The origin of magma encountered during drilling of geothermal well KJ-39, Krafla, Iceland and the relationship with the nearby IDDP-1 magma

• Main Author: Rule, Georgina
• Format: Other/Unknown Material
• Language: English
• Published: University of Canterbury 2020
• Subjects: Krafla; Iceland
• Online Access: https://hdl.handle.net/10092/101052; https://doi.org/10.26021/10115

In July 2008, well KJ-39 encountered magma at 2571 metres below the surface while drilling into the geothermal system within the Krafla central volcano, north-eastern Iceland. The magma was returned to the surface as quenched glass along with a resorbed mineral assemblage consisting of bytownite with some anorthite and augite with minor pigeonite and orthopyroxene lamellae. In June 2009, the better-known IDDP-1 well, located 2.5 km north of KJ-39, also intercepted rhyolitic melt and partially molten felsite rich in quartz, plagioclase and alkali feldspar. Because of the proximity of the two drill holes, I investigate whether the magma encountered in KJ-39 is from the same source as IDDP-1. This will have important implications for the dimensions of the magma chamber and thus future geothermal drilling operations. Major elements, S, Cl and F have been measured in KJ-39 glass chips using electron probe micro-analysis (EPMA) and H2O and CO2 have been measured using Fourier-transform infrared spectroscopy (FTIR). The KJ-39 chips have a striking range in composition, from andesitic to rhyolitic (SiO2 58-75 wt%), the H2O content ranges from 0.64-3.02 wt% with some samples showing evidence of partial or total hydration and CO2 measurements range from 5- 40 ppm, although most measurements were masked by atmospheric CO2. S contents range from below detection to 1373 ppm and show a strong negative correlation with SiO2, as does F, while Cl shows a positive correlation with SiO2. Based on the chemistry and petrology of the KJ-39 samples, there is evidence for formation of a high-SiO2 rhyolite by partial melting of hydrated basalt, while the andesitic and dacitic glass compositions presented here suggest partial melting of a more mafic source, an andesite, likely melted by the same heat source. When this mechanism for formation is compared with that for IDDP-1 both are to some degree formed by the partial melting of hydrated basalt. The wide range of glass compositions in the KJ-39 sample compared to IDDP-1 suggests that ...