HR: 17:45h
AN: V52F-08    [PDF]
TI: Decoupling of $\delta$Lu/Hf and $^{230}$Th/$^{238}$U in the Galapagos Lavas: The Who is Who in the Garnet Signature
AU: * Saal, A E
EM: asaal@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912 United States
AU: Blichert-Toff, J
EM: jblicher@ens-lyon.fr
AF: Ecole Normale Sup‚rieure de Lyon, 46, Allee d'Italie, Lyon, 69364 France
AU: Kurz, M
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Whole Road, Woods Hole, MA 02543 United States
AB: High $\delta$Lu/Hf and ($^{230}$Th/$^{238}$U) disequilibrium have been used as indicator of mantle melting in the garnet stability field. In contrast to our expectations, the Galapagos samples with high $^{230}$Th excess have low $\delta$Lu/Hf. 30 samples previously analyzed for Th isotopes (ranging from 0.98 to 1.484) have been analyzed for Hf isotopes ($\epsilon$Hf ranging from 8.49 to 13.16; $\delta$Lu/Hf 0.75 to 0.2). $\epsilon$Hf and $^{230}$Th excess increase, while $\delta$Lu/Hf and (Sm/Yb)$_{PM}$ ratios decrease with increasing distance from the center of the plume (considered to be Fernandina based on $^{3}$He/$^{4}$He isotopes). The increase in $\epsilon$Hf and decrease in $\delta$Lu/Hf and (Sm/Yb)$_{PM}$ ratios has previously been explained by a decrease in the depth of mantle melting, from the garnet to the spinel stability field, associated with an increase in the asthenospheric component in the source of melts erupted at increasing distance from the center of the plume. However, this hypothesis cannot explain the increase in $^{230}$Th excess. Evaluation of trace element composition and Sr, Nd, Pb, Hf, He and Th isotopes of Galapagos lavas indicates that magma mixing between plume and asthenosphere melts has been the main process responsible for the geochemical variation observed in the archipelago. The correlation between isotopes and trace element ratios (such as Nb/La, Nb/Zr, Ba/La, K/Rb and Ba/Ce) indicates that magma mixing controls most of the trace element variations in the basalts. The correlations between ($^{230}$Th /$^{232}$Th) and Sr, Nd, Hf and Pb isotopes confirm that the Th isotopic ratios are mainly source-controlled and are the result of magma mixing between the plume and asthenosphere melts. The high ($^{230}$Th/$^{238}$U) disequilibrium found in Galapagos lavas indicates that they originated total or partially in the garnet stability field. The decoupling between the ($^{230}$Th/$^{238}$U) excess and $\delta$Lu/Hf can be accounted for by mixing of low extent of melting from a plume source in the garnet stability field with magmas produced by relatively large extents of melting from the asthenosphere in the spinel stability field. This mixing model illustrates that the mixture carries a garnet signature in the most incompatible elements (such as U, Th) even though the variation of the MREE (and Hf) to HREE does not reveal any indication of melting in the garnet stability field.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting