HR: 1340h
AN: V53A-0602    [Abstracts]
TI: U-Series Whole-Rock and Mineral Geochemistry of Recent Bimodal Eruptive Products From the Torfaj\"{o}kull/Veidiv\"{o}tn Volcanic System, South-Central Iceland
AU: * Zellmer, G F
EM: gzellmer@hawaii.edu
AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa 1680 East West Road, Honolulu, HI 96822 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa 1680 East West Road, Honolulu, HI 96822 United States
AU: Gr\"{o}nvold, K
EM: karlgr@hi.is
AF: Nordic Volcanological Institute, University of Iceland, Reykjavik, IS-101 Iceland
AU: Jurado-Chichay, Z
EM: zinzuni@hawaii.edu
AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa 1680 East West Road, Honolulu, HI 96822 United States
AU: Hern\'{a}ndez-Tr\'{e}vi\~{n}o, T
EM: tht@tonatiuh.igeofcu.unam.mx
AF: Instituto de Geofisica, UNAM, Mexico, DF 04510 Mexico
AB: Together, the Torfaj\"{o}kull central volcano and the Veidiv\"{o}tn Fissure Swarm immediately to the north represent a loosely associated volcanic system that has in historic and post-glacial times produced numerous coeruptive rhyolite and basalt deposits on the southern end of Iceland's eastern rift, the Eastern Volcanic Zone (EVZ). In order to decipher the rates and processes of bimodal magma generation in this area, we have studied major, trace and U-series isotope geochemistry of Torfaj\"{o}kull rhyolite lavas and associated Veidiv\"{o}tn basalts (lavas and tephras) of the 871 AD and 1477 AD bimodal eruptions. The Torfaj\"{o}kull magmas are glassy high-K rhyolites (SiO$_{2}$ = 71%, K$_{2}$O/Na$_{2}$O = 0.9). Minerals in both rhyolites and basalts are dominantly phenocrysts that display mild normal zoning. Xenocrysts are rare, but crystals of composition similar to those of the Veidiv\"{o}tn basalts make up a small percentage of the Torfaj\"{o}kull rhyolite mineral assemblage. ($^{238}$U/$^{232}$Th) activity ratios in basalts range from 0.99 to 1.02, while rhyolites yield significantly lower values of ~0.90. ($^{230}$Th/$^{232}$Th) activity ratios are more similar, ranging between 1.03 and 1.06 (\pm0.01, 2$\sigma$), resulting in larger Th excesses in the rhyolites. A pyroxene mineral separate from the 1477 Breidavatn basalt flow has $^{238}$U excess and ($^{230}$Th/$^{232}$Th) = 1.06, yielding a zero age whole rock-pyroxene U-Th isochron. (Analysis of other mineral phases and Ra concentrations is in progress.) Torfaj\"{o}kull is therefore distinct from other volcanoes within or just off the EVZ, such as Askja, Krafla and Hekla, which typically have 7-15% higher ($^{230}$Th/$^{232}$Th) activity ratios in basalts compared to rhyolites. We speculate that the rhyolitic melt at Torfaj\"{o}kull was generated from a source similar to that of the Veidiv\"{o}tn basalts by rapid differentiation (partial melting and/or fractional crystallization), affecting Th/U ratios but not ($^{230}$Th/$^{232}$Th) activity ratios. Crustal melting has been advocated by other authors as a process to generate rhyolitic magmas in Iceland, and the data presented here suggests that if this process operates at Torfaj\"{o}kull, it is rapid compared to the half-life of $^{230}$Th and capable of generating significantly greater $^{230}$Th-$^{238}$U disequilibria than occur in the basalts. The heat of fusion required to melt Icelandic crust may have been provided by the injection of Veidiv\"{o}tn basaltic parental magma into upper crustal levels (accompanied by uptake of few crystals into the newly generated rhyolitic melt), simultaneously leading to rapid crystallization of the basaltic magma with its correspondingly young U-Th crystallization age.
DE: 8434 Magma migration
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting