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Title: Petrochemistry of the Northern Rift Zone, NE Iceland : plumbing dyamics and source variations
Author: Pernet-Fisher, John François
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2012
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In this study 156 post, inter- and intra- glacial basalts sampled from the Northern Rift Zone (NRZ), Iceland, have been shown to exhibit a wide range of chemical and isotopic compositions. In the majority of cases fractional crystallisation has been shown to be the dominant process in controlling major and trace element variations. In addition crystallisation pressures have been estimated resolving location and depth of the main bodies of crystallisation under individual rift systems. The pressures calculated from these samples and their relationships with eruption ages has indicated that whist multiple stacked chambers exist under all central volcanos, only one chamber is responsible for generating the majority of basaltic lava flows during individual eruptive episodes. This precludes the need for complex polybaric crystallisation models during rifting episodes and suggests that a simple one chamber model of magma evolution may be valid. d 18O values were determined by laser fluorination on a small number olivine phenocrysts, all showing values below normal mantle. Crystallisation depths indicate that most magma chambers are situated in the lower crust, as such it is difficult to reproduce these light δ¹⁸O values by assimilation of lower crustal cumulates. Large proportions of gabbroic assimilation are required (>60%), inconsistent with the primitive nature of the samples analysed. The light δ¹⁸O values from this study therefore support the hypothesis that Iceland has lower than normal mantle δ¹⁸O (Thirlwall et al, 2006). Radiogenic isotopes indicate that a mixture of depleted and enriched sections of recycled crust as well as a recycled sediment component is present in the mantle source of NRZ lavas. Double spike corrected Pb isotope ratios shows that the recycled crust is ~450 Ma, consistent with observations from other Icelandic rift zones (Thirlwall et al, 2004). Within individual rift segments lavas form very tight correlations suggesting these mantle components are able to fully homogenise in order to form the tight 'pseudo-binary' mixing trends observed. Overall the influence of the sediment components increase towards the plume centre, coupled with higher Dy/YbN and Na2O/TiO2 ratios in these samples imply that that these components are tapped by a deep fusible component within the mantle. Based on similar observations from other OIBs these recycled crustal components are expected to be present in the mantle as olivine free pyroxenites, however Ni, Ca and Fe/Mn values of olivines phenocryst from across the rift zone show little evidence for such a pyroxenite evolvement.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: mantle geochemistry