HR: 1330h
AN: V42B-0353    [PDF]
TI: Experimental Constraints on the Magma Chamber Conditions and Degassing of Arenal Volcano, Costa Rica
AU: * Szramek, L A
EM: ftlas@uaf.edu
AF: Geophysical Institute and Dept.of Geology and Geophysics, Univ. Alaska Fairbanks, Fairbanks, AK 99775 United States
AU: Gardner, J
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin 1 University Station C1100, Austin, TX 78712-0254 United States
AU: Larsen, J
EM: faust@gi.alaska.edu
AF: Geophysical Institute, Univ. Alaska Fairbanks, Fairbanks, AK 99775 United States
AB: Continuously erupting since 1968, Arenal Volcano's frequent activity displays differing styles, from highly explosive events to passive lava flows. The erupted products are geochemically similar, yet show a complex history of magma chamber processes that are not fully understood. Given the temporal records and geophysical data collected from Arenal, a comparison with petrologic data and carefully controlled experiments may enable us to better understand the processes occurring at this highly active volcano. We performed phase equilibria experiments on sample AR99-1, from a 1999 eruption of Arenal, at water-saturated conditions with {\it f}O$_{2}$ buffered at NNO, in TZM pressure vessels. The purpose of these experiments is to determine the pre-eruptive P and T conditions of the Arenal magma as a starting point for decompression experiments, which will be used to help determine magmatic ascent rates. Most samples from Arenal contain plagioclase, pyroxene, and magnetite in a matrix of variably crystallized microlite-rich groundmass. Because ilmenite is absent, we first had to iterate in P and T until we obtained the correct assemblage that matches the starting material. Water content has not been obtained due to the lack of appropriate samples (mostly degassed lavas and degassed glass inclusions). Preliminary results indicate that the magma was stored at between 950$^{\circ}$C and 1000$^{\circ}$C and at a depth of 3-5 km prior to eruption, assuming water saturation. These experiments will continue, including those that are water under saturated. In order to determine magmatic ascent rates, we plan to perform a series of decompression experiments at various rates, starting from our determined pre-eruptive conditions. Our goal is to match the microlite textures observed in rapidly quenched Arenal samples, such as those from explosively produced pyroclastic flows, as a way to resolve the ascent rates. This information can be compared to geophysical data collected in the 1990's in order to link pre-cursory geophysical signals with magmatic ascent rates and petrologic observations.
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting