Geochemical signatures of underground waters in a basaltic tropical environment. Experimental approach

Abstract

Abstract Introduction The Reunion Island is an oceanic basaltic island (2500 km2) located in the Indian Ocean under tropical climate (fig. 1). It comprises two shield volcanoes (i) the Piton des Neiges (3069 m omsl), active between 2.1 Ma and 12,000 y and now inactive; (ii) the Piton de la Fournaise (2631 m omsl), active still 1 Ma. This volcanic system presents the complete alkaline magmatic series from the most alkaline end-members to the most differentiate ones: oceanite, aphyric basalt, cotectic basalt, zeolitic basalt, plagioclase-rich basalt and mugearite (fig. 2). The humid climate, with a maximum annual rainfall around 12,000 mm on the eastern coast, favours an intense weathering of rocks and probably the formation of impermeable deep levels in the massif [Courteaud et al., 1997], originating a large number of springs in altitude. The chemical composition of these spring waters indicates a variable chemical signature as a function of the lithology of the concerned drainage basins [Join et al., 1997; Hoareau, 2001] : the Mg/Ca ratio in natural waters is respectively 0.3 and 4.3 for plagioclase-rich basalt aquifer and oceanite aquifer (tab. I). This study is therefore devoted to an experimental approach of the origin of these geochemical signatures. Method Experimental weathering was conducted in double, in closed systems (batch) during 200 hours per experiment, at 20oC, using pure water (750 ml). The reacting rock samples had a given parallelepipedic shape (12 mm x 3 mm x 57 mm) and represented the six types of basalts mentioned above for the complete magmatic differentiated series (fig. 2). CO2 gas was bubbled through each reactor in order to simply reproduce (i) natural conditions in soils or (ii) possible deep CO2 contaminations [Nicolini et al., 1991], without studying in detail the effect of a variable CO2 partial pressure. The aqueous samples were collected during each experiment when a significant variation of electrical conductivity was detected. Results The analytical results clearly show that each type of basaltic rock produces in the aqueous reacting fluid a specific geochemical signature (fig. 4b). This signature reflects first the mineralogical composition of the weathered rock and secondly the distribution of the chemical elements in the different mineral phases of the rocks (particularly between crystalline or vitreous phases). The aqueous solutions produced by experimental weathering reach the domain of composition of natural waters, having altered similar rocks, after about thirty five hours when they reach a quasi steady state of concentration for major cations and anions (fig. 3), while the silica concentration still increases (fig. 5). Discussion The intensity of weathering, illustrated first by the electrical conductivity of natural waters (fig. 3), decreases significantly in the following order of reacting basalts: mugearite (112 μS.cm−1), zeolitic basalt (90 μS.cm−1), cotectic basalt (40 μS.cm−1), and the three remaining basalts (30 μS.cm−1). This gives an idea of the total salts (TDS) dissolved by experimental weathering. The potential weathering sequences have been confirmed by a geochemical modelling of the basalt-water interaction [Hoareau, 2001] using a computer code based on thermodynamic options [the KINDIS code, Madé et al., 1994] without using here kinetic laws not enough documented for basaltic rock-forming minerals. These results allowed to suggest what are the possible geological environments for the underground major water circulation pathways, deduced from the characteristics of natural solutions in springs on the volcano. Such an approach can also be used as an indirect prospecting approach of the geology of aquifers in complex geological environments, sometimes very difficult to approach in the field of the Reunion Island, because of their depth. Conclusion The experimental weathering of basaltic rocks representative from the basaltic underground environments in the Reunion Island has shown interesting results: – the hydrolysis seems to be more important in differentiated basalts. As an example, mugearite alteration produced the solutions with the highest total dissolved salts (TDS); – the transfer of sodium and potassium from basalts to solutions is highest when these elements are located in the crystals (fig. 6) more than in the glass fraction. In this way the percentage of sodium in plagioclasic rich basalt (crystal) reaches 16.43% and only 10.86% in the lixiviating solution. In the case of mugearite (glass fraction) the percentages are respectively 42.88% and 5.21%; – the extraction of calcium and magnesium from the basalts is the highest when these elements are located in the vitreous part (mesostase) of the rock, as in mugearite and oceanite (fig. 6). The results underline the fact that the ability of a rock to liberate some elements is strongly related to its petrology. This study also allows to precise more in detail the geochemical signature of the solutions produced by weathering of the basalt as a function of the basalt-forming mineral phases for a given type of basalt : the differentiation of rocks and the proportion of phenocrysts, microlites and glass play obviously an important role in the control of the chemical composition of weathering solutions. Basalt dissolution is clearly incongruent and dependent on the distribution of chemical elements in the different phases of the rock: (glass, mesostase, crystals) as shown by Crovisier et al. [1990] and Legal [1999]. The comparison between computed chemical compositions of weathering solutions (fig. 4b) and compositions of natural spring waters (fig. 4d) is very encouraging and validate the geochemical model for the interpretation of these geochemical signatures, even without using kinetic laws in a first approach. However some discrepancies still exist, probably due to the fact that the reacting rocks in the experiments represent only one type of basalt, while the natural solutions are generally reacting with different types of basalts in complex hydrogeological systems in large drainage basins. The study of these mixing phenomena is now an interesting perspective for future studies. This study may also be completed using geochemical computer codes combining thermodynamic and kinetic approaches of solutions-basalts interactions in coupled mass-transfer models. This is now under development.