Volcanic spherules condensed from supercritical fluid in the Payenia volcanic province, Argentina

Spherules can be formed by high temperature processes during volcanic eruptions, lightning strikes and meteorite impacts. Here we report four different types of spherules and spheroidal particles associated with tephra deposits from two separate volcanic fields in the southern Payenia province of Argentina. These silicate and carbonate spherules represent <0.01 % of the sampled material with individual spherules <200 µm in size. Thirty particles have been imaged, only the transparent spherules are smooth, perfect spheres. Other morphologies include ellipsoids and aggregated dumbbells, and the spheroids are hollow or solid. Major-element analyses show that the spherules and spheroids have silica-rich, iron-rich, carbonate and basaltic compositions. Chemical analysis of the carbonate spheroids shows some variability in trace element content between the cores and rims suggesting element mobility and loss towards the margins. All analysed carbonate spheroids have elevated Sr/Y, La/Y and La/Ce, outside of the range of sedimentary carbonate. All four spherule types are considered volcanic in origin, with the excess CO2 required for the formation of carbonate spherules potentially sourced from basement lithologies. Based on major- and trace-element analyses we conclude that the silica-rich and carbonate spherules formed by instantaneous condensation from supercritical CO2-rich hydrous fluids saturated with dissolved silicates.