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Cristina Perinelli, University of Pisa (Italy)
Giovanni Battista Andreozzi, University of Rome "Sapienza" (Italy)
Pietro Armienti, University of Pisa (Italy)
Aida Maria Conte, C.N.R. (Italy)
Gabriele Giuli, University of Camerino (Italy)
Sigrid Griet Eeckhout, ESRF (France)
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Oxygen fugacity (fO2) is an important parameter in many geochemical processes in the mantle, magma genesis, chemical differentiations and metasomatism. It has also an active role in the stability conditions of hydrous minerals being the speciation of volatiles (C-O-H-S) strictly linked to fO2. Spinel peridotites included in Cenozoic alkaline basalts of northern Victoria Land (Antarctica) have revealed a remarkable compositional and mineralogical heterogeneity in the mantle linked to the coupled action of partial melting and metasomatism. In particular, samples from two adjacent localities (Baker Rocks - BR - and Greene Point - GP-, about 80km distant) show distinct metasomatic features: in Baker Rocks the metasomatism is proved by the occurrence of amphibole (diffuse or in vein) and by major (Fe and Ti) and trace elements enrichments; in Greene Point instead, the metasomatism is cryptic and evidenced by variable enrichments in incompatible elements of clinopyroxenes. In order to identify possible relationships between the two kind of metasomatism and fO2, we estimated the redox state of the upper mantle of this area by the spinel-orthopyroxene-olivine oxybarometer, using the ferric iron content of spinels, measured by 57Fe Mössbauer spectroscopy. The resultant oxygen fugacities yield values which range from 0.5 to 1.8 below the fayalite-magnetite-quartz FMQ buffer (Δlog fO2 FMQ). The Fe3+/Fetot ratio of spinel has been also measured by Fe K-edge XANES spectroscopy. We have tested this technique because of the very low amount of material required for the analysis. In fact, it is often difficult separating from a xenolith a spinel quantity sufficient for Mössbauer analyses, being it a minor phase in the upper mantle. The correlation coefficient between the results from the two approaches is R2 = 0.90 XANES technique provided Fe3+/Fetot values slightly lower than those measured by Mössbauer spectroscopy, shifting the fO2 range between 0.7 and 1.9 below FMQ buffer. The data obtained, viewed as a whole, show that there is no correlation between fO2 and the presence of hydrous phases: in fact, the most oxidised samples contain no hydrous phases and belong to Greene Point collection (-0.88 Δlog fO2 FMQ and -0.52Δ log fO2 FMQ for amphibole-bearing BR218 and GP6, respectively). Within individual suites there is no clear correlation between fO2 and degree of depletion, as indicated by spinel Cr#. In the same way, no clear correlation is found between fO2 and degree of incompatible elements enrichments in clinopyroxenes among GP xenoliths; on the contrary in Baker Rocks suite the most enriched samples are also the most oxidised samples. The heterogeneity of fO2 obtained for northern Victoria Land upper mantle indicates that silicate melts responsible of the last metasomatic event recorded by xenoliths, are an effective oxidiser of the depleted, shallow, upper mantle.
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