Geochemical and Structural Constraints on Magma Storage at Llaima Volcano, A Support for Shallow Dikes Reservoirs
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Llaima volcano (38.6°S , 71.6° W, 3179 m.a.s.l.) is a large compound stratovolcano (up to 400 km3 in volume) located in The Central South Volcanic Zone (CSVZ) in Chile. It is the most active volcano in the CSVZ zone, with more than 50 eruptions in the last 500 years (Petit-Breuilh, 2006). In this work, the hypothesis that the eruptive products from Llaima volcano are the result of fractional crystallization from a parental basaltic melt under isobaric cooling is tested. The whole-rock chemistry analyses from samples from the volcano have been compared to the results of phase equilibria fractionation calculations using MELTS from 1250°C to 900°C, under 500 bars along the QFM+2 oxygen fugacity buffer. The parental melt composition is associated with the most primitive chemical composition found among the samples from different units erupted at Llaima volcano, a basaltic melt (50.94 wt% SiO 2 , 6.28 wt% MgO), with 1 wt% H 2 O. The calculation provides a resulting volcanic suite that is similar to the actual calc-alkaline suite, and a coherent formation of mineral assemblage of plagioclase, clinopyroxene, olivine and spinel. This insight into Llaima magmatic system allows estimation of the spatial constraints on the plumbing system enabling an estimation of the evolution of the magma. Results estimated crystallization at a depth of 1600 to 2020 m under the summit crater, which indicate a very shallow reservoir since the crater is 2400 m high above the basement. Furthermore, structural setting analysis of a restricted perimeter around the edifice with a focus on the parasitic cones distribution and their morphologic characteristic contributes to refine the internal structure of the volcano and suggests a major SW-NE orientation trend for the emplacement of a dike-like reservoir.