Using Crystal Size Distributions (CSDs) of Fe-Ti oxides to Determine the Origins of the Hjallin Lens in Northwestern Iceland

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

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Abstract:

The Skagi peninsula of Northwestern Iceland is the location of the Snæfellsnes-Húnaflói paleo-rift axis, active from ~3.3-16 mya, resulting in a regional flexure zone. The regional flexure is disrupted by the Hjallin lens, a ~150 m thick, ~3 km long extrusive body. Originally mapped as an intrusion, the lens exhibits well-developed columnar jointing, a flow-banded chilled basal contact, and an underlying sedimentary layer containing organics. Field relationships suggest the lens in a syn- (or post-) subsidence extrusion emplaced into a local bowl-shaped depression. Despite its thickness, the Hjallin Lens is nearly uniform in composition and mineralogy. The tholeiitic basalt is relatively evolved (FeO* ~14.7 wt.%, MgO ~5.0 wt.%) and Ti-rich (TiO₂ ~3.6 wt.%), non-vesiculated and aphanitic, with an intergranular groundmass of plagioclase and clinopyroxene in approximately equal abundances. Fe-Ti oxides are present in significant (~15%) amounts. Field work was conducted to collect samples across the body and base of the Lens with the goal of exploring systematic compositional and textural changes, and to answer the question, how can a thick extrusive body cool quickly enough to form a fine-grained basalt with uniform mineralogy and composition?  

20 Samples were made into polished thin sections and imaged by microscope and SEM-EDS. SEM-EDS methods will be used to acquire elemental data to determine oxide compositions and backscattered images for Crystal Size Distribution (CSD) analysis of Fe-Ti oxides. To maximize the quality of the CSD analysis, we will use photomosaics of entire thin sections, which will be made in Fiji by stitching individual photomicrographs.  Fiji software will be used to determine crystal boundaries and area percentages of thin section. Preliminary results show up to 2,500 Fe-Ti oxides per thin section and ranges from 10-16% of the thin section area. This information, alongside individual crystal measurements will be used in CSDSlice and CSDCorrections1.6 to determine CSD parameters which will be used to calculate cooling rates. Results will be compared with previously determined plagioclase CSDs from the same samples that suggest cooling rates exceed those estimated from conductive cooling alone. If emplaced as a single body, unique conditions in the lens must have enhanced crystal nucleation while maintaining constant composition. If the lens is a paleo-lava lake, it is unlike other well documented case-studies of thick extrusive bodies.