Оn mechanisms of the accessory chromian spinel formation in plastic deformed crystals of enstatite from Kraka massifs (Urals Ophiolite Belt)
Saveliev D. E., Musabirov I. I.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 2. P. 28-49
Tiny chromian spinel precipitations associated with plastic deformed enstatite crystals from spinel peridotite were studied. It is defined that there are several localizations of the precipitations: 1) in recrystallization zones where they coexist with olivine and enstatite neoblasts, 2) in the volume of polygonized primary enstatite crystals as lamellae and holly leaf grains, 3) in lamellae and neoblasts of diopside and pargasite which were formed during a plastic deformation of primary orthopyroxene. In the first two cases, we suggest the solid-state mechanism of their formation. First, it is a result of an impurities segregation on the structure defects (dislocation → subgrain boundary → grain boundary). Second, it is a result of the nucleation and growth of the new-formed mineral phases (spinel, forsterite, enstatite II) in most distorted places of porphyroclasts. In the third case, it is possible that chromian spinel were appears either as a result of the solid-state crystallization or as a cumulate of «diopside» or «pargasite» melts formed by frictional melting of orthopyroxene porphyroclasts in micro-chambers.
Key words: chromian spinel, enstatite, plastic deformation, recrystallization, ultramafic rocks, the Southern Urals