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The Magmatic Evolution of Krafla, NE Iceland

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The Magmatic Evolution of Krafla, NE Iceland. Hugh Nicholson Thesis submitted for the degree of Doctor of Philosophy University of Edinburgh DECLARATION I declare that this thesis is my own work except where otherwise stated. Hugh Nicholson ABSTRACT Krafla is an active central volcano in the NE axial rift zone of Iceland associated with a fissure swarm which trends NNE-SSW. Lava/hyaloclastite samples were collected from the volcanic system, covering the last 4 interglacial and 3 glacial periods. A siratigraphic framework for the volcanic system has been established by use of the Hekla tephra layers and the distinctive volcanic products of glacial and interglacial periods. Recent volcanic activity has shown that there is a shallow magma reservoir beneath the central volcano, which supplies magma laterally to the fissure swarms forming dykes and to the surface directly for eruption. The Krafla rocks contain the following phenocryst assemblages: ol + plag, ol + plag + cpx, plag + cpx ± ol, plag + cpx + opx + FeTi oxides, plag + FeTi oxides ± cpx ± fayalitic ol. These distinct assemblages suggest that the Krafla suite evolves predominantly by fractional crystallisation. They are also similar to the assemblages - found in other mid-ocean ridge basalt (MORB) suites. Most samples, excluding rhyolites, contain plagioclase xenocrysts, implying that there has been at least 2 stages of magma mixing prior to eruption. The absence of plagioclase xenocrysts in the rhyolites suggests that they formed in isolation from the most primitive magmas, with or without crustal assimilation. Fractional crystallisation using the observed phenocryst assemblage explains many aspects of the major-element compositions of the suite. Least-squares modelling confirms this observation, although it also suggests that the rhyolites contain higher concentrations of K 20 than would be predicted by closed-system fractional crystallisation. This discrepancy may be explained by open system fractionation and/or by crustal assimilation. Numerical modelling of phase equilibria suggests that fractional crystallisation occurs over a range of pressures from 1-3.5 kbar. As was suspected, magma mixing also appears to be a significant process in the Krafla system and may explain some of the scatter on major-element variation diagrams. The Krafla suite is more differentiated than the majority of MORB suites, consistent with the development of long-lived magma reservoirs. The suite also has higher average values of total iron oxide and lower average values of Na 2O (for a given MgO content), which are explicable by larger degrees of melting athigher than average pressures than for the majority of MORBs. Trace-element compositions also confirm the role of fractional crystallisation. Although the trace-element data also show that the highly incompatible trace elements (e.g. Th,U,Rb) show have high concentrations in the rhyolites like K20. As mentioned above, this enrichment in the most incompatible elements may be explained by either open system effects and/or by crustal assimilation. However, the temporal variation in rock composition shows no evidence for enrichment in the more incompatible elements when compared with the less incompatible elements, effectively reducing the role of open system fractionation. This suggests that the enrichment in incompatible elements occurs through crustal assimilation of possibly basaltic wall-rock which has undergone relatively small degrees of melting. Ratios of incompatible trace elements also suggest that the Krafla basalts are distinctive from most MORBs in that they appear to be derived from a less incompatible element enriched mantle source. The major- and the trace-element compositions of the most primitive Krafla basalts show evidence for variable degrees of melting of mantle or possibly variations of the source. In particular, the major-element concentrations, when compared with data from experimental studies, suggest that the basalts derived from the smallest degrees of melting are also produced at the highest pressures. The variation in the incompatible trace elements appears to be "coupled" to that seen in the major elements. The least-differentiated Krafla compositions are also compared with melt compositions predicted from the parameterisation of melting experiments (McKenzie & Bickle 1988). After correcting for the effects of fractional crystallisation, the Krafla melts are most consistent with model compositions produced by a value of 1480C for the mantle potential temperature. Discrepancies between model compositions and the Krafla compositions may possibly be explained by the dynamic effects of a mantle plume on the melting processes. The Krafla lavas show anomalously low 8180 values compared with the majority of MORBs. The 8180 values of the lavas correlate positively with the MgO content. This correlation is consistent with crustal assimilation of low- 180 hydrothennally-altered country-rock. The assimilation process appears to be coupled to fractional crystallisation, although the linearity of the 8180 vs MgO plot appears to confirm that magma mixing is also operating. The ( 230Th/232Th) values of postglacial rocks also correlate positively with MgO content, consistent with their origin by the same crustal assimilation process. The 0- and Th-isotope ratios may be modelled by an assimilation with fractional crystallisation (AFC) model, providing the 6180 value of the assimilant is assumed. An assimilant produced at the top of the magma reservoir is likely to have a ö'O value of about 100/ oo. If this value of 8 180 is used for the assimilant, then a single-stage AFC model requires that the assimilant has to contain between 3-5 ppm Th and r=0.15-0.20. A 2-stage model, however, gives a better fit with the O-Th isotopic data, with an assimilant containing about 1 ppm Th in the first stage of differentiation and 7 ppm in the second stage. Both models can generate the high values of the most incompatible trace elements in the rhyolites. iv ACKNOWLEDGEMENTS Firstly I would like to thank my supervisors Godfrey Fitton and Brian Upton for supervision and friendly guidance during the last three years. Assistance in the field proved vital to the project and particular thanks must go to Karl Grönvold and Kristjan Saemundsson who patiently put up with my requests for yet more samples. Kristjan is particularly thanked for his help in establishing the stratigraphy of the volcanic system. I am grateful to Karl for many things; notably for my introduction to Icelandic petrology and geochemistry and to Icelandic life in general. The trip to Kverkfjoll with Karl, Konni and Ritva will remain in my memory for a long time. The support of the Nordic Volcanological Institute in the form of 4WD-transport and accommodation is gratefully acknowledged. Brian Upton is also thanked for his "surprise visit" to Myvatn; for his help in planning the collection of samples and teaching me, amongst other things, a good hammering technique. There are many people whose help proved invaluable in the laboratory. In semi-chronological oider they are: Godfrey Fitton and Dodie James for XRF; Pete Hill and Stuart Kearns for the microprobe; Adrian Boyce, Terry Donnelly, Julie Gerc, Alison McDonald and Graeme Rogers for isotope work at East Kilbride; Michel Condomines, Catherine Daniel, Jean Sérange and Olgeir Sigmarsson for U-series disequilibrium work at Clennont-Ferrand, whilst not forgetting my introduction to this technique by Frances Lindsay and Graham Shimmield in Edinburgh; and finally to all the people at the Geochemistry Lab. at Royal Holloway and Bedford New College (London), especially Jan Barker, Nick Walsh, Alison Warren, Anne, Claire and Nigel. Invaluable discussion has been had with many people over the last 3 years (of course not forgetting Godfrey and Brian), especially with Paul Beattie, Michel Condomines, John Dixon, Tony Faffick, Martin Fisk, Karl Grönvold, Bjöm Hardarsson, Dodie James, Gawen Jenkins, Dave Latin, Dave McGarvie, Sue Mingard, Graeme Rogers, Olgeir Sigmarsson, John Valley, Sue Walls and Martin Wilding. Others not mentioned above have been vital in making these 3 years enjoyable ones in the Grant Institute. I must thank my fellow rats in the Genome building; especially my office-mates Bjöm, Dave, Mike, Sue, and Martin (especially for his performance at the 1989 Christmas Party!) and for many others in the Department. Especial thanks go to Dave Latin for introducing me to the "Dan Clan", for unravelling McKenzie's FORTRAN programs and for critically reading several chapters.Sue Mingard is also thanked for wading through several chapters. iY1 Additionally my sanity was preserved (!) by support from my flatmates and friends; in particular Dave, Susan and Ingrid (who supplied much valuable literature not in the reference list) and not forgetting Pete and the Hairies (see Dymoke 1988) and my home group at St. Paul's & St. George's. Finally my family is thanked for love and support. My father is especially thanked for enthusiastically reading much of the first-draft of the script and my mother for proof-reading at the end of the write-up. I gratefully acknowledge receipt of a NERC research studentship. vi TABLE OF CONTENTS Chapter 1 Introduction 1.1 Regional setting 1 1.2 Previous work 4 1.2.1 General work on Iceland 4 1.2.2 Previous work on Krafla 5 1.3 Aims of the project 6 Chapter 2 Fieldwork and recent geophysical evidence 8 2.1 Aims of the fieldwork 8 2.2 Stratigraphic Framework 8 2.3 Volcanic history of Krafla 12 2.3.1 General
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