HR: 14:55h
AN: V23D-06    [Abstracts]
TI: Degassing Mechanisms and Timescales of Implied by ($^{210}$Pb) Values for Andesites Erupted from Arenal Volcano
AU: * Reagan, M K
EM: mark-reagan@uiowa.edu
AF: U. Iowa, Dept. of Geoscience, Iowa City, IA 52242 United States
AU: Tepley, F J
EM: ftepley@emerald.ucsc.edu
AF: UCSC, Dept. of Earth Sciences, Santa Cruz, CA 95064 United States
AU: Gill, J B
EM: jgill@emerald.ucsc.edu
AF: UCSC, Dept. of Earth Sciences, Santa Cruz, CA 95064 United States
AU: Lundstrom, C
EM: lundstro@uiuc.edu
AF: U. Illinois, Dept. of Geology, Urbana, IL 61801 United States
AB: The ongoing eruption of Arenal, which began in 1968, is an ideal laboratory for investigating magmatic processes that occur over short time periods during eruptions. To identify and place time constraints on these processes, lavas from throughout this eruption have been analyzed for ($^{210}$Pb) (t$_{1/2}$ = 22.6 y). Because Pb is both incompatible and only weakly volatile, variations in ($^{210}$Pb)/($^{226}$Ra) largely monitor decade-scale fluxes of $^{222}$Rn through magmas. At present, only one lava has been analyzed for $^{226}$Ra, and the following discussion assumes that Ra varies in concert with other highly incompatible elements in Arenal lavas. By meeting time, additional whole rock $^{226}$Ra values will be available to further constrain this discussion. The eruption has been divided into two principal stages based on variations in bulk composition (Ryder, C., 2004, MS Thesis, UCSC). The first stage lasted from the beginning of the eruption until the early 1970s when Pb isotopes shifted. This shift marked the end of the eruption from one reservoir and the appearance of a new magma presumably from a deeper chamber. Lavas and tephras erupted in 1968 have small excesses in $^{210}$Pb over calculated $^{226}$Ra values, whereas those erupted in 1969 have $^{210}$Pb deficits. These data are consistent with decade-scale transfer of $^{222}$Rn from the less-differentiated lower portion of the original magma reservoir to the more differentiated and more phenocryst poor upper reservoir. This could occur either by diffusion of $^{222}$Rn through the melt within the chamber or by transfer in a separate gas phase in vapor saturated magma. In 1971, just before the shift in Pb isotopes, lavas erupted with an approximately 2-fold $^{210}$Pb excess over $^{226}$Ra, which suggests that the deeper magma that eventually erupted and shifted Pb-isotope values contributed volatiles to the lower portion of the original reservoir. If the excess in 1971 was due to $^{222}$Rn fluxing since 1968, then the average ($^{222}$Rn)/( $^{226}$Ra) in the lava-gas mixture must have been about 10-fold. Such large magnitude excesses can only occur if the Rn in streaming gasses was generated by ingrowth and extracted from a larger volume of magma than for other volatiles. By 1973, just before a several month pause in the eruption, ($^{210}$Pb)/( $^{226}$Ra) approached equilibrium values showing that these lavas did not received the sustained flux of $^{222}$Rn. Upon resumption of eruption, $^{210}$Pb excesses remained small but systematically climbed until 1996. By 1999, these excesses decreased again. These excesses are explainable by a constant source of excess $^{222}$Rn through the rising magmas from 1974 until about 1996 when the source of the Rn was terminated by a slackening recharge (see Ryder, 2004). The recent decrease in excess $^{210}$Pb could be a precursor for a different mode of activity for Arenal. Continued crystallization combined with lessened recharge could increase viscosity, which could terminate the eruption or cause it to become more explosive.
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting