Geological and geothermal data from the recently drilled 2054 m deep well, RN-10, in the Reykjanes high-temperature field are discussed. The strata range from probable pillow basalt formations at the deepest level to shallower tuffaceous volcanic succession intercalated with reworked shallow marine fossil-rich sediments, and lastly pillow basalt and subaerial lavas. The formations are relatively high-porosity and low permeability, and the aquifers encountered in the well are largely related to fractures along sub-vertical dyke intrusions. The largest aquifer near the bottom is related to a sub-vertical fracture. The hydrothermal alteration shows that the well enters into the high-temperature system below about 500 m depth, where it shows a progressive alteration zonation ranging from smectite-zeolite > chlorite > chlorite-epidote > epidote-actinolite zone. The sequence of mineral deposition in rock cavities indicates that the geothermal system has from its initial stage been progressively heating up. The highest bottom temperature logged is about 320?C. Th-measurements in fluid inclusions show a good correlation with alteration and measured formation temperatures, while Tm-measurements show a wide salinity range, irrespective of depth, from fresh to seawater compositions, the latter being near to the present salinity of the field. Evidence suggests that well RN-10 is sited further away from an upflow zone than RN-9. However, at >1000 m depth, temperatures in well RN-10 are up to 20?C higher than found elsewhere in the reservoir, reaching a maximum of about 320?C.

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