HR: 17:45h
AN: V54A-07    [Abstracts]
TI: Secondary Hawaiian volcanism formed by flexural arch decompression
AU: * Bianco, T A
EM: rixctf@hawaii.edu
AF: Dept. of Geology and Geophysics, Univ. Hawaii, 1680 East-West Rd., POST 810 , Honolulu, HI 96822 United States
AU: Ito, G
EM: gito@hawaii.edu
AF: Dept. of Geology and Geophysics, Univ. Hawaii, 1680 East-West Rd., POST 810 , Honolulu, HI 96822 United States
AU: Becker, J M
V54A-07 AF: Dept. of Geology and Geophysics, Univ. Hawaii, 1680 East-West Rd., POST 810 , Honolulu, HI 96822 United States
AU: Garcia, M O
V54A-07 AF: Dept. of Geology and Geophysics, Univ. Hawaii, 1680 East-West Rd., POST 810 , Honolulu, HI 96822 United States
AB: We propose that secondary volcanism at Hawaii is a direct consequence of flexural uplift caused by the viscoelastic response of the lithosphere to a growing volcanic load. Beneath a growing volcano lithosphere flexes downward as the plume experiences partial melting in the hotspot melting zone. Away from the volcanic load, lithosphere flexes upward, decompressing the plume material below (which is near its solidus and at critical porosity) to cause a small amount of melting. The model successfully predicts the observed timing and positioning of onshore (Hawaiian rejuvenation) and offshore (Hawaiian arch) secondary volcanism with the distance to the flexural arch predicted for an effective elastic plate thickness of 25-35 km. We model flow in the asthenosphere beneath the arch as an axisymmetric, isoviscous half-space. Upwelling rate is greatest near the lithosphere-asthenosphere boundary, and this result is employed in the melting model. The melt model predicts realistic magma fluxes if magma is focused to individual eruption sites from an area two to ten times the eruption area. To address the isotopic distinction between secondary and shield stage volcanism, we assume that plume material is chemically and isotopically heterogeneous. Since flow driven by flexural uplift is greatest in the shallow melting zone, secondary volcanism mostly melts shallow melting (or depleted) components, which we assume are relatively depleted in incompatible elements, have a high 143Nd/144Nd, and a low 87Sr/86Sr. In contrast, buoyancy-driven asthenospheric flow beneath the shield is most rapid deep in the melting zone, and thus shield volcanism mostly melts components that begin melting deepest, which we assume are relatively enriched in incompatible elements, have a low 143Nd/144Nd, and a high 87Sr/86Sr. Thus the model predicts a mean high 143Nd/144Nd and low 87Sr/86Sr for secondary lava and a mean low 143Nd/144Nd and high 87Sr/86Sr of shield stage lava by melting the same plume material in different melt zone dynamics. With the same model, we predict that secondary magmas are formed by a lower mean degree of partial melting than shield stage magmas, which is consistent with the alkalic versus tholeiitic distinction observed between secondary and shield stage eruptions of Hawaii volcanoes. To collectively predict isotopic and mean extent of partial melting characteristics consistent with observations, the plume must contain some pyroxenite that must begin melting deeper than dry peridotite.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 3075 Submarine tectonics and volcanism
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005