Tracing the circulation of groundwater in volcanic systems using the 87Sr/86Sr ratio: Application to Mt. Etna

Abstract The 87Sr/86Sr ratio was investigated in groundwater circulating in the volcanic edifice of Mt. Etna in order to estimate the possible contribution of deep brines circulating in the sedimentary basement. Samples from 14 sites were collected and analyzed for their chemical composition and Sr-isotope ratios. While the most common approach of coupling 87Sr/86Sr ratios with the concentration of dissolved Sr is not effective in distinguishing between the deep brine and seawater contributions, we suggest that the Sr/Cl ratio is a useful complementary parameter that needs to be considered when attempting to clearly identify the Sr sources. The obtained data indicate that the Sr-isotope signature of groundwater is determined by the volcanics hosting the aquifer. The volcanic isotopic signature is modified by very small amounts of brines (< 1%), characterized by a high concentration of Sr and a 87Sr/86Sr ratio typical of sedimentary environments, but only at sites where the groundwater circulates almost in contact with the sedimentary basement. Conversely, the contribution of seawater is completely ruled out since this should produce a higher concentration of chloride. The proposed approach is potentially very effective for tracing the circulation of groundwater not only at Mt. Etna but also at volcanic edifices that overlie a bedrock with different 87Sr/86Sr ratios as well as at volcanic islands where freshwater overlies seawater.