Thesis
Geochemical variations in off-axis alkaline glasses - Snaefellsnes Peninsula, Iceland
University of Iowa
Master of Science (MS), University of Iowa
Spring 2018
DOI: 10.17077/etd.7zqdp7ln
Abstract
<p>Magmatism on Iceland is dominated by the eruption of tholeiitic lavas along two NE-SW trending rift zones that represent the loci of new crustal formation. Small volumes of transitional to alkaline magmas erupted along the ~120 km long Snæfellsnes Peninsula in western Iceland. The Snæfellsnes Peninsula is broken into three distinct volcanic systems; Ljósufjöll, Lýsuskard, and Snæfellsjökull. This study examines the geochemical compositions and depths of crystallization of alkaline basaltic glass to see if these features vary spatially along the peninsula. Glass MgO values range from 3.4 to 9.0 wt %, TiO2 ranges from 0.9 – 4.8 wt. % increasing westward along the peninsula. Estimations of crystallization pressures, based on the pressure dependence of ol±plag±cpx, average 4.6 ± 2.8 kbar, translating to depth a depth of 16.1 ± 10 km generally increasing with distance from the main rift zones. In some cases, cox-melt barometry was also used as a check on the glass cotectic pressures. Crustal thickness beneath Snæfellsnes has been estimated to be ~25 km, suggesting mid to lower crustal magma storage depths. Calculated depths in Snæfellsjökull and Lýsuskard are consistent and deep (22 ± 2.4 km) whereas the Ljósufjöll system is more varied. The Ljósufjöll system displays two groupings of depths, split lengthwise. The northern group exhibits depth between 15-26 km, like the rest of the peninsula, while the southern group has no discernable pattern. Previous data on post-glacial lavas reveals broad spatial trends in REEs across the peninsula. Basaltic glasses exhibit a wide range of La/Sm (1 – 5.6) suggesting a variation in melting percentage over a small area. Samples west of 22.33⁰W have higher Dy/Yb (>2.1) indicating a deeper melting depth due to residual garnet as well as a higher La/Sm (>10) indicative of a smaller degree of melting and/or an enriched source. These data have shown that melt storage occurs at near Moho depths along the peninsula but is highly variable in the eastern Ljósufjöll.</p>
Details
- Title: Subtitle
- Geochemical variations in off-axis alkaline glasses - Snaefellsnes Peninsula, Iceland
- Creators
- Alex Maruszczak - University of Iowa
- Contributors
- David Peate (Advisor)Ingrid Ukstins (Committee Member)C. Tom Foster (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Spring 2018
- DOI
- 10.17077/etd.7zqdp7ln
- Publisher
- University of Iowa
- Number of pages
- xii, 116 pages
- Copyright
- Copyright © 2018 Alex Maruszczak
- Language
- English
- Description illustrations
- color illustrations, color maps
- Description bibliographic
- Includes bibliographical references (pages 77-82).
- Public Abstract (ETD)
On Iceland, small volumes of transitional to alkaline magmas have erupted along the ~120 km long Snæfellsnes Peninsula. This magmatism is located >150 km west of the main rift zones where 95% of Icelandic magmatism occurs due to separation of the North American and Eurasian tectonic plates and decompression melting of the underlying mantle. The main questions are why melting occurs in this off-axis setting and where melt storage and transport in the crust takes place. Basaltic glasses from 75 centers (sub-glacial hyaloclastites and post-glacial scorias) were collected in order to address such questions. Crystallization depths were estimated from major element glass compositions relying on the co-crystallization of olivine, plagioclase, and clinopyroxene. The resulting calculations showed a wide range of crystallization depths of 2 - 36 km, but with a majority of samples falling in mid to lower crustal depths (10 - 20 km). By using the trace element compositions of glasses, it is possible to establish spatial variations in mantle composition and depths of melting. This data indicates a broad spatial pattern in the incompatible element ratios (La/Sm, K/Ti) suggesting a difference in melt generation in across a small area. The western half of the peninsula potentially samples three different parental sources while the eastern portion samples just two. The pressures and incompatible element ratios suggest that the Snæfellsnes Peninsula is spatially complex exhibiting different crystallization depths, degrees of melting, and source compositions over a short area only 120 km long.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9983776723102771
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