Silicic Magma, 238U-series Disequilibria and Krafla Geothermal System
Olgeir SIGMARSSON, Abdelmouhcine GANNOUN and Kristján JÓNASSON
The discovery of silicic magma at approximately 2 km depth during the IDDP-1 drilling revealed the presence of crustal melts in Krafla geothermal system, N-Iceland. Crustal melts formed at depth and emplaced at shallower level where they crystallize as felsic intrusions could be an important heat source for the geothermal system. However, the extent of such crustal melt is unknown, as is the age of the IDDP-1 rhyolite and its connections to other silicic magmas produced at Krafla central volcano. Lower 18O/16O in silicic rocks compared to contemporaneous basalts suggests partial melting of hydrothermally altered basalts to be at the origin of silicic melts in the rift-zones of Iceland. The silicic formations erupted at Krafla all plot close to the low-pressure minimum point in the Qz-Or-Ab petrogenetic residual system, suggesting melt transfer from the crustal source towards the surface without noticeable mixing or mingling with basaltic magma. The 238U-decay series radioactive disequilibria in the IDDP obsidian were measured and compared with silicic pumice from the 1724 CE Víti eruption from the Mývatn Fires and pre-Holocene rhyolite ridge of Hrafntinnuhryggur as well as those from the silicic lava domes of Hlíðarfjall, Jörundur and Hágöng. Both (230Th/238U) and (230Th/232Th) are indistinguishable in the IDDP obsidian and the 1724 Víti pumice, whereas other silicic formations have different ratios. The IDDP-1 drill hole, which was located close to the Víti explosive crater, thus sampled crustal melt that ran out of gas and can be considered as left-overs from the Mývatn Fires. The IDDP-1 rhyolite was also analysed for 230Th-226Ra and 226Ra-210Pb disequilibria. Partial crustal melting can only fractionate these highly incompatible elements if the degree of melting is small. The 5% Ra excess over Th is inconsistent with large melting of felsite veins generating the IDDP-1 rhyolite. The timing of the crustal melting is constrained to be younger than 8000 yrs from (230Th) less than (226Ra) and older than 100 yrs from (210Pb) = (226Ra) consistent with IDDP-1 rhyolite being residual melt from 1724 CE.
|        Topic: Geology||Paper Number: 12023|