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

Abstract

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 ∥ ( 2 2 ¯ 0 ) Chr , ( 2 ¯ 00 ) Cpx ∥ (111) Chr and (010) Opx ∥ ( 2 2 ¯ 0 ) Chr , (200) Opx ∥ ( 2 2 ¯ 0 ) Chr , indicating an exsolution origin. The abundant presence of pyroxene exsolution lamellae in the centre of the host chromites implies the incorporation of Si 4+ and Ca 2+ cations in the precursor chromite, a CaFe 2 O 4 -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. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists