HR: 13:55h
AN: V23D-02 INVITED     [Abstracts]
TI: Closed to Open-System Crustal-level Differentiation During Eruption of Andesite: Arenal, Costa Rica, 1968-2003
AU: * Gill, J
EM: jgill@es.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AU: Ryder, C
EM: carrie@evje.com
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AU: Tepley, F
EM: ftepley@es.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AU: Ramos, F
EM: framos@es.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AB: Arenal volcano has erupted $\sim$ 0.5 km3 of medium-K tholeiitic basaltic andesite continuously for $>$35 years. We report new high precision whole rock major and trace element and Sr-Nd-Hf-Pb isotope data for $>$50 samples from throughout the eruption. Lava compositions during the first half of the eruption by volume can be related by $\sim$ 20% closed system fractional crystallization of the phenocryst minerals (Opx$>$Plag$>$Cpx$>$Mt) with $\Sigma$r2 = 0.01 for major elements, and acceptable D's for all trace elements. Ratios of isotopes and incompatible trace elements are constant. MELTS models best approximate this at water-undersaturated conditions: P=4 kb, T=1190oC, H2O = 2.5 wt%, and fO2 = QFM+2. The match of melt and mineral compositions is imperfect, but results can be reconciled qualitatively by polybaric crystallization extending to water-saturated conditions at $<$3 km. The differentiates erupted first from the shallowest part of the storage system and progressed to less differentiated compositions. Differentiation processes subsequently changed. Although some differentiation indices have returned to or exceeded initial values, compatible elements have remained constant or declined only slightly, and Pb isotope and initially-constant trace element ratios have changed. The change has been continuous in some cases but lasted for only several years in others (Pb isotopes, Th/U, Pb/Ce). In all cases, the more recent magmas have less of a ``subduction signature'' (lower Ba/La, Pb/Ce, excess 238U). We attribute these trends to an evolving balance between recharge, crystallization, and eruption. The recharging magma reflects less flux melting of an isotopically similar source that is transitional between that of central Costa Rica and Nicaragua.
DE: 8439 Physics and chemistry of magma bodies
DE: 8145 Physics of magma and magma bodies
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting