HR: 11:05h
AN: V22A-04    [Abstracts]
TI: Leucite Crystallization in 79AD Vesuvius Magmas Inferred from Decompression Experiments
AU: * Shea, T
EM: tshea@hawaii.edu
AF: Dept. of Geology & Geophysics, Univ. of Hawaii, Honolulu, HI 96822, United States
AU: Larsen, J
EM: faust@gi.alaska.edu
AF: Geophysical Institute, Univ. of Alaska Fairbanks, Fairbanks, AK 99775, United States
AU: Gurioli, L
EM: gurioli@hawaii.edu
AF: Dept. of Geology & Geophysics, Univ. of Hawaii, Honolulu, HI 96822, United States
AU: Houghton, B
EM: bhought@soest.hawaii.edu
AF: Dept. of Geology & Geophysics, Univ. of Hawaii, Honolulu, HI 96822, United States
AB: Small leucites are ubiquitous in the phonolite and tephriphonolite magmas erupted in 79 AD at Vesuvius. They show a narrow size range (27±5 μm) throughout most of the eruptive sequence and their habit is generally euhedral. While plagioclase in calc-alkaline magmas may crystallize during ascent (e.g. Mt St Helens), the mechanism of leucite formation in peralkaline magmas is unclear. Decompression experiments on 79 AD samples may yield valuable information on their origin. In this study, single step (SSD) and multiple step decompression (MSD) experiments were conducted using rapid-quench-capable cold seal vessels with natural EU1 and EU2 phonolitic pumices as starting material. Prior to decompression, samples were equilibrated under water-saturated conditions at 150 MPa and 800, 825, and 850°C for 5-7 days, which is above leucite stability curves for both EU1 and EU2. The SSD runs involved rapidly decompressing the experiments followed by holding at 25 MPa, within the leucite stability field for all temperatures, for 5 min-7 days. The MSD runs involved decompressing the samples in a step-wise manner, where each 5 MPa pressure drop was followed by hold time of 20 seconds, replicating an ascent rate of 10 m/s. All SSD runs contain leucites with skeletal habits, regardless of the length of the hold time at 25 MPa, which suggests diffusion-limited growth and crystallization in a nucleation dominant regime after a very large effective undercooling. In contrast, the MSD runs using EU1 starting material at 800 and 825°C produced euhedral leucites similar to those in natural samples. At 850°C, the MSD runs failed to crystallize leucite in either EU1 or EU2. This suggests that leucites in the 79AD magmas could have formed either in the magma chamber, or during ascent. However, if they formed in the chamber, P and T conditions could not have exceeded 100 to 150 MPa and 800 to 850°C, assuming water saturation. Alternatively, if they formed during ascent, magma temperature could not have exceeded 825°C, assuming a mostly linear decompression path. More work comparing leucite size distributions between the experiments and natural samples will provide constraints on their formation in the 79 AD phonolite.
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting