Reconstructing the plumbing system of an off-rift primitive alkaline tuya (Vatnafell, Iceland) using geothermobarometry and CSDs

The Snæfellsnes Peninsula of western Iceland is a region of transitional to alkaline Quaternary (<1.5 Ma) intraplate magmatism. Vatnafell, a sub-glacial tuya (~410 ka), is centrally located within the peninsula, between the Helgrindur/Lýsuskarð and Ljósufjöll volcanic systems, ~120 km from the divergent Western Volcanic Zone plate boundary. The Vatnafell edifice comprises a basal hyaloclastite unit capped by a ~150 m thick supraglacial lava flow that is highly porphyritic (~15% clinopyroxene, olivine, and plagioclase macrocrysts). We use textural and compositional data to investigate magma transport and storage in this unique strike-slip tectonic setting (minimal extension and old c. 7 Ma crust) that is distinct from all other volcanic zones in Iceland. Textural analysis was done using petrographic observations and by constructing crystal size distributions (CSDs) for clinopyroxene, olivine, and plagioclase. All three phases show kinked CSDs, consistent with macrocryst incorporation through disaggregation of crystal mushes. In detail, the Vatnafell unit contains two distinct macrocryst associations: a dominant population of wehrlitic glomerocrysts (clinopyroxene+olivine), and less common gabbroic-troctolitic glomerocrysts (plagioclase+olivine±clinopyroxene). The wehrlitic glomerocrysts contain the most primitive populations of clinopyroxene (Mg# 84 ± 3) and olivine (Fo% 83 ± 1), while the gabbroic-troctolitic glomerocrysts are dominated by plagioclase (An% 80-89), with more evolved olivine (Fo% 79 ± 1) and minor clinopyroxene (Mg# 76). LA-ICP-MS REE data on the wehrlitic clinopyroxenes indicates an origin from an alkaline melt, similar in composition to the host lava, but slightly more primitive. A range of geothermometers (olivine-clinopyroxene Fesbnd Mg exchange; olivine-spinel Al exchange; clinopyroxene composition; clinopyroxene-melt equilibrium) give broadly consistent crystallization temperatures of ~1220 °C for the wehrlitic glomerocrysts. Clinopyroxene-based geobarometers give pressures of ~6 kbar for the wehrlitic glomerocrysts and <4 kbar for the rare gabbroic-troctolitic glomerocrysts that indicate polybaric fractional crystallization at a range of crustal depths. Geophysical estimates of the crustal thickness beneath Vatnafell are ~25 km, and the dominant wehrlitic crystal population likely experienced magma storage at ~20 km in the lower crust near the Moho.