A SIMS investigation of trace elements in well RN-17 at Reykjanes geothermal system (Reykjanes Peninsula, SW Iceland)

Secondary ion mass spectrometry (SIMS) has become an essential tool to investigate complex materials at small scale. The possibility to use an ion beam to measure the in-situ concentration of trace elements with high precision and accuracy, minimum consumption of material, limited sample preparation and virtually null contamination is a formidable weapon in the geologist's arsenals. Here we present preliminary results of an investigation of well RN-17 at the Reykjanes geothermal system (Reykjanes Peninsula, SW Iceland). We show that the adoption of micro spot SIMS in combination with bulk-rock analysis techniques (ICP-MS, ID, TIMS) carried out on 10 basaltic drill cuttings from over 3,000 m depth allowed us to obtain insights into the geochemical characteristics of the well. Whole rock and mineral phases (plagioclase, clinopyroxene, epidote and amphibole) indicate that the cuttings were affected by hydrothermal alteration, a heterogeneous magmatic source and magmatic differentiation. The alteration, due to a progressive decrease of seawater/rock interaction with depth has caused significant modification of the Sr isotope ratios and the distribution of K, Rb, Cs, Ba and B in the shallow and intermediate cuttings, while the occurrence of Na-rich plagioclase, epidote and amphibole down to the well root are evidence of persistent fluid/rock interaction at depth.