Geochemistry of ash leachates from the 1993 Lascar eruption, northern Chile. Implication for recycling of ancient evaporites

Volcanic formations in the central Andes overlie extensive amounts of ancient evaporites of Cenozoic age. Recycling of buried salts in present closed basins through the hydrological cycle is a well-known process. Another salt recycling process through volcanic activity has been detected in a recent eruption of the Lascar volcano in northern Chile. About 109 metric tons of air-fall tephra was deposited over the surrounding closed basins. Leaching experiments were carried out on ashes deposited near the summit and in a lower neighbouring valley. Summit ash leachates are almost pure CaSO4 solutions, whereas the valley ashes leachates are enriched in minor components. Leachates of the summit ash are near-neutral, whereas those of the valley ash have pH as high as 10.5. Sulfur isotope composition of the leached sulfate is consistent with the volcanic recycling of Tertiary gypsum present in the sedimentary section beneath the volcanic cordillera. As a rough estimate, about 700,000 tons of easily soluble CaSO4 were added in only three days to the surrounding closed basins. The high pH of the valley ash leachates is due to the hydration of alkali and alkaline-earth metal oxides formed during pyrolysis of vegetation by hot ash. Efflorescent salts encrusted on plants and dissolution of volcanic glass provided some of the minor components. The almost pure CaSO4 composition of the leach waters is compositionally distinct from any present inflow waters in the central Andes. Addition of CaSO4 may explain some of the differences between the observed brine compositions and those predicted by simulating the evaporation of regular inflows. The recycling of sedimentary gypsum is probably not restricted to Lascar volcano. The recycling of ancient salts, and possibly also the pyrolysis of vegetation, may have been crucial processes in the salt balance of closed basins during the intense volcanic activity of the Tertiary.