Abstract
This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.
Highlights
Dominance of Ru-Os-Ir minerals over other platinum-group minerals (PGM) is considered a typical feature of podiform chromitites within the residual mantle in ophiolite-type dunite-harzburgite massifs worldwide [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15], among others
This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia)
The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle olivine (Fo 88–93)
Summary
Dominance of Ru-Os-Ir minerals over other platinum-group minerals (PGM) is considered a typical feature of podiform chromitites within the residual mantle in ophiolite-type dunite-harzburgite massifs worldwide [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15], among others. The previously collected Os-isotope data for primary Ru-Os-Ir alloys and Ru-Os sulfides from the Verkh-Nevinsk massif revealed a wide range of sub-chondritic 187Os/188Os values (from 0.1162 to 0.1346 [49,50]), whereas first S-isotope data for Ru-Os sulfides showed a restricted range of δ34S values (from 0.2 to 2.3‰ [51]). Both datasets were interpreted as being consistent with a deep-seated source of highly siderophile elements (HSE) and sulfur. The PGM grains analyzed in this study are nuggets (crystals and aggregates) that fall within a size range between 0.5 and 1.5 mm
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