Chemical and isotopic signatures of Na/HCO 3 /CO 2 ‐rich geofluids, North Portugal
2006; Hindawi Publishing Corporation; Volume: 6; Issue: 4 Linguagem: Inglês
10.1111/j.1468-8123.2006.00144.x
ISSN1468-8123
AutoresJosé M. Marques, Milca Rodrigues Vieira de ANDRADE, Paulà M. Carreira, Hans Eggenkamp, R. C. Graça, Luís A. Aires-Barros, M. ANTUNES DA SILVA,
Tópico(s)earthquake and tectonic studies
ResumoAbstract Geochemical and isotopic studies have been undertaken to assess the origin of CO 2 ‐rich waters issuing in the northern part of Portugal. These solutions are hot (76°C) to cold (17°C) Na–HCO 3 mineral waters. The δ 2 H and δ 18 O signatures of the mineral waters reflect the influence of altitude on meteoric recharge. The lack of an 18 O‐shift indicates there has been no high temperature water–rock interaction at depth, corroborating the results of several chemical geothermometers (reservoir temperature of about 120°C). The low 14 C activity (up to 9.9 pmC) measured in some of the cold CO 2 ‐rich mineral waters (total dissolved inorganic carbon) is incompatible with the presence of 3 H (from 1.7 to 4.1 TU) in those waters, which indicates relatively short subsurface circulation times. The δ 13 C values of CO 2 gas and dissolved inorganic carbon range between −6‰ and −1‰ versus Vienna‐Peedee Belemnite, indicating that the total carbon in the recharge waters is being diluted by larger quantities of CO 2 ( 14 C‐free) introduced from deep‐seated (upper mantle) sources, masking the 14 C‐dating values. The differences in the 87 Sr/ 86 Sr ratios of the studied thermal and mineral waters seem to be caused by water–rock interaction with different granitic rocks. Chlorine isotope signatures (−0.4‰ < δ 37 Cl < +0.4‰ versus standard mean ocean chloride) indicate that Cl in these waters could be derived from mixing of a small amount of igneous Cl from leaching of granitic rocks.
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