ISSN 1075-7015, Geology of Ore Deposits, 2016, Vol. 58, No. 8, pp. 16–23. © Pleiades Publishing, Ltd., 2016. Original Russian Text © S.Yu. Chazhengina, Z.P. Rybnikova, S.A. Svetov, 2015, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2015, No. 6, pp. 94–106.
ANALYTICAL TECHNIQUES OF MINERALS, ROCKS, AND ORES
Scanning Electron Microscopy and Raman Spectroscopy as Combined Methods for Studying Zoning in Minerals: The Case of Spinels from Archean Komatiites S. Yu. Chazhengina*, Z. P. Rybnikova, and S. A. Svetov Institute of Geology, Karelian Research Center, Russian Academy of Sciences, ul. Pushkinskaya 11, Petrozavodsk, 185910 Russia *e-mail:
[email protected] Received April 20, 2015
Abstract—Several types of both magmatic and metamorphic spinels have been found in Archean komatiites of the Sovdozero and Kostomuksha greenstone belts in the eastern part of the Fennoscandian Shield. Scanning electron microscopy and Raman spectroscopy revealed relics of cores of primary magmatic chrome-spinels with high Cr and Al contents. In the Sovdozero structure, the relics are better retained than those in the Kostomuksha structure, which is caused by a different degree of metamorphic transformation. The comparable 100 · Cr/(Al + Cr) values of spinel cores from Sovdozero and Kostomuksha reflect similar conditions of partitional melting in the mantle. These data agree with the fact that both komatiite complexes belong to the Al-undepleted petrogenic type. Wide variations in the Cr and Al contents in primary chrome-spinel cores together with a constant Mg/(Fe2+ + Mg) ratio correspond to low oxygen fugacity during magma crystallization. In general, the composition of these primary chrome-spinels is similar to that of accessory phases in peridotites from suprasubduction zones and agrees with hypothesis of komatiite complex formation in back-arc basins. Keywords: spinels, chromite, komatiites, Archean, Raman spectroscopy, mineral zoning DOI: 10.1134/S1075701516080067
electron and laser microscopy, Raman spectroscopy, LA-ICP-MS) assigned for detailed study of accessory chrome-spinels (Ahmed et al., 2005; Ghosh et al., 2013; Chen et al., 2015; Liu et al., 2015). We used some of these techniques in the study of chrome-spinels from Archean komatiites of the Sovdozero (3.0–2.9 Ga) and Kostomuksha (2.9–2.8 Ga) greenstone belts in the eastern Fennoscandian Shield, which are the best retained fragments of the Archean oceanic crust (Puchtel et al., 1998; Svetov, 2005). Accessory spinels from ultramafic rocks of the Precambrian complexes in the Fennoscandian Shield (in picrite and komatiite) were previously studied in the Kola Province (Vrevsky, 1989: Smol’kin, 1992) and within the Karelian Craton (Puchtel et al., 1998; Kulikov et al., 2011; Rybnikova et al., 2014); however no comprehensive mineralogical and geochemical study of chromite was performed.
INTRODUCTION The main problem in studying Archean volcanic rock is that the essential metamorphic transformation of the rocks (under conditions of greenschist to amphibolite facies) results in replacement of primary mineral paragenetic assemblages by secondary metamorphic ones (Komatiites…, 1998; Barnes, 1998, 2000; Svetov, 2005; Komatiite, 2008). In these cases, reconstruction of the melt generation conditions is possible based on the chemical composition of rocks (distribution of immobile major and trace elements) (Polat et al., 1999; Svetov and Smol’kin, 2003; Svetov, 2005; Komatiite., 2008; Manikyamba et al., 2008). The study of accessory minerals, which are universal and informative petrological indicators, is another reliable way to limit the modes of magma generation (Barnes, 1998; Page and Barnes, 2009). In komatiite magmas, this is accessory chrome-spinel (chromite), frequently as the only retained primary constituents (Barnes, 1998, 2000; Barnes and Roeder, 2001). The identification of relict grains of the primary accessory spinel is not a trivial task because of the essential metamorphic transformation of Archean komatiites. The modern method of attack comprises a set of physicochemical techniques (optical, scanning
SUBJECTS AND METHODS The paper discusses the results of accessory chrome-spinels from the Archean volcanic rocks of the Vedlozero–Segozero (Sovdozero structure, Central Karelia) and Gimol–Kostomuksha (Kostomulsha structure, Northwestern Karelia) greenstone belts 16
SCANNING ELECTRON MICROSCOPY AND RAMAN SPECTROSCOPY
referred to komatiites according to the classification of Arndt (Komatiite, 2008) and reported in detail by Komatiites…, (1988), Puchtel et al. (1998), and Svetov (2005). Komatiites were sampled from massive and upper parts of cumulative zones of thin (
