The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets

The Lutetium-Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu-Hf system parameters need to be more tightly constrained, in particular the Lu-Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu-Hf and Sm-Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5. and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu-Hf isotopic composition of CHUR. Our new determinations of Lu-Hf CHUR parameters are Lu-176/Hf-177=0.0336 +/- 1 and Hf-176/Hf-177=0.282785 +/- 11 (2 sigma(m)), which are higher than previous estimates and, together with average Sm-Nd chondrite compositions of unequilibrated chondrites Of Sm-147/Nd-144=0.1960 +/- 4 and Nd-143/Nd-144=0.512630 +/- 11(2 sigma(m)), now provide firm constraints on the chondritic parameters for both Lu-Hf and Sm-Nd isotopic systems. A comparison of Lu-Hf and Sm-Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu-Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the Lu-176 decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history. (C) 2008 Elsevier B.V. All rights reserved.