Comparative Analysis of Plagioclase in the Eyjafjöll Ankaramites
Total Page：16
File Type：pdf, Size：1020Kb
Comparative analysis of plagioclase in the Eyjafjöll ankaramites Þrúður Helgadóttir Faculty of Earth Science University of Iceland 2016 Comparative analysis of plagioclase in the Eyjafjöll ankaramites Þrúður Helgadóttir 10 ETCS thesis submitted in partial fulfilment of Baccalaureus Scientiarum degree in Geology Advisor Enikő Bali Faculty of Earth Science School of Engineering and Natural Sciences University of Iceland Reykjavík, May 2016 Comparative analysis of plagioclase in the Eyjafjöll ankaramites Analysis of plagioclase in Eyjafjöll ankaramites 10 ECTS thesis submitted in partial fulfilment of Baccalaureus Scientiarum degree in Geology Copyright © 2016 Þrúður Helgadóttir All rights reserved Faculty of Earth Science School of Engineering and Natural Sciences University of Iceland Askja, Sturlugata 7 101 Reykjavík Telephone: 525 4000 Registration information: Þrúður Helgadóttir, 2016, Comparative analysis of plagioclase in the Eyjafjöll ankaramites, BSc thesis, Faculty of Earth Science, University of Iceland, 67 pages. Printing: Háskólaprent Reykjavík, May 2016 Útdráttur Ankaramít er einkennisbergtegund Eyjafjallajökuls. Það er útbasískt, dílótt, basanít berg með miklu magni klínópýroxen- og ólivíndíla ásamt miklu magni plagioklas í grunnmassa, en einnig er talsvert magn af oxíðum í grunnmassa. Ankaramít bergsýni í þessari rannsókn eru frá Hamragarðaheiði, Hvammsnúpi, Brattaskjóli og Arnarhóli í Eyjafjöllum. Handsýni voru skoðuð lauslega og plagioklas kristallar voru rannsakaðir í þunnsneiðum með bergsmásjá og efnagreiningar framkvæmdar í rafeindasmásjá. Steindasamsetning grunnmassa var fundin með CIPW-norm útreikningum út frá meðalefnasamsetningu grunnmassa. Grunnmassasamsetning Arnarhóls er 60.5% plagioklas, 11% pýroxen, 24.9% ólivín og 5.8% oxíð, Hamragarðaheiði 54.5% plagioklas, 35.1% pýroxen, 3.3% ólivín og 5.3% oxíð, Brattaskjóls 50.5% plagioklas, 33.3% pýroxen, 13.3% ólivín og 4.1% oxíð og Hvammsnúpi 57.5% plagioklas, 28.4% pýroxen, 8.6% ólivín og 4.4% oxíð. Öll sýni voru dílótt. Sýni frá Arnarhóli og Hamragarðaheiði eru mjög plagioklas dílótt, sýni frá Hvammsnúpi eru með mun minna af plagioklas dílum og Brattaskjól með enga slíka. Samkvæmt efnagreiningu á plagioklas í grunnmassa sem framkvæmd var fyrir þetta verkefni var anorþíthlutfall í sýni frá Brattaskjóli An73-77. Plagioklas í grunnmassa bergs frá Arnarhóli og Hvammsnúpi er með svipað anorþíthlutfall, á bilinu An58-72 og An41-70 á meðan hlutfallið er á bilinu An51-60 í bergsýnum frá Hamragarðaheiði. Anorþíthlutfall í plagioklas dílum í sýnum frá Arnarhóli, Hamragarðaheiði og Hvammsnúpi er An79-90, An79-82 og An87-94. Efnagreining var gerð á ólivíndílum í sýni frá Brattaskjóli og borið saman við gögn frá Kristjánssyni (2015) fyrir Hamragarðaheiði og Hvammsnúp. Þessi þrjú sýni voru með forsteríthlutfall á bilinu Fo85-89, Fo67-78 og Fo70-70. Abstract Eyjafjallajökull is the type locality for ankaramite in Iceland. This is an ultrabasic, porphyritic melanocratic basanite rock with abundant phenocrysts of clinopyroxene and olivine and plagioclase rich groundmass which also contains a considerable amount of oxides. Rock samples used in this study are from the ankaramitic outcrops in Hamragarðaheiði, Hvammsnúpur, Brattaskjól and Arnarhóll in Eyjafjöll. Hand specimens were briefly examined and plagioclase in thin sections were analysed by polarization microscope and SEM-EDS. Groundmass mineral assemblage according to CIPW norm calculation from average groundmass compositions showed Arnarhóll containing 60.5% plagioclase, 11% pyroxene, 24.9% olivine and 5.8% oxides, Hamragarðaheiði 54.5% plagioclase, 35.1% pyroxene, 3.3% olivine and 5.3% oxides. Brattaskjól contains 50.5% plagioclase, 33.3% pyroxene, 13.3% olivine and 4.1% oxides while Hvammsnúpur contains 57.5% plagioclase, 28.4% pyroxene, 8.6% olivine and 4.4% oxides. All samples were porphyritic. Arnarhóll and Hamragarðaheiði rock samples are rich in plagioclase phenocrysts, Hvammsnúpur samples have plagioclase phenocrysts in much less quantities and Brattaskjól sample had none. According to plagioclase chemical analysis in this study, anorthite content for plagioclase in the groundmass of the Brattaskjól samples is An73-77. Arnarhóll and Hvammsnúpur groundmass contain similar ranges, An58-72 and An41-70 respectively while the range is An51-60 in Hamragarðaheiði. Anorthite content range for plagioclase phenocrysts in Arnarhóll, Hamragarðaheiði and Hvammsnúpur samples is An79-90, An79-82 and An87-94 respectively. Olivine phenocryst chemical analysis was made for Brattaskjól and compared to olivine phenocryst data from Kristjánsson (2015) for Hamragarðaheiði and Hvammsnúpur. These three samples contained forsterite content range of Fo85-89, Fo67-78 and Fo70-70 respectively. Table of Contents List of figures ...................................................................................................................... xi List of tables ...................................................................................................................... xiii Abbreviations ..................................................................................................................... xv Acknowledgements .......................................................................................................... xvii 1 Introduction ................................................................................................................... 19 1.1 Plagioclase ............................................................................................................. 19 1.2 Ankaramite ............................................................................................................ 21 2 Geological setting .......................................................................................................... 23 2.1 Localities ............................................................................................................... 26 3 Methods .......................................................................................................................... 29 3.1 Hand specimen ...................................................................................................... 29 3.2 Microscopic analysis ............................................................................................. 29 3.3 SEM analysis ......................................................................................................... 29 4 Results ............................................................................................................................ 31 4.1 Hand specimen ...................................................................................................... 31 4.2 Thin sections.......................................................................................................... 34 4.2.1 Arnarhóll ...................................................................................................... 35 4.2.2 Hamragarðaheiði .......................................................................................... 36 4.2.3 Brattaskjól .................................................................................................... 37 4.2.4 Hvammsnúpur .............................................................................................. 38 4.3 SEM results ........................................................................................................... 39 4.3.1 Arnarhóll ...................................................................................................... 39 4.3.2 Hamragarðaheiði .......................................................................................... 42 4.3.3 Brattaskjól .................................................................................................... 47 4.3.4 Hvammsnúpur .............................................................................................. 50 5 Discussion ...................................................................................................................... 53 6 Conclusions .................................................................................................................... 59 References........................................................................................................................... 61 Appendix A ......................................................................................................................... 63 Appendix B ......................................................................................................................... 67 ix x List of figures Figure 1: Volcanism and tectonics in Iceland. .................................................................... 23 Figure 2: Geologic map of the western Eyjafjöll ................................................................ 25 Figure 3: Map of Eyjafjallajökull volcano and localities. ................................................... 26 Figure 4: Geologic map of the ankaramite locality in Hvammsnúpur. ............................... 28 Figure 5: Stereomicroscopic images of hand specimens ..................................................... 31 Figure 6: Arnarhóll hand specimen. .................................................................................... 32 Figure 7: Hamragarðaheiði hand specimen. ........................................................................ 32 Figure 8: Brattaskjól hand specimen. .................................................................................. 33 Figure 9: Hvammsnúpur hand