Chemical composition of surface- and groundwater in fast-weathering silicate rocks in the Seiland Igneous Province, North Norway

The geochemical composition of nepheline-syenite, carbonatite, ultramafic rocks and gabbro samples from the Seiland Igneous Province is presented in terms of whole-rock chemistry, major and selected trace elements, and mineralogy. Additionally, the hydrochemistry of around 100 surface water, groundwater, rainfall and seawater samples from the area is presented. Surface water and groundwater solute chemistry can be explained in terms of reaction of precipitation with bedrock minerals: the waters thus acquire a clear chemical signature dependent on the catchment’s lithology. The
mineralisation of the waters (total dissolved solids or TDS) is generally low, although it is somewhat elevated in carbonatite- or nepheline-syenitedominated terrains, in comparison with ultramafic, gabbro or granitic terrains. Additionally, each lithology exhibits its specific water type. Thus, waters in a nepheline-syenite-dominated terrain tend to be Ca–Na–HCO3 rich with relatively high pH, whereas waters in carbonatite terrains are Ca–HCO3 dominated with only slightly elevated pH. Waters in ultramafic rocks exhibit strongly elevated Mg concentrations with circumneutral pH, whereas waters from gabbros are variable in terms of their major cation (Mg, Ca, Na) composition. Mineral stability diagrams aid our understanding of the evolution of rainwater by chemical interaction with rocks in each lithology. In gabbro and ultramafic terrains, such diagrams suggest that kaolinite is likely to be the primary alteration product and that saturation with respect to calcite will not be reached; instead, the waters will evolve until saturation with respect to stilbite (a Na–Ca zeolite mineral) is attained. Waters in ultramafic rocks will then follow this stable phase boundary until saturation with respect to chlorite is achieved. The gabbro waters, on the other hand, could also follow the stable phase-boundary to stilbite until saturation with respect to paragonite (a sodium mica) is reached, depending on the plagioclase-to-clinopyroxene ratio (Plg/Cpx). Surprisingly, it is found that olivine
(i.e., forsterite) does not control the water chemistry in gabbros and ultramafic rocks..

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