||Helga Margret HELGADOTTIR, Hjalti FRANZSON, Niels OSKARSSON, Karl GRONVOLD, Sigurdur STEINTHORSSON
||Hellisheidi, geothermal, alteration, olivine, pyroxene, Iceland
||World Geothermal Congress
||Extensive drilling at the Hellisheiði geothermal field within the Hengill volcanic complex in SW-Iceland has yielded vast amount of data on hydrothermal alteration and alteration zones within an active rift-zone. The majority of rock formations in the area are of olivine-tholeiite composition and are mostly hyaloclastite formations formed subglacially. Basalt lava flows from interglacial periods and from the Holocene occupy the lowlands and are therefore less common in the volcanic centre. This study involved characterising the primary clinopyroxene in the Hellisheiði geothermal wells and their subsequent alteration by electron microprobe analyses. The intention was to compare the progressive alteration of buried lava flows, where temperatures have risen with increased burial depth, with the alteration of intrusive rocks that should record the primary alteration of the prevailing temperature conditions in their surroundings. The results suggest that the geothermal alteration of primary pyroxene in olivine tholeiite is dominated by dissolution-precipitation reactions. They also show that there is a significant difference in the composition of actinolite replacing pyroxene depending on the origin of the primary mineral within a lava flow or an intrusive rock. This is possibly attributed to the less oxidised intrusive rock but more significant, however, may be the different modes of alteration: in the lava flows the actinolite may form from the already altered pyroxene (via chlorite) whereas in the intrusive rocks it forms directly from the primary pyroxene.