U-Th-Ra disequilibria and the time scale of fluid transfer and andesite differentiation at Arenal volcano, Costa Rica (1968-2003)

Frank J. Tepley, Craig C. Lundstrom, James B. Gill, Ross W. Williams

Research output: Contribution to journalArticlepeer-review

Abstract

To evaluate the time scale of fluid transfer and andesite differentiation at Arenal volcano in Costa Rica, we have measured trace-element concentrations and U-series disequilibria in whole rocks and mineral separates (pyroxene, plagioclase, magnetite) from lavas of the current eruption (1968 to 2003) by HR-ICP-MS, TIMS and PIMMS techniques. Whole rock and mineral separate analyses show a small but measurable variation in (230Th)/(232Th) (1.10 to 1.18). In contrast, (230Th)/(238U) range from 0.91 to 1.04 reflecting the moderate spread in Th/U. Stage 1 (1968-1971) whole rocks and mineral separates have both higher (230Th)/(232Th) and (238U)/(232Th) than to younger stage 2 lavas (1971 to present), which have lower, nearly constant (230Th)/(232Th) and lower, slightly variable (238U)/(232Th). 226Ra excesses exist in both whole rocks and mineral separates with (226Ra)/(230Th) ranging between 0.94 and 4.8. Whole rock (226Ra)/(230Th) are largest early in the eruption and decrease in the later lavas, which are influenced by newer recharge material. 238U-230Th whole rock and mineral data produce an inclined array on an equiline diagram, which we interpret to represent progressive melting of a variably fluxed mantle wedge and a Nicaraguan sediment component, and subsequent mixing. 238U-230Th internal isochrons suggest that minerals grew instantaneously with respect to the half-life of 230Th. Whole rock and mineral separate (226Ra)/(230Th) data indicate that melts were produced, transported, differentiated and erupted in < 8 kyr. Mineral (226Ra)/Ba-(230Th)/Ba model ages are calculated and corrected for melt inclusions and glass adherents in the mineral fractions, and for the differential partitioning of Ra and Ba. Plagioclase model ages and 238U-230Th isochron ages suggest that plagioclase could be as young as a few years or as old as several centuries upon eruption.

Original languageEnglish (US)
Pages (from-to)147-165
Number of pages19
JournalJournal of Volcanology and Geothermal Research
Volume157
Issue number1-3
DOIs
StatePublished - Sep 15 2006

Keywords

  • Arenal volcano
  • crystal model ages
  • differentiation time scale
  • fluid transfer U-Th-Ra disequilibria

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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