HR: 14:10h
AN: V32F-03    [PDF]
TI: Sediment/Fluid Contributions and Element Transport Deduced from Elemental and Isotopic Data From the Luzon-Bicol arc Systems, Philippines
AU: * Du Frane, S A
EM: radium@unm.edu
AF: Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 United States
AU: Asmerom, Y
EM: asmerom@unm.edu
AF: Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 United States
AU: Mukasa, S B
EM: mukasa@umich.edu
AF: Department of Geosciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Dreyer, B
EM: dreyer@levee.wustl.edu
AF: Department of Earth $ Planetary Science, Washington University, Saint Louis, MO 63130 United States
AU: Morris, J D
EM: jmorris@levee.wustl.edu
AF: Department of Earth $ Planetary Science, Washington University, Saint Louis, MO 63130 United States
AB: Melting and mass transfer processes in arcs are poorly understood. The Luzon/Bicol arc system, Philippine archipelago, provides the opportunity to gain insight into these processes. Volcanism in the Bicol arc (eastern arc) is associated with the westward subduction of the Philippine Sea Plate, with thin pelagic sediment cover, while volcanism of the Luzon arc (western arc) is associated with the eastward subduction of the South China Sea Plate, including a thick terrigenous sediment cover from Eurasia. Here we present U-series, Nd and Sr isotopic and trace element data of lavas from both arcs: Pinatubo and Taal [Luzon arc], Bulusan, Iriga and Mayon [Bicol arc]. Lavas from both volcanic fronts have typical (230Th/238U) $<$ 1 associated with arc volcanism, except for Mayon volcano, which has up to 15% 230Th enrichment. Bicol arc lavas have 87Sr/86Sr = 0.70371-0.70400 and ŸONd = + 7.9 to +6. Lavas from the Luzon arc have higher 87Sr/86Sr = 0.704227-0.70458 and lower 143Nd/144Nd, ŸONd = + 3.4 to +6.6. Bicol arc lavas have consistently higher Ba/Th = 122-208, than Luzon arc lavas, Ba/Th = 42-133. Consistent with previous inferences, the isotopic data support strong sediment involvement in Luzon arc lavas. Lavas from the Bicol arc reflect more complex melting histories. Mayon lavas, showing 230Th over 238U enrichment and the most radiogenic Nd isotopic values ($\sim$ +8), most likely reflect the result of dynamic melting of the mantle wedge under the Bicol arc. The combination of high Ba/Th ratio ($\sim$ 200, in contrast to MORB or Bulk Earth values of $\sim$ 75) and lack of 238U enrichment suggest that the wedge was modified long before the recent melting events. In contrast to Mayon, lavas from the other Bicol arc volcanoes show some fluid and possibly some sediment involvement. Although the contrast in the U-series data among volcanoes from the Bicol arc may reflect differences in melting styles (dynamic melting vs. hydration melting), the cause of the difference is not clear.
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting