HR: 13:40h
AN: V23D-01    [Abstracts]
TI: The Geochemical and Petrological History of Arenal Volcano, Costa Rica
AU: * Bolge, L L
AF: Rutgers University, Department of Geological Sciences 610 Taylor Rd, Piscataway, NJ 08854 United States
AU: Carr, M J
AF: Rutgers University, Department of Geological Sciences 610 Taylor Rd, Piscataway, NJ 08854 United States
AU: Feigenson, M D
AF: Rutgers University, Department of Geological Sciences 610 Taylor Rd, Piscataway, NJ 08854 United States
AU: Alvarado, G E
AF: Instituto Costarricense de Electricidad, P.O. Box 10032, San Jos‚, 1000 Costa Rica
- AB: The eruptive history of Arenal spans approximately 7,000 years, consisting of both explosive (tephra) eruptions and more effusive (lava) eruptions. Over the past 4,000 years Arenal has produced tephras layers roughly every 300 years, although there are two 500-600 year gaps in volcanic activity between 3000 and 1800 years ago. This study collected samples from 20 tephra and 10 lava units from the past 4,000 years and one lava and tephra unit dated at 7,000 years. The purpose of this study is to examine the geochemical evolution of the magma erupting at Arenal. Tephras from Arenal divide into two groups on the basis of SiO2 content. Mafic tephras have up to 53 wt.% SiO2, whereas felsic tephras have at least 56 wt.% SiO2. The gap in the SiO2 contents of the tephras is filled by lavas, which have SiO2 contents of 53-56 wt.%. The tephras that erupted prior to a hiatus at 3000 years ago consist mostly of mafic tephras. After the gap, tephras alternate between mafic and felsic. At approximately the time of this gap, the degree of melting, estimated from Zr/Hf and Nb/Ta, and the slab signal, estimated from Ba/La and Pb/Ce, also changed at Arenal. Prior to the gap the degree of melting is increasing along with the slab signal, both then begin decreasing after the gap. Over the past 4,000 years the mafic explosive tephras, which all have similar SiO2 contents, show an increase in compatible elements and a decrease in incompatible elements. This can also been seen in the felsic tephras, though not as well, since these tephras are not present throughout the whole eruptive history. This change in compatible and incompatible elements over time demonstrates fractionation occurring in the magma chamber prior to the entire eruptive sequence. The lavas, however, do not show this change in compatible and incompatible elements with time.
DE: 8400 VOLCANOLOGY
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting