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Silicic Magma Genesis in Basalt-dominated Oceanic Settings: Examples from Iceland and the Canary Islands
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The origin of silicic magma in basalt-dominated oceanic settings is fundamental to our understanding of magmatic processes and formation of the earliest continental crust. Particularly significant is magma-crust interaction that can modify the composition of magma and the dynamics of volcanism. This thesis investigates silicic magma genesis on different scales in two ocean island settings. First, volcanic products from a series of voluminous Neogene silicic centres in northeast Iceland are investigated using rock and mineral geochemistry, U-Pb geochronology, and oxygen isotope analysis. Second, interfacial processes of magma-crust interaction are investigated using geochemistry and 3D X-ray computed microtomography on crustal xenoliths from the 2011-12 El Hierro eruption, Canary Islands.

The results from northeast Iceland constrain a rapid outburst of silicic magmatism driven by a flare of the Iceland plume and/or by formation of a new rift zone, causing large volume injection of basaltic magma into hydrated basaltic crust. This promoted crustal recycling by partial melting of the hydrothermally altered Icelandic crust, thereby producing mixed-origin silicic melt pockets that reflect the heterogeneous nature of the crustal protolith with respect to oxygen isotopes. In particular, a previously unrecognised high-δ18O end-member on Iceland was documented, which implies potentially complex multi-component assimilation histories for magmas ascending through the Icelandic crust. Common geochemical traits between Icelandic and Hadean zircon populations strengthen the concept of Iceland as an analogue for early Earth, implying that crustal recycling in emergent rifts was pivotal in generating Earth’s earliest continental silicic crust.

Crustal xenoliths from the El Hierro 2011-2012 eruption underline the role of partial melting and assimilation of pre-island sedimentary layers in the early shield-building phase of ocean islands. This phenomenon may contribute to the formation of evolved magmas, and importantly, the release of volatiles from the xenoliths may be sufficient to increase the volatile load of the magma and temporarily alter the character and intensity of an eruption.

This thesis sheds new light on the generation of silicic magma in basalt-dominated oceanic settings and emphasises the relevance of magma-crust interaction for magma evolution, silicic crust formation, and eruption style from early Earth to present.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 54 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1338
Keyword [en]
Silicic magmatism, Iceland, magma-crust interaction, proto-continental crust, early Earth, zircon geochronology and geochemistry, oxygen isotopes, 2011-2012 El Hierro eruption, crustal xenoliths, 3D X-ray μ-CT, volatiles
National Category
Geochemistry Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-272318ISBN: 978-91-554-9454-4 (print)OAI: oai:DiVA.org:uu-272318DiVA: diva2:893923
Public defence
2016-03-03, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2016-02-11 Created: 2016-01-13 Last updated: 2016-02-19
List of papers
1. Iceland's best kept secret
Open this publication in new window or tab >>Iceland's best kept secret
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2014 (English)In: Geology Today, ISSN 0266-6979, E-ISSN 1365-2451, Vol. 30, no 2, 54-60 p.Article in journal (Refereed) Published
Abstract [en]

The ‘forgotten fjords’ and ‘deserted inlets’ of NE-Iceland, in the region between Borgarfjörður Eystri and Loðmundarfjörður, are not only prominent because of their pristine landscape, their alleged elfin settlements, and the puffins that breed in the harbour, but also for their magnificent geology. From a geological point of view, the area may hold Iceland's best kept geological secret. The greater Borgarfjörður Eystri area hosts mountain chains that consist of voluminous and colourful silicic rocks that are concentrated within a surprisingly small area (Fig. 1), and that represent the second-most voluminous occurrence of silicic rocks in the whole of Iceland. In particular, the presence of unusually large volumes of ignimbrite sheets documents extremely violent eruptions during the Neogene, which is atypical for this geotectonic setting. As a group of geoscientists from Uppsala University (Sweden) and the Nordic Volcanological Center (NordVulk, Iceland) we set out to explore this remote place, with the aim of collecting material that may allow us to unravel the petrogenesis of these large volumes of silicic rocks. This effort could provide an answer to a long-standing petrological dilemma; the question of how silicic continental crust is initially created. Here we document on our geological journey, our field strategy, and describe our field work in the remote valleys of NE-Iceland.

National Category
Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-213020 (URN)DOI: 10.1111/gto.12042 (DOI)
Available from: 2013-12-17 Created: 2013-12-17 Last updated: 2017-12-06
2. Rapid generation of high-silica magmas in basalt-dominated rift settings
Open this publication in new window or tab >>Rapid generation of high-silica magmas in basalt-dominated rift settings
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(English)Manuscript (preprint) (Other academic)
National Category
Geology Geochemistry
Research subject
Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-272262 (URN)
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-02-19
3. Zircons from northeast Iceland analogous to those from early Earth
Open this publication in new window or tab >>Zircons from northeast Iceland analogous to those from early Earth
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(English)Manuscript (preprint) (Other academic)
National Category
Geochemistry
Identifiers
urn:nbn:se:uu:diva-272264 (URN)
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-02-19
4. Origin of high whole-rock δ18O values in rhyolites from northeast Iceland
Open this publication in new window or tab >>Origin of high whole-rock δ18O values in rhyolites from northeast Iceland
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(English)Manuscript (preprint) (Other academic)
National Category
Geochemistry
Identifiers
urn:nbn:se:uu:diva-272315 (URN)
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-02-19
5. Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption
Open this publication in new window or tab >>Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption
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2012 (English)In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 3, no 1, 97-110 p.Article in journal (Refereed) Published
Abstract [en]

A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e. g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e. g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro.

Place, publisher, year, edition, pages
Copernicus Publications, 2012
National Category
Geology Geochemistry
Identifiers
urn:nbn:se:uu:diva-171483 (URN)10.5194/se-3-97-2012 (DOI)000309884400008 ()
Funder
Swedish Research Council
Note

Correction in Solid Earth. Vol. 3(2) p. 189. DOI: 10.5194/se-3-189-2012

Available from: 2012-03-19 Created: 2012-03-19 Last updated: 2017-05-03Bibliographically approved
6. Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography
Open this publication in new window or tab >>Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography
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2016 (English)In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 12, 85Article in journal (Refereed) Published
Abstract [en]

During the first week of the 2011 El Hierro submarine eruption, abundant light-coloured pumiceous, high-silica volcanic bombs coated in dark basanite were found floating on the sea. The composition of the light-coloured frothy material ('xeno-pumice') is akin to that of sedimentary rocks from the region, but the textures resemble felsic magmatic pumice, leaving their exact mode of formation unclear. To help decipher their origin, we investigated representative El Hierro xeno-pumice samples using X-ray computed microtomography for their internal vesicle shapes, volumes, and bulk porosity, as well as for the spatial arrangement and size distributions of vesicles in three dimensions (3D). We find a wide range of vesicle morphologies, which are especially variable around small fragments of rock contained in the xeno-pumice samples. Notably, these rock fragments are almost exclusively of sedimentary origin, and we therefore interpret them as relicts an the original sedimentary ocean crust protolith(s). The irregular vesiculation textures observed probably resulted from pulsatory release of volatiles from multiple sources during xeno-pumice formation, most likely by successive release of pore water and mineral water during incremental heating and decompression of the sedimentary protoliths.

Keyword
El Hierro, Xeno-pumice, X-CT imaging, Vesicle morphologies, Vesicle size distribution, Heterogeneous vesiculation, Sedimentary ocean crust
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-272316 (URN)10.1007/s00445-016-1080-x (DOI)000394130700001 ()
Funder
The Royal Swedish Academy of SciencesSwedish Research Council
Note

The manuscript version of this article was used under the name "Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by synchrotron μ-CT" in the following thesis: Silicic Magma Genesis in Basalt-dominated Oceanic Settings: Examples from Iceland and the Canary Islands http://uu.diva-portal.org/smash/record.jsf?pid=diva2:893923

Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2017-11-30Bibliographically approved
7. Hur bildades kontinenterna?
Open this publication in new window or tab >>Hur bildades kontinenterna?
2014 (Swedish)In: Geologiskt Forum, ISSN 1104-4721, Vol. 84, 12-13 p.Article in journal (Other (popular science, discussion, etc.)) Published
Place, publisher, year, edition, pages
Jönköping: Geologiska Föreningen, 2014
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-272317 (URN)
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-02-19

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