HR: 1330h
AN: V42B-0358    [PDF]
TI: Volatile Release and Eruption Dynamics of a Basaltic Plinian Eruption From Masaya Caldera, Nicaragua
AU: * Wehrmann, H
EM: hwehrmann@geomar.de
AF: SFB 574 / Geomar Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Freundt, A
EM: afreundt@geomar.de
AF: SFB 574 / Geomar Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Kutterolf, S
EM: skutterolf@geomar.de
AF: SFB 574 / Geomar Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Schmincke, H
EM: hschmincke@geomar.de
AF: SFB 574 / Geomar Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Strauch, W
EM: wil.gf@ineter.gob.ni
AF: Ineter, Direccion Geofisica, Managua, 1 Nicaragua
AB: Our project is part of SFB 574 "Volatiles and Fluids in subduction zones", and focusses on degassing dynamics of highly-explosive arc volcanoes. Masaya Caldera in west-central Nicaragua is part of the Central American volcanic arc at the convergent boundary of the Cocos and Carribean plates. A basaltic plinian eruption of VEI 6 occurred at Masaya Caldera in the Late-Pleistocene, depositing a widespread fan of scoria lapilli, named Fontana Tephra. We have constrained parameters of the Fontana eruption by extensive isopach and isopleth mapping. Total erupted tephra volume is $>$0.83 km$^3$ (about $10^{12}$ kg DRE). The eruption columns reached 30 to 35 km height at an average discharge rate of 1.3*$10^8$ kg/s. This violent eruption was not continuous but proceeded in distinct pulses evident by the well-bedded deposit. An initial sequence of numerous highly explosive but short pulses formed a well-bedded layer of very highly vesicular, hawaiian-type lapilli, possibly representing a gas-enriched top zone of the magma reservoir. The following series of longer-duration plinian events, interupted by weak phases of ash emission, formed beds of highly vesicular scoria lapilli. The eruption ceased with abundant short-lived pulses of lower-energy subplinian activity. We estimate volatile emissions during the eruption from the differences in volatile concentration between matrix glass and glass inclusions in minerals, considered to represent degassed and undegassed melt, respectively. Concentrations of fluorine of about 7000 ppm are about equal in matrix glass and glass inclusions, indicating little degassing of fluorine during eruption. Chlorine contents amount to 1200 ppm in the inclusions, and to about 1000 ppm in matrix glass. The concentration difference, multiplied by erupted magma mass, suggests a total chlorine emission of 16 Mt. Apparently only little chlorine exsolved in the initial eruption phase, but degassing strongly increased during the plinian phase. Sulphur concentrations in glass inclusions range between 220 ppm and 680 ppm while reaching values as low as 20 ppm in matrix glass. Sulphur emission totals at least 24 Mt, of which the largest portion had been released during late stages of the eruption. This is a minimum estimate of sulphur emission since it is not known if a sulphurous fluid phase already existed in the magma reservoir. The Fontana eruption provides an impressive example of a highly-explosive, large-magnitude plinian eruption of basaltic composition. The eruption was driven by degassing of magmatic volatiles and evolved through numerous eruptive pulses, of which high-frequency short-duration phases bracket low-frequency long-lasting episodes.
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting