Except for the first 2 years since July 29, 1968, Arenal volcano has
continuously erupted compositionally monotonous and phenocryst-rich
(similar to35%) basaltic andesites composed of plagioclase (plag),
orthopyroxene (opx), clinopyroxene (cpx), spinel 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 minimum of four
crystallization environments. We suggest three distinct crystallization
environments crystallized low Mg# (<78) silicate phases from andesitic
magma but at different physical conditions, such as variable pressure of
crystallization and water conditions. The dominant environment, i.e.,
the one which accounts for the majority of minerals and overprinted all
other assemblages near rims of phenocrysts, cocrystallized clinopyroxene
(Mg# similar to71-78), orthopyroxene (Mg# similar to71-78),
titanomagnetite and plagioclase (An(60) to An(85)). The second
environment cocrystallized clinopyroxene (Mg# 71-78), olivine
(<Fo(78)), titanomagnetite, and very high An (similar to90) plagioclase,
while the third cocrystallized clinopyroxene (Mg# 71-78) with high (>7)
Al/Ti and high (>4 wt.%) Al2O3, titanomagnetite with considerable Al2O3
(10-18 wt.%) and possibly olivine but appears to lack plagioclase. A
fourth crystallization environment is characterized by clinopyroxene
(e.g., Mg#=similar to78-85; Cr2O3=0.15-0.7 wt.%), Al-, Cr-rich spinel
olivine (similar toFo(80)), and in some circumstances high-An (>80)
plagioclase. This assemblage seems to record mafic inputs into the
Arenal system and crystallization at high to low pressures.
Single crystals cannot be completely classified as xenocrysts,
antecrysts (cognate crystals), or phenocrysts, because they often
contain different parts each representing a different crystallization
environment and thus belong to different categories. Bulk compositions
are mostly too mafic to have crystallized the bulk of ferromagnesian
minerals and thus likely do not represent liquid compositions. On the
other hand, they are the cumulative products of multiple mixing events
assembling melts and minerals from a variety of sources. The driving
force for this multistage mixing evolution to generate erupting basaltic
andesites is thought to be the ascent of mafic magma from lower crustal
levels to subvolcanic depths which at the same time may also go through
compositional modification by fractionation and assimilation of country
rocks. Thus, mafic magmas become basaltic andesite through mixing,
fractionation and assimilation by the time they arrive at subvolcanic
depths. We infer new increments of basaltic andesite are supplied nearly
continuously to the subvolcanic reservoir concurrently to the current
eruption and that these new increments are blended into the residing,
subvolcanic magma. Thus, the compositional monotony is mostly the
product of repetitious production of very similar basaltic andesite.
Furthermore, we propose that this quasi-constant supply of small
increments of magma is the fundamental cause for small-scale,
decade-long continuous volcanic activity; that is, the current eruption
of Arenal is flux-controlled by inputs of mantle magmas. (C) 2004
Elsevier B.V. All rights reserved.