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Exceptionally high whole-rock δ18O values in intra-caldera rhyolites from Northeast Iceland

Published online by Cambridge University Press:  29 May 2018

Sylvia E. Berg*
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Askja, 101 Reykjavik, Iceland
Valentin R. Troll
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
Chris Harris
Affiliation:
Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
Frances M. Deegan
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
Morten S. Riishuus
Affiliation:
Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Askja, 101 Reykjavik, Iceland
Steffi Burchardt
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
Michael Krumbholz
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden

Abstract

The Icelandic crust is characterized by low δ18O values that originate from pervasive high-temperature hydrothermal alteration by 18O-depleted meteoric waters. Igneous rocks in Iceland with δ18O values significantly higher than unaltered oceanic crust (~5.7‰) are therefore rare. Here we report on rhyolitic intra-caldera samples from a cluster of Neogene central volcanoes in Borgarfjörður Eystri, Northeast Iceland, that show whole-rock δ18O values between +2.9 and +17.6‰ (n = 6), placing them among the highest δ18O values thus far recorded for Iceland. Extra-caldera rhyolite samples from the region, in turn, show δ18O whole-rock values between +3.7 and +7.8‰ (n = 6), consistent with the range of previously reported Icelandic rhyolites. Feldspar in the intra-caldera samples (n = 4) show δ18O values between +4.9 and +18.7‰, whereas pyroxene (n = 4) shows overall low δ18O values of +4.0 to +4.2‰, consistent with regional rhyolite values. In combination with the evidence from mineralogy and rock H2O contents, the high whole-rock δ18O values of the intra-caldera rhyolites appear to be the result of pervasive isotopic exchange during subsolidus hydrothermal alteration with 18O-enriched water. This alteration conceivably occurred in a near-surface hot spring environment at the distal end of an intra-caldera hydrothermal system, and was probably fed by waters that had already undergone significant isotope exchange with the country rock. Alternatively, 18O-enriched alteration fluids may have been produced during evaporation and boiling of standing water in former caldera lakes, which then interacted with the intra-caldera rock suites. Irrespective of the exact exchange processes involved, a previously unrecognized and highly localized δ18O-enriched rock composition exists on Iceland and thus probably within the Icelandic crust too.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Jason Harvey

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Exceptionally high whole-rock δ18O values in intra-caldera rhyolites from Northeast Iceland
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