HR: 1330h
AN: V12A-0559    [PDF]
TI: Unraveling Complex Crystal Populations of the Current Eruption of Arenal Volcano, Costa Rica: Key to Understanding 35 Years of Continuous Eruption of Monotonous Basaltic Andesites
AU: * Streck, M J
EM: streckm@pdx.edu
AF: Dept. of Geology, Portland State University, Portland, OR 97207
AU: Dungan, M A
AF: Dept of Mineralogy, Univ. of Geneva, Geneva, 1211 Switzerland
AU: Bussy, F
AF: Inst. of Mineralogy \& Geochemistry, Univ. of Lausanne, Lausanne, 1015 Switzerland
AU: Malavassi, E
AF: OVSICORI, Universidad Nacional, Heredia, 2346 Costa Rica
AB: Except for the first two years since July 29, 1968, Arenal volcano has been continuously erupting compositionally monotonous and phenocryst-rich ($\sim$35%) basaltic andesites composed of similar proportions of plagioclase (plag), orthopyroxene (opx), clinopyroxene (cpx), spinel $\pm$ olivine. Detailed textural and compositional analyses of phenocrysts, mineral inclusions, and microlites reveal comparable complexities in any given sample and identify mineral components that require a series of growth environments that not only compositionally varied but also likely in P-T-X conditions. Frequency distributions of mafic mineral compositions indicate the current eruption has been mainly tapping a reservoir containing low Mg\# silicate phases ($<$ 78) and mainly titanomagnetite which crystallized in at least three different environments to account for the following assemblages: a) low Al$_{2}$O$_{3}$ ($<$3 wt.%) cpx + opx, b) high Al$_{2}$O$_{3}$ (4 to 6 wt.%) cpx (+ opx ?), and c) low Fo olivine + cpx but without opx. Ubiquitous high Mg\# (80-$>$86) mafic silicate phases (mainly cpx) with Cr-, Al-rich spinel inclusions indicate that some portions of crystals are derived from more mafic (basaltic?) magmas occurring mainly as growth zones between cores to rims of euhedral phenocrysts. Plagioclase has a wide compositional range (An$_{95-55}$) and inclusion data in cpx hosts suggest high An plag (An$_{90-80}$) crystallized from liquids with variable Mg\# while low An plag crystallized only from low Mg\# ($<$ 47) liquids. Multi-step mixing processes likely facilitated assembling various mineral components as mafic magma from lower crustal levels ascended to subvolcanic depths undergoing at the same time evolution to erupting compositions. The mineralogical record is consistent with ascent-driven evolution of mafic magmas concurrently with the current eruption leading to new increments of basaltic andesite and that may ultimately lead to the continuous volcanic activity and compositional monotony since replenishments may be small and quasi-continuous.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting