HR: 0800h
AN: V31B-0486    [Abstracts]
TI: Textural Comparison of Natural and Experimental Vesicles in the 79 AD Vesuvius Pumice
AU: * Gurioli, L
EM: gurioli@hawaii.edu
AF: Dept. of Geology & Geophysics, University of Hawaii, Honolulu, HI 96822, United States
AU: Shea, T
EM: tshea@hawaii.edu
AF: Dept. of Geology & Geophysics, University of Hawaii, Honolulu, HI 96822, United States
AU: Larsen, J
EM: faust@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, United States
AU: Houghton, B
EM: bhought@soest.hawaii.edu
AF: Dept. of Geology & Geophysics, University of Hawaii, Honolulu, HI 96822, United States
AB: This study focuses on the products of the opening stage (EU1) and the Plinian fallouts (EU2 and EU3) of the 79 AD Vesuvius eruption, Italy, and the comparison of their textural features with those from decompression experiments. EU1 and EU2 (white phonolitic pumices) show evidence of continuous bubble nucleation, unrestricted growth and coalescence, as well as degassing-induced crystallization. In contrast, pumices from the base to the top of the grey tephriphonolitic fallout sequence show a gradation from lower to higher vesicularities, and a transition from abundant coarser vesicles (EU3b) to an absence of coarse bubbles and the highest vesicle number densities (EU3t). EU3b textures indicate either strain-induced bubble collapse due to incomplete mixing, or early nucleation within the magma chamber, whereas EU3t textures point to fast ascent. Decompression experiments were conducted using EU1 and EU2 pumices as starting material. Series A experiments involved equilibrating the powdered samples for 4-5 days at 825 and 850°C and 150 MPa. Series B experiments used solid slugs of hydrated EU2 glass synthesized at 1000°C and 100 MPa for 3 days to remove pore spaces from the starting powders, then re-equilibrated at 100 MPa and 800-850°C for 4-5 days. After a 30 min reheating step at 800-850°C at 100 and 150 MPa, samples were decompressed at a constant rate of 0.25 MPa/s. In all experiments, bubble nucleation starts after decompression by 25 to 50 MPa. Likewise, all experiments show onset of coalescence when samples reach a pressure of 25 MPa. In EU1 bubbles grow continuously whereas in EU2 bubble growth is sluggish and discontinuous. A comparison between series A and B experiments shows that large pore spaces do not significantly influence vesiculation. After coalescence begins, any textural differences from early nucleation and growth at pressures <25 MPa are progressively erased. This offers yet another explanation for the textural homogeneity of natural EU1 and EU2 samples. Additional experiments involving EU3 are en route to determine whether strain-induced collapse, early bubble nucleation and/or fast ascent can be recorded and preserved after decompression.
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting