Mineralogical and isotopic peculiarities of high-Cr chromitites: Implications for a mantle convection genesis of the Bulqiza ophiolite

Submicroscopic to micro-diamonds, silicates, wüstite, platinum group minerals (PGM), sulfides and ultra-high pressure (UHP) mineral inclusions have been recovered from podiform and banded chromitites of the Bulqiza ophiolite in Albania. The alignment of acicular silicate inclusions with crystallographic planes of magnesiochromite suggests an exsolution origin. Positive and negative-crystal faces of cubic silicate inclusions are consistent withs a high-temperature paragenesis. Micro-inclusions and nanoparticles (<500 nm) of Ru–Os bisulfides (laurite; RuS₂), Os–Ir, and Os-Ir-Ru alloys, tolovike, irarsite, Ir–Ni sulfides, and millerite (NiS) are also common in the Bulqiza chromitites. Dispersed spherical, nano-inclusions of Ir–Pt and Os–Ir may reflect formation of the Ir–Pt and Os–Ir particles by sulfide liquid immiscibility. The high Cr# of the chromitites, their evolved whole-rock PGE abundances, and IPGE/PPGE ratios (≥ 10) suggest extensive partial melting, likely in a supra-subduction zone (SSZ) environment. This interpretation is supported by the variable ¹⁸⁷Re/¹⁸⁸Os (0.0103–0.0271) and ¹⁸⁷Os/¹⁸⁸Os (0.12600–0.13014) ratios and the low γOs isotope values (mean + 0.01) of the chromitites, reflecting derivation from heterogeneously depleted mantle materials. The presence of remarkably zoned zircons with U–Pb ages (164–3354 Ma) represents the addition of crustal materials. Discrete micro-diamonds, PGM, and sulfide inclusions point to a super-reducing (SuR), ultra-high pressure (UHP) origin. We propose a genetic model that involves incorporation of crustal and UHP phases into the mantle by convection or subduction. The UHP and crustal silicates are consistent with a subduction-related origin, but the presence of super-reduced (SuR) inclusions suggests equilibration under subduction-disparate mantle conditions.