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

摘要

The Ecuadorian Quaternary volcanic arc is characterized by about 60 volcanoes many of which are active ornpotentially active. This volcanic activity is the result of the subduction processes of the Nazca Plate beneathnthe north-western part of South America. The geochemical signature of the discharged fluids from these vol-ncanic systems gives an important contribution to the comprehension of the subduction processes in thenSouth-American region. In this work, we present the first systematic geochemical characterization of dis-ncharged fluids from the entire Ecuadorian volcanic arc on the basis of the chemical and isotopic compositionnof 56 samples of thermal and cold waters, as well as 32 dissolved and 27 bubbling gases collected fromnnorth to south across the arc. The isotopic composition of waters reveals a mainly meteoric origin, while thenchemistry of the dissolved gases is characterized by He and CO2 contents, 2–3 orders of magnitude highernthan the air saturated water values, which implies very active gas–water interaction processes with deep flu-nids. Moreover, both dissolved and bubbling gases’ isotopic signature shows a wide compositional range, withnhelium ranging between 0.1 and 7.12 R ⁄ Ra and carbon ranging from )1.75 to )10.50& d13nC(TDIC). Such iso-ntopic features may be related to the presence of at least two distinct end-member sources: the mantle andnthe crust. Finally, this geochemical study clearly reveals the two distinct geographic parts of the arc, showingndifferent isotopic characteristics of fluids for the Quaternary active volcanism, (north of 2u0002S), and for the inac-ntive arc, (south of 2u0002S).