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The use of The use of δδ3737Cl Cl to explain origin and production of salt from the saline spring “Fonte da Pipa” in
Rio Maior (Central Portugal)
DEVELOPMENT OF A CONCEPTUAL MODELDEVELOPMENT OF A CONCEPTUAL MODELVery close to the saline spring several fresh water Very close to the saline spring several fresh water wells are found. Cl isotopes of these fresh waters wells are found. Cl isotopes of these fresh waters are compared to the Cl isotope composition of the are compared to the Cl isotope composition of the saline spring. The wells to the west of a NE-SW saline spring. The wells to the west of a NE-SW running fault have, just like the saline spring, running fault have, just like the saline spring, δδ3737ClCl values which are positive relative to SMOC, while values which are positive relative to SMOC, while the well the well to the east have a negativeto the east have a negative δ37Cl.This information, combined with knowledge on the topography and the geology allowed us to develop a preliminary conceptual model of this system.
HGM Eggenkamp1, JM Marques1, H Graça2
1Centro de Petrologia e Geoquímica, Instituto Superior Técnico, Lisboa, Portugal; contact: [email protected]
2Centro Hospitalar das Caldas da Rainha, Caldas da Rainha, Portugal
INTRODUCTIONThe saline spring “Fonte da Pipa” (or “Fonte da Bica”, as there is some dispute on the name) is the only remaining actively exploited terrestrial salina in Portugal. The spring contains some 137 gram per liter dissolved solids of which 97% is NaCl. The salt is extracted from this brine in a two-stage process and commercialized by the “Cooperativa Agrícola dos Produtores de Sal de Rio Maior”.
Despite its importance to the local economy, both as a natural resource and a tourist attraction its geochemistry has never been studied beyond one preliminary water analysis in 1936.In this study we have used stable Cl isotopes to develop a model for the origin of the saline spring. As this spring contains nearly pure NaCl it also is a perfect location to study progressive Cl isotope fractionation during evaporation processes.
DEVELOPMENT OF δ37Cl DURING EVAPORATION OF THE BRINE
Isotope fractionation of the chloride ion during salt precipitation is dependent on the coprecipitating cation. While a NaCl precipitate has a δ37Cl which is 0.26‰ more positive than the brine, KCl and MgCl2.6H2O have
slightly more negative δ37Cl (-0.09‰ and -0.06‰ respectively). In the case of seawater, which contains considerable amounts of K and Mg which will precipitate as kainite, carnallite and bischofite, a minimum δ37Cl of about -0.5‰ can be expected (assuming an original seawater brine with a δ37Cl of 0‰ vs. SMOC). In the “Fonta da Pipa” which contains only very small amounts of K and Mg, a (very) low δ37Cl is expected.
VERY NEGATIVE δ37Cl VALUES?δ37Cl values in progressively more evaporated salt pans show indeed more negative values. It is also nicely shown that the δ37Cl of the precipitate is more positive that the co-existing brine.
This research was funded by a grant from the Portuguese Ministry of Health, Centro Hospitalar das Caldas da Rainha, Portugal, under the Research Contract HIDROCALDAS No. 1577 and the Portuguese Foundation for Science and Technology (FCT). We thank the “Cooperativa Agrícola dos Produtores de Sal de Rio Maior” for giving us the opportunity and much support to sample on their property. We thank the Institut de Physique du Globe de Paris for the opportunity to measure the preliminary Cl isotopes of the evaporation experiment and to print this poster.
1
2
3
4
Jurassic
Neogene
Cretaceous
Palaeogene
+0.09‰-0.22‰-0.34‰
-0.25‰
-0.21‰
?
β
P4
?
NaCl
Marl
Karstic Limestone
Miocene Sand and LimestoneJurassic
PlioceneLignite and Diatomite
Salinespring
P1, P2, P3
? ? ?
?
EVAPORATION EXPERIMENTTo determine if it is possible to obtain even lower δ37Cl values under laboratory conditions we are currently applying an evaporation experiment with one of the more advanced samples taken (the sample with evaporation order 8). Preliminary results indicate that indeed lower data can be obtained. This experiment will be continued.
1 2 3 4 5 6 7 8 9 10-1.0
-0.8
-0.6
-0.4
-0.2
Evaporation experiment
δ37C
l