The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

Abstract The mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it underlined by dunitic transition zone (DTZ) can reach few hundred meters thickness host abundant chromitite ore bodies. dunites have been deciphered as essentially mantle-derived most ophiolitic massifs; is, reactional residues of interactions between peridotite percolating melt(s). Although both dunite the focus many studies, reasons for their systematic association remain unclear. In this study we explored inclusion content chromite grains disseminated from DTZ exposed Maqsad area Oman ophiolite where former asthenospheric diapir exposed. Similarly to bodies, contain great diversity silicate inclusions. Based minor element composition 1794 single inclusions chromites 285 samples associated rocks DTZ, infer formed similar ‘metallogenic’ process chromitites, that, whole, actually represent low-grade end-member single, giant body. nature (amphibole mica among others) enclosed precludes crystallization an anhydrous primitive basaltic melt, rather calls melt hybrid common mafic melts more exotic Si-, Na- volatile-rich fluids. parent medium chromitites results interaction (the mid-ocean ridge basalt source), colder, altered lithospheric lid hydrothermal fluids responsible alteration. excess silica provided incongruent dissolution enstatite mantle harzburgite and/or moderate degree partial melting gabbroic crust. variable when than suggesting greater variability fluid fractions involved genesis ores. Finally, be viewed metamorphic contact aureole episodically rising diapirs formerly accreted axial lids. Our conclusion about chicken egg dilemma linking (i.e. do form response formation or conversely?) dunitization itself potential release Cr its re-concentration ores, turn competition orthopyroxene (± plagioclase) fractionation during fluid–melt–peridotite reaction mineralogical dunites.