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- Title
Pre-eruptive magma mixing and crystal transfer revealed by phenocryst and microlite compositions in basaltic andesite from the 2008 eruption of Kasatochi Island volcano.
- Authors
NEILL, OWEN K.; LARSEN, JESSICA F.; IZBEKOV, PAVEL E.; NYE, CHRISTOPHER J.
- Abstract
The August 7-8, 2008, eruption of Kasatochi Island volcano, located in the central Aleutians Islands, Alaska, produced abundant, compositionally heterogeneous basaltic andesite (52-55 wt% SiO2) that has been interpreted to result from pre-eruptive magma mixing. The basaltic andesite contains two populations of plagioclase phenocrysts. The first, volumetrically dominant population consists of oscillatory-zoned phenocrysts with an overall normal zonation trend toward comparatively sodic rims (An55-65), interrupted by dissolution features and spikes in calcium content (up to ~An85). The second population consists of phenocrysts with highly calcic compositions (~An90). These phenocrysts contain sharp decreases in calcium content close to their rims (reaching as low as ~An60), but are otherwise texturally and compositionally homogeneous. Groundmass plagioclase microlites are generally much more calcic than rims of the first phenocryst population, with more than 50% of measured microlites containing >An80. Major, minor, and trace element concentrations of plagioclase microlites and phenocrysts indicate that oscillatory-zoned phenocrysts derived from cooler (800-950 °C), more silicic mixing magma, while unzoned, calcic phenocrysts were associated with hotter (900-1050 °C), mafic magma. The mixing of these magmas just prior to eruption, followed by decompression during the eruption itself created high effective undercoolings in the mafic end-member, and lead to the nucleation of high-An microlites. MgO and FeO concentrations of plagioclase microlites and high-An phenocryst rims (up to ~0.4 and ~1.3 wt%, respectively) provide further evidence for high mixing- and eruptioninduced effective undercoolings.
- Subjects
KASATOCHI Volcano (Alaska); ALEUTIAN Islands (Alaska); VOLCANISM; MAGMAS; PLAGIOCLASE
- Publication
American Mineralogist, 2015, Vol 100, Issue 4, p722
- ISSN
0003-004X
- Publication type
Article
- DOI
10.2138/am-2015-4967