H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland

Portnyagin, Maxim, Hoernle, Kaj, Storm, Sonja, Mironov, Nikita, van den Bogaard, Christel and Botcharnikov, Roman (2012) H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland Earth and Planetary Science Letters, 357/358 . pp. 337-346. DOI 10.1016/j.epsl.2012.09.047.

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Abstract

Silicic Icelandic magmas are widely believed to contain low to moderate H2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO2=57–72 wt%, K2O=1.3–2.6 wt%) in Fe-rich olivines (Fo2–42) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H2O contents (3.3–6.2 wt%). The silicic Hekla melts originate primarily by extensive (∼90%) crystal fractionation of H2O-poor (∼0.6 wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO2 (ΔQFM ∼−0.1 to −0.4) and have as high H2O contents as significantly more oxidized island-arc magmas (ΔQFM≥1). This demonstrates that H2O and ΔQFM do not correlate in silicic magmas from different tectonic settings, and that fO2, not H2O content, shows a major difference between silicic ocean-island (e.g., Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H2O activity and low fO2 expand the field of olivine stability in silicic melts. Low fO2 and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H2O-rich silicic parental magmas crystallized at low fO2. Finally, the high H2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma–ice interaction.

Document Type: Article
Additional Information: WOS:000312621600033
Keywords: melt inclusions; volatiles; olivine; H2O; oxygen fugacity; explosive volcanism; CALC-ALKALINE DIFFERENTIATION; SUBDUCTION ZONE MAGMAS; SKAERGAARD INTRUSION; GLASS INCLUSIONS; RHYOLITIC MELTS; TRACE-ELEMENTS; SOURCE REGIONS; ERUPTION
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
DOI etc.: 10.1016/j.epsl.2012.09.047
ISSN: 0012-821X
Projects: Future Ocean
Date Deposited: 29 Oct 2012 09:52
Last Modified: 06 Mar 2017 10:30
URI: http://eprints.uni-kiel.de/id/eprint/16564

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