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 176Lu/177Hf = 0.0336 ± 1 and 176Hf/177Hf = 0.282785 ± 11 (2σm), which are higher than previous estimates and, together with average Sm-Nd chondrite compositions of unequilibrated chondrites of 147Sm/144Nd = 0.1960 ± 4 and 143Nd/144Nd = 0.512630 ± 11 (2σm), 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 176Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.