Iceland Deep Drilling Project (IDDP): (3) Oxygen and Hydrogen Stable Isotope Composition of a Rhyolite Magma, Krafla Geothermal System

The Krafla geothermal system, located within the Krafla caldera in the active rift zone of NE Iceland, was host to the first Iceland Deep Drilling Project drillhole (IDDP-1), which drilled into rhyolite magma at a depth of 2104m in June 2009. This was sampled at the surface as quenched glass shards in the drill cuttings. Separates of cuttings from the glass-rich samples were analyzed for hydrogen and oxygen stable isotopes, and are compared to isotope data for the hydrothermal alteration mineral epidote, separated from the bulk rock cuttings of several geothermal wells in the Krafla region. Twelve hydrogen isotope analyses of the IDDP-1 glass range from -124 to -117‰, with an average value of -121 ± 2‰ (1σ deviation). Oxygen isotope values from four analyses range from 3.1 to 3.2‰. The water content of these glasses range from 1.9 to 2.0%. The hydrogen isotope composition of the IDDP-1 glass is similar to measured δD values for hydrothermal epidote from the Krafla system. The δD of epidote in wells K-17, -20, -25, -26, -32, -34 and -39 are between -127 and -108‰. However, δ18O values of Krafla epidotes are significantly lower than the IDDP-1 glass, with values between -13.0 and -9.7‰ from wells K-20, -26 and -34. Rather, the δ18O composition of the IDDP-1 glass is comparable to unaltered surface basalts at Krafla, and whole rock samples from the IRDP core in Reydarfjördur, Iceland. Our analyses indicate that the shallow intrusive melts in Krafla that were tapped by drilling the IDDP-1 appear to be the product of partial melting of hydrothermally altered basalts within this meteoric water dominated geothermal system. The δD values preserved in the recovered glasses are entirely a relic of the hydrous alteration minerals of the geothermal system, whereas the oxygen isotope signal of the glasses is dominated by the basalt host-rock composition. Similar processes may occur along mid-ocean ridges, where partial melting of shallow, hydrothermally altered material is assimilated into rising melts.