HR: 14:10h
AN: V22E-03    [PDF]
TI: The Coupled Hf-Nd Isotopic Perspective on Crust-Mantle Evolution
AU: * Vervoort, J D
EM: vervoort@wsu.edu
AF: Dept. Geol., Washington State Univ., Pullman, WA 99164 United States
AU: Patchett, P J
EM: patchett@geo.arizona.edu
AF: Dept. Geosci., Univ. Arizona, Tucson, AZ 85721 United States
AU: S”derlund, U
EM: ulf@geo.arizona.edu
AF: Dept. Geosci., Univ. Arizona, Tucson, AZ 85721 United States
AB: The Sm-Nd and Lu-Hf isotope systems have been valuable tools in trying to understand the differentiation and evolution of bulk silicate Earth (BSE). Together these systems allow us to constrain important processes such as the evolution of the crust and mantle as well as provide a basis for quantifying bulk Earth isotopic mass balance. Linking these systems is especially important for examination of the early Earth as they complement each other and provide independent records in rocks with long and complex tectonothermal histories. Ultimately, the utility of these systems in this regard will depend on how well we know both the decay constants for $^{147}$Sm and $^{176}$Lu as well as the Lu-Hf and Sm-Nd isotopic composition of BSE. Both sets of parameters, however, have been the subject of recent debate and this uncertainty has prevented these systems from achieving their full potential. Recent work on the $^{176}$Lu decay constant by cross-calibration of U-Pb and Lu-Hf isotope systems on mineral isochrons in terrestrial rocks [1,2] have determined values (1.865 x 10$^{-11}$y$^{-1}$) considerably lower than the value currently in use (1.93 x 10$^{-11}$y$^{-1}$) and lower still than the value determined from examination of various meteorite groups [3,4]. The value of 1.865 x 10$^{-11}$y$^{-1}$, if true, would result in initial $\epsilon$$_{Hf}$ values for the Early Archean Greenland gneisses [5] to be on average chondritic instead of having a depleted mantle signature ($\epsilon$$_{Hf}$=$^{+}$2-$^{+}$4). While this is certainly plausible, it conflicts with the dominantly depleted signature in the Nd isotopic record for these rocks. More significantly the lower value for $\lambda^{176}$Lu would result in the Jack Hills detrital zircons [6] having initial $\epsilon$$_{Hf}$ of $^{-}$2 to $^{-}$4 at 4.0 Ga which would have profound implications for the existence of continental crust at or near 4.4 Ga. One notable aspect of the existing Hf-Nd isotopic record from Earth's reservoirs is the apparent mismatch of $\sim$3 $\epsilon$$_{Hf}$ units between the current BSE value (based on an average of ordinary and carbonaceous chondrites) and the center of the Hf-Nd terrestrial array, which would seem to require a hidden reservoir in order to achieve isotopic mass balance [7]. Recent work [8] has demonstrated a large range in Lu-Hf isotopic composition in chondrites and a systematic difference between carbonaceous and ordinary chondrites. Therefore, unlike the Sm-Nd system, there is considerable latitude in how the Lu-Hf chondritic parameters are chosen. The average of carbonaceous chondrites are $\sim$7 $\epsilon$$_{Hf}$ units higher than ordinary chondrites and $\sim$3 $\epsilon$$_{Hf}$ units higher than the current chondritic value. If BSE is closer to that of carbonaceous chondrites, the BSE point would lie in the middle of the terrestrial array and solve the Hf-Nd isotopic mass balance problem. A higher $^{176}$Hf/$^{177}$Hf value, however, would have little effect on Archean initial Hf values because of the correspondingly higher chondritic $^{176}$Lu/$^{177}$Hf. The lack of coherence between the Hf and Nd records for the early Archean demonstrates that there are still some critical unresolved issues that need to be addressed. [1] Scherer et al., 2001., Science, 293: 683-686. [2] S\"{o}derlund et al., in review, EPSL. [3] Bizzarro et al, 2003, Nature, 421: 931-933. [4] Blichert-Toft et al., 2002, EPSL, 204: 167-181. [5] Vervoort and Blichert-Toft, 1999, GCA, 63:533-556. [6] Amelin et al., 2000, GCA, 64: 4205-4225. [7] Blichert-Toft and Albar\`{e}de, 1997, EPSL, 148: 243-258. [8] Patchett et al., in review, EPSL.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting