A SIMS study of lithium, boron and chlorine in basalts from Reykjanes (southwestern Iceland)

Iceland is one of the few places on earth where it is possible to study on land the effects of hydrothermal alteration on basaltic rocks from oceanic crust. In this frame, we have carried out a micro-analytic study on determining the concentrations of some of the so-called hydrothermal markers, such as the light lithophile (Li, B) and halogen (Cl) elements. They were investigated by the ion microprobe on magmatic phases (plagioclase and clinopyroxene) in three drill-cutting samples, selected on the basis of different depths (400-3000m), in one unaltered lava from the well RN-17, and one core sample (dolerite dyke) from well RN-19, all of them located within the Reykjanes geothermal fields in SW Iceland. Additionally, B and Li on whole rock were then utilized to better constrain the hydrothermal processes. Whole rock Li and B data have shown that the samples investigated were affected mainly by seawater hydrothermal alteration. On the contrary, the discrepancy between the B concentrations in the whole rock and mineral phases (plagioclase and clinopyroxene) as well as that for Cl between single minerals could be ascribed to the occurrence of secondary phases (such as epidote and amphibole) related to hydrothermal alteration, along with the effects of fluid circulation in fracture zones.