Scanning electron microscopy and Raman spectroscopy as the complex techniques to investigate the zoning of minerals (by example of spinels from archean komatiites)

Chazhengina S. Yu,, Rybnikova Z, P., Svetov S. A.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 6. P. 94-106

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Language: Russian

Abstract

Coexistence of several generations of accessory spinels as of magmatic so of methamorphic origin has been revealed in Archean komatiites from Sovdozero and Kostomuksha greenstone belts in the eastern part of Fennoscandian Shield. Scanning electron microscopy and Raman spectroscopy detected relics of cores of the primary magmatic chrome-spinels with high contents of chromium and aluminum. Spinel relics in Sovdozero structure preserved the better integrity than these minerals from Kostomuksha due to difference in degree of metamorphic transformation over their komatiite complexes. Ratios 100Cr/(Al+Cr) have comparable values for primary spinel cores as from Sovdozero, so from Kostomuksha, reflecting similar conditions of partitional melting in the mantle. These data coincide with the fact that both komatiite complexes belong to the Al-undepleted petrogenic type. Wide variations of Cr and Al contents in primary chrome-spinel cores, together with the constant Mg/(Fe²⁺+Mg) ratio, correspond to the low oxygen fugacity during the magma crystallization. In general, these primary chrome-spinels have the composition similar to accessory phases in peridotites from supra-subduction zones, and it is in agreement with hypothesis of the komatiite complex formation in back-arc basins.

Key words: spinels, chromite, komatiites, Archean, Raman spectroscopy, mineral zoning.