Timescales of Magmatic Processes from Diffusional Profiles Recorded in Minerals of the 2016-2017 Eruption of Bogoslof Island, Alaska.

The most recent eruption of Bogoslof volcano began in December 2016 and continued until August 2017. Prior to the eruption, only two detailed petrologic studies existed for this backarc stratovolcano. The eruption was comprised of three distinct lithologies: basaltic scoria, dense trachyandesitic dome lava, and trachytic pumice.

The scoria and the trachyandesite contain a plagioclase-amphibole-clinopyroxene mineral assemblage; the trachyte contains plagioclase and clinopyroxene with relic amphibole structures. Within all three lithologies, many of the crystals exhibit a variety of compositional zoning. Our results focus on mineral zoning within clinopyroxene.

In basaltic scoria, clinopyroxene composition ranges from 31 to 45 mol. % enstatite, with resorbed cores of either amphibole or En ₃₃ clinopyroxene, and with rims with relatively constant compositions at En ₄₁ , which occasionally exhibit subtle normal zoning.

In trachyandesites, clinopyroxenes frequently exhibit two dissolution surfaces at which enstatite content decreases from En ₃₆ to En ₃₃ . Conversely, Ti increases within clinopyroxene at dissolution boundaries.

In trachyte pumice, clinopyroxenes possess relatively constant compositions, oscillating between En ₃₆ and En ₃₈ .

The changes in composition across zones, coupled with experimentally derived diffusion coefficients for clinopyroxene, can provide time estimates on the diffusion of elements across chemical boundaries, and yield constraints on the environment in which the crystals were growing. Examples of magmatic processes that have the potential to generate chemically distinct zones in a crystal include events such as temperature increase due to basaltic recharge or a decrease in pressure caused by an eruption. Ongoing analyses and modeling of these patterns of mineral zoning provide insight on the reaction and recovery time of crystals to various magmatic events.