Abstract
Studies that track and compiled transformations of ilmenite and magnetite under conditions of different metamorphic grades are still very limited. The Sedova Zaimka intrusion in Western Siberia (Russia) is a good example to examine the effect of contact metamorphism from greenschist to low-amphibolite facies on magmatic Fe-Ti oxide minerals, as this small mafic-ultramafic body is located within contact metamorphic aureole of a large granite pluton. In the Sedova Zaimka, ilmenite with little to no magnetite is present as an accessory dissemination throughout metamorphosed rocks. Ilmenite is variably enriched in MnO (1 to 13.3 wt%) and depleted in MgO (less than 0.3 wt%), suggesting that its primary magmatic composition has been unevenly modified by diffusion re-equilibrium with coexisting metamorphic silicates. The elevated content of ZnO (up to 0.5 wt%) and NiO (up to 0.4 wt%) in ilmenite suggests that both ZnO and NiO, like MnO, must be strongly partitioned into ilmenite relatively to silicate minerals under the reducing contact metamorphic conditions, if chromite is absent. The textural observations of ilmenite–sulfide and ilmenite–titanite–sulfide relationships indicate that Ti-magnetite, in contrast to ilmenite, is an unstable phase in the presence of sulfur-rich reduced metamorphic fluids and is completely replaced by sulfides, with the development of specific “octahedral meshes” of ilmenite in sulfides.
Highlights
The effect of metamorphism on Fe-Ti oxide assemblages is insufficiently studied and poorly compiled and analyzed, despite the fact that ilmenite, magnetite, hematite, and rutile are common accessory minerals in both igneous and metamorphic rocks
In olivine metagabbro from the upper part of the intrusion, three morphological types of ilmenite were identified: (i) predominant elongated, euhedral to subhedral grains with exsolution textures of thin parallel-oriented lamellae of magnetite (Figure 2A); (ii) rare euhedral to subhedral grains of ilmenite-magnetite intergrowing crystals (Figure 2B); and (iii) occasional rounded or sub-rounded grains consisting of ilmenite cores and magnetite rims (Figure 2C)
Some ilmenite grains are in association with sulfides, which occasionally replace lamellar magnetite
Summary
The effect of metamorphism on Fe-Ti oxide assemblages is insufficiently studied and poorly compiled and analyzed, despite the fact that ilmenite, magnetite, hematite, and rutile are common accessory minerals in both igneous and metamorphic rocks (see, e.g., [1,2,3,4,5,6,7,8,9,10,11]). Studies that track the textural and compositional transformations of ilmenite or magnetite under conditions of different metamorphic grades are still very limited. Cassidy et al [22] highlighted that the MnO enrichment in ilmenite accompanied by MgO depletion, due to diffusion re-equilibrium reactions with silicate minerals, is an important indicator of metamorphic overprint of ilmenite. It was Minerals 2020, 10, 253; doi:10.3390/min10030253 www.mdpi.com/journal/minerals. Ti-magnetite are observed in metamorphosed mafic-ultramafic rocks of the Zaimka occasional Ti-magnetite are observed in metamorphosed mafic-ultramafic rocks of theintrusion, Sedova with ilmenite crystallizing Ti-magnetite.
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