An Alternative Scenario on the Origin of Ultra-High Pressure (UHP) and Super-Reduced (SuR) Minerals in Ophiolitic Chromitites: A Case Study from the Mercedita Deposit (Eastern Cuba)

Abstract

The origin of the assemblage of ultra-high pressure (UHP), super-reduced (SuR) and several crustally derived phases in ophiolitic chromitites is still hotly debated. In this paper, we report, for the first time, this assemblage of phases in ophiolitic chromitites of the Caribbean. We studied the Mercedita chromitite deposit in the eastern Cuban ophiolitic complexes. The mineral phases were characterized using microRaman spectroscopy, energy-dispersive spectroscopy with a scanning electron microscope (SEM-EDS), X-ray microdiffraction and electron microprobe analyses. Mineral concentrates were prepared using hydroseparation techniques. We have identified oriented clinopyroxene lamellae in chromite, oriented rutile lamellae in chromite, moissanite hosted in the altered matrix of the chromitite, graphite-like amorphous carbon, corundum and SiO2 hosted in healed fractures in chromite grains, and native Cu and Fe–Mn alloy recovered in heavy-mineral concentrates obtained by hydroseparation. This assemblage may correspond to UHP-SuR conditions, implying recycling of chromitite in the mantle or formation of the chromite grains at deep mantle depths, followed by emplacement at a shallow level in the mantle. However, the chromitite bodies contain gabbro sills oriented parallel to the elongation of the chromitite lenses, and these show no evidence of HP/UHP metamorphism. Therefore, the identified “exotic” phases may not be indicative of UHP. They formed independently as oriented clinopyroxene lamellae in chromite during cooling (clinopyroxene and rutile), in super-reduced microenvironments during the serpentinization processes, and by transference of subducted crustal material to the mantle wedge via cold plumes.

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

“…Geochemical compositions and mineral chemistry characteristics of peridotites and chromitites from Greece have shown that the partial melting degree of primitive mantle and the hydrous nature of parent magma are major factors that control the compositional variations in them [10,18,19,42,45,56,57]. In addition, the known widespread geochemical heterogeneities in the bulk-rock compositions of chromitites from the Balkan Peninsula (Table 1) would be inconsistent with their evolution through simple partial melting processes, and suggest that their original geochemical compositions may have been modified by subsequent, post-magmatic processes taking place at shallow mantle depths [8,[66][67][68][69].…”

“…Recently, the investigation of microdiamonds and graphite in chromite from ophiolite-type chromitites hosted in the Tehuitzingo serpentinite (southern Mexico), have been attributed a secondary origin and process related to the retrograde evolution of the respective chromitite. These processes apparently took place at relatively low temperatures (520-670 • C), at low pressures and in shallow depths [69]. The presence of minerals indicative of super-reducing phases, such as graphite-like, Fe-Ni-Cr alloys, awaruite (Ni 3 Fe), and heazlewoodite (Ni 3 S 2 ) [44,45,82,83], in the chromitite ores from the islands of Othrys and Skyros may be suggestive of the introduction of continental crust derived, reduced C-O-H fluids, during post-magmatic alteration processes.…”

“…The presence of minerals indicative of super-reducing phases, such as graphite-like, Fe-Ni-Cr alloys, awaruite (Ni 3 Fe), and heazlewoodite (Ni 3 S 2 ) [44,45,82,83], in the chromitite ores from the islands of Othrys and Skyros may be suggestive of the introduction of continental crust derived, reduced C-O-H fluids, during post-magmatic alteration processes. These were probably facilitated during brittle deformation stages in the shallow crust and in the subducted oceanic slab [69,84,85]. The association of graphite with sperrylite and sulphides in magnetite ore from the Skyros ophiolite complex has also been attributed to its deposition from a CO 2 − and CH 4 − dominated reducing fluid at low oxygen fugacity at relatively low temperatures (500-300 • C) during serpentinization [58].…”

“…In summary, the original geochemical compositions of chromitites may have been modified by subsequent, post-magmatic processes taking place at shallow mantle depths [35][36][37][38]56,57,68,69,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89]97]. The wide range of δ 53 Cr values, from positive, slightly negative to the most negatively fractionated signatures, even present in individual relatively large chromitite deposits, together with negative trends observed between δ 53 Cr values and Cr/(Cr + Al) ratios in chromitites, may reflect a control of Cr isotope compositions in chromitites by degrees of partial melting and by magma fractionation.…”

A shallow origin for diamonds in ophiolitic chromitites: REPLY

“…In our view, the finding of diamonds is not stand-alone evidence for UHP unless additional UHP minerals are found. We would like to raise a warning flag to readers that many other minerals or features that are commonly used as indicators of UHP in ophiolitic chromitites (e.g., oriented lamellae of pyroxene in chromite) and associated super-reduced conditions in the deep mantle (e.g., moissanite, native elements, and alloys) did not necessarily form at these conditions (e.g., Pujol-Solà et al, 2018). Pujol-Solà et al reported these textures and minerals in chromitite bodies hosting (assimilating) unmetamorphosed gabbro sills in Cuban ophiolites, and concluded that these minerals did not form in the deep mantle, but as the result of cooling and serpentinization processes.…”