HR: 0800h
AN: V41C-1473    [Abstracts]
TI: High Resolution, Pb Isotope Variability Within Historic Eruptions of the Cumbre Vieja Volcano, La Palma, Canary Islands
AU: * Locke, J A
EM: lockej@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Peterson, B T
EM: boork@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Nelson, B K
EM: bnelson@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AB: The range of time-scales over which ocean island basalt (OIB) magmatism taps different mantle heterogeneities is a fundamental dynamic of mantle plumes. The variability of long-lived radiogenic isotopes in OIB magmas erupted on time scales less than 100 years has been addressed primarily for Hawaiian magmas (e.g., Pietruszka et al., 2001). Similar data are relatively sparse for hot spots with low buoyancy fluxes. The Canary Islands have low eruption rates and have been historically active. The Cumbre Vieja volcano in southern La Palma, Canary Islands, has six, well-mapped, historic eruptions spanning the entire southern rift zone. We have investigated Pb isotope compositional variations expressed in magmas erupted in a series of events spanning 500 years (the 1480, 1585, 1677, 1712, 1949, and 1971 eruptions), and sampled in detail two of these events (the 1677 and 1712 eruptions) to document isotopic variability at the month to year time-scale as well as the 100-year time scale. Previous Pb isotope investigations of Cumbre Vieja did not reveal systematic variations (e.g., Marcantonio et al., 1995 and Ovchinnikova et al., 1995). With denser sampling (40 samples) and higher precision MC-ICP-MS analyses, we observe that radiogenic Pb isotope compositions over the 500 year eruptive history decrease systematically with time (²⁰⁶Pb/²⁰⁴Pb =19.669 -- 19.611, ²⁰⁷Pb/ ²⁰⁴Pb = 15.618 -- 15.602, ²⁰⁸Pb/²⁰⁴Pb = 39.530 -- 39.430). Detailed Pb isotope analyses of the 1677 and 1712 eruptions indicate isotopically homogeneous magmas within a single eruptive episode. However, samples from both the 1677 and 1712 eruptions display mineralogic evidence for magma mixing: 1677 samples include isotopically distinct gabbroic xenoliths, and both magmas have reversely- zoned clinopyroxene phenocrysts with corroded cores of Na-rich salite, and zoned overgrowths of Al-rich salite. With time, an increasing proportion of partial melt from a less radiogenic end-member within a heterogeneous plume explains the 500 year trend. Alternatively, higher level mixing of two magmas would require sequential recharge of a single magma chamber that feeds the entire rift zone. A single chamber is structurally unlikely, and not consistent with geochemical and petrographic trends (Klü gel, 1999). From the decompressing plume, batches of melt with homogeneous Pb isotope ratios are extracted periodically. To generate mineralogic disequilibrium, each batch must segregate into a zoned magma chamber or multiple, isolated pockets and differentiate at multiple levels. Prior to eruption, magma from these pockets may remix, producing reversely-zoned clinopyroxene phenocrysts. Isotopic homogeneity is preserved within a given magma batch. In the Canary Islands, the minimum time period for eruptive basalts to reflect resolvable mantle isotope heterogeneity is on the order of 50 - 100 years. A. Klügel, K. A. Hoernle, H.-U. Schmincke, J. D. L. White, J. Geophys. Res. 105(B3), 5997 (2000). F. Marcantonio, A. Zindler, T. R. Elliot, H. Staudigel, Earth Planet. Sci. Lett. 133, 397 (1995). G. V., Ovchinnikova, B. V., Belyatskii, I. M., Vasil'eva, L. K., Levsky, A. F., Grachev, V., Arana, I. J., Mitjavila, Petrologiya, 3, 195 (1995). A. J. Pietruszka, K. H. Rubin, M. O. Garcia, Earth Planet. Sci. Lett. 186, 15 (2001).
DE: 1040 Radiogenic isotope geochemistry
DE: 3618 Magma chamber processes (1036)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005