CLINOPYROXENE GEOBAROMETRY OF OFF-AXIS BASALTIC LAVAS FROM THE SNÆFELLSJÖKULL VOLCANO, ICELAND

Snæfellsjökull is a stratovolcano located on the western tip of the Snaefellsnes Peninsula, about 150 km west of the main rift zone. The focus of this study are the mafic basaltic cinder cones and lava flows found on the flanks of the edifice, in particular unravelling details of the magmatic plumbing system in this off-axis location. The studied lavas are alkaline and primitive (MgO 7 to 12 wt%), and are porphyritic with macrocrysts of olivine, clinopyroxene, and plagioclase. Samples contain two populations of clinopyroxene, Cr-rich and high mg# green crystals and lower Cr and mg# black crystals. Whole rock trace element analyses show enrichments in more incompatible trace elements compared to the rift tholeiites (e.g. La/Sm 4.3-4.6 vs. < 3.0). Clinopyroxene-melt geothermobarometry was used to infer depths of magma storage for these mafic flows, using the Neave & Putirka (2017) model (precision of +/- 1.4 kbar). Clinopyroxene and glass compositions were analyzed by electron microprobe from three lava samples (one with ~7 wt% MgO, and two with 10-12 wt% MgO). Calculated clinopyroxene crystallization pressures were 6-10 kbar for the two primitive flows, and 3-7 kbar for the more evolved flow. Crustal thickness at Snæfellsjökull has been estimated at 26-29 km, based on receiver function analysis data, although the position of the Moho is not well defined to +/- 2-5 km. Therefore, the clinopyroxene-melt geobarometry results are consistent with magma storage in the lower crust.