HR: 1340h
AN: V13B-0557    [Abstracts]
TI: The 226Ra-230Th-238U disequilibria of enigmatic magmas from Piton de la Fournaise Volcano, Reunion Island (1950-1998)
AU: * Pietruszka, A J
EM: apietrus@geology.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, San Diego, CA 92182 United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Albarede, F
EM: albarede@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, Laboratoire des Sciences de la Terre, Lyon, 69364 France
AU: Blichert-Toft, J
EM: jblicher@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, Laboratoire des Sciences de la Terre, Lyon, 69364 France
AB: We present a detailed study of the 226Ra-230Th-238U disequilibria and selected incompatible trace-element abundances of lavas from Piton de la Fournaise, a frequently active ocean-island volcano, using high-precision MC-ICP-MS. The samples erupted between 1950-1998 from vents within the summit caldera (Enclos Fouque) or along the rift zones of the volcano. The lavas display a significant range in 226Ra-230Th disequilibria (25-33% excess 226Ra) and 230Th-238U disequilibria (14-20% excess 230Th). The (230Th/232Th) ratios of the lavas are relatively constant (0.9% range) and do not correlate with either Th/U or (230Th/238U). A strong correlation (R2=0.97) between the (230Th/238U) and Th/U ratios suggests that most of the Th-U fractionation in these lavas occurred recently compared to the 76 kyr half-life of 230Th [because the effects of this fractionation are not yet reflected in the (230Th/232Th) ratios]. In contrast, the 226Ra-230Th disequilbria do not correlate with the Ba/Th ratios of the lavas (a geochemical analog). Thus, the range in Ba/Th (6.1%) and (226Ra/230Th) ratios probably results from either more than one magmatic process or a single process operating over a time scale that is longer than the half-life of 226Ra (1600 years). Unlike Hawaiian shield volcanoes, which display systematic temporal geochemical variations on a time scale of decades to centuries, the fluctuations in lava chemistry at Piton de la Fournaise do not display any coherent trends over the last 50 years. Some geochemical parameters [e.g., Ba/U, Nd/Sm or (230Th/238U)] do vary systematically over a period of a few years, but these trends are not observed consistently for the same samples and different geochemical parameters. The origin of these complex variations in lava chemistry at Piton de la Fournaise is enigmatic, but probably includes a combination of mantle (Albarede and Tamagnan, 1988, J. Petrol. v. 29) and crustal (Sigmarsson et al., 2005, EPSL v. 234; Vlast‚lic et al., 2005, J. Petrol. v. 46) processes. Possible explanations for the data will be explored at the meeting.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005