Petrography and mineral chemistry of Rosário-16 Kimberlite: an inference of P, T, and fO2 for an ultramafic alkaline assemblage in Southern Brazil

Rosário-16 (R-16) is a hypabyssal transitional kimberlite, divided into two volcanic pipes, R-16-A and R-16-B, within the Kimberlite Province of Rosário do Sul (KPRS), southeast of the Paraná Basin (PB), southeast Brazil. The saprolite outcrops, within a NE/SW structure, occur next to the Rosário-6 alnöite, comprising microcrysts of olivine, Mg-chromite, Mg-ilmenite, diopside, phlogopite, fluorapatite, and perovskite. R-16 also contains microxenocrysts, potentially derived from peridotitic macrocrysts, characterized by electron microprobe analyses. Calculation of pressure, representative temperature (based on averaged mineral compositions), and oxygen-fugacity conditions using geothermobarometers for olivine and spinel, and an oxygen barometer for perovskite, indicates pressures of ≥ 4 GPa, a representative temperature of ~1135 °C, and ƒO2 (oxygen fugacity) values between ΔNNO = –3.92 and –1.35. Chemical analyses indicate that CO2 metasomatism in R-16 was less than Rosário-6 alnöite. We found that CO2 reactions occurred in the mantle at higher pressures in R-16, resulting in the generation of kimberlitic magma through silicate interaction with carbonatitic liquids. The similarity between the ƒO2 versus temperature profiles of R-16 and high/low-Ti PB tholeiites suggest a heterogeneously metasomatized mantle source for the KPRS with the emplacement of R-16 favored by the Gondwana breakup.