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

doi: 10.7185/geochemlet.1924 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. Received 19 May 2019 | Accepted 28 August 2019 | Published 22 October 2019 1. Institute of Geosciences, University of Bonn, Germany 2. Earth Sciences Department, University of Durham, UK 3. Russian Academy of Sciences, Institute of Geology and Geochemistry Ural Division, Moscow, Russia 4. Helmholtz Centre Potsdam, GeoForschungs Zentrum, Section 3.5 Surface Geochemistry, Potsdam, Germany * Corresponding author (email: ambre.luguet@uni-bonn.de) 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; Meibom and Frei, 2002; Pearson et al., 2007; Coggon et al., 2012). 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; Mochalov et al., 2016, 2018). 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). 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) hosted in the dunitic core of the Kondyor ZUC. Our Pt-alloys are a different subset from those investigated for the 190Pt-4He isotope system. 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