CRYSTAL SIZE DISTRIBUTION AND TEXTURAL ANALYSIS AT THRIHNUKAGIGUR VOLCANO, ICELAND

The subaerial extension of the Mid-Atlantic Ridge on the Reykjanes provides an accessible location to study the processes attendant to the genesis and emplacement of oceanic basalts. Thríhnúkagígur, located on the Reykjanes, records two eruptive styles from a single magma plumbing system. A large tephra body exposed fortuitously at a depth of ~100m within the lava pile indicates an early Strombolian style of eruption while a subsequent eruption produced a more effusive, fissure-eruption style of compositionally analogous basalt. Crystal size distribution (CSD) and textural analysis of plagioclase feldspars reveals consistent trends that we interpret as evidence of two different magma evolution pathways. CSD slopes for the tephra and lava samples are generally kinked indicating a disturbance in the growth stage of crystals within the respective magma bodies. Additional signs of thermal disequilibrium include abundant plagioclase feldspar crystals that display sieving, scoured edges, holes within the lattice, and rounded corners. Likely disturbances include magma mixing, convection or reheating of the magma chamber, addition of assimilated basaltic country rock through partial melting, addition or degassing of volatiles, or an overall change in bulk chemistry from magmatic recharge, all of which can drastically alter the formation of crystals within a melt. Two different morphologies of plagioclase crystals (stubby and elongate) are present in both size populations of plagioclase feldspar, and indicate a similar residence time for both morphological populations. The smaller populations of crystals in both eruptions have calculated residence times of ~100 years and the larger crystals indicate residence times of 200-300 years.