Petrographic characterization of material ejected during eruptive phases of Copahue volcano (Argentina) in 2012

CASELLI A; DAGA R; ALBITE J; AGUSTO M.

Kagoshima

Congreso; IAVCEI 2013 Scientific Assembly; 2013

The Volcanological Society of Japan

Copahue volcano is an Andean active basaltic-andesitic stratovolcano. The crater currently hosts an hot and acidiccrater lake. On July 19, 2012 a phreatic explosion occurred with emission of pyroclastic material, producing a smallplume that extended 18 km on ESE direction. A sample of this event, recovered from the crater, was constitutedmainly by sulphur pyroclasts, with low proportion of pumice and scoriae fragments, irregular argillaceous whitematerial and accidental/accessory fragments. Sulphur pyroclastic material have variable sizes (3-4 mm), mostlyglobular morphology with vesicles, like perfect spheres or elongated forms in some cases, and like deformed ?drops?in others. On December 22, 2012 a new magmatic pulse initiated with a rapid vaporization of the Crater Lake anda phreatic eruption. Minutes later it became in phreatomagmatic with convective cloud generation and ballisticemission of blocks and bombs, and pyroclastic fall of ash, mainly dark brown pumice fragments, from the plume.Several hours later the eruptive phase turned to strombolian style, with emission of elongated volcanic bombs.The emitted material sampled 9 km far from the volcano corresponds to lapilli-sized tephra. It is mainly composedby irregularly to subrounded shaped dark gray scoria with moderate vesicularity. Another abundant componentcorresponds to dark brown pumice, with both fluidal and vesicular textures and high vesicularity degree. Bothscoria and brown pumice contain mafic and felsic crystals in the glassy matrix. An argillaceous white material isalso present in the sample, less abundant than the previous components. Accidental/accessory fragments togetherwith mafic and felsic crystals and crystal fragments are present in the sample. The andesitic volcanic bombsreach 1.5 km away and have elongated shapes with moderate vesicularity; microscopically they are characterizedby a porphyritic texture. The observed phenocrysts are euhedral oligoclase, clinopyroxene and alkali feldspars;the hialopilitic matrix is almost completely made of glass with microlites of plagioclase, alkali feldspars andclinopyroxenes. The dark brown pumiceous fragments are flat and reach up to 10 cm length. They are characterizedby light weight because of their high vesicularity. Under the microscope, the dark brown pumice showed porphyritictexture, where phenocrysts represent less than 1 Percent, all euhedral and fragmented phenocrysts (oligoclase andclynopiroxene). Due to its light weight, ability to float, and mineralogy it might be named pheno-andesitic scoria.According to field observations, this ejects were thrown up by a grey plume that preceded the strombolian event.According to the preliminary study of the emitted material and the observations made during the eruption, we caninfer an andesitic composition for the magma and vesiculated pumice fragments, which correspond to the initialphase, accumulated at the top of the conduit.