Spinel and amphibole record oxygen fugacity in the lithospheric mantle of Northern Victoria Land (Antarctica).

Northern Victoria Land (Antarctica) mantle, sampled by spinel peridotite xenoliths found in basalts of Mt. Melbourne Volcanic Province, brings evidence of a metasomatic event related to Cenozoic volcanism. This metasomatic signature varies across the volcanic province: at Baker Rocks (BR) metasomatism is characterized by the occurrence of amphibole as disseminated grains or in veins hosted in lherzolite and harzburgite xenoliths carried by alkali basalt; at Greene Point (GP, about 80km north of BR) cryptic metasomatism provides variable enrichments of incompatible elements in clinopyroxenes from lherzolite/harzburgite xenoliths that were transported to the surface by a nephelinite-basanite lava. In this work we check the effects of the two styles of metasomatis on oxygen fugacity, estimating the redox state of anhydrous xenoliths with different metasomatic signatures, by means of the spinel-orthopyroxene-olivine oxybarometer. At BR, we estimated also the redox state of the metasomatic agent (the crosscutting amphibole-bearing veins) by applying the Popp et al. (2006) method to the crystal-chemical data obtained from amphiboles crystallized in the veins. Xenolith mineral chemistry was determined by electron microprobe, and the ferric iron content of spinels and amphibole were measured by 57Fe Mössbauer spectroscopy. Amphiboles were also characterized by single-crystal X-ray structure refinement, which allowed quantification of dehydrogenation by the method proposed by Oberti et al. (2007). Results show that oxygen fugacities recorded by the xenoliths range from -0.2 to -1.6 log-bar units (?logfO2) with respect to the fayalite-magnetite-quartz (FMQ) buffer. The ?logfO2 values calculated from amphiboles are about -0.6 log-bar units, thus in the range found for metasomatic amphibole of mantle xenoliths from the same area of northern Victoria Land (?logfO2 = -1.4 : -0.4 log-bar units, Nazzareni et al. 2010). In detail, fO2 data shows that i) beneath Greene Point, metasomatism increased fO2 by ~ 1 ?log fO2 log-bar units, whereas in Baker Rocks lithospheric mantle the metasomatic melt/rock interactions did not produce any significant change in fO2; ii) within each suite, there is no evident correlation between fO2 and the degree of mantle depletion, as indicated by spinel Cr#; iii) in GP xenoliths no direct correlation is found between fO2 and the enrichment in incompatible elements in clinopyroxenes. In conclusion, as evidenced by Baker Rocks xenoliths, mantle metasomatism leading to amphibole formation does not necessarily imply mantle oxidation; in contrast, the variability of fO2 obtained for Greene Point xenoliths highlights that the silicate melts responsible for the last metasomatic event recorded by xenoliths acted as an effective oxidiser of the depleted shallow upper mantle. Nazzareni, S., Bonadiman, C., Comodi, P., Faccini, B., Zanazzi, P.F., Coltorti, M. & Giuli, G. (2010): 89° Congresso SIMP, 146. Oberti, R., Hawthorne, F.C., Cannillo, E. & Cámara, F. (2007): Rev. Mineral. Geochem., 67, 125-171. Popp, R.K., Hibbert, H.A. & Lamb, W.M. (2006): Am. Mineral., 91, 54-66.