUC) than the Pt-He isotopic system.
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By agreeing with Luguet et al. (2019), Yakubovich et al. (2022) signal that they do not understand the implication of their own assertion, rendering their comment to Luguet et al. (2019) obsolete.
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Furthermore, the 190Pt/188Os ratios of the Pt alloys range from 0.06 to 19.89 (Table S-2 of Luguet et al., 2019), encompassing those of the Pt−, Pt > Ir, Pt > Os and Pt > Pd Pt-alloys (0.24 to 9, Table S-2 of Yakubovich et al., 2022).
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Zaccarini, F., Garuti, G., Pushkarev, E., Thalhammer, O. (2018) Origin of Platinum Group Minerals (PGM) Inclusions in Chromite Deposits of the Urals. Minerals 8, 379. https://doi.org/10.3390/min8090379
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Osmium exsolution lamellae observed in Pt alloys from the Uralian Alaskan-type ZUC (Garuti et al., 2002; Zaccarini et al., 2018) and from the Kondyor ZUC (Luguet et al., 2019; Malitch et al., 2020) likely reflect the low fS2 of a given magmatic system, where Os partitions into the Pt alloy structure unable to form Os sulfides (Garuti et al., 2002)
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Figures
Figure 1 (a) Normal probability plot of the 190Pt/188Os composition of the Kondyor Pt alloys analysed by Luguet et al. (2019)
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. Geochemical Perspectives Letters 11, 44–48.
and revealing compositional clusters/groups (a to d). (b) 190Pt-188Os isochrons of the Kondyor Pt alloys (global and compositional clusters) determined using IsoplotR. Isochronal ages (±1 sigma uncertainty) obtained on the compositional clusters overlap with each other as well as with the isochron calculated on the 28 analyses of Luguet et al. (2019)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. Geochemical Perspectives Letters 11, 44–48.
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