Petrological constraints on the origin of enclaves from Kasatochi Volcano, Aleutian Islands, Alaska

Abstract

Products of the 2008 eruption of Kasatochi volcano in the Aleutian Islands, Alaska, include mafic and ultramafic crystalline intrusive rocks. I analyzed 42 such samples that were collected from Kasatochi in 2012. The mafic enclaves are hornblende gabbros that are texturally and compositionally layered and are composed of plagioclase (An���⁉�܉����), pargasitic hornblende, clinopyroxene, and accessory magnetite. The ultramafic samples include variable amounts of clinopyroxene and olivine with interstitial pargasitic hornblende and accessory spinel. Modal mineral abundances in the ultramafic enclaves were used to classify the samples as wehrlite, olivine-clinopyroxenite, and clinopyroxenite. Compositions of interstitial hornblende in the pyroxenite samples are the same as those for the gabbro, implying that the ultramafic enclaves resided within the same magma as the cumulate gabbro long enough to begin re-equilibration. The results of both whole-rock X-ray fluorescence and mineral electron microprobe analyses of the pyroxenite and peridotite samples indicate a non-mantle source: clinopyroxene Cr# is less than 25, Ni-in-olivine is less than 1600 ppm, and spinel is Cr# is less than 50. The presence of disequilibriurn textures, such as resorption in clinopyroxene, intimate a prolonged residence time in the host magma. Bulk and mineral compositions for representative mafic and ultramafic samples show that the two sample suites are chemically unrelated and therefore have different igneous origins. The mafic samples display textures and compositions that reflect a cumulate origin related to magma stored below Kasatochi, both of which were brought to the surface during the 2008 eruption. I also interpret the ultramafic enclaves as cumulates, but from a separate, unrelated magmatic source. The ultramafic and mafic enclaves from Kasatochi share mineralogical and compositional similarities with inclusions from neighboring Adak Island, suggesting that similar igneous processes occur at both volcanic centers.