Strontium isotope shift in clay minerals, epidote and geothermal fluid in the Hellisheiði Geothermal Field, SW-Iceland

• Main Author: Gísli Örn Bragason 1984-
• Other Authors: Háskóli Íslands
• Format: Thesis
• Language: English
• Published: 2012
• Subjects: Jarðfræði; Háhitasvæði; Ummyndunarsteindir; Jarðhitavatn; Iceland
• Online Access: http://hdl.handle.net/1946/11774

The large difference in the isotopic ratio 87Sr/86Sr between sea-water-derived meteoric water (0.70916) and that of the original reservoir rocks (0.70314) allows the detection of small amounts of Sr derived from meteoric water in alteration minerals formed. The broad picture confirmed here is that meteoric Sr is largely removed in the upper clay and zeolite zones but enough survives into the epidote-dominated zones. This causes a small but significant shift in the Sr-isotopic ratio in epidote downhole to the bottom of the geothermal wells. A few samples were found to have significantly higher 87Sr/86Sr than the surrounding samples and these samples coincide with the main aquifers feeding the geothermal wells. This suggests that the aquifer water flows fast enough through the system for equilibrium not to be attained with the bulk of the reservoir rocks. Epidote is an abundant alteration mineral in high-temperature geothermal fields and one of the main index minerals used for the definition of temperature-dependent alteration zoning. In this study epidote from two drillholes, HE-50 and HE-51 in the Hellisheiði Geothermal Field was analyzed in detail by microprobe. As previously observed the epidote shows ubiquitous zoning of individual crystals with Fe-richer cores and Al-richer rims. Individual crystals at different depths span the entire compositional range observed throughout the geothermal wells. The interpretation of this zoning is that the early Fe-rich epidote forms in the chlorite-epidote alteration zone in association with wairakite-prehnite-andradite. The growth of the Fe-poorer outer zone forms in the mineral assemblage hematite-prehnite-epidote. Epidote is a significant host for Sr within the deeper levels of the thermal fields while clay minerals and zeolites are the main hosts in the upper levels. Eight samples of geothermal fluid from different sub-regions of the Hellisheiði geothermal fields were analyzed for 87Sr/86Sr and showed significant variation. Five samples showed ratios similar to the ...