1. Two-pyroxene intergrowth exsolution textures in ophiolitic chromites: implications for the deep mantle origin of the Mirdita ophiolite, Albania.
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Wu, Weiwei, Yang, Jingsui, Dilek, Yildirim, Das, Souvik, Lian, Dongyang, Cai, Pengjie, Wang, Yun, Yang, Yu, and Wen, Yuanyun
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CHROMITE ,TRANSMISSION electron microscopy ,OPHIOLITES ,PYROXENE - 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 ∥ (22¯0)Chr , (2¯00)Cpx ∥ (111)Chr and (010)Opx ∥ (22¯0)Chr , (200)Opx ∥ (22¯0)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 CaFe2 O4 -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 [ABSTRACT FROM AUTHOR]- Published
- 2024
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