HR: 14:25h
AN: V23D-04 INVITED     [Abstracts]
TI: Support from Mineral and Melt-Inclusion Data for a Flux-controlled Ascent Model to Explain Longevity, Magnitude, and Composition of the Current Arenal Eruption
AU: * Streck, M J
EM: streckm@pdx.edu
AF: Dept. of Geology, Portland State University, Portland, OR 97207 United States
AU: Costa, F
EM: Fidel.Costa-Rodriguez@rub.de
AF: Institut f\"{u}r Geologie, Mineralogie & Geophysik, Ruhr-Universit\"{a}t Bochum, Bochum, 44780 Germany
AB: Arenal volcano is famous for its ongoing, small-scale, continuous activity ever since it reawakened in July of 1968. Although activity levels have generally declined from early in the eruption to now, multiple daily explosions and associated lava production have persisted and continue to be characteristic for Arenal's current eruption. The combination of small eruption volumes, decades-long continuous activity, and eruption products that are remarkable compositionally similar, phenocryst-rich basaltic andesites provoke pressing questions about eruption driving force and petrogenetic history. Our key results indicate all eruption products exhibit significant but comparable complexities in mineral compositions, zoning and distributions requiring multi-stage mixing. Bulk compositions are mostly too mafic to have crystallized the majority of ferromagnesian minerals yet appropriate liquids are preserved in melt inclusions from tephras. Furthermore, no progressive changes in zoning near rims of phenocrysts over the course of the current eruption are apparent which complicates scenarios in which one, evolving magma batch has been tapped for the last 36 years. Evidence for replenishment events with more primitive magma is often found in cpx phenocrysts in form of discrete growth bands of high Mg\#, Cr-rich cpx bound by low Mg\# cpx. Modeling the diffusive equilibration of Fe-Mg gradients across bands yielded times of the latest recharge within a crystal prior its eruption of $<$ 1 to $\sim$200 years suggesting variable residence times of phenocrysts and that mafic recharge can closely predate eruption. Our results fit best a model in which similar basaltic andesites are repeatedly generated from mantle magma batches during their ascent as they mix with resident magmas, fractionate, and assimilate crystals. We infer that new increments of basaltic andesite are continuously blended into the eruption-feeding reservoir concurrently to the current activity. We propose that observed longevity, magnitude, and monotonous composition of the current Arenal eruption mirror the combined result of duration, mass, and frequency of mantle inputs moving through the crust.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting