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Building Ice-Age Askja: Processes, Products and Paleoclimate

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Building Ice-Age Askja: Processes, Products and Paleoclimate BUILDING ICE-AGE ASKJA: PROCESSES, PRODUCTS AND PALEOCLIMATE by Alison Hollomon Graettinger BS, University of Puget Sound, 2005 MSc, University of Waikato, 2008 Submitted to the Graduate Faculty of Dietrich School of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2012 gG112 UNIVERSITY OF PITTSBURGH DIETRICH SCHOOL OF ARTS AND SCIENCES This dissertation was presented by Alison Hollomon Graettinger It was defended on June 25, 2012 and approved by Thomas Anderson, PhD, Professor Emeritus, University of Pittsburgh Charles Jones, PhD, Lecturer, University of Pittsburgh David McGarvie, PhD, Staff Tutor, The Open University, UK Committee Co-Chair: Michael Ramsey, PhD, Professor, University of Pittsburgh Dissertation Adviser: Ian P. Skilling, PhD, Senior Lecturer, University of Glamorgan, UK ii Copyright © by Alison Hollomon Graettinger 2012 iii BUILDING ICE-AGE ASKJA: PROCESSES, PRODUCTS AND PALEOCLIMATE Alison Hollomon Graettinger, PhD University of Pittsburgh, 2012 Austurfjöll is the largest glaciovolcanic construct at Askja Volcano, the best exposed and largest basaltic central volcano in Iceland. The massif records the repeated interaction of basaltic fissure-dominated eruptions with a 600-900 m thick Pleistocene ice sheet in Iceland. The Austurfjöll deposits serve as an important proxy record for ice presence and thickness, supplementing the limited terrestrial glacial record in Iceland. The model of the construction of the 3.62 km3 glaciovolcanic massif is the first to outline in detail and date the growth and evolution of a long-lived polygenetic ice-confined central volcano. The model is based on lithologic descriptions, petrologic investigations, textural studies, unspiked K-Ar dating, volatile saturation pressures based on FTIR analysis of water content in glass, and remote sensing-based mapping. The massif is composed of basal basaltic pillow lava sheets, dominantly subaqueously-deposited vitriclastic deposits erupted from overlapping fissure ridges, and accumulations of gravity-driven deposits in inter-ridge depositional centers. The ridges are locally capped by emergent to subaerial tephra and subaerial lava flows. Detailed textural studies of sequences of in-situ transitions from pillow lavas through breccias to overlying lapilli tuffs are interpreted as ex amples of phreatomagmatic explosions triggered by initial magmatic exsolution and fragmentation at water depths > 600 m. A stratigraphy for Austurfjöll is established and consists of one interglacial unit, six glaciovolcanic units, and two glaciogenic sedimentary units established through chemostratigraphy and field mapping. Eruptive units are numbered chronologically, with glacial units designated Dm: Unit 1 (A and B), Dm1, Unit 2, Dm2, Unit 3, Unit 4, Unit 5, Unit 6 and Unit 7. Diamictite deposits and emergent facies are described for the first time at Austurfjöll. Two eruptive units were dated radiogenically by iv unspiked K-Ar methods to 71 +/- 7 ka (Unit 2) and 29 +/- 8 ka (Unit 3). Ice presence is inferred from glacial, subaerial, and subglacial lithofacies including coherent margined volcaniclastic dikes (CMVDs) that are interpreted as the result of basaltic intrusions into ice-cemented sediments. The deposits described from Austurfjöll reflect a history of interglacial, ice-confined subaqueous, subglacial and emergent eruptions with a dynamic Pleistocene ice sheet over at least 40 ka. v TABLE OF CONTENTS PREFACE .......................................................................................................................... XXXVII 1.0 INTRODUCTION ........................................................................................................ 1 1.1 INTRODUCTION TO THE GEOLOGIC SETTING AND HISTORY OF ERUPTIVE ACTIVITY OF ASKJA VOLCANO, ICELAND ........................................ 2 1.2 REGIONAL SETTING ....................................................................................... 8 1.3 GLACIAL HISTORY ....................................................................................... 11 1.4 GEOLOGY OF ASKJA VOLCANO ............................................................... 12 1.5 STRUCTURE AND RELATIVE AGE OF ASKJA COMPONENTS ......... 14 1.6 HOLOCENE ERUPTIVE HISTORY ............................................................. 17 1.7 HISTORIC ERUPTIONS FROM ASKJA ...................................................... 20 1.7.1 Eruption of 1875 ............................................................................................ 20 1.7.2 Eruptions of the 1920s ................................................................................... 21 1.7.3 Eruption of 1961 ............................................................................................ 22 1.8 AREA OF INTEREST: AUSTURFJÖLL....................................................... 22 1.9 DEFORMATION AND MONITORING OF ASKJA VOLCANO .............. 23 2.0 ARCHITECTURE OF AUSTURFJÖLL MASSIF, ASKJA, ICELAND: LITHOLOGY, PETROGRAPHY............................................................................................. 27 2.1 METHODOLOGY ............................................................................................ 29 vi 2.2 LITHOLOGY .................................................................................................... 30 2.2.1 Lavas ............................................................................................................... 32 2.2.2 Pillowed lavas ................................................................................................. 37 2.2.2.1 Geomorphology of pillowed units ...................................................... 38 2.2.2.2 Non-pillowed lavas .............................................................................. 43 2.2.3 Rhyolite lava ................................................................................................... 46 2.2.4 Fragmental Units ........................................................................................... 48 2.2.4.1 Breccias ................................................................................................ 48 2.2.4.2 Diamictites ........................................................................................... 50 2.2.4.3 Tuffs...................................................................................................... 51 2.2.5 Intrusions ........................................................................................................ 59 2.2.6 Xenoliths ......................................................................................................... 61 2.3 PETROGRAPHY .............................................................................................. 62 2.4 STRUCTURE AND VENTS ............................................................................. 68 2.5 INTERPRETATION OF LITHOFACIES...................................................... 79 2.6 ARCHITECTURE ............................................................................................. 89 2.6.1 Fissure ridges ................................................................................................. 89 2.6.2 Lava dominated features ............................................................................... 91 2.6.3 Depo-centers ................................................................................................... 92 2.6.4 Mantling deposits ........................................................................................... 94 2.7 SUMMARY ........................................................................................................ 96 3.0 GEOCHEMISTRY AND RADIOMETRIC DATING OF AUSTURFJÖLL MASSIF, ASKJA, ICELAND .................................................................................................... 98 vii 3.1 METHODOLOGY ............................................................................................ 98 3.2 WHOLE ROCK GEOCHEMISTRY ............................................................ 104 3.3 PHENOCRYST COMPOSITION ................................................................. 114 3.4 RADIOGENIC DATING ................................................................................ 117 3.5 DISCUSSION ................................................................................................... 118 3.5.1 Major and Minor element trends ............................................................... 119 3.5.1.1 Establishment of units ...................................................................... 120 3.5.2 Mineralogy ................................................................................................... 122 3.5.3 Pearce element ratios ................................................................................... 123 3.6 CONTEXT OF ASKJA ................................................................................... 128 3.7 PETROGENESIS ............................................................................................ 132 3.8 CONCLUSIONS .............................................................................................. 137 4.0 ARCHITECTURE AND EVOLUTION OF AUSTURFJÖLL MASSIF: IMPLICATIONS FOR ASKJA VOLCANO, ICELAND ..................................................... 138 4.1 VOLUME CALCULATIONS ........................................................................ 140 4.1.1 Construction of Austurfjöll......................................................................... 141 4.1.2 Eruptive
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