Chaotic Mixing Experiments at High Temperature: Towards Unravelling a Large Magmatic Province

The Paraná-Etendeka Magmatic Province (PEMP) is the largest outpour of magma on the Earth surface during lower Cretaceous times (~133 Ma). Basalts (SiO2~50%, both high-Ti and low-Ti members) predominate over an estimated volume of 7·105 km3. However, ca. 2.5% of the volcanic products, are chemically more evolved (SiO2>63%). Their genesis is still under debate. This work aims a first attempt to experimentally reproduce the impact of underplating basaltic melt into a pre-existing continental crust. Mixing dynamics is thought to greatly influence the formation/contamination conditions of the high-Ti acid member (Chapecó-type). We used a chaotic mixing protocol (Journal Bearing System) at 1,350 °C and following end-members: KS-700 basalt (high-Ti Pitanga-type from PEMP; η1350 = 8.78 Pa.s; ρ1350 = 2.47 g/cm3) and LMC-027 granite (syenogranite from Capão Bonito Stock; η1350 = 1.22·105 Pa s; ρ1350 = 2.29 g/cm3). Homogenized glasses from the basalt and the granitic basement were used in an 80/20 proportion, respectively. The experiment was performed during 212 min in total, i.e., two periods of: (i) two clockwise rotations of outer cylinder (35 min); (ii) six anticlockwise rotations of inner cylinder (18 min). The independent and non-simultaneous movements of the two cylinders guaranteed the chaotic flow. The obtained mixed glass was cut in slices of 3 mm perpendicular to the rotation axes and two of the sections reproduced Poincaré patterns, which are theoretically expected to be resulted for this kind of dynamic mixing. With the development of stretching and folding processes, chaotic trajectories are distributed off-centered as lamellar lens-like structures in the mixed system. Vortex structures are comparable to those produced by mixing Fe-free silicate melts, however with much higher fractal dimension (Df). These sections were preliminarily analyzed for the changes in morphology by comparing the Df using binary images (ImageJ software). Obtained Df’s (≈1.79) are close to those from similar experiments with natural melts, although widespread orbicular structures found along all basaltic morphological domains are thought to enhance the complexity. Further experiments varying the granitic end-member are planned. Raman, microprobe and LA-ICP-MS investigations will be performed to compare chemical behavior of major/minor oxides and trace elements. Furthermore, numerical simulations will follow.