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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)

Abstract

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.

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Acknowledgments

This study was supported by Russian Foundation for Basic Research (grant 12-05-01166 to IYuB) and the Uralian Division of Russian Academy of Sciences (project No 12-P-5-1020). Valuable assistance in obtaining heavy mineral concentrates by V.V. Knauf (NATI Research JSC, St. Petersburg, Russia) and help with drafting Fig. 1 by Svetlana M. Tuganova are gratefully acknowledged. We thank Dr. Hazel Prichard and Dr. Brian O’Driscoll for constructive review comments. Editorial input by the Editor in Chief, Prof. Johann G. Raith, is greatly appreciated.

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Correspondence to Kreshimir N. Malitch.

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Editorial handling: J.G. Raith

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Badanina, I.Y., Malitch, K.N., Lord, R.A. et al. Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland). Miner Petrol 107, 963–970 (2013). https://doi.org/10.1007/s00710-013-0271-9

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  • DOI: https://doi.org/10.1007/s00710-013-0271-9

Keywords

  • Chromite
  • Podiform Chromitites
  • Mineral Chemistry Data
  • Ophiolite Chromitites
  • Mantle Tectonite