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RHYOLITE VOLCANISM AT ÖRÆFAJÖKULL VOLCANO, S.E. ICELAND - A WINDOW ON QUATERNARY CLIMATE CHANGE
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
Öræfajökull is an ice-capped stratovolcano situated in the south east of Iceland which has developed throughout the mid to late Quaternary. It has erupted basaltic and rhyolitic lavas during interglacial and glacial periods, many of which display strong physical evidence of volcano-ice interaction. This makes Öræfajökull an ideal location to reconstruct terrestrial palaeo-environments. The area of Goðafjall and Hrútsfjall is one of a small number of rhyolitic depositional centres situated on the south west flanks of the volcano and is the first rhyolitic area of Öræfajökull to be mapped in detail.The relatively high K content of the rhyolitic units make them good candidates for 40Ar/39Ar dating, yielding eruption ages that provide a temporal constraint on the development of the stratovolcano.40Ar/39Ar dating of young rocks (<1 My) is challenging and many of the samples were found to contain both excess and atmospheric argon. A small number exhibited a fractionated argon source with a sub-atmospheric 40Ar/36Ar ratio that could not be explained by a single episode of mass fractionation. Soret thermal diffusion has been suggested as a possible mechanism for fractionation, although further investigation is required.Two dominant rhyolite eruptions have been identified by detailed field mapping supported by the geochemical application of chemostratigraphy.The stratigraphically lower group of lavas outcrop between 100 to 380 m.a.s.l. and were erupted subaerially into a relatively ice-free environment at the base of the edifice. A 40Ar/39Ar age of 202 ± 9 ka implies that eruption occurred during the interglacial MIS 7.Conversely the upper group of lavas show strong evidence of volcano-ice interaction, suggesting that they were erupted subglacially and confined by ice with a minimum ice surface elevation of at least 800 m.a.s.l. These lavas yield a 40Ar/39Ar age of 116 ± 14 ka, which implies that eruption occurred during the transitional period between the MIS5e interglacial and colder sub-stages prior to MIS 4.At least two further glacial advances have occurred since the emplacement of the subglacial rhyolite unit. In addition, an ice confined trachydacite flow from the Vatnafjall ridge situated 20 km north east of Goðafjall has also been dated yielding a 40Ar/39Ar age of 95 ± 7 k. This lava was emplaced at an elevation of over 700 m in the presence of an adjacent valley fill glacier was at least 700 m thick.Ice thickness has varied dramatically throughout the evolution of Öræfajökull and glacial erosion has played an important role in its topographic development.