Active Basalt Alteration at Supercritical Conditions in IDDP-2 Drill Core, Reykjanes, Iceland

Abstract

The IDDP-2 drill hole is located in a seawater-recharged, active hydrothermal system on the landward extension of the Reykjanes Ridge. Spot cores recovered from 3648 to 4659 meters depth consist of pervasively altered sheeted dikes. The shallowest cores have an alteration mineral assemblage similar to the producing geothermal reservoir with calcic plagioclase, hornblende, actinolite, epidote, chlorite and quartz. Samples from depths below 3865 m are dominated by intermediate plagioclase and hornblende and do not contain epidote or chlorite. The deepest cored section (4634-4659 m) generally retains a diabasic texture, but all igneous minerals are replaced or have changed composition. Plagioclase retains its igneous texture but now has composition ranging from An30 to An99. Igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Recrystallized magnetite and ilmenite are ubiquitous, along with pyrrhotite. Hydrothermal biotite and olivine (Fo58-64) occur in some samples. Multiple independent mineral geothermometers indicate that peak hydrothermal alteration occurred at ~800°C. Geothermometry on the paragenetically latest alteration suggests that in situ temperatures at the bottom of the hole are approximately 600°C. Co-existence of vapor-rich and salt + vapor-rich fluid inclusions suggest that in situ conditions are in the brine liquid - vapor field, consistent with the presence of brine in pores indicated by precipitation of halite + K-Fe-Cl salts on the surface of some cores as they dried.