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Geochemical and isotopic characterization of volcanic and geothermal fluids discharged from the Ecuadorian volcanic arc

Abstract:

The Ecuadorian Quaternary volcanic arc is characterized by about 60 volcanoes many of which are active or potentially active. This volcanic activity is the result of the subduction processes of the Nazca Plate beneath the north-western part of South America. The geochemical signature of the discharged fluids from these volcanic systems gives an important contribution to the comprehension of the subduction processes in the South-American region. In this work, we present the first systematic geochemical characterization of discharged fluids from the entire Ecuadorian volcanic arc on the basis of the chemical and isotopic composition of 56 samples of thermal and cold waters, as well as 32 dissolved and 27 bubbling gases collected from north to south across the arc. The isotopic composition of waters reveals a mainly meteoric origin, while the chemistry of the dissolved gases is characterized by He and CO2 contents, 2-3 orders of magnitude higher than the air saturated water values, which implies very active gas-water interaction processes with deep fluids. Moreover, both dissolved and bubbling gases' isotopic signature shows a wide compositional range, with helium ranging between 0.1 and 7.12 R/Ra and carbon ranging from -1.75 to -10.50‰δ13C(TDIC). Such isotopic features may be related to the presence of at least two distinct end-member sources: the mantle and the crust. Finally, this geochemical study clearly reveals the two distinct geographic parts of the arc, showing different isotopic characteristics of fluids for the Quaternary active volcanism, (north of 2°S), and for the inactive arc, (south of 2°S). © 2010 Blackwell Publishing Ltd.