Mantle-derived carbon in Hercynian granites. Stable isotopes signatures and C/He associations in the thermomineral waters, N-Portugal

作者：Carreira Paula M^*; Marques Jose M; Carvalho M Rosario; Capasso Giorgio; Grassa Fausto

来源：Journal of Volcanology and Geothermal Research, 2010, 189(1-2): 49-56.

DOI：10.1016/j.jvolgeores.2009.10.008

• CO(2)-rich thermomineral waters
• mantle volatiles
• isotopes
• Chaves geothermal system
• N-Portugal

Na-HCO(3)-CO(2)-rich thermomineral waters issue in the N of Portugal, within the Galicia-Tras-os-Montes region, linked to a major NNE-trending fault, the so-called Penacova-Regua-Verin megalineament. Along this tectonic structure different occurrences of CO(2)-rich thermomineral waters are found: Chaves hot waters (67 degrees C) and also several cold (16.1 degrees C) CO(2)-rich waters. The delta(2)H and delta(18)O values of the thermomineral waters are similar to those of the local meteoric waters. The chemical composition of both hot and cold mineral waters suggests that water-rock reactions are mainly controlled by the amount of dissolved CO(2) (g) rather than by the water temperature. Stable carbon isotope data indicate an external CO(2) inorganic origin for the gas. delta(13)C(CO2) values ranging between -7.2 parts per thousand and -5.1 parts per thousand. are consistent with a two-component mixture between crustal and mantle-derived CO(2). Such an assumption is supported by the (3)He/(4)He ratios measured in the gas phase, are between 0.89 and 2.68 times the atmospheric ratio (Ra). These ratios which are higher than that those expected for a pure crustal origin (approximate to 0.02 Ra), indicating that 10 to 30% of the He has originated from the upper mantle. Release of deep-seated fluids having a mantle-derived component in a region without recent volcanic activity indicates that extensive neo-tectonic structures originating during the Alpine Orogeny are still active (i.e., the Chaves Depression).

• 出版日期2010-1-1