Gas geochemistry of hydrothermal fluids of the S. Miguel and Terceira Islands, Azores

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• 作者：Stefano Caliro^a ; ^{stefano.caliro@ingv.it" class="auth_mail" title="E-mail the corresponding author} ; Fá ; tima Viveiros^b ; Giovanni Chiodini^c ; Teresa Ferreira^b
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2015
• 出版时间：1 November 2015
• 年：2015
• 卷：168
• 期：Complete
• 页码：43-57
• 全文大小：2374 K

文摘

We present here for the first time a complete dataset of the chemical and isotopic compositions of fumarolic fluids collected on the S. Miguel (Fogo and Furnas volcanoes) and Terceira (Pico Alto volcano) Islands. The data are analyzed and discussed, to provide both a comprehensive picture of the thermodynamic conditions of the hydrothermal systems on these two islands, and to give new insight into the origins of these fluids, for a better understanding of the geodynamic context of the Azores archipelago. For S. Miguel Island, the gas equilibria in the H₂–CO₂–CO–CH₄–H₂O system suggest temperatures of the hydrothermal system reservoirs from 190 °C to 280 °C for the Fogo volcano fumaroles, which are 30–35 °C higher than those measured for geothermal wells. The equilibrium temperatures estimated for the Furnas volcano hydrothermal system are from 200 °C to 275 °C. Equilibrium vapor at a temperature of ∼190 °C is inferred for the fumarolic fluids discharged on Terceira Island.

The He isotopic composition of ∼9.6 R/Ra measured in the fumaroles of Terceira suggests that a plume-like source, presumably from the lower mantle, feeds this hydrothermal system. The relatively low ³He/⁴He ratios (from 5.21 to 5.35 R/Ra) and higher CO₂/³He ratios of S. Miguel fluids suggest an addition of ∼45% of radiogenic He and ∼30% of crustal CO₂ to a plume-like composition as at Terceira.

A mainly meteoric origin is inferred for the fumarolic water, whereas the unreactive gas species (He, Ar, N₂) arise from mixing processes between an atmospheric-like component and a magmatic component. On S. Miguel Island, the estimated magmatic fluid composition in terms of the ratios of N₂/⁴⁰Ar (62 ± 6) suggests a plume-like mantle source. Deep derived N₂ isotope compositions characterized by very negative 未¹⁵N values (未¹⁵N 猢?#xA0;−14‰) are inferred for the fluids of the Terceira Island. Also S. Miguel fluids are compatible with the same source with a possible involvement of crustal-derived Nitrogen. These values are more negative than the typical compositions of both the upper mantle and plume-like mantle, which thus supports the possible presence of a ¹⁵N-depleted source, presumably from the lower mantle, in the region.