Origin and redox conditions of the Rosário-6 alnöite of southern Brazil: Implications for the state of the mantle during Gondwana breakup
The Rosário-6 alnöite is an alkaline occurrence that belongs to the Rosário do Sul kimberlitic field, situated in the south-eastern edge of the Paraná Basin, in the South of Brazil, and erupted concomitant or just after the volcanism of the Paraná-Etendeka Large Igneous Province (LIP). Following recent published nomenclature, Rosário-6 was classified as a kimberlite from a deep mantle source with a distinctive inequigranular texture resulting from the presence of olivine macrocrysts set in a finer-grained matrix. Trace element compositions of olivine, monticellite, spinel, phlogopite, perovskite and apatite show an enrichment of Nb, Ce, Ta and U, which implies that the Rosário-6 mantle source was enriched by recycled oceanic crust. The positive anomalies of Rb, Ba and Sr, the enrichment in LREE, and the negative anomalies of HREE in the Rosário-6 minerals, are indicative of a metasomatic process in the mantle source that could be caused by fluids from recycled oceanic crust. Temperature, pressure and redox conditions (fO2) of Rosário-6 crystallization are estimated from olivine, spinel, perovskite and monticellite compositions: Rosário-6 crystallization temperatures using olivine-spinel geothermobarometry were around 1390(±56)°C at a pressure of 2 GPa, and 1405(±56)°C at 3 GPa with ΔNNO = 2.8, at pressures constrained by the silica activity limited by the crystallization of monticellite. Using a perovskite oxybarometer, we obtained a larger range of ΔNNO (from -2.8 to 3.4), whereas the monticellite oxybarometer results in fO2 of -2.6 to -0.8 ΔNNO units. The fO2 indicate that the mantle source of Rosário-6 at the time of crystallization was possibly oxidized by materials from ancient subduction, which may be the cause for Rosário-6's low potential to carry and preserve diamonds. Horizontal tomographic images derived from P-wave velocity data constrain the thickness of the lithosphere in this region and the overall information indicates that mantle cooling at depths below 200 km may have resulted of an accumulation of oceanic plate slabs from old subduction. The geochemical data in conjunction with the geophysical characterizes the conditions of Rosário-6 mineral crystallization and also the mantle of this part of South America during Gondwana breakup.