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Exceptionally high whole-rock delta O-18 values in intra-caldera rhyolites from Northeast Iceland
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik. Univ Iceland, Nord Volcanol Ctr, Inst Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
Univ Cape Town, Dept Geol Sci, ZA-7701 Rondebosch, South Africa.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
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2018 (engelsk)Inngår i: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, nr 5, s. 1147-1168Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
MINERALOGICAL SOC , 2018. Vol. 82, nr 5, s. 1147-1168
Emneord [en]
Northeast Iceland, high(18)O values, hydrothermal alteration, intra-caldera lakes
HSV kategori
Identifikatorer
URN: urn:nbn:se:uu:diva-372902DOI: 10.1180/mgm.2018.114ISI: 000452016700011OAI: oai:DiVA.org:uu-372902DiVA, id: diva2:1277988
Forskningsfinansiär
Swedish Research CouncilThe Royal Swedish Academy of SciencesTilgjengelig fra: 2019-01-11 Laget: 2019-01-11 Sist oppdatert: 2019-01-11bibliografisk kontrollert

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