Iceland possesses numerous subglacial volcanoes (tuyas and tindars) that act as geological records of the thick ice sheets that once spanned across the country. Flat topped volcanic summits, which were able to penetrate through the covering ice, provide an estimate of the ice sheet's minimum thickness at the time the volcano was in its final eruptive stage. Iceland not only provides a large number of potential subglacial edifices to sample but is geologically interesting as it sits above a magmatic hotspot and spreading ridge, providing a long term record of volcanism during considerable climate variability. However, it is unknown exactly in which glacial cycle these volcanic edifices formed, due to the lack of accurate and reliable eruption ages.Helium isotopes are valuable tracers of the mantle source for the origin of volcanic rocks. Helium isotopes are comprised of a mixture of magmatic helium trapped whilst in the mantle, cosmogenic helium built up by interaction with cosmic rays during surface exposure and radiogenic helium produced from the in situ decay of U and Th. These helium components yield invaluable information on the mantle source, time of eruption and length of exposure respectively. The principle aim of this study is to determine whether reliable and accurate eruption ages of basaltic subglacial volcanoes in Iceland are possible, by applying a relatively new geochronological tool: U-Th/He dating. These eruption ages combined with cosmogenic derived 3He surface exposure ages of the summits helps to establish ice surface elevation and thickness fluctuations over time.Olivine bearing basalt samples were acquired from 14 tuyas, 4 tindars and 1 shield volcano across the Western and Northern Volcanic Zones in Iceland (WVZ and NVZ). Samples were collected from the base of each edifice for U-Th/He dating and the summits of 3 tuyas for cosmogenic 3He surface exposure dating. Olivine phenocrysts were hand-picked and analysed on two noble gas mass spectrometers (VG5400 and Helix-MC instruments). As part of the analytical procedure for these techniques, magmatic helium trapped in the olivine grains is also analysed.The mantle helium isotope ratios indicate that nearly all edifices sampled obtain a MORB-like source (1-18RA) except for an OIB-like (24RA) centrally located tuya, Bláfell. The helium ratios acquired in the NVZ are lower (4-11RA) than those in the WVZ (8-20RA). There are no significant differences seen in the mantle components between the different types of edifice sampled. Mantle helium isotope ratios confirm the heterogeneity of the mantle beneath Iceland. The cosmogenic 3He exposure ages of Hlöđufell and Bláfell in the WVZ and Gaesafjöll in the NVZ are determined as 0.5±7.4ka, 67±0.7ka and 108±46ka respectively. This study presents the first 3He cosmogenic exposure age determined to be over 20ka in Iceland. The low concentrations of U and Th observed in Icelandic olivines (0.002-0.04 nmol/g and 0.01-0.1 nmol/g respectively) prevent any reliable disequilibrium ages to be established. Minimum secular equilibrium U-Th/He ages are determined and provide a range of erroneously large eruption ages (0.4 - 80.4 Ma), due to potential helium implantation and under-detection of 3He released during sample analysis. Progress can be made towards determining accurate eruption ages for Icelandic basalts if certain factors are adhered to and various sources of helium are separated effectively.
|Date of Award||1 Aug 2017|
- The University of Manchester
|Supervisor||Raymond Burgess (Supervisor) & Simon Brocklehurst (Supervisor)|
- noble gas
The Timing of Volcano-Ice Interactions and Deglaciation in Iceland
Street, K. (Author). 1 Aug 2017
Student thesis: Phd