Insight from Inside the Volcano: Genesis and eruption of Thríhnúkagígur volcanics, Reykjanes Peninsula, Iceland

Open Access
Author:
Hudak, Michael Ryan
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 08, 2015
Committee Members:
  • Maureen Feineman, Thesis Advisor
  • Peter Christopher Lafemina, Thesis Advisor
Keywords:
  • fissure; eruption; basalt; mantle sources; assimilation; magma s
Abstract:
Thríhnúkagígur is the main cone of a small 3500 ka fissure eruption in the Brennisteinsfjöll fissure swarm on the Reykjanes Peninsula. Beneath the summit of the main cone is a 120 m deep cave exposing a large part of the uppermost magmatic plumbing system and a buried tephra cone. The exposure of this structure offers a unique view into a non-unique eruptive process. Basaltic fissure eruptions are common in many tectonic and volcanic settings. Direct observations in the cave in addition to major element, trace element and Sr-Nd-Pb isotope analyses allow a fissure eruption on the Reykjanes Peninsula to be investigated from melt generation in the mantle to eruption. Field relationships between the main dike that fed the eruption and the unconsolidated buried tephra suggest that the tephra was entrained by the erupting magma. Assuming the cave represents the volume of tephra entrained and removed from the system, estimates of the erupted volume of lava produce maximum estimates of ~29% tephra content in the lavas. Furthermore, the location of the buried tephra directly beneath the main cone of Thríhnúkagígur suggest that the buried tephra acted as a local asperity in the crust forming a preferential pathway for the magma to ascend to the surface. The main vents were developed here because flow was focused by the buried tephra. However, the vertical sheet-like structure of the dike indicates that the far field stresses dictate the orientation of fissure despite the contrast in competency between the buried tephra and overlying basaltic lava flow. The Sr-Nd-Pb isotope systematics illustrate that Thríhnúkagígur lavas are consistent with other relatively enriched compositions observed on the Reykjanes Peninsula, with radiogenic Pb and Sr, and relatively unradiogenic Nd. These isotopes suggest that Thríhnúkagígur magmas are derived from two main components – an enriched plume source and a depleted mantle source – with possible minor contributions from an EM1-like mantle source. Anomalously high ratios of Nb/U at Thríhnúkagígur suggest that despite isotopic similarities between the Reykjanes Peninsula and the Eastern Volcanic Zone (EVZ), the enriched plume source at the Reykjanes is unique from the EVZ in its enriched Nb. Decoupling of the Sr isotopes from Pb and Nd isotopes suggests that the source of radiogenic Sr at Thríhnúkagígur may be assimilation of basaltic crust that has been hydrothermally altered by seawater. The tephra entrainment hypothesis is tested with immobile trace element mixing models. Models of La/Yb vs. Nb/Zr yield maximum estimate 28-35% of the tephra component in the lavas, consistent with results from the geophysical analysis. However, large uncertainty in the initial magma composition limits the precision of this model and some samples may not contain any entrained tephra. Finally, the variability in the major and trace element geochemistry and the Sr isotopes, particularly in the uppermost magmatic plumbing system, suggest that the small volume of erupted magma was not homogenized prior to eruption. In the absence of a central volcano at Brennisteinsfjöll, we invoke a network of dikes and sills that are quasi-isolated until the eruption, allowing for variable geochemical evolution. This model both preserves magma heterogeneity from the mantle and facilitates the evolution of magmas with variable compositions, consistent with the variation in geochemistry observed at multiple levels in this single eruption.