HR: 0800h
AN: V41C-1471    [Abstracts]
TI: Spatial and Temporal Isotopic Gradients in the Western Canary Islands
AU: * Nelson, B K
EM: bnelson@u.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences, Seattle, WA 98195 United States
AU: Carracedo, J C
EM: jcarracedo@ipna.csic.es
AF: Estacion Volcanologica de Canarias, IPNA-CSIC, La Laguna, 38206 Spain
AU: Badiola, E R
EM: mcner12@mncn.csic.es
AF: Museo Nacional de Ciencias Naturales, CSIC, Madrid, 28006 Spain
AU: Hamilton, A
EM: superam@u.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences, Seattle, WA 98195 United States
AU: Guetschow, H
EM: guetschhwork@yahoo.com
AF: University of Washington, Dept. Earth and Space Sciences, Seattle, WA 98195 United States
AB: The westernmost Canary Islands (La Gomera, La Palma and El Hierro) are erupted on Jurassic oceanic crust away from chemical influence by African continental crust or its sedimentary apron. These islands are in their shield-building or post-shield gap stage of growth, and expose thick volcanic sections that have excellent absolute age constraints. Although on a reconnaissance scale magmas from these islands are isotopically homogeneous, relatively dense sampling and new Nd-Sr-Hf and Pb isotope data indicate subtle temporal and spatial shifts in isotopic compositions that have not been resolved previously. The overall range in isotopic composition is small (e.g., ε_Nd = +4.5 to +6.5). All Pb isotope compositions plot below the Northern Hemisphere Reference Line, with ²⁰⁶Pb/²⁰⁴Pb ranging from 19.40 to 20.10. However, especially in the high-precision MC-ICP-MS Pb data (>100 new analyses), we observe some changes in average composition with age. The oldest subaerial eruptions on La Palma (Garafia and Taburiente volcanism; 1.5 - 0.4 Ma) have ²⁰⁷Pb/²⁰⁴Pb less radiogenic than subsequent eruptions (Bejenado and Cumbre Vieja; 0.4 Ma to present). At a finer scale, within a single volcano, there are some systematic temporal variations at the millennial or hundred-year time scale (Taburiente and Cumbre Vieja, respectively) but it is not consistent throughout the life of the volcano. La Palma magmas show no resolvable temporal patterns in Nd and Sr compositions. Spatial variability is manifested in basalt from El Hierro, which erupted contemporaneously with Cumbre Vieja. El Hierro basalts are slightly, though consistently, more radiogenic in Nd (approximately 1 ε-unit) for a given Sr composition, and on average have less radiogenic Pb compositions than La Palma basalt. La Gomera has a more restricted range in Hf and a larger range in Sr isotope compositions than La Palma and El Hierro. The overall isotopic homogeneity of these islands contrasts strongly with hot spots such as Hawaii and Iceland. The variation that is resolvable is either on a large spatial scale (distinct sources for El Hierro, La Palma and La Gomera) or on a very fine scale when short (<100 kyr) stratigraphic sections are examined. This is consistent with the Hoernle and Schmincke (1993) model of individual ``blobs'' within the plume feeding different volcanoes approximately simultaneously. In addition, there must be some chemical heterogeneity within these individual volumes that may be sampled in systematically changing proportions with progressive melting to generate isotopic trends on a short (10 - 10⁵ yr) time scale. K. Hoernle and H.-U. Schmincke, J. Petrol. 34, 599 (1993).
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005