High temperature metamorphism in the conductive boundary layer adjacent to a rhyolite intrusion in the Krafla geothermal system, Iceland

▶ We studied drill cuttings of granoblastic hornfelses recovered adjacent to the intrusive contact in IDDP-1. ▶ Two pyroxene geothermometry records temperatures in the range of 800–950°C. ▶ The heat flow across the conductive boundary layer is calculated to be a minimum of 23Wm−2. A rhyolite magma body within the Krafla geothermal system that was encountered at a depth of 2.1km during drilling of the IDDP-1 borehole is producing high temperature metamorphism within a conductive boundary layer (CBL) in adjacent host rocks. Cuttings recovered during drilling within a few meters of the intrusive contact in IDDP-1 are mainly comprised of granoblastic hornfelses, the rock type which confirms the presence of the CBL at the base of the IDDP-1 bore hole. The two pyroxenes in these hornfelses record temperatures that are in the range of 800–950°C. The minimum heat flow across the CBL is 23Wm−2. Country rocks at distances beyond 30m of the intrusive contact are essentially unaltered, implying that they have been emplaced very recently and/or as yet unaffected by hydrothermal fluid flow.