Geochemical and isotope characterization of geothermal spring waters in Sri Lanka: Evidence for steeper than expected geothermal gradients

摘要

Seven geothermal springs from the Precambrian high-grade metamorphic terrain of Sri Lanka were investigated to assess their formation processes and to determine reservoir temperatures based on their chemical compositions. Silica-based geothermometric calculations for the Marangala and Nelumwewa springs showed the highest average reservoir temperatures of 122°C and 121°C, respectively. Samples of low temperature (<35°C) groundwater from nearby springs, piezometers and open wells were also collected for comparison. All samples were analyzed for their major and trace element compositions as well as stable isotope ratios ~2H/~1H and ~(18)O/~(16)O (expressed as δ~2H_(H2O) and δ~(18)O_(H2O)). Discharge temperatures of the thermal waters varied from 39-62°C. These waters showed low concentrations of selected trace elements (Fe<0.09; Mn<0.04; Cu<0.01; Cr<0.01; As<0.025mg/L)) and were also comparable to that of non-geothermal groundwaters. Stable isotope compositions of geothermal waters ranged from -6.5 to -5.0‰ for δ~(18)O_(H2O) and between -39 ‰ to -28 ‰ for δ~2H_(H2O). In the non-geothermal waters, the isotope values were almost identical within the analytical uncertainties of 0.1‰ and 1‰ for δ~2H_(H2O) and δ~(18)O_(H2O), respectively. In addition, all isotope ratios of geothermal and non-geothermal water samples scattered around the local meteoric water lines for the dry and intermediate climatic regions of Sri Lanka, thus indicating origin from precipitation without further influences of evaporation or water rock interaction. This similarity to the local meteoric water lines also makes influences of seawater an unlikely factor. Close matches of geochemical and isotope data from geothermal and corresponding non-geothermal waters confirm the hypothesis of a common source. The proposed model for Sri Lanka subsurface waters is that rainfall from the dry and/or intermediate climatic zones percolates with little time delay downward through structurally weaker zones in high-grade metamorphic rocks. They are subsequently heated by steep or heterogeneous geothermal gradients that are most likely associated with the Highland-Vijayan thrust zone.