Iceland Deep Drilling Project Explores the Magma-Hydrothermal Interface

The primary goal of the Iceland Deep Drilling Project (IDDP) is to develop new sources of power from supercritical hydrothermal fluids. Two of three planned deep drill holes have been completed and both recovered material that records partial melting of hydrothermally altered basaltic rocks to form silicic melt. The IDDP-1 hole was drilled in the Krafla geothermal field on the North Iceland Rift. The IDDP-1 well intersected a high silica (76.5 Wt. %), low δ¹⁸O (3.1‰) rhyolite melt at a depth of 2104m. The melt is nearly aphyric with sparse phenocrysts of plagioclase, augite, pigeonite, and titanomagnetite, interpreted to have formed by partial melting of hydrothermally altered basalt at depth. It was emplaced into a crystalline felsite intrusion of similar composition at a temperature near 900° C. Quenched glass in cuttings of the felsic host rock includes rhyolite formed by in situ partial melting with a composition near granite minimum melt. Some glass-rich fragments preserve partial assimilation of host rock forming mixed rhyolite melts.

The IDDP-2 hole was drilled to a depth of 4650m in the Reykjanes Geothermal field where the Mid-Atlantic Ridges comes ashore. Core recovered from >4500m records high temperature alteration of a sheeted dike complex. The igneous minerals are completely replaced by hydrothermal calcic plagioclase, hornblende, clinopyroxene, orthopyroxene, ilmenite, magnetite, pyrrhotite, ± biotite and olivine. Hydrothermal orthopyroxene and olivine do not show hydrous alteration suggesting the in situ temperature is above 550°C. Co-existing hydrothermal clinopyroxene-orthopyroxene pairs record temperatures of formation of 1090° to 701°C, and have locally developed granoblastic textures. A quartz-filled vug in a sample with granoblastic pyroxene is lined with eutectic intergrowths of quartz-plagioclase that show dendritic growth into the vug that we interpret as an incipient melting of hydrated basalt. Titanium in quartz from eutectic intergrowths gives temperatures from 745° to 800°C. Hydrothermal fluid at Reykjanes is modified seawater and therefore unlikely to produce low δ¹⁸O rhyolite commonly observed in Iceland, but the cores record the transition from magmatic to hydrothermal conditions, including partial melting due to hydration of hot hydrothermally altered basalt.