The bulk composition of magma erupted from Volcan Arenal has remained
nearly constant (SiO2 = 53.6-54.9 wt%; MgO = 5.0-4.5 wt%) during
almost 30 years of continuous activity (1969-1996). None the less,
clinopyroxene (cpx) phenocrysts and their spinel inclusions record a
much more complex open-system evolution in which steady-state production
of the erupted basaltic andesitic magma is linked to episodic injections
of basalt into Arenal's magma conduit/reservoir system. High-resolution
major element zoning profiles (electron microprobe) on a large number of
phenocrysts (>14,000 analyses), tied to back-scattered electron (BSE)
images, have been used to assess the compositional characteristics of
the magmatic end members as well as the timing and dynamics of magma
replenishment events. No two cpx phenocrysts have exactly the same
zoning profile. The vast majority of our analyses record the
crystallization of cpx (Cr2O3 < 0.12 wt%; Mg# = 65-79; Al/Ti = 2-7)
from a liquid comparable to or more evolved than erupted magma
compositions. However, half of all cpx grains are cored by high-Cr cpx
(Cr2O3 = 0.2-0.72 wt%) or contain similar basaltic compositions as
abrupt growth bands in phenocrysts with and without high-Cr cores;
phenocrysts with high-Cr cpx occur throughout the ongoing activity. In a
few cases, high-Cr cpx occurs very near the outer margin of the grain
without an apparent growth hiatus, particularly in 1968/69 and 1992/93.
The main conclusions are: (1) all basaltic andesitic lavas erupted at
Arenal during the ongoing activity that began in July, 1968, are the
products of magma mixing, (2) clinopyroxenes record multiple
replenishment events of basaltic magma in contrast to the near constancy
of erupted bulk compositions, (3) some phenocrysts preserve records of
multiple interactions with basaltic magmas requiring magmatic processes
to operate on time-scales shorter than residence times of some
phenocrysts, (4) multiple occurrences of clinopyroxene with high-Cr rims
suggest that basalt replenishment events have occurred with sub-decadal
frequency and may predate eruption by months or less. From this we infer
that Arenal volcano is underlain by a continuously active, small-volume
magmatic reservoir maintained in quasi-steady state by basalt recharge
over several decades. The monotony of erupting Arenal magmas implies
that fractionation, recharge, ascent, and eruption are well balanced in
order for magmas to be essentially uniform while containing phenocrysts
with vastly different growth histories at the time of eruption.