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Exceptionally high whole-rock delta O-18 values in intra-caldera rhyolites from Northeast Iceland
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Univ Iceland, Nord Volcanol Ctr, Inst Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Univ Cape Town, Dept Geol Sci, ZA-7701 Rondebosch, South Africa.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
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2018 (English)In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1147-1168Article in journal (Refereed) Published
Abstract [en]

The Icelandic crust is characterized by low delta O-18 values that originate from pervasive high-temperature hydrothermal alteration by O-18-depleted meteoric waters. Igneous rocks in Iceland with delta O-18 values significantly higher than unaltered oceanic crust (similar to 5.7 parts per thousand) are therefore rare. Here we report on rhyolitic intra-caldera samples from a cluster of Neogene central volcanoes in Borgarfjorour Eystri, Northeast Iceland, that show whole-rock delta O-18 values between +2.9 and +17.6 parts per thousand (n = 6), placing them among the highest delta O-18 values thus far recorded for Iceland. Extra-caldera rhyolite samples from the region, in turn, show delta O-18 whole-rock values between +3.7 and +7.8 parts per thousand (n = 6), consistent with the range of previously reported Icelandic rhyolites. Feldspar in the intra-caldera samples (n = 4) show delta O-18 values between +4.9 and +18.7 parts per thousand, whereas pyroxene (n = 4) shows overall low delta O-18 values of +4.0 to +4.2 parts per thousand, consistent with regional rhyolite values. In combination with the evidence from mineralogy and rock H2O contents, the high whole-rock delta O-18 values of the intra-caldera rhyolites appear to be the result of pervasive isotopic exchange during subsolidus hydrothermal alteration with O-18-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, O-18-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 delta O-18-enriched rock composition exists on Iceland and thus probably within the Icelandic crust too.

Place, publisher, year, edition, pages
MINERALOGICAL SOC , 2018. Vol. 82, no 5, p. 1147-1168
Keywords [en]
Northeast Iceland, high(18)O values, hydrothermal alteration, intra-caldera lakes
National Category
Geology
Identifiers
URN: urn:nbn:se:uu:diva-372902DOI: 10.1180/mgm.2018.114ISI: 000452016700011OAI: oai:DiVA.org:uu-372902DiVA, id: diva2:1277988
Funder
Swedish Research CouncilThe Royal Swedish Academy of SciencesAvailable from: 2019-01-11 Created: 2019-01-11 Last updated: 2019-01-11Bibliographically approved

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Berg, SylviaTroll, Valentin R.Deegan, FrancesBurchardt, SteffiKrumbholz, Michael

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