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

Abstract

Platinum Group Minerals are typically dated using the 187 Re-187 Os and 190 Pt-186 Os isotope systems and more recently using the 190 Pt-4 He geochronometer. The 187 Re-187 Os and 190 Pt-186 Os 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 187 Os/ 188 Os and most radiogenic 186 Os/ 188 Os 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 190 Pt-4 He 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.

Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019)

“…Mochalov et al (2016) obtained 190 Pt-4 He ages of multiple individual Pt alloy grains to be 112-129 Ma, that was consistent with regional geology, sequence of mineral formation and independent age estimations. Subsequently Luguet et al (2019) reported a 190 Pt-186 Os isochron age of 240-250 Ma. This age discrepancy led Luguet et al (2019) to conclude that the 190 Pt-4 He ages of the Pt grains reflected "open system" behaviour, essentially arguing that the radiogenic 4 He generated by Pt decay had diffused out of the grains since their formation.…”

“…Subsequently Luguet et al (2019) reported a 190 Pt-186 Os isochron age of 240-250 Ma. This age discrepancy led Luguet et al (2019) to conclude that the 190 Pt-4 He ages of the Pt grains reflected "open system" behaviour, essentially arguing that the radiogenic 4 He generated by Pt decay had diffused out of the grains since their formation. This explanation is extremely difficult to reconcile with the experimental low diffusion rate of radiogenic 4 He in Pt-Fe alloys (Shukolyukov et al, 2012a,b), the retention of extremely high concentrations of cosmogenic 3 He by Pt-Fe alloys (Yakubovich et al, 2019) and with the theory of helium behaviour in metals in general (Trinkaus and Singh, 2003).…”

“…These isotope systems have closure temperatures that range from ∼300 °C (biotite Ar-Ar; Harrison et al, 1985) to over 1150 °C (clinopyroxene Sm-Nd; Van Orman et al, 2001), implying that Pt mineralisation was simultaneous with intrusion, and that post-intrusion cooling of the complex was instantaneous, within our ability to resolve it. Luguet et al (2019) have ignored the evidence for the Cretaceous age of alkaline magmatic complexes, and associated ore deposits, in the Aldan shield region (Yarmolyuk et al, 2019 and references therein). In support of the Early Triassic age of the Kondyor ultramafic complex they remark on the similarity with detrital zircon grains from the Lena river and Mohe-Upper Amur basin.…”

“…The Luguet et al (2019) study failed to put the study material into context. The formation of platinum mineralisation within the Kondyor massif was polycyclic (Mochalov, 2019).…”

“…This resulted in the formation of five genetically distinct types of PGMs. The Pt alloys analysed by Luguet et al (2019) belong to the later generations of PGM that were formed due to the recrystallisation and remobilisation of earlier PGM at temperatures not higher than 650-850 °C (see Supplementary Information; Table S-2). This indicates that the PGE chemistry of fluids that were responsible for the formation of these Pt alloys evolved with time.…”