GEOTHERMOBAROMETRY AND TEXTURAL ANALYSIS OF FLANK-ZONE ALKALI BASALTS FROM SNAEFELLSNES, ICELAND

Post-glacial (<12 Kya) volcanism on the Snaefellsnes peninsula (west Iceland) is dominated by low-volume monogenetic eruptions from point source cinder cones in contrast to the larger-volume fissure eruptions occurring in the main rift zones (>100 km away) that dominate Icelandic volcanism. These flank-zone lavas are mainly primitive (>8wt% MgO) alkali basalts, with macrocrysts of olivine ± clinopyroxene ± plagioclase.

The focus of this research is to use mineral chemistry and quantitative textural analysis to infer details of the “magmatic plumbing” systems beneath the Snaefellsnes peninsula, and to investigate how these are influenced by the different tectonic setting of Snaefellsnes (limited extension, thicker lithosphere) compared to the main rift zones.

The petrographic texture of the Budahraun flow (near Snaefellsjökull) was quantified by determining crystal size distributions (CSDs) for olivine and clinopyroxene. Both phases show kinked CSDs that indicate open-system magmatism and accumulation of larger crystals, likely from a crystal mush. Evidence for multiple chemically distinct crystal populations is apparent from an Mg X-ray map that shows two types of olivine macrocrysts and also from the presence in hand specimen of ‘green’ and ‘black’ clinopyroxene macrocrysts.

Clinopyroxenes were analyzed from the Budahraun flow as well as from lavas in the Ljosufjoll system just to the east. Preliminary geobarometry calculations reveal that clinopyroxenes crystallized near the Moho (at ~25 km), and throughout the lower crust (up to depths of 10 km). Sector zonation present in some clinopyroxenes indicates variable compositions crystallizing at the same time within a phenocryst, and brings with it the ability to test the resolution of geothermobarometric calculations within the magmatic system. A difference of ~8 km calculated from different zones within the same phenocryst shows the importance of disentangling disequilibrium effects vs. different crystallization depths when conducting geobarometric calculations.