Boron, Sr, O, and H isotope geochemistry of groundwaters from Mt. Etna (Sicily)—hydrologic implications
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文摘
Combined B, O, H, and Sr isotopic studies of groundwaters from Mt. Etna provide new constraints on their origin and the consequences of fluid-rock interaction within this hydrologic system. Variations in δ18O (−9.7 to −7.2‰) and δD (−62 to −23‰) mainly lie along the regional meteoric water line and suggest that most waters originated as local precipitation. However, small shifts in δ18O, and variable 87Sr/86Sr (0.70355 to 0.70879) and δ11B (−5.2 to +25.8‰) indicate that subsequent interactions occurred between the fluids and local rocks. High B/Cl ratios in all samples seemingly preclude direct involvement of seawater in the hydrologic system despite the proximity to the coast and, in some samples, elevated 87Sr/86Sr and δ11B. Two general ‘end-member’ water types are recognized on the basis of their chemistry. These apparently are produced by interactions of local meteoric waters with the dominant reservoir rocks—either basaltic lavas of Etna or the underlying sediments, respectively; high δ11B in the sediment-hosted end-member points to a significant marine carbonate contribution. Subsequent mixing between these or similar end-members produced a range of intermediate composition groundwaters. Certain anomalous water compositions require the presence locally of a distinct component with high B and moderate δ11B (ca. +10‰) but relatively low 87Sr/86Sr; an anthropogenic source for this component is plausible. One unusual sample has B and Sr isotopic compositions similar to the other volcanic rock-hosted waters, but anomalously high Cl content that likely reflects local magmatic outgassing near this sampling locality. In general, this study indicates that groundwater B and Sr isotopic compositions are rock-dominated; these data provide useful constraints on the origin and evolution of groundwaters.