Possible recycled origin for ultrahigh-pressure chromitites in ophiolites

Abstract

Podiform chromitites have been interpreted as cumulates by harzburgite/melt interaction and related melt mixing at the upper mantle to the Moho transition zone. Recent discovery of diamond and other ultrahigh -pressure (UHP) minerals from some podiform chromitites, especially those from Tibet, however, has raised a question about the depth of their formation. These UHP chromitites are possibly of deep recycling origin; they had been originally formed at the upper mantle before sinking down to deeper mantle, and upwelling again to the shallowest mantle by convection. Diamond is formed by reduction/oxidation of fluidal carbon species (e.g., CO 2 or CH 4 ) obtained during the convection history, and has survived oxidation because of strong encapsulation in metal alloys further included by chromian spinel. Exsolved silicates (diopside and coesite) in chromian spinel from some UHP chromitite are possibly derived from hydrous mineral inclusions in chromian spinel formed at low pressures, which have been decomposed/molten during sinking and solved in the UHP chromian spinel phase. Both the UHP chromitites and ordinary low -pressure ones could be present in the upper mantle derived from the mid -oceanic ridge, one of the main ends of the mantle upwelling flow. The mantle recycling issue unraveled through chromitite thus can be one of the targets of deep oceanic mantle drilling including the Mohole.

Fluid-present deformation aids chemical modification of chromite: Insights from chromites from Golyamo Kamenyane, SE Bulgaria

“…Scientific interest in chromite arises from the fact that within peridotitic rocks, chromites in general are better preserved than silicates, as they are more resistant to fluid-related processes; this has led to the widespread use of chromitites to track the evolution of the Earth's mantle convection (e.g. Arai, 2010;González-Jiménez et al, 2012Miura et al, 2012;Walker et al, 2002). Chromitites in ophiolites are commonly hosted within discordant or subconcordant dunite bodies, within the shallow mantle part of the oceanic lithosphere (e.g.…”

Zincian chromite inclusions in diamonds: Possibility of deep recycling origin

“…It is possible that the zincian chromite inclusions in diamonds are peculiar deep recycled materials that have been apparently intact, possibly in a way similar to some recycled podiform chromitites (Arai, 2010). This is in contrast to other recycled materials such as the mafic oceanic crust, which has been subducted and mixed with peridotitic mantle (e.g., Allegre and Turcotte, 1986).…”

Alteration of chromitites from the Voidolakkos and Xerolivado mines, Vourinos ophiolite complex, Greece: implications for deformation-induced metamorphism

“…Upper mantle rocks dominate the central and easternmost part of the ophiolitic outcrop. A 300-500 m-thick discontinuous sheared meta-sedimentary sliver between the basal harzburgites and the Pelagonian Platform corresponds to the Vourinos mélange (Ghikas et al, 2007;2010). The dunites frequently host economically feasible chromitite bodies, which can be locally enriched in noble metals (Konstantopoulou and Economou-Eliopoulos, 1991).…”

“…In terms of deformation the investigated chromitites display a multistage structural evolution, although they are highly resistant against early plastic (Arai, 2010) and later ductile-brittle (Li et al, 2002) deformation. They show peculiar igneous structures such as orbicular, nodular and anti-nodular texture that are indicative of their formation under upper mantle conditions (1200-1300°C; Li et al, 2002).…”

“…Ross et al, 1980;Roberts, 1988;Nicolas, 1989). Yang et al, 2007;Yamamoto et al, 2009;Arai, 2010;Miura et al, 2012). Yang et al, 2007;Yamamoto et al, 2009;Arai, 2010;Miura et al, 2012).…”