HR: 15:10h
AN: V23D-07    [Abstracts]
TI: Phase Equilibrium Experiments at 0.5 GPa and 1100-1300 deg. C on a Basaltic Andesite From Arenal Volcano, Costa Rica.
AU: * Pertermann, M
EM: mperter@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 245 NHB, 1301 W Green Street, Urbana, IL 61801 United States
AU: Lundstrom, C C
EM: lundstro@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 245 NHB, 1301 W Green Street, Urbana, IL 61801 United States
AB: The on-going eruption of Arenal volcano has undergone complex changes in eruptive behavior. To investigate pre-eruptive crustal processes, we conducted phase equilibrium experiments on a basaltic andesite. The anhydrous synthetic starting mix has a Mg\# of 52 and concentrations of SiO$_{2}$, Na$_{2}$O and K$_{2}$O of 55, 3.0 and 0.6 wt.%, respectively. Al(OH)$_{3}$ was used to obtain bulk water concentrations of 2 and 4 wt.%. Starting mixes were loaded into Pt/C capsules and each run contained three capsules with nominal water contents of 0, 2, and 4 wt.%. Experiments were run in a piston-cylinder apparatus with NaCl/pyrex/MgO assemblies and S-type thermocouples. Water loss is a potential problem in wet experiments: the actual water contents in quenched glasses remain to be quantified and results presented here are only from runs in which melt fractions systematically increased with initial water content at a given constant temperature. At $1300\deg$C the dry composition yields glass coexisting with traces of spinel and plag, and both hydrous compositions are above the liquidus. Glass, plag (An$_{75-82}$) and traces of spinel are present for all three compositions at $1250\deg$C. At $1200\deg$C, cpx is present in the dry composition with plag and glass; increasing water content enhances the plag stability (An$_{77-69}$) at the expense of cpx, which is replaced by low-Ca pyroxene. Traces of spinel are present and are found in all experiments at lower temperatures, independent of water content. Cpx, low-Ca pyroxene, plag and andesitic glass are present at $1150\deg$C in the dry and 2% water compositions, but the 4% water composition yields only cpx and no low-Ca pyroxene. Due to the overall small grain size (­š10 $\mu$m) in our experiments, low-Ca pyroxene may have been overlooked, yet the lower plag mode and its decreasingly anorthitic character (An$_{64-59}$) may account for the absence of low-Ca pyroxene. At $1100\deg$C, glasses become dacitic to rhyolitic and coexist with low-An plag and cpx, and in the case of both hydrous compositions, also ilmenite. Olivine was not identified in any experiment. The plag range seen at and above $1150\deg$C covers the lower end of the plag range in erupted lavas but fails to account for An$_{85-95}$ phenocrysts also present in Arenal lavas.
DE: 8439 Physics and chemistry of magma bodies
DE: 8450 Planetary volcanism (5480)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting