Hydrogeochemistry and stable isotope geochemistry of geothermal fluids discharged from volcanoes in São Miguel (Azores, Portugal)

Abstract:

Hydrochemical and isotopic composition of hydrothermal fluids discharged from 3 different volcanic areas of São Miguel has been analysed to create a conceptual model for the evolution of a geothermal system in a basaltic ocean island with still pronounced volcanic activities. Besides the fluids, associated solid-phase precipitates and local volcanic rocks were collected for geochemical investigations. Thermal discharges were divided into springs (up to 75 °C) and boiling pools. Major classification based on dominant dissolved ions divided fluids in Na-SO₄, Na-HCO₃ and Na-Cl types. Temperatures estimated from water hydrochemistry (e.g. Na-K, Na-K-Ca cationic geothermometers) let us assume that the in-situ solutions with discharging temperatures exceeding 32°C were close to thermodynamic equilibrium with respect to local rocks. The aqueous solutions have their origin in meteoric water with a δ¹⁸O(H₂O) value of -3 ±1 ‰ (V-SMOW) and a δD(H₂O) value of -13 ±7 ‰ (V-SMOW). Only the δ¹⁸O and δD values of Ferraria reveal mixing of meteoric with sea water. Dissolved inorganic carbon (DIC) is chemically evolved by the uptake of volcanic carbon with δ¹³C(DIC) = -5 ±3 ‰ (V-PDB). The isotopic composition of sulphate with δ³⁴S(SO₄) = 0.4 ± 0.3‰ (V-CDT) is close to typical igneous rock values. The extent of basaltic rock leaching depends on the availability of volcanic CO₂ and on temperature. The molar Mg/Ca ratio (0.8) of all thermal discharges reflects leaching of the local basalt. In Furnas, at higher temperatures, the concentrations of dissolved Si(OH)₄, K⁺ and Sr²⁺ increased due to intensive leaching of basaltic rocks, whereas in contrast Al³⁺ and ΣFe decreased due to the precipitation of e.g. alunite. Annual supply of dissolved CO₂ in Furnas springs was calculated to be about 61 t. In Fogo the Na-SO₄ type dominated and at higher altitude in particular CO₂ rich (up to 23 mmol L^-1) water at T ≈ 20 °C occurs. It is further suggested that the isotope signature and hydrochemistry of the fluids can be used to estimate the distance to the magma sources and as a general indicator of volcanic activity.