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

A Rosetta stone linking melt trajectories in the mantle to the stress field and lithological heterogeneities (Trinity ophiolite, California)

“…The nature and distribution of the inclusions in Sp that recrystallized during the reaction show that the dissolution of Px was initiated by a waterand alkali-rich melt. We drew a similar conclusion for the origin of the dunitic mantle-crust transition zone in the Oman ophiolite (Rospabé et al, 2017(Rospabé et al, , 2021. We call this melt a "pioneer melt" because it contributed to the initial stage of reaction leading to the formation of dunite.…”

Formation of Ultra‐Depleted Mantle Peridotites and Their Relationship With Boninitic Melts: An Example From the Kamuikotan Unit, Hokkaido, Japan

“…(2017). Although the formation of spinel‐hosted inclusions is controversial, it allows us to estimate the melt compositions involved in the system (e.g., Arai et al., 2022; Rospabé et al., 2021; Tamura et al., 2014). The calculated melts equilibrated with the TH amphiboles are more depleted compared to the melt equilibrated with amphibole from the IBM forearc dunite (Morishita et al., 2011).…”

“…We also calculated melts in equilibrium with amphiboles using the partition coefficients from Ozawa (2001) and Shimizu et al (2017). Although the formation of spinel-hosted inclusions is controversial, it allows us to estimate the melt compositions involved in the system (e.g., Arai et al, 2022;Rospabé et al, 2021;Tamura et al, 2014).…”

Geochemical Characterization of the Oman Crust‐Mantle Transition Zone, OmanDP Holes CM1A and CM2B

“…(f) The dunites' impregnation characteristic appears to be different in Maqsad (e.g., mostly pl and clinopyroxene (Abily & Ceuleneer, 2013; Boudier & Nicolas, 1995; Koga et al., 2001) but also widespread opx and amph impregnations in the higher level of the transition zone (Rospabé et al., 2017, Rospabé, Benoit, & Candaudap, et al., 2018; Rospabé, Benoit et al., 2019) compared to CM Hole dunites (pl and clinopyroxene impregnations only). Similarly, exotic silicate inclusions (opx, amph, mica for the more abundant) enclosed in disseminated chromite grains in the dunites from the Maqsad area as well as in associated chromitite ore bodies (Lorand and Ceuleneer, 1989; Leblanc and Ceuleneer, 1991; Schiano et al., 1997; Borisov et al., 2012; Rollinson et al., 2018; Zagrtdenov et al., 2018; Rospabé, Ceuleneer et al., 2019, 2020, 2021), suggest the involvement of a fluid or fluid‐rich melt in the melt/rock reactions, which were not investigated in existing work on cores CM1A and CM2B ‐ a few chromite schlierens have been sampled along the cores but not studied in details yet. Further investigation of CM ultramafic rock mineral chemistry is need to evaluate these discrepancies between the two CM and Maqsad sites.…”