Golowin, Roman (2012) Major, trace elements and volatiles in meld inclusions in minerals from the 1362 AD Öraefajökull erpution (Iceland) : implications fro the origin of a large volume homogeneous rhyolitic magma in Iceland. (Master thesis), Christian-Albrechts-Universität zu Kiel, Mathematisch-Naturwissenschaftliche Fakultät, Kiel, 73 Bl. pp.

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Abstract

Rhyolitic magmas in Iceland are believed to be generated, either by extensive fractional crystallisation of basaltic melts, or by remelting of basaltic, hydrothermally altered crust. Several volcanoes on Iceland have produced rhyolitic melts in highly explosive eruptions in historical time. The high explosivity of these rhyolitic eruptions has been explained by either interaction of magma with glacier/meteoric water, or with the fact that rhyolitic melts may contain very high primary H2O contents. The 1362 AD Öræfajökull eruption was the largest, plinian, silicic eruption in Iceland in historical time. Compositions of glass inclusions in olivine, clinopyroxene and plagioclase from the 1362 AD Öræfajökull tephra are very homogeneous (67.61- 72.42 wt% SiO2, 3.19-3.53 wt% K2O). Comparison of melt inclusion data with the composition of the matrix glasses showed that these melt inclusions were trapped during crystallisation of the phenocryst phases in a rhyolitic parental magma, which was compositionally almost similar to the matrix glass composition. Nevertheless, compositional variations show a clear differentiation trend from the most primitive melt inclusions in olivine to the most evolved inclusions in plagioclase. FTIR analyses of Ol-hosted melt inclusions showed high primary H2O contents (2.37-5.43 wt%) within this rhyolitic melt. Thermobarometrical calculations revealed a relatively reduced oxidation state of thatlarge-volume rhyolite magma(ΔQFM between -0.40 and -0.92). Comparison of the high H2O contents and the reduced oxidation state of that magma with recent silicic Hekla eruptions (Portnyagin et al., 2012) may confirm thederivation of the rhyolitic magma through extensive fractional crystallisation of a basaltic melt. Comparison of trace element data from rhyolitic glasses from this study with data for basaltic rocks from Öræfajökull from earlier studies (Prestvik, 1980), as well as isotope data from earlier studies (Prestvik et al., 2001), supports the model of fractional crystallisation for the origin of the silicic, large-volume 1362 Öræfajökull eruption. Calculations of initial pressures before the entrapment of Ol-hosted melt inclusions, based on the highest H2O contents in melt inclusions, reveal aminimum depth of the magma chamber in 5.5-7.0 km beneath Öræfajökull Volcano. Calculated partition coefficients for rare earth elements (REE) between the Fe-rich olivines and the rhyolitic meltindicate KD’s for heavy rare earth’selements (HREE) reaching values of almost 1,surprisingly showing an almost compatible behaviour of these elements between Fe-rich olivines and the rhyolitic melt. Further, modelling of fractional crystallisation using COMAGMAT showed, that it is possible to generate evolved meltsfrom a basaltic source by ~90% of extensive, fractional crystallisation. Nevertheless, differences in analysed and modelled major element contentsshowed, that the model does not perfectly predict the origin of such highly evolved melts like it is the case for the rhylolitic melt from the 1362 AD Öræfajökull eruption.

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