HR: 17:20h
AN: V44B-06    [Abstracts]
TI: Mantle Plume Upwelling Rates: Evidence from U-Series in Young Ocean Island Basalts
AU: * Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: Lab. Geochimie Cosmochimie IPGP-CNRS, 4 Place Jussieu, Paris, 75252 France
AU: Turner, S P
EM: sturner@els.mq.edu.au
AF: GEMOC, Dept of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109 Australia
AU: Stracke, A
EM: stracke@mpch-mainz.mpg.de
AF: Abt. Geochemie, Max-Planck-Institut fur Chemie, Mainz, 55128 Germany
AU: Saal, A E
EM: Alberto_Saal@brown.edu
AF: Dept of Geological Sciences, Brown Universit, Providence, RI 02912 United States
AB: U-series disequilibria measured in recent lavas at intraplate volcanoes provide a powerful probe to examine the validity of the plume model. U-Th and U-Pa fractionation produced during melting is a function of the melting rate. In turn, this parameter should scale with mantle upwelling velocities. Simply stated, a larger melting rate (larger mantle upwelling velocity) yields smaller Th and Pa excess relative to their parent nuclides. A number of observations supports this approach: (1) there is a negative correlation between $^{230}$Th excess and buoyancy fluxes (2) based on new measurements of $^{231}$Pa in the Azores, Iceland and the Galapagos and literature data, we show here that there is also a well defined correlation between $^{231}$Pa excess and buoyancy flux (3) For Hawaii, Iceland and the Azores, $^{230}$Th excess (or $^{231}$Pa excess) increases as a function of the distance from the centre of the `hotspot'. These observations suggests that `hotspot' buoyancy fluxes are associated with a greater melt production per unit of time and that the centre of `hotspot' corresponds to a faster mantle upwelling velocity than its periphery. This is therefore in strong support of a model where ocean islands are associated with faster upwelling at depth. However, there is in fact not a simple relationship between melt productivity and upwelling velocities. Notably, the presence of volatiles, of mafic lithologies or of variably enriched peridotitic source could all affect melting rate and hence U-Th-Pa fractionation. We have considered these issues in great detail using a large data base for the Azores islands. While there are clear variations in mantle source composition, they cannot explain the observations of increasing $^{231}$Pa/$^{235}$U ratio with distance from the centre of the Azores hotspot . If we take into account the effect of water in the source of the Azores, it clearly affects the scaling between U-series fractionation and upwelling velocity but not the overall conclusions.
DE: 8450 Planetary volcanism (5480)
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting