HR: 0830h
AN: S21E-0360    [PDF]
TI: Origin of Upper Mantle Mid-Ocean Ridge Basalt Source Isotope Signatures: Hafnium and Lead Isotope Constraints
AU: * Hanan, B
EM: bhanan@mail.sdsu.edu
AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182-1020 United States
AU: Blichert-Toft, J
EM: jblicher@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, 46 Allee d'Italie, Lyon Cedex 7, 69364 France
AU: Pyle, D
EM: pyled@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd, Honolulu, HI 96882 United States
AU: Christie, D
EM: dchristie@coas.oregonstate.edu
AF: Oregon State University, 104 Ocean Admin Building, Corvallis, OR 97331 United States
AB: The origins of the geochemical heterogeneity of the upper mantle are fundamental constraints for mantle dynamics. An important requirement to understanding the composition and origin of mantle plumes is a knowledge of the extent and origin of MORB source heterogeneity. The Australian-Antarctic Discordance is a geochemical boundary between Indian-type and Pacific-type upper mantle provinces that is unaffected by plume-ridge interaction. The juxtaposition of the two distinct mantle domains makes this an excellent location to explore and contrast the nature and extent of upper mantle geochemical heterogeneity. Indian and Pacific Ocean MORB define two distinct bisecting arrays in a diagram of $^{206}$Pb/$^{204}$Pb versus $\epsilon$Hf. Basalts with ultra-depleted trace element signatures (e.g., low La/Sm) from the Indian side of the boundary represent one end-member of the Indian MORB array. These Indian ultra-depleted MORB have a unique isotope signature, with very high $\epsilon$Hf and $\epsilon$Nd and low $^{206}$Pb/$^{204}$Pb. Similarly, ultra-depleted Pacific EPR-MORB define one end-member of the Pacific MORB array. Although these Pacific ultra-depleted MORB have very similar $\epsilon$Nd and $^{206}$Pb/$^{204}$Pb to the ultra-depleted Indian-type, they have significantly lower $^{87}$Sr/$^{86}$Sr, $\epsilon$Hf and $^{207}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb (for a given $^{206}$Pb/$^{204}$Pb). All of these ultra-depleted MORB may represent re-melting of a heterogeneous source that previously supplied MORB melt to the adjacent mid-ocean ridges. The divergence of the isotope signatures of Indian- and Pacific-type MORB, including the ultra-depleted basalts, requires long-term differences in Rb/Sr, U/Pb, Th/Pb, Sm/Nd, and Lu/Hf parent-daughter ratios. The isotopic contrasts could be explained by different source compositions and/or ages of origin. Alternatively, the crossing quasi-linear isotopic trajectories are consistent with different pollution mechanisms of the upper mantle by recycled components. The Indian-type isotope signature may result from delamination and stirring of garnet facies continental material during rifting, while the Pacific-type may originate from processes associated with slab subduction, or by pollution with plumes containing recycled slab material.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Seismology [S]
MN: 2003 Fall Meeting