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- Mössbauer spectroscopy (1)
- V/Sc (1)
- metasomatism (1)
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The multi-valence nature of vanadium means that its geochemical behaviour will be ƒO2-dependent, so that its concentration or V/Sc (or V/Ga), can serve as proxies for oxidation state in mantle peridotites. Compared to Fe3+/Fe2+-based equilibria, such trace elements may be less sensitive to metasomatic processes. To investigate these systematics, we have measured V, Sc, Ga and Fe3+ contents in clinopyroxene from well-characterised spinel peridotite xenoliths from the Massif Central, France. These samples were metasomatised by a variety of agents with different oxidation states.V contents can be modified by metasomatic interactions, and other geochemically similar elements including Sc and Ga can also be added, removed or remain constant. A link between V/Sc and Fe3+-Fe2+ equilibria is apparent. Partial removal of V is caused by different metasomatic agents; the common factor is that all agents were significantly more oxidised than the initial ambient mantle peridotite. This extraction can be understood by a decreasing partition coefficient for V for ΔlogƒO2 > ~FMQ-2. Considering that mineral/melt partitioning of V decreases similarly for all peridotite minerals, the bulk-rock V/Sc will also change during relatively oxidising metasomatic interactions and mirror the results obtained for clinopyroxene.
Over the last several decades, spinel-structured minerals with the chemical formula AB2O4 (where A and B stand for divalent and trivalent cations, respectively) have attracted more and more attention, particularly with regards to their breakdown at high pressures and temperatures and the nature of the so-called "post-spinel" phases. Spinel-structured phases with different endmember compositions, like magnetite (Fe3O4), hercynite (FeAl2O4) or spinel (MgAl2O4), are known to breakdown differently at high pressure-temperature conditions (e.g., Akaogi et al. 1999; Schollenbruch et al. 2010; Woodland et al. 2012). Such phases are of particular interest when they incorporate ferric (Fe3+) and ferrous (Fe2+) cations as this makes their stability sensitive to redox conditions. Since magnetite and magnesioferrite (MgFe3+ 2O4) have been found as inclusions in diamond (e.g., Stachel et al. 1998; Harte et al. 1999; Wirth et al. 2014; Palot et al. 2016; Jacob et al. 2016), understanding their phase relations is important for setting constraints on the conditions of their formation.
This study aimed to experimentally investigate the phase relations of Fe-Mg spinel-structured phases at conditions of the deep upper mantle and transition zone. Exploring the stability of new post-spinel phases and their characterization were also major goals of this study. Approaching a pyrolitic mantle composition by adding amounts of SiO2 in the system allowed constraints on the relevance of Fe-Mg post-spinel phases coexisting with mantle silicates to be made. ...