HR: 1340h
AN: V13E-0603    [Abstracts]
TI: Evidence for Shallow-level Assimilation of the Submarine Volcanic Complex Underlying the NW Rift Zone of Tenerife, Canary Islands
AU: * Hamilton, A
EM: superam@u.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, 351310, Seattle, WA 98195
AU: Nelson, B
EM: bnelson@u.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, 351310, Seattle, WA 98195
AU: Carracedo, J
EM:
AF: Estacion Volcanologica de Canarias, IPNA-CSIC, La Laguna, Tenerife, 38206 Spain
AB: Evidence for shallow-level assimilation of the volcanic edifices underlying ocean islands has been identified in volcanoes such as Hawaii and Iceland that have high fluxes and well developed magma chambers. In the low magma flux setting of the Canary Islands, high level assimilants appear restricted to oceanic crust and associated sediment. The exceptions are Quaternary formations from Tenerife (Wolff and Palacz, 1989; Wolff et al., 2000), and phonotephrites from La Palma's 1949 eruption (Klugel et al., 2000). From less differentiated magmas dispersed along the NW rift zone of Tenerife, we report evidence for assimilation of the submarine phase of one of three underlying volcanic massifs of Tenerife. Sampled NW rift zone activity comprises basaltic to phonolitic eruptions over approximately the last 20,000 years. MgO concentrations range from 0.85 to 8.26%. Samples are enriched in incompatible elements, La averages 303x chondrites, (La/Yb)_N averages 20.5. Major and trace element data are consistent with evolution only by fractional crystallization; however, high precision Pb isotope coupled with Sr-Nd isotope data indicate assimilation and mixing. ²⁰⁶Pb/²⁰⁴Pb ranges from 19.725 to 19.783, with rare samples as low as 19.680 and as high as 19.820, with increasingly radiogenic values accompanying differentiation. Possible assimilants include Jurassic ocean crust, overlying oceanic sediment, and early-stage Tenerife volcanic products. Teno, an older volcanic massif underlying the NW rift zone, has Pb isotope compositions typically more radiogenic than the NW rift zone samples (Simonsen et al., 2000), consistent with the modeled assimilant. Assimilation of this volcanic massif is consistent with shallow fractionation (<15 km) required by the presence of plagioclase phenocrysts (Hoernle and Schmincke, 1993). Assimilation of Teno can also explain the seawater hydrothermal signal observed in the Sr-Nd isotope trend. A subset of seven rift zone samples range to more radiogenic Sr ratios (0.70303 to 0.70355) relative to other Tenerife samples, but have nearly constant Nd compositions; ε_Nd ranges only from +4.1 to +4.9. During its growth, the seamount stage of Teno underwent varying degrees of hydrothermal alteration by interaction with seawater (Ancochea et al., 1990); NW rift zone magmas appear to have inherited this signature by assimilation. There is no evidence for a syn-eruptive hydrothermal system or post-eruptive alteration. Assimilation of Teno restricts the crystallization process to a shallow-level within the massif, with a depth no greater than ~6 km below sea level (Ranero, et al., 1995). Isotopic identification of Teno as an assimilant constrains part of the magmatic system to depths above the Jurassic oceanic crust. References: 1. Wolff J. A., Palacz, Z.A., Min. Mag 53, 519 (1989). 2. Wolff J. A., Grandy, J.S., Larson, P.B., J. Vol. Geo. Res 103, 343 (2000). 3. Klugel A., Hoernle, K.A., Schmincke, H., White, J., J. Geophys. Res 105, 5997 (2000). 4. Simonsen S. L., Neumann, E.R., Seim, K., J. Vol. Geo. Res 103, 299 (2000). 5. Hoernle K., Schmincke, H.-U., J. Pet. 34, 599 (1993). 6. Ancochea E., Fuster, J.M., Ibarrola, E., Cendrero, A., Coello, J., Hernan, F., Cantagrel, J.M., Jamond, C., J. Vol. Geo. Res 44, 231 (1990). 7. Ranero C. R., Torne, M., Banda, E., Mar. Geophys. Res 17, 519 (1995).
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005