Two-pyroxene intergrowth exsolution textures in ophiolitic chromites: implications for the deep mantle origin of the Mirdita ophiolite, Albania

Increasing mineralogical and textural evidence from podiform chromitites in ophiolites suggests their ultra-high-pressure origin (>150 km), challenging conventional models for their formation under low-pressure conditions (<60 km) in the upper mantle. However, this challenge remains controversial due to a lack of in situ mineralogical evidence. Here, we report new data and observations from the Skenderbeu massif in the Mirdita ophiolite, Albania. Transmission electron microscopy shows that these chromitites (Cr# = 41.8-43.2) have numerous exsolution lamellae of diopsidic clinopyroxene and orthoenstatite. These lamellae have a crystallographic topotaxy relationship with the host chromite, i.e. (020)(Cpx) parallel to (220)Chr, (200)Cpx parallel to (111)(Chr) and (010)(Opx)parallel to (220)Chr, (200)(Opx) parallel to (220)Chr, indicating an exsolution origin. The abundant presence of pyroxene exsolution lamellae in the centre of the host chromites implies the incorporation of Si4+ and Ca2+ cations in the precursor chromite, a CaFe2O4-structured high-pressure polymorph that is stable at >12.5 GPa (i.e. 380 km deep). These in situ nanoscale observations and their geological occurrence, together with previously discovered ophiolitic diamonds in the Mirdita ophiolite, suggest a much deeper origin for ophiolitic chromitites than conventional interpretations and provide a valuable opportunity to understand the composition of the deep mantle.